Category 6™

Gulf of Mexico's Hermine Finally Gets its Name; Hurricane Madeline Lashing Hawaii

By: Jeff Masters and Bob Henson , 10:50 PM GMT on August 31, 2016

Tropical Depression Nine in the Gulf of Mexico finally got its act together enough to deserve a name, the NOAA Hurricane Hunters discovered on Wednesday afternoon. They found top sustained winds of 45 mph in Tropical Storm Hermine, ending a week-long drama that left us all wondering if someone had cast a “hold” spell on the storm. However, the aircraft found that the storm’s central pressure remained a fairly high 1004 mb, and Hermine has a lot of organizing to do before it can become a hurricane. Late Wednesday afternoon, the strong winds from Hermine were already creating storm surge heights over 1’ along the entire Gulf Coast from New Orleans, Louisiana to Naples, Florida. The maximum surge was just over 2’ at Cedar Key, Florida on Wednesday afternoon. Satellite images on Wednesday afternoon showed a much more organized storm, with heavy thunderstorms building near the storm’s center and some significant low-level spiral bands forming. Wind shear continued to be a moderate 10 - 15 knots on Wednesday afternoon, but water vapor satellite imagery still showed plenty of dry air to the storm’s north and west; the combination of these factors is likely slowing down the intensification process. Sea surface temperatures (SSTs) near Hermine’s center remained favorable for development, near 30.5°C (87°F).


Figure 1. MODIS visible satellite image of Hermine on Wednesday afternoon, August 31, 2016. Note the large area of intense thunderstorms to the northeast of the storm, over the west coast of Florida. This chunk of moisture and its associated spin are from a non-tropical low pressure system, and appear poised to get entrained into the circulation of Hermine. When this happens, Hermine will expand considerably in size, and may dump heavier rains than are currently forecast. Image credit: NASA.

Intensity forecast: Hermine may become a Category 1 hurricane
The SHIPS model on Wednesday afternoon predicted moderately favorable conditions for intensification, with wind shear staying a moderate 10 - 15 knots through landfall on Thursday evening. SSTs will be a very warm 30 - 30.5°C (86 - 87°F), and mid-level relative humidity was predicted to be a reasonably moist 65%. Our three best intensity models--the HWRF, DSHIPS and LGEM models--were in good agreement with their latest runs available Wednesday afternoon, with landfall intensities for Hermine ranging from 75 - 80 mph—Category 1 hurricane strength. NHC is going with a forecast of a high-end 70 mph tropical storm at landfall. The Gulf Coast of Florida is highly vulnerable to large storm surges, due to the extensive stretch of shallow continental shelf waters offshore that extend up to 90 miles from the coast. On Wednesday afternoon, NHC increased their maximum storm surge forecast to 4 - 6’ above ground along a stretch of the Florida coast to the right of where the center is expected to make landfall.

Track forecast for Hermine: a Florida Gulf Coast landfall, followed by a run up the Southeast coast
The latest Wednesday morning runs of our top models are in solid agreement that Hermine will make landfall along the Florida Big Bend coast north of Tampa on Thursday evening. In their 5 pm EDT Wednesday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from Hermine along the Gulf Coast of Florida were 76%, 74%, and 64%, respectively, for Apalachicola, St. Marks and Panama City, Florida.

After landfall, Hermine will begin transitioning to a powerful extratropical storm as it sweeps northeastward along the coast, and begin deriving energy from atmospheric dynamics, rather than from the heat energy of the ocean. This extra energy source should allow Hermine to maintain tropical storm intensity as it traverses the Southeast U.S. In their 5 pm EDT Wednesday Wind Probability Forecast, NHC gave odds of at least 25% for tropical storm-force winds affecting the entire U.S. coast from northern Florida to Delaware, including Washington D.C.


Figure 2. Radar estimated rainfall between August 29 - and 6:08 pm EDT August 31, 2016 from the Tampa radar. Swaths of 2 - 4” of rain (yellow colors) were common over Florida, with 4 - 6” near Tampa.

Extremely rich moisture available to Hermine
Near record-warm ocean temperatures in the Gulf of Mexico are evaporating near-record amounts of water vapor into the atmosphere for Hermine to feed off of. At 8 am EDT Wednesday, the upper-air balloon sounding at Tampa, Florida measured 2.5” of total precipitable water (TPW)—the amount of water that would result if one condensed all the water vapor in a column above and precipitated it out. This value ranked in the upper 1% of all TPW measurements taken at the site since 1948. According to the National Weather Service, Tampa’s all-time greatest precipitable water sounding was 2.85” on September 6, 2004, when Hurricane Frances was crossing Florida (though SPC lists one higher value around 3.1”, year unknown). TPW values close to that record level were analyzed by satellite over the Gulf of Mexico on Wednesday afternoon, so it is possible Tampa may challenge its TPW record in the next day or two (thanks go to Sheldon Kusselson for some of these links).

Very heavy rains have already occurred in many locations across Florida with Hermine. A more concentrated band of 2” - 6” rains, with local totals perhaps exceeding 10”, can be expected across far northern Florida and far southeast Georgia as Hermine moves ashore. Beyond that point, the divergent model guidance keeps us guessing, but there is a good chance of very heavy rain along the Southeast coast, perhaps extending into central parts of the Carolinas. If Hermine approaches the mid-Atlantic coast and slows as much as some model runs are predicting, very heavy rains are possible in the Delmarva area (Delaware, eastern Maryland, and eastern Virginia). This area could extend north over the holiday weekend depending on Hermine’s eventual track.

Figure 3. Rainfall totals from Hurricane Agnes, which slogged through the eastern United States in late June 1972.Image credit: NOAA/NWS Weather Prediction Center.
 

In June 1972, Hurricane Agnes (only briefly a hurricane) took a largely-inland track from the Florida Panhandle into the mid-Atlantic, then moved back offshore before a second landfall near New York City. As it interacted with a strong midlatitude trough, Agnes produced massive flooding, at least 128 deaths, and $3 billion in damage (1972 dollars), making it the most expensive U.S. hurricane on record up to that point. The point is not that Hermine would mirror Agnes precisely (the upper-level forecasts are far from identical, and Agnes was much larger than Hermine), but that even a minimal hurricane or tropical storm that spends much of its life inland can cause huge amounts of rain if it moves very slowly or interacts with other weather features. We will need to be ready for a potentially significant rain/flood event if Hermine decides to slow significantly and hang out near the U.S. East Coast.

A weakening Madeline edges toward Hawaii
Hawaii’s Big Island is catching a break as Hurricane Madeline—a Category 4 storm at its peak— has been losing much of its power en route to the island. Located about 95 miles southeast of Hilo at 5 pm EDT (10 am HST) Wednesday, Madeline has weakened dramatically over the last 24 hours, with top sustained winds now down to minimal hurricane strength, 75 mph. That trend is expected to continue until Madeline is safely past Hawaii, although some restrengthening is possible as Madeline approaches Johnston Atoll. Northeast winds were gusting to 30-35 mph just after 11 am HST at Discovery Harbor, near the south tip of the Big Island. At an airport near Waimea (elevation 2600 feet), storm force winds of 43 mph, gusting to 52 mph, were reported.


Figure 4. Rainbands arc around the center of Madeline, well southeast of the Big Island, at 1716Z (5:16 pm EDT and 11:16 am HST) Wednesday, August 31, 2016.

Madeline is moving just south of due west at 13 mph, a trajectory that should take it south of the Big Island but within 100 miles of it. A tropical storm warning remains in effect for the islands of Hawaii, Maui, Molokai, Lanai and Kahoolawe. Even the weakened version of Madeline could deliver quite a punch to Hawaii, especially the Big Island. Surf as high as 25 feet can be expected along the Big Island’s east- and south-facing coastlines. Torrential rains could lead to flooding and mudslides in some locations as Madeline’s winds slam against the Big Island’s mountainous terrain. Rainfall amounts could exceed 15” along east-facing slopes.

The best place to find frequently updated local statements on Madeline’s expected impact is at a dedicated website maintained by the National Weather Service office in Honolulu. See our Wednesday morning post for more background on Hawaii’s hurricane history and what the future may have in store.


Figure 5. Enhanced infrared image of Hurricane Madeline (left) and Hurricane Lester (right) at 2013Z (4:13 pm EDT and 10:13 am HST) Wednesday, August 31, 2016. Image credit: NASA/MSFC Earth Science Office.

Category 4 Lester still a threat to Hawaii
About 1000 miles east of Hilo, Hurricane Lester continues to blast the open ocean with top sustained winds of 130 mph as of the 5 pm EDT advisory from NHC. (Advisories on Lester will be issued by CPHC starting at 11 pm EDT, as the hurricane moves west of 140°W into that agency’s area of responsibility.). Computer models agree that Lester’s westward path will start bending toward the west-northwest by Thursday, with the hurricane gradually weakening as it encounters greater wind shear and waters churned up by Madeline. The NHC outlook has Lester as a Category 1 hurricane on Saturday, as it approaches the longitude of the Big Island.


Figure 6. The official NHC forecast track of Hurricane Lester as of 2100Z (5:00 pm EDT) Wednesday.

Models agree that Lester should parallel the Hawaiian island chain as it moves west-northwest, but they disagree on how close it may come to the state. The 12Z Wednesday runs of the GFS and UKMET models bring Lester very near Oahu and Kauai on Sunday. The 12Z ECMWF keeps the storm about 100-200 miles north of the islands, as do the HWRF and GFDL models. Lester will most likely be a strong tropical storm or a Category 1 hurricane by this point. One positive factor is that a track north of the islands would put them on the weaker left-hand (south) side of Lester, reducing the potential impact. Huge surf is a safe bet.

It’s still uncertain exactly how close Lester’s path will be to Hawaii, especially Oahu, so residents throughout the state need to keep tabs on this powerful storm. On Wednesday afternoon, the NCAR/NSF Gulfstream-V jet was gathering data on the environment around Lester. The data will feed into the next round of computer model guidance (00Z Thursday), hopefully giving us more clarity on Lester’s weekend track.


Figure 7. The NCAR/NSF Gulfstream-V is assisting in high-altitude hurricane monitoring acoss the Atlantic and Pacific through mid-October while NOAA’s Gulfstream-IV undergoes unscheduled maintenance. Image credit: UCAR, photo by Carlye Calvin.

Links
Regional Hawaii radar
Mauna Kea weather (elevation 13,796’)
Weather on Mauna Kea
Live stream from KHON2 TV in Honolulu
Central Pacific Hurricane Center
2-km resolution WRF model output from the University of Hawaii for Hawaii
Storm surge maps for Oahu
Storm info from Tropical Tidbits
NWS Honolulu


Figure 8. MODIS visible satellite image of Invest 92L on Wednesday morning, August 31, 2016. The tropical wave was embedded in a large area of African dust to its west and north. Image credit: NASA.

92L off the coast of Africa embedded in dry air
A large tropical wave with plenty of spin that emerged from the coast of Africa on Monday was designated Invest 92L by NHC, but NHC is no longer issuing their suite of model forecasts for the system, due to the system’s lack of potential for development. The wave was just west of the Cabo Verde Islands on Wednesday, and was embedded in a major area of dust and dry air from the Sahara Desert. This dry air will greatly interfere with development over the coming days as 92L heads west at 15 - 20 mph across the tropical Atlantic. The latest 12Z Wednesday runs of our three top models for forecasting tropical cyclone genesis--the GFS, European and UKMET models—had one of the three, the UKMET, showing development of the system over the next five days. A strong and persistent ridge of high pressure should keep 92L on a fairly straightforward west to west-northwest path, and the storm will likely move through or just north of the Lesser Antilles Islands on Sunday. In their 2 pm EDT Tuesday Tropical Weather Outlook, NHC reduced their 2-day and 5-day development odds to 0% and 30%, respectively.

We’ll be back with a new post late Thursday morning.

Bob Henson and Jeff Masters

Hurricane

Hurricane Watch Continues for TD 9; Hawaii Hunkers Down for Madeline

By: Bob Henson and Jeff Masters , 4:20 PM GMT on August 31, 2016

A Hurricane Watch continues for the Florida Gulf Coast from the Anclote River to Indian Pass, and Tropical Storm Watches have been issued for portions of the Florida and Georgia Atlantic coasts as Tropical Depression Nine sits nearly stationary in the Gulf of Mexico, deciding what to do next. Satellite images on Wednesday morning showed a significant change to TD 9’s organization, with a huge blow-up of heavy thunderstorms over the storm’s center, which created a central dense overcast (CDO) of high cirrus clouds. A CDO is typically the classic sign of a tropical cyclone undergoing significant intensification, but this particular cloud signature is different from the classic CDO, and is often associated with a storm in an arrested state of development (see the discussion of "Central Cold Cover" after Figure 7 in the original 1984 satellite classification paper by Dvorak, 1984.)

There have been no hurricane hunter missions into TD 9 since Tuesday night, so NHC elected not to upgrade TD 9 to Tropical Storm Hermine. The Hurricane Hunters were scheduled to fly a number of missions into TD 8 and TD 9 since last night, but no missions flew. They may be over-extended and suffering mechanical issues, due to the grueling requirements of flying three simultaneous storms over a multi-day period in both the Atlantic and Pacific. Starting this week, the National Center for Atmospheric Research is sending the NSF/NCAR Gulfstream-V aircraft to sample the large-scale environment around Atlantic storms through October while NOAA’s Gulfstream-IV undergoes maintenance. As of 1 pm EDT Wednesday, a NOAA hurricane hunter aircraft had begun a mission into TD 9.

Buoy 42003, located about 200 miles to the northeast of TD 9’s center, has seen a steady increase in wind speed on Wednesday morning. At 11 am EDT, winds at the buoy were 31 mph, gusting to 38 mph. The strong winds from TD 9 were already creating a two-foot storm surge at Cedar Key, Florida on Wednesday morning. A nearby ship measured sustained winds of 42 mph at 8 am EDT, but this measurement may have been made at a height much higher than the standard 10 meters above the surface used for classifying systems as tropical storms. Wind shear continued to be a moderate 10 - 15 knots on Wednesday morning, but water vapor satellite imagery still showed plenty of dry air to the storm’s north and west; the combination of these factors is likely slowing down the intensification process. Sea surface temperatures (SSTs) near TD 9’s center remained favorable for development, near 30.5°C (87°F). 


Figure 1. The view of TD 9 from the cockpit of NOAA hurricane hunter aircraft N43RF (Miss Piggy) on an August 30, 2016 flight. Image credit: NOAA/AOML/HRD Twitter feed.


Figure 2. Radar estimated rainfall between August 29 - August 31, 2016 from the Tampa radar. Swaths of 2 - 4” of rain (yellow colors) were common over Florida.

Track forecast for TD 9: a Florida Gulf Coast landfall, followed by a run up the Southeast coast
The latest Wednesday morning runs of our top models are in solid agreement that TD 9 will make landfall along the Florida Big Bend coast north of Tampa on Thursday afternoon or evening. In their 11 am EDT Wednesday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from TD 9 along the Gulf Coast of Florida were 59%, 56%, and 55%, respectively, for Apalachicola, St. Marks and Cedar Key, Florida.


Figure 3. Projected 7-day rainfall from 12Z (8:00 am EDT) Wednesday, August 31, through 12Z Wednesday, September 7, 2016. Rainfall amounts of 5 - 10” are expected along TD 9’s path across Florida and along the Southeast U.S. coast. Heavy rains from the system may also affect Southeast Massachusetts on Labor Day. Image credit: NOAA/NWS Weather Prediction Center.

A Labor Day spoiler for the Mid-Atlantic and Northeast U.S.?
Once TD 9 crosses over Florida and southern Georgia and enters the Atlantic again, the storm will be directly over the axis of the very warm Gulf Stream current. This will make the storm resist weakening, despite the expected presence of high wind shear in excess of 20 knots. The storm will also begin transitioning to a powerful extratropical storm at that time, and begin deriving energy from atmospheric dynamics, rather than from the heat energy of the ocean. In their 11 am EDT Tuesday Wind Probability Forecast, NHC gave odds of at least 25% for tropical storm-force winds affecting the Southeast U.S. coast from Daytona Beach, Florida northwards to Cape Hatteras, North Carolina--including the entire coasts of Georgia and South Carolina.

The official NHC forecast at 11 am EDT Wednesday had TD 9 headed northeastward out to sea, caught in the steering flow of a trough of low pressure, after clearing the coast of North Carolina on Saturday. TD 9 is likely to become a powerful extratropical storm with 60 - 65 mph winds by Sunday. However, the extratropical storm expected to emerge from TD 9 may turn north and spoil the Labor Day plans of millions of people in the Northeast U.S. The latest 06Z (2 am EDT) and 12Z (8 am EDT) Wednesday runs of the GFS model predicted that the trough pulling TD 9 to the northeast would quickly be replaced by a ridge of high pressure, resulting in a northward turn by TD 9 followed by a stall after passing by North Carolina. This track would threaten the Mid-Atlantic coast with heavy rain on Saturday and Sunday, spreading into the Northeast coast on Sunday and Monday. In their latest 0Z Wednesday (8 pm EDT Tuesday) runs, the European and UKMET models did not go along with this idea as strongly--predicting that TD 9 would stay well away form the Mid-Atlantic coast--but they had the storm potentially threatening Southeast Massachusetts on Monday (Labor Day.)

The bottom line: there is an unusually large amount of uncertainty with TD 9’s possible track over the weekend, and the storm might be capable of bringing heavy rain and strong winds--potentially at tropical storm force--to the New England and Mid-Atlantic coasts.


Figure 4. Screen shot of NHC’s interactive Storm Surge Probability product from 5 am EDT Wednesday, August 31, 2016, showing the probability of inundation in excess of 4’ above ground level from TD 9. A 100-mile stretch of the Florida Gulf Coast to the right of where the center of TD 9 is expected to make landfall is predicted to have a 60 - 70% chance of getting a inundation in excess of four feet (orange colors). The odds were around 40% near Savannah, Georgia. The graphic is based upon an ensemble of Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model runs created using the current National Hurricane Center (NHC) official hurricane advisory. Storm surge probabilities depend on the historical accuracy of NHC's forecasts of hurricane track, and wind speed, and an estimate of storm size.

Intensity forecast: TD 9 likely to stay below hurricane strength
The SHIPS model on Wednesday morning predicted moderately favorable conditions for intensification, with wind shear staying a moderate 10 - 15 knots through landfall on Thursday afternoon. SSTs will be a very warm 30 - 30.5°C (86 - 87°F), and mid-level relative humidity was predicted to be a reasonably moist 65%. Our three best intensity models--the HWRF, DSHIPS and LGEM models--were in reasonable agreement with their latest runs available late Wednesday morning, with landfall intensities for TD 9 ranging from 55 - 70 mph. NHC is going with a forecast of a 65 mph tropical storm at landfall. The Gulf Coast of Florida is highly vulnerable to large storm surges, due to the extensive stretch of shallow continental shelf waters offshore that extend up to 90 miles from the coast. On Wednesday morning, NHC was calling for a maximum storm surge of 3 - 5’ above ground from TD 9 along a stretch of the Florida coast to the right of where the center is expected to make landfall. The other main hazard from TD 9 is heavy rain--rainfall amounts of 5 - 10” are expected along TD 9’s path across Florida, with 15” possible near the coast where the storm makes landfall. We also cannot rule out a few tornadoes from the storm.


Figure 5. Infrared satellite image of Hurricane Madeline at 1400Z (10:00 am EDT) Wednesday, August 31, 2016. Image credit: NOAA/NESDIS.

Weakening Madeline still on track to sweep past Hawaii’s Big Island
Hurricane Madeline is taking a turn for the better as far as Hawaiians are concerned. Madeline has weakened dramatically over the past 24 hours, losing its well-defined eye and becoming much less organized (see image above). After topping out with 135-mph sustained winds on Tuesday morning, Madeline’s top winds were down to 80 mph as of the 11 am EDT advisory from the Central Pacific Hurricane Center (CPHC). Now a Category 1 storm, Madeline is ingesting fairly dry mid-level air (relative humidities of 30-40%) in moderate wind shear of 15-20 knots. All of these factors should keep Madeline’s intensity on a downward path, even as it moves over steadily higher SSTs.

Another piece of good news: the much-anticipated leftward swing in Madeline’s path now appears more likely to keep the center from making landfall on the Big Island. All of the major computer models now keep Madeline’s track just south of the island. Madeline was only about 140 miles east-southeast of Hilo at 5:00 am HST (11:00 am EDT). The latest CPHC outlook brings Madeline within about 50 miles of the southeast coast of the Big Island this afternoon as a minimal hurricane. Since hurricane-force winds extend ony about 10 miles from Madeline’s center, such a track would keep those winds offshore, although gusts could still reach 80 - 100 mph in a few spots. The most likely outcome for the bulk of the Big Island is widespread tropical-storm-force winds sustained at 40 - 70 mph, strongest toward the south. A Hurricane Warning remains in effect for the Big Island, and a Tropical Storm Warning is now in effect for the islands of Maui, Molokai, Lanai and Kahoolawe.

Potential impacts from Madeline
The steep topography along the shore of the Hawaiian Islands tends to minimize storm surge while maximizing surf height. Surf as high as 25 feet can be expected along the Big Island’s east- and south-facing coastlines. A storm surge of 1-3 feet is possible along parts of Hawaii’s southern and eastern coast. Storm surge expert Hal Needham provided background on Hawaii’s surge and surf risk from tropical cyclones in a blog post on Tuesday afternoon. Heavy rains are already beginning to nudge into the Big Island, and torrential rains, landslides, and flooding are a distinct possibility. Localized rainfall amounts could exceed 15” along east-facing slopes as Madeline’s winds slam against the Big Island’s mountainous terrain. The best place to find frequently updated local statements on Madeline’s expected impact is at a dedicated website maintained by the National Weather Service office in Honolulu.

Overall, Madeline’s impact could match or exceed that of Hurricane Iselle (2014), another Category 4 storm that weakened quickly while approaching Hawaii. Iselle struck on the Big Island as a moderate tropical storm--the strongest landfall on record for the island--and caused 1 fatality and inflicted nearly $80 million in damage across Hawaii, qualifying as an agricultural disaster. This year’s Hurricane Darby came ashore on the Big Island as a 40-mph tropical storm on July 23, with widespread flooding but no deaths or major damage.

Hawaii’s hurricane history and its potential hurricane future
Apart from Iselle and Darby, only three other tropical cyclones have made landfall anywhere in Hawaii since records began in 1949:

--The state’s worst hurricane by far, Hurricane Iniki, which hit Kauai as a Category 4 hurricane, killed 6 and caused $1.8 billion in damage (1992 dollars.)

--Hurricane Dot, which hit Kauai as a Category 1 hurricane, caused 6 indirect deaths and $6 million in damage (1959 dollars.)

--An unnamed 1958 storm that brought sustained winds of 50 mph at landfall to the Big Island killed one person and caused $0.5 million in damage.

The region around Hawaii has seen a lot of tropical activity over the past four years, including a number of near-misses. Partly this is a result of El Niño, which warmed the waters of the tropical Central and Eastern Pacific where Hawaii-heading cyclones are born. However, the uptick may also be a harbinger of things to come. See the August 2014 post, Climate Change May Increase the Number of Hawaiian Hurricanes.

The winds at 13,000 feet on Mauna Kea
The weather on top of the highest point in Hawaii, the Big Island's Mauna Kea, elevation 13,796' (4,205 m), will be interesting to follow as Madeline passes the Big Island. Winds will rise steadily today, and there are six anemometers on top of the mountain to watch. However, beware of the data from the Canada - France - Hawaii Telescope (CFHT). The Mauna Kea webcam page says that those winds are highly exaggerated due to location of the anemometer tower between two large telescope domes. You can see this tower on Google Maps.

Links
Regional Hawaii radar
Weather on Mauna Kea
Live stream from KHON2 TV in Honolulu
Central Pacific Hurricane Center
2-km resolution WRF model output from the University of Hawaii for Hawaii
Storm surge maps for Oahu
Storm info from Tropical Tidbits
NWS Honolulu



Figure 6. Infrared satellite image of Hurricane Lester at 1400Z (10:00 am EDT) Wednesday, August 31, 2016. Image credit: NOAA/NESDIS.

Lester still lurking farther east of Hawaii
While Madeline has lost ground in the past day, Hurricane Lester remains a powerful and well-organized Category 4 storm. Lester was located about 1085 miles east of Hilo as of the 11 AM EDT Wednesday advisory from NHC. (Advisories on Lester will be issued by CPHC once the hurricane moves west of 140°W.) Lester’s core of intense storms has shrunk and become more asymmetric over the last few hours, but its top sustained winds have dropped only slightly, down to 130 mph in the most recent advisory from a Tuesday night peak of 140 mph.

Computer models agree that Lester’s westward path will start bending toward the west-northwest by Thursday, with the hurricane gradually weakening as it encounters greater wind shear and waters churned up by Madeline. Computer models are in close agreement on taking Lester just north of Hawaii during the weekend on a path closely paralleling the island chain. The NHC outlook positions Lester about 100 miles north of Oahu early Sunday as a strong tropical storm. It’s still uncertain exactly how close Lester’s path will be to Hawaii, especially Oahu, so residents throughout the state need to keep tabs on this powerful storm. One good thing: assuming no major southward shift in the path, Hawaii will lie on the weaker left-hand (south) side of Lester, which would reduce the potential impact. Huge surf is a safe bet.


Figure 7. Visible satellite image of Hurricane Gaston at 1015Z (7:15 am EDT) Wednesday, August 31, 2016. Image credit: NOAA/NESDIS.

Spectacular Gaston heads toward the Azores
For a good contrast with the underperforming, disheveled TD 9, you need look no further than Category 3 Hurricane Gaston (above), which has become a marvel of persistence and organization. Gaston didn’t hit its stride until it reached the subtropics, around 30°N. Here, Gaston lingered for days in very weak wind shear atop unusually warm waters (1-2°C above average), gathering strength even as it churned up deeper, cooler water. Gaston became a Category 3 storm for the second time on Tuesday night, matching its peak sustained winds of 120 mph from Sunday night. Fortunately, Gaston has remained well northeast of Bermuda, posing a threat only to shipping thus far. At 11 am EDT Wednesday, Gaston was located about 1150 miles west of Faiail Island in the Azores with top sustained winds of 115 mph.

Gaston has become a classic example of an annular hurricane--the type with a huge eye surrounded by a solid ring of thunderstorms, with little or none of the spiral banding typical of strong hurricane. Annular storms develop when enough mixing occurs between the eye and eyewall to allow the eye to expand. Because of their resilient structure, annular storms are less vulnerable to disruption and often maintain intensity longer than their peers. Gaston could remain a major hurricane into Thursday, when it will begin accelerating east-northeast over cooler waters amid increasing wind shear. The latest NHC outlook brings Gaston across the Azores late Friday into early Saturday as a strong tropical storm. NHC will likely be issuing a Hurricane Watch or Tropical Storm Watch for parts of the Azores later Wednesday Given its annular nature and the rarity of hurricanes in the Azores--only about one per decade, on average--Gaston could pack a punch should it pass over or near any of the widely scattered Azores islands. This year’s first Atlantic storm, Hurricane Alex, struck the island of Terceira in the central Azores on January 15 as a bizarrely out-of-season tropical storm in January.

92L off the coast of Africa embedded in dry air
A large tropical wave with plenty of spin that emerged form the coast of Africa on Monday was designated Invest 92L by NHC, but they are no longer issuing their suite of model forecasts for the system, due to the system’s lack of potential for development. The wave was just west of the Cabo Verde Islands on Wednesday, and was embedded in a major area of dust and dry air from the Sahara Desert. This dry air will greatly interfere with development over the coming days as 92L heads west at 15 - 20 mph across the tropical Atlantic. The latest 0Z Wednesday runs of our three top models for forecasting tropical cyclone genesis--the GFS, European and UKMET models--did not show any development of the system over the next five days. A strong and persistent ridge of high pressure should keep 92L on a fairly straightforward west to west-northwest path, and the storm will likely move through or just north of the Lesser Antilles Islands on Sunday. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 0% and 40%, respectively.

We’ll be back with a new post late Wednesday afternoon.

Bob Henson and Jeff Masters


Figure 8. Surface winds around Gaston late Tuesday, August 30, 2016, as estimated by the Advanced Scatterometer (ASCAT) aboard the European MetOp satellite. Image credit: NESDIS/STAR, via Grant Wise, @wise_wx.

Hurricane

Hurricane Warning for Hawaii, a Watch for Florida; TD 9 Headed Towards NE U.S.?

By: Jeff Masters and Bob Henson , 10:51 PM GMT on August 30, 2016

NHC has issued a Hurricane Watch for the Florida Gulf Coast from the Anclote River to Indian Pass, and a Tropical Storm Watch for the Florida Gulf Coast west of Indian Pass to the Walton/Bay County line. You’d wouldn’t guess from Tropical Depression Nine’s appearance on satellite imagery, though, that the storm could become a hurricane by Thursday. TD 9 struggled with dry air and wind shear all day Tuesday, and a NOAA hurricane hunter aircraft on Tuesday afternoon found that top sustained winds had remained near 35 mph, and the central pressure had remained constant at 1004 mb. TD 9 continued to bring heavy rains to western Cuba during the day Tuesday, though; Santa Lucia in Pinar Del Rio province reported a 36-hour rainfall total of 317.4 mm (12.50”) ending at 8 am EDT. Additional heavy rains of 3 - 5” are likely over western Cuba before TD 9 finally pulls away on Wednesday.


Figure 1. MODIS visible satellite image of Tropical Depression Nine in the southern Gulf of Mexico on Tuesday afternoon, August 30, 2016. Image credit: NASA.

Satellite images on Tuesday showed little change to TD 9 during the day, with only a small amount of heavy thunderstorm activity near the center of circulation, and a circulation center that was still partially exposed to view—the typical look of a tropical cyclone struggling with wind shear and dry air. Wind shear was a moderate 10 - 15 knots, but water vapor satellite imagery showed plenty of dry air to the storm’s north and west. Sea surface temperatures (SSTs) near TD 9’s center remained favorable for development, near 30  - 30.5°C (86 - 87°F).

Track forecast for TD 9: a Florida Gulf Coast landfall, followed by a run up the Southeast coast
The latest 12Z Tuesday (8 am EDT) runs of our top models are a little father to the north with their landfall locations, bringing TD 9 to the Florida Big Bend coast north of Tampa on Thursday afternoon. In their 5 pm EDT Tuesday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from TD 9 along the Gulf Coast of Florida were 52%, 51%, and 50%, respectively, for Cedar Key, Apalachicola and St. Marks, Florida.

A Labor Day spoiler for the Mid-Atlantic and Northeast U.S.?
Once TD 9 crosses over Florida (and potentially southern Georgia) and enters the Atlantic again, the storm will be directly over the axis of the very warm Gulf Stream current. This will make the storm resist weakening, despite the expected presence of high wind shear of 20 knots. In their 5 pm EDT Tuesday Wind Probability Forecast, NHC gave odds of at least 30% for tropical storm-force winds affecting the Southeast U.S. coast from Daytona Beach, Florida northwards to Cape Hatteras, North Carolina—including the entire coasts of Georgia and South Carolina. The official NHC forecast at 5 pm EDT Tuesday had TD 9 headed northeastward out to sea, caught in the steering flow of a trough of low pressure, after clearing the coast of North Carolina on Friday. However, that outcome is in doubt.


Figure 2. Wind forecast for TD 9 made at 12Z (8 am EDT) Tuesday, August 30, 2016. Both the European model (left) and GFS model (right) were predicting that TD 9 would be a tropical storm just off the Northeast U.S. coast over Labor Day weekend.

For over a day, the GFS model has been predicting that the trough pulling TD 9 to the northeast would quickly be replaced by a ridge of high pressure, resulting in a northward turn by TD 9 followed by a stall after passing by North Carolina. This track would threaten the Mid-Atlantic and Northeast U.S. coasts on Sunday and Monday with heavy rain and high surf. In their latest 12Z (8 am EDT) Tuesday runs, the European and UKMET models have also jumped on this bandwagon, with the European model showing a very strong ridge building over TD 9 that forces a sharp left turn by the storm, back towards New England, after TD 9 approaches the Canadian maritime provinces. Thus, it looking increasingly possible that TD 9 could bring heavy rain and strong winds—potentially at tropical storm-force—to a large swath of the U.S. East Coast over the busy Labor Day weekend. Even if the storm’s rains and winds don’t do any damage, the lost business from a holiday weekend washout would run into the tens of millions of dollars. The uncertainties are high at this point, though—the diameter of NHC’s cone of uncertainty for their 5-day track forecast is about 550 miles.

Intensity forecast: TD 9 likely to stay below hurricane strength
TD 9’s failure to organize on Tuesday gives support to the thought that the storm will never significantly intensify in the Gulf of Mexico, but the latest 12Z (8 am EDT) Tuesday runs of the GFS and European models show a little more development of TD 9 compared to their previous ones. These latest runs show TD 9 as a tropical storm with 45 - 60 mph winds at the time of landfall on the Florida Gulf Coast on Thursday afternoon. The SHIPS model on Tuesday afternoon predicted moderately favorable conditions for intensification, with wind shear staying a moderate 10 - 15 knots through Thursday morning. SSTs will be a very warm 30 - 30.5°C (86 - 87°F), and mid-level relative humidity was predicted to be a reasonably moist 65%. Our three best intensity models—the HWRF, DSHIPS and LGEM models—were in reasonable agreement with their latest runs available late Tuesday afternoon, with landfall intensities for TD 9 ranging from 55 - 75 mph. NHC is going with a forecast of a 60 mph tropical storm at landfall. The Gulf Coast of Florida is highly vulnerable to large storm surges, due to the extensive stretch of shallow continental shelf waters offshore that extend up to 90 miles from the coast. On Tuesday afternoon, NHC was calling for a maximum storm surge of 2 - 4’ above ground from TD 9 along a 30-mile stretch of the Florida coast to the right of where the center is expected to make landfall. The other hazard from TD 9 is heavy rain: rainfall amounts of 5 - 10” are expected along TD 9’s path across Florida, with 15” possible near the coast where the storm makes landfall.

92L off the coast of Africa embedded in dry air
A large tropical wave with plenty of spin that emerged from the coast of Africa on Monday evening was designated Invest 92L by NHC. The wave was moving through the Cabo Verde Islands on Tuesday and will potentially develop into a tropical depression later in the week. SSTs and wind shear would be favorable for development during the coming five days, but 92L is embedded in a major pulse of dust and dry air from the Sahara that left the coast at the same time, just to the north of 92L. This dry air will greatly interfere with development over the coming days as 92L heads west at 15 - 20 mph across the tropical Atlantic. The latest 12Z Tuesday (8 am EDT) runs of two of our three top models for forecasting tropical cyclone genesis—the GFS and UKMET models—developed 92L, but not until five days from now. A strong and persistent ridge of high pressure should keep 92L on a fairly straightforward west to west-northwest path, and the storm will likely move through or just north of the Lesser Antilles Islands on Sunday. In their 2 pm EDT Tuesday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 0% and 40%, respectively.


Figure 3. Enhanced infrared satellite image of Hurricane Madeline as of 2100Z (5:00 pm EDT) Tuesday, August 30, 2016. Hawaii is outlined at far left. Image credit: NOAA/NESDIS.

Hurricanes Madeline and Lester barreling toward Hawaii
A Tropical Storm Warning and Hurricane Watch is in effect for the Big Island of Hawaii in advance of Category 3 Hurricane Madeline. A Tropical Storm Watch is also in effect for the islands of Maui County (Maui, Molokai, and Lanai). As of the 5:00 pm EDT advisory from the Central Pacific Hurricane Center (CPHC), Madeline was located about 370 miles east of Hilo, moving due west at 10 mph. Madeline’s top sustained winds were 115 mph, down from their peak of 135 mph on Monday night, and its once-clear eye became obscured by clouds on Tuesday afternoon. An Air Force Hurricane Hunter mission on Tuesday afternoon measured top flight-level winds of 100 knots (115 mph), with 115-knot (132-mph) surface winds detected by radiometer. As vertical wind shear increases to the 10-20 knot range, some fairly dry air will be pushed into the storm (mid-level relative humidities around 40-45%). Thus, Madeline’s strength should gradually drop over the next couple of days, despite SSTs warming slightly to 27-28°C. Madeline is expected to make its closest approach to Hawaii as a Category 1 hurricane.

Madeline should angle toward the west-southwest over the next couple of days, which may keep the center just south of the Big Island on Thursday morning. The official CPHC forecast brings Madeline less than 50 miles south of the southern tip of island. This track will put the island in the more dangerous right-hand (north) side of Madeline, slamming moisture-laden air and very strong winds against high mountains. Because of the inherent uncertainty in a 36-hour track forecast, there remains a possibility that Madeline will make landfall as the Big Island’s first hurricane in records going back to 1949.


FIgure 4. Tracking map for Hurricane Madeline as of 5:00 pm EDT (11:00 am HST) Tuesday, August 30.

Potential impacts from Madeline
Madeline’s area of hurricane-force winds is quite small, only about 25 miles in radius, but tropical storm force winds (sustained winds of 39 mph or greater) extend out to 125 miles. Thus, whether or not Madeline makes landfall as a hurricane, we can expect much or most of the Big Island—and perhaps parts of neighboring islands—to experience tropical storm conditions from late Wednesday into early Thursday, with very high winds, torrential rain (up to 15” in localized parts of the Big Island, with up to 4” in Maui County), and the potential for flooding and landslides. On the Big Island, some winds could be strong enough to produce considerable damage to trees, power lines, and roofs. Surf there is expected to reach at least 15 to 25 feet on east-facing shores by Wednesday, with significant damage to roads and coastal properties possible. For more details, refer to the local statements that are compiled on a CPHC website; these will be updated as Madeline approaches.

The steep topography along the shore of the Hawaiian Islands tends to minimize storm surge while maximizing surf height. The highest storm surge on record for Hawaii was produced by Category 4 Hurricane Iniki (1992), which pushed waters 6 feet above normal astronomical tides and produced 30-foot surf. In a blog post on Tuesday afternoon, storm surge expert Hal Needham noted:

“On the Big Island, where Madeline’s impact will be felt the strongest, Hurricane Diana in 1972 generated surf levels that also reached 30 ft along the Puna coast, while storm surge levels at Hilo ranged from 4 to 5 ft (Central Pacific Hurricane Center 1972). Although this may not sound impressive, modest storm surges combined with tremendous wave heights can still be destructive, because waves are riding on top of the storm surge, enabling them to push a destructive force of water inland, well beyond the storm surge water limit. Diana’s combined storm surge and high surf swept four homes from their foundations and eroded 200 feet of a private road (Central Pacific Hurricane Center 1972).” He added: “The biggest concern for coastal flood potential will be on the southeastern shores of the Big Island, near the villages Naalehu and Pahala, where hurricane-force winds may wrap around the ‘back side’ of Madeline after its closest approach. Such winds come on strong from the opposite direction than they were previously blowing, quickly changing a low-water event into a sudden storm surge event, which can be surprisingly destructive.”


Figure 5. Satellite image of Hurricanes Madeline (left) and Lester (right) as of 2100Z (5:00 pm EDT) Tuesday, August 30, 2016. Image credit: NASA/MSFC Earth Science Office.

Next up for the weekend: Hurricane Lester
Hawaii has another storm to keep an eye on: Hurricane Lester. Located about 1280 miles east of Hilo and about 900 miles east of Madeline, Lester had top sustained winds of 120 mph as of the 5:00 pm EDT Tuesday advisory from NHC. Very favorable atmospheric conditions and modestly supportive SSTs around 27°C have allowed Lester to maintain its strength. From Thursday into Friday, Lester will be rolling directly over cooler waters churned up by Madeline, which could put a dent in its power. Wind shear is predicted by the SHIPS model to remain very low for at least the next 2-3 days before gradually increasing. The official NHC outlook brings Lester’s sustained winds down to minimal Category 1 strength (65 mph) by Saturday and strong tropical storm strength (70 mph) by Sunday.

Lester’s current westward track is expected to bend to the west-northwest as it approaches Hawaii, which should produce a track over the weekend roughly parallel to the island chain. The official NHC outlook continues to keep Lester 100-200 miles north of the islands on Saturday and Sunday. However, there is an increased note of uncertainty in the latest (12Z Tuesday) model guidance on Lester’s track. The UKMET and the averaged GFS ensemble runs (GEFS) bring Lester across the islands over the weekend, while the Euro’s path runs just north of Oahu and Kauai. Other models keep Lester farther north by various margins. The increased spread among model solutions is a reminder that paths can change by a large amount over a 4- or 5-day period. An upper-level high to the north of Lester will become quite strong later this week, implying that a southward track closer to the islands is certainly plausible. Tropical storm-force winds are predicted to extend outwards 90 miles to the southwest of Lester this weekend, putting most of the island chain at at the edge of getting sustained 40-mph winds should Lester follow the path predicted by CPHC.


Figure 6. Tracking map for Hurricane Lester as of 5:00 pm EDT (11:00 am HST) Tuesday, August 30.

The tracks of both Lester and Madeline may be influenced by the Fujiwhara effect, which causes two hurricanes that get within about 800 miles of each other to rotate around a common point in between, with that motion superimposed upon the primary storm motion. In a case like this, the easterm storm (Lester) would angle northward and the western storm (Madeline) would angle southward.

Wunderblogger Steve Gregory has an excellent update on the tropics in his Tuesday afternoon post, TD 9 Gets Better Organized/TD 8 to Skirt NC Coast.

We’ll be back with a new post late Wednesday morning.

Jeff Masters and Bob Henson


Hurricane

Twin Major Hurricanes Menace Hawaii; Little Change to Atlantic's TD 8 and TD 9

By: Jeff Masters and Bob Henson , 3:59 PM GMT on August 30, 2016

Powerful Hurricane Madeline continues edging toward Hawaii’s Big Island, where a Hurricane Watch remains in effect. An astounding 36-hour burst of intensification peaked early Tuesday, with top sustained winds of 135 mph at 5 am EDT making the hurricane a Category 4 on the Saffir-Simpson Scale. As of 11 am EDT Tuesday, Madeline’s top winds were down to 120 mph. The formerly distinct eye has become obscured in infrared imagery over the last few hours, another sign of weakening. Wind shear will increase to the moderate range (15 - 20 knots) by Wednesday, so we can expect at least gradual weakening to continue.


Figure 1. Hurricanes Madeline (left) and Lester (right), as captured by the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi NPP spacecraft on Monday, August 29, 2016. Image credit: NASA.

Located about 450 miles east of Hilo as of 11 am EDT, Madeline was moving west at 10 mph. Computer models agree that a strengthening ridge to the north of Madeline will help induce a leftward bend in Madeline’s track. This bend, plus interactions with Hurricane Lester (see below), may be enough to put Madeline’s track just south of the Big Island and well south of the rest of Hawaii, but there is still some uncertainty about this. The 00Z UKMET and the 06Z HWRF and GFS model runs keep the hurricane about 100-200 miles south of the island, while the 00Z European and 06Z GFDL run suggest a landfall on the Big Island at hurricane strength. No hurricane has ever struck the Big Island in records going back to 1949. The official track from the Central Pacific Hurricane Center brings Madeline within 50 miles of the south tip of the Big Island on Thursday morning, putting the island on the more dangerous right-side (north) side of the storm.


Figure 2. Tracking map for Hurricane Madeline as of 11:00 am EDT (5:00 am HST) Tuesday.

Although models diverge on how quickly Madeline will weaken, the 11 am EDT Tuesday outlook from CPHC projects that Madeline will be a Category 1 hurricane by the time it reaches the vicinity of Hawaii. Even a brush just to the south of the Big Island at this strength would push very strong tropical-storm-force winds into the east side of the island, slamming upslope against towering mountainsides. The result would be torrential rains of 5” - 15” or more and the potential for flooding and landslides. Winds could be strong enough to produce considerable damage to trees, power lines, and roofs. Surf is expected to reach 15 to 25 feet on east-facing shores by Wednesday, with significant damage to roads and coastal properties possible. For more details, refer to the local statements that are compiled on a CPHC website; these will be updated as Madeline approaches.


Figure 3. Infrared satellite image of Hurricanes Madeline (center) and Lester (right) as of 1200Z (8:00 am EDT) Tuesday, August 30, 2016. Image credit: NASA/MSFC Earth Science Office.

Next up for the weekend: Hurricane Lester
The 50th state has another storm to keep an eye on: Hurricane Lester. Peaking on Monday night with top sustained winds of 140 mph, Lester weakened slightly overnight, but on Tuesday morning it was rebuilding a very solid and well-structured core of convection. Located about 1350 miles east of Hilo and about 900 miles east of Madeline, Lester was packing top sustained winds of 120 mph as of the 11 am EDT advisory from NHC.

Lester’s current westward track is expected to bend to the west-northwest as it approaches Hawaii, which should produce a track over the weekend roughly parallel to the island chain and most likely 100-200 miles to its north. Model guidance is tightly clustered around this track, which is reflected in the official NHC outlook. Among the 00Z Tuesday runs of the leading models, only the UKMET produces a landfall in Hawaii, and that occurs with a much-weakened Lester. This gives us increasing confidence that Hawaii will dodge the Lester bullet, although there is still enough error in 4- and 5-day outlooks to keep most of the state in the “cone of uncertainty” for now (see image below). Lester is expected to weaken very gradually over the next several days, with more significant weakening possible as wind shear increases toward the weekend. Lester will also be moving over waters churned up by Madeline, which could further dampen its strength.


Figure 4. Tracking map for Hurricane Lester as of 11:00 am EDT (5:00 am HST) Tuesday.

Only five tropical storms or hurricanes have made landfall on a Hawaiian island since records began in 1949, and two of those have been in the last three years (see our Monday post for details). Given Hawaii’s limited experience with tropical cyclones, both Madeline and Lester need to be taken very seriously. An unprecedented deployment of hurricane hunter resources to Hawaii kicked off on Monday afternoon, when three Air Force Hurricane Hunter aircraft were sent to the islands. Beginning on Tuesday afternoon, these aircraft will provide regular fixes every 12 hours on Madeline, and will also begin flying into Lester when it draws closer to Hawaii.

Near-perfect storms
The remarkable strengthening of both Madeline and Lester since Sunday could serve as textbook examples of hurricane intensification. SSTs along the paths of both storms have been on the low side--just above the 26°C benchmark for tropical development—yet both storms have taken full advantage of exceptionally supportive atmospheric conditions, including very low wind shear and very strong upper-level outflow channels. According to output from the SHIPS model, the maximum potential intensity (MPI) for both Madeline and Lester as of Tuesday morning was about 130 knots (150 mph). MPI values take into account the ability of a tropical cyclone to create a “heat engine” based on SSTs and the atmospheric temperature structure. Most hurricanes never reach their MPI value, so that fact that two hurricanes so close to each other have managed this feat testifies to the extremely favorable atmospheric dynamics in play.

Figure 5. The Fujiwhara effect causes two tropical cyclones near each other to rotate around a common midpoint. This motion is on top of the preexisting movement of each cyclone. Image credit: Hong Kong Observatory.

Ironically, the coexistence of Madeline and Lester may help keep either one from a direct landfall on Hawaii, thanks to the Fujiwhara effect, which was discovered nearly a century ago by Japanese researcher Sakuhei Fujiwhara. When two tropical cyclones get within about 800 miles of each other, the interaction tends to make the pair rotate around a common point in between, with the effect superimposed on the storm’s preexisting motions. In a case like this, the easterm storm (Lester) would angle northward and the western storm (Madeline) would angle southward. Both effects would tend to drive Madeline and Lester away from Hawaii.

TD 9 still battling dry air and wind shear
The endless struggle of Tropical Depression Nine with dry air and wind shear continued overnight. The storm’s winds were below tropical storm strength on Tuesday morning, confirmed a NOAA hurricane hunter aircraft, though the central pressure was down 3 mb since Monday morning, to 1004 mb. TD 9 brought heavy rains to Cuba on Monday; In Central Cuba, Trinidad reported 252.4 mm (9.94”) in the 24 hours ending at 8 pm EDT Monday, while Santa Lucia in Western Cuba reported 228.3 mm (8.99”). Additional heavy rains of 2 - 4” are likely over western Cuba on Tuesday. Rainfall amounts of 1 - 2” were common over South Florida on Monday, with a few spots of 3+”.

Satellite images on Tuesday morning showed TD 9’s heavy thunderstorms building, though the circulation center was still partially exposed to view—the telltale sign of a tropical cyclone struggling with wind shear and dry air. Wind shear was a moderate 10 - 15 knots, and water vapor satellite imagery showed plenty of dry air to the storm’s north and west. Sea surface temperatures (SSTs) near TD 9’s center remained favorable for development, near 30 - 30.5°C (86 - 87°F).


Figure 6. Projected 5-day rainfall from 12Z (8:00 am EDT) Tuesday, August 30, through 12Z Sunday, September 4, 2016. Rainfall amounts of 5 - 10” are expected along TD 9’s path across Florida, with up to 12” possible near the coast where the storm makes landfall. Image credit: NOAA/NWS Weather Prediction Center.

Track forecast for TD 9: a Florida Gulf Coast landfall, followed by a run up the Southeast coast
The latest 0Z Tuesday (8 pm EDT Monday) runs of our top models continue to bring TD 9 to a landfall on the Florida coast north of Tampa on Thursday. The models have converged on their timing of this landfall, which is expected to occur late morning or early afternoon on Thursday. In their 11 am EDT Tuesday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from TD 9 along the Gulf Coast of Florida were 49%, 39%, and 33%, respectively, for Cedar Key, Tampa, and Apalachicola, Florida. Tropical storm-force winds may also occur on the east coast of Florida near where the storm exits the coast after crossing the state: NHC gave odds of tropical storm-force winds of 25% or higher to the Florida coast from Cocoa Beach to Georgia, and to all of the Georgia coast. Lower odds were posted for the South Carolina coast. It is possible the storm could turn north over the weekend just offshore of the Mid-Atlantic coast, as suggested by recent runs of the GFS model. This would bring high surf to beaches from North Carolina to Massachusetts, but the heaviest rains would stay offshore.


Figure 7. Screen shot of NHC’s interactive Storm Surge Probability product from 5 am EDT Tuesday, August 30, 2016, showing the probability of inundation in excess of 4’ above ground level from TD 9. A 30-mile stretch of the Florida Gulf Coast to the right of where the center of TD 9 is expected to make landfall is predicted to have a 50 - 60% chance of getting a inundation in excess of four feet. The graphic is based upon an ensemble of Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model runs created using the current National Hurricane Center (NHC) official hurricane advisory. Storm surge probabilities depend on the historical accuracy of NHC's forecasts of hurricane track, and wind speed, and an estimate of storm size.

Intensity forecast: TD 9 likely to stay below hurricane strength
TD 9 has defied predictions that it would intensify its entire way across the Atlantic as tropical wave 99L, and now as TD 9, so there is reason to be dubious that the storm will ever significantly intensify. Indeed, the latest 00Z Tuesday runs of the GFS and European models show TD 9 as a tropical depression or weak tropical storm with 45 mph winds at the time of landfall on the Florida Gulf Coast on Thursday morning, likely due to the continued presence of dry air at middle and upper levels of the atmosphere. However, we cannot totally dismiss the possibility that TD 9 might put on a sudden show of intensification into a strong tropical storm before landfall on Thursday. The SHIPS model on Tuesday morning predicted moderately favorable conditions for intensification, with wind shear staying a moderate 10 - 15 knots through Thursday morning. SSTs will be a very warm 30 - 30.5°C (86 - 87°F), and mid-level relative humidity was predicted to be a reasonably moist 65%. Our three best intensity models—the HWRF, DSHIPS and LGEM models—were in reasonable agreement with their latest runs available late Tuesday morning, with landfall intensities for TD 9 ranging from 50 - 65 mph. NHC is going with a forecast of a 60 mph tropical storm at landfall. The Gulf Coast of Florida is highly vulnerable to large storm surges, due to the extensive stretch of shallow continental shelf waters offshore that extend up to 90 miles from the coast. On Tuesday morning, NHC was giving a 50 - 60% chance that the maximum height of the storm surge above ground from TD 9 would exceed four feet along a 30-mile stretch of the Florida coast to the right of where the center is expected to make landfall. The other hazard from TD 9 is heavy rain—rainfall amounts of 5 - 10” are expected along TD 9’s path across Florida, with 15” possible near the coast where the storm makes landfall.

TD 8: Tropical Storm Warning continues for the Outer Banks of North Carolina
A Tropical Storm Warning continues for the Outer Banks of North Carolina as Tropical Depression Eight chugs north-northwest at 5 mph towards Cape Hatteras. TD 8 has not strengthened into a tropical storm yet, confirmed an Air Force hurricane hunter aircraft on Tuesday morning, as the plane found top winds in the storm near 35 mph, with a central pressure holding steady at 1011 mb. Satellite images and long-range Morehead City, North Carolina radar on Tuesday morning showed TD 8 continued to struggle to hold onto its heavy thunderstorms in the face of very dry air (45 - 50% relative humidity at mid-levels of the atmosphere), combined with moderate wind shear of 10 - 15 knots. Sea surface temperatures (SSTs) near TD 8’s center remained favorable for development, though, near 29°C (84°F).

Forecast for TD 8: grazing the Outer Banks of North Carolina
The computer models are in excellent agreement that TD 8 will continue on its current north-northwest track through Tuesday afternoon, then make a turn to the north Tuesday night, and then turn to the northeast on Wednesday. These steering currents should bring the center of TD 8 very close to Cape Hatteras, North Carolina on Tuesday night and Wednesday morning. A storm surge of 1 - 2 feet can be expected along the Outer Banks, along with heavy rains of 1 - 3 inches. In their 11 am EDT Tuesday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from TD 8 were 43% for Cape Hatteras, North Carolina. Dry air and moderate wind shear will continue to affect TD 8 through Wednesday, but there is still the possibility it could become a weak tropical storm by Wednesday.


Figure 8. MODIS visible satellite image of Invest 92L on Tuesday morning, August 30, 2016. The tropical wave was embedded in a large area of African dust to its west and north. Image credit: NASA.

92L emerges from the coast of Africa
A large tropical wave with plenty of spin emerged from the coast of Africa on Monday evening, and was immediately designated Invest 92L by NHC. The wave will move through the Cabo Verde Islands on Tuesday and potentially develop into a tropical depression later in the week. While the latest run of the SHIPS model predicted that SSTs and wind shear would be favorable for development during the coming five days, 92L emerged from the coast at the same time that a major pulse of dust and dry air from the Sahara also left the coast, just to the north. This dry air will greatly interfere with development over the coming days as 92L heads west at 15 - 20 mph across the tropical Atlantic.The latest 0Z Tuesday (8 pm EDT Monday) runs of our three top models for forecasting tropical cyclone genesis, the GFS, UKMET and European models, did not develop 92L over the next five days—though the more recent 06Z (2 am EDT) Tuesday run of the GFS model did develop 92L five days from now. A strong and persistent ridge of high pressure should keep 92L on a fairly straightforward west to west-northwest path, and the storm will likely move through or just north of the Lesser Antilles Islands on Sunday. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 0% and 40%, respectively.

We’ll be back with a new post late Tuesday afternoon.

Bob Henson and Jeff Masters

Hurricane Watch for Hawaii’s Big Island; TDs 8, 9 May Become Tropical Storms

By: Bob Henson and Jeff Masters , 10:33 PM GMT on August 29, 2016

In a dual scenario unprecedented in hurricane recordkeeping, two major hurricanes are heading toward Hawaii, and both could affect the island with high surf, torrential rain, and potential high winds over the next week. Hurricane Madeline is the closer of the two, located about 630 miles east of Hilo, HI, as of the 5 pm EDT advisory from the Central Pacific Hurricane Center. Now moving west-northwest at 10 mph, Madeline has been rapidly intensifying, growing from tropical storm to Category 3 strength in just 24 hours. As of 5 pm EDT, Madeline’s top sustained winds were at 115 mph. CPHC is projecting Madeline to move on a leftward-arcing path that would take it just south of the Big Island as a Category 1 hurricane by Wednesday night (see Figure 1 below). A Hurricane Watch is now in effect for all of the Big Island (Hawaii County). Update (11:30 pm EDT Monday]: Madeline's top sustained winds had increased to 125 mph as of the 11 pm EDT CPHC advisory.


Figure 1. Tracking map for Hurricane Madeline as of 5:00 pm EDT (11:00 am HST) Monday.

Madeline could become the first hurricane on record in the Big Island
At its closest, the center of Madeline is projected to be roughly 100 miles south of the Big Island. Given that this is more than two days out, we cannot yet entirely rule out the possibility that Madeline will stay far enough north to produce the first-ever hurricane strike on the Big Island in records going back to 1949. The 12Z Monday HWRF and GFDL model runs bring Madeline into the Big Island at hurricane strength, while the European and GFS models keep Madeline south of the island. Even if the latter occurs, very strong northeast winds rotating around the hurricane could produce torrential rains, flooding, and huge surf on the east side of the island. Tropical-storm-force winds currently extend out up to 115 miles from Madeline, and that envelope may expand by Wednesday as the hurricane matures.


Figure 2. Resembling the eyes of some ghostly sea creature, Hurricanes Madeline (left) and Lester (right) were charging westward toward Hawaii (far left) at 1800Z (2:00 pm EDT) Monday, August 29, 2016. Image credit: NASA/MSFC Earth Science Office.

About 1000 miles east of Madeline, Hurricane Lester remains impressive, now packing minimal Category 4 winds of 130 mph as of the 5 pm EDT advisory from NHC. As its shield of thunderstorms grows larger, Lester is taking on more of the characteristics of an annular hurricane--the type that features a large eye and a single broad ring around that eye, as opposed to spiral bands. Annular hurricanes tend to be slow to weaken, which raises the odds of Lester remaining strong enough to affect Hawaii as a hurricane. NHC predicts that Lester will be roughly 150 miles northeast of Hilo on Saturday morning as a Category 1 hurricane, on a northwestward-angling track that could keep the storm just north of the islands. There is enough error in five-day tracks to put most of the Big Island within NHC’s “cone of uncertainty” for Saturday, and a direct strike from Lester on one or more islands cannot yet be ruled out.

One factor that could influence both tracks is the Fujiwhara effect, in which hurricanes within about 800 miles of each other begin to rotate around a center of gravity in between them. Lester is slowly catching up to Madeline as it moves west at 14 mph, vs. Madeline’s 10 mph motion. If the two hurricanes get close enough, the Fujiwhara effect will tend to angle Madeline’s path toward the south and Lester’s toward the north--in both cases, exactly what you would want to reduce the chance of a direct hit on Hawaii. Such an outcome is by no means guaranteed, though. Given Hawaii’s limited experience with tropical cyclones (see this morning’s post for more details), both of these systems need to be taken very seriously.

An unprecedented deployment of hurricane hunter resources to Hawaii kicked off on Monday afternoon, when three Air Force Hurricane Hunter aircraft were sent to the islands. Beginning on Tuesday afternoon, these aircraft will provide regular fixes every 12 hours on Madeline, and will also begin flying into Lester when it draws closer to Hawaii.


Figure 3. Tracking map for Hurricane Lester as of 5:00 pm EDT (11:00 am HST) Monday.

TD 9 struggling with dry air and wind shear
Tropical Depression Nine continues to struggle with dry air and wind shear, and has not strengthened into a tropical storm yet, confirmed a NOAA hurricane hunter aircraft on Monday afternoon. TD 8 was headed to the west-northwest at 5 mph on a track just north of western tip of Cuba. The storm’s top winds remained near 35 mph, with a central pressure holding steady at 1007 mb.


Figure 4. MODIS visible satellite image of Tropical Depression Nine in the Florida Straits on Monday afternoon, August 29, 2016. Image credit: NASA.

Satellite images on Monday afternoon showed a slow increase in the intensity and areal coverage of TD 9’s heavy thunderstorms, though late in the day the circulation center became exposed to view—the telltale sign of a tropical cyclone struggling with high wind shear. Long-range Key West radar showed heavy rain over western Cuba, where up to 12” of rainfall had likely fallen, and a few scattered rain showers over the Florida Keys, but little in the way of low-level spiral bands. The main factor keeping TD 9 from developing was wind shear that was a moderate 10 - 15 knots. TD 9 was also struggling with dry air, as seen on water vapor satellite imagery. Sea surface temperatures (SSTs) near TD 9’s center remained favorable for development, though, near 30  - 30.5°C (86 - 87°F).

Track forecast for TD 9: a Florida Gulf Coast landfall, followed by a run up the Southeast coast
The latest 12Z (8 am EDT) Monday runs of our top models continue to bring TD 9 to a landfall on the Florida coast north of Tampa on Thursday. There is significant spread in the timing of TD 9’s landfall in Florida, with the HWRF model predicting a 1 am strike, European model an 11 am strike, and the GFS model an 8 pm strike. In their 5 pm EDT Monday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from TD 9 along the Gulf Coast of Florida were 43%, 36%, and 36%, respectively, for Cedar Key, Tampa, and Apalachicola, Florida. Tropical storm-force winds may also occur on the east coast of Florida near where the storm exits the coast after crossing the state: NHC gave odds of tropical storm-force winds of 25% or higher to Orlando, The Villages, Daytona Beach, Gainesville and Jacksonville in Florida, and to Kings Bay in Georgia. The latest run of the European model showed TD 9 scooting along the coasts of Georgia and South Carolina early Friday morning, and it is possible the storm could spread sustained winds near 40 mph to the coast there.

One potential monkey wrench in the track forecast: TD 9 could undergo a relocation of its center to point more than 50 miles south of its current center, so that it is closer to the heaviest thunderstorms near the western tip of Cuba. If this occurs, a southward shift in the predicted track of TD 9 may be required.


Figure 5. The 12Z (8 am EDT) Monday run of the HWRF model predicted that TD 9 would have top winds of 50 mph at landfall, and would bring copious rains of 8 - 16” along its track across Florida. The HWRF rainfall amounts are likely too high, as the official NHC forecast at 5 pm EDT Monday called for 3 - 7” of rain along TD 9’s track, with isolated amounts of up to 10” along the coast near its landfall location. Image credit: NOAA/EMC.

Intensity forecast for TD 9 becoming clearer
Once TD 9 pulls away from Cuba, a round of steady intensification is likely, with the system reaching tropical storm strength by Tuesday morning. The SHIPS model on Monday afternoon predicted moderately favorable conditions for intensification, with wind shear staying a moderate 10 - 15 knots Monday afternoon through Wednesday. SSTs will be a very warm 30 - 30.5°C (86 - 87°F), and mid-level relative humidity was predicted to be a reasonably moist 65 - 70%. There is a significant amount of dry air at middle and upper levels of the atmosphere that may interfere with development, though. Our three best intensity models—the HWRF, DSHIPS and LGEM models—were in reasonable agreement with their latest runs available on Monday afternoon, with landfall intensities for TD 9 ranging from 50 - 75 mph. NHC is going with a forecast of a 65 mph tropical storm at landfall, noting that increasing wind shear in the final day before landfall may stop the intensification process. TD 9 could be a Category 1 hurricane at landfall, as suggested by the DSHIPS intensity model, and residents along the Gulf Coast of Florida should anticipate this possibility. This portion of the coast is highly vulnerable to large storm surges, due to the extensive stretch of shallow continental shelf water offshore that extend up to 90 miles from the coast. A worst-case Category 1 hurricane hitting at high tide can cause a storm surge that will inundate the Florida Gulf Coast north of Tampa to a depth to 9 - 10 feet, as seen in SLOSH model imagery available in WU’s storm surge pages.

Tropical Storm Warning for the Outer Banks of North Carolina
A Tropical Storm Warning is up for the Outer Banks of North Carolina as Tropical Depression Eight chugs northwest at 6 mph towards the state. TD 8 has not strengthened into a tropical storm yet, confirmed an Air Force hurricane hunter aircraft on Monday afternoon, as the plane found top winds in the storm near 35 mph, with a central pressure holding steady at 1011 mb.


Figure 6. Radar image of TD 8 from the long-range Morehead City, North Carolina radar at 4:39 pm EDT August 29, 2016. TD 8 was a spirally-looking thingy.

Satellite images and long-range Morehead City, North Carolina radar on Monday afternoon showed TD 8 had a vigorous circulation and a modest but growing area of heavy thunderstorms. Development was being slowed to very dry air (45 - 50% relative humidity at mid-levels of the atmosphere), combined with moderate wind shear of 15 - 20 knots. Sea surface temperatures (SSTs) near TD 8’s center remained favorable for development, though, near 29°C (84°F).

Forecast for TD 8: grazing the Outer Banks of North Carolina
The computer models are in excellent agreement that TD 8 will continue on its current northwest track through Tuesday morning, then make a sharp turn to the north and northeast on Tuesday afternoon after getting caught in the steering flow of a trough of low pressure passing to the north. These steering currents should bring the center of TD 8 very close to Cape Hatteras, North Carolina on Tuesday night and Wednesday morning. A storm surge of 1 - 2 feet can be expected along the Outer Banks, along with heavy rains of 1 - 3 inches. In their 5 pm EDT Monday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from TD 8 were 42% for  Cape Hatteras, North Carolina. Dry air and moderate wind shear will continue to affect TD 8 through Wednesday, and it is unlikely this storm will be stronger than a 50 mph tropical storm at the time of its closest approach to the coast on Tuesday night/Wednesday morning. As TD 8 accelerates away from the coast on Wednesday and Thursday, more significant strengthening may occur.

A new tropical wave worth watching is leaving the coast of Africa
A large tropical wave with plenty of spin was emerging from the coast of Africa on Monday afternoon, will move through the Cabo Verde Islands on Tuesday, and potentially develop into a tropical depression later in the week as it heads west at 15 - 20 mph across the tropical Atlantic. The latest 12Z (8 am EDT) Monday runs of two our three top models for forecasting tropical cyclone genesis, the GFS and European models, continue to agree that this wave will develop into a tropical depression late in the week. The UKMET model dropped development in its 12Z Monday run after supporting development in its previous few runs. The wave should remain on a fairly straightforward west to west-northwest path through the week, arriving near or just north of the Lesser Antilles Islands by Sunday evening. In their 2 pm EDT Monday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 0% and 50%, respectively.

Wunderblogger Steve Gregory has an excellent update on the tropics in his Monday afternoon post, TD 9 Getting Better Organized as TD 8 Heads for NC Coast. See our morning post for more on the other significant storm of the hour, Hurricane Gaston, which is spinning harmlessly in the middle of the North Atlantic.

We’ll be back with a new post late Tuesday morning.

Bob Henson and Jeff Masters

Hurricane

TD 9 Organizing in the Gulf of Mexico; TD 8 Headed Towards North Carolina

By: Jeff Masters and Bob Henson , 3:39 PM GMT on August 29, 2016

Tropical Depression Nine has not strengthened into a tropical storm yet, confirmed a NOAA hurricane hunter aircraft early on Monday morning, as the storm headed to the west at 7 mph on a track just north of western Cuba. The storm’s top winds remained near 35 mph, with a central pressure holding steady at 1007 mb. The strongest winds observed at a surface station on Monday morning were at Pulaski Shoals Lighthouse, located about 70 miles west of Key West, Florida, which recorded sustained winds of 31 mph, gusting to 35 mph, at 10 am EDT.


Figure 1. Latest satellite image of Tropical Depression Nine.

Satellite images on Monday morning showed a slow increase in the intensity and areal coverage of TD 9’s heavy thunderstorms. Long-range Key West radar showed heavy rain over western Cuba, where up to 12” of rainfall was expected, and a few scattered rain showers over the Florida Keys, but little in the way of low-level spiral bands. The main factor keeping TD 9 from developing was wind shear that was a moderately high 15 - 25 knots. TD 9 was also struggling with dry air, as seen on water vapor satellite imagery. Proximity to western Cuba was also interfering with development, as the storm’s counterclockwise flow of air pulled air across the mountains of Cuba into the storm. As this air descends to the ocean after crossing Cuba, warming and drying of the air occurs, robbing TD 9 of an important moisture source. Sea surface temperatures (SSTs) near TD 9’s center remained favorable for development, though, near 30 - 30.5°C (86 - 87°F).


Figure 2. Projected 5-day rainfall from 12Z (8:00 am EDT) Monday, August 29, through 12Z Saturday, September 3, 2016. Rainfall amounts of 3 - 7” are expected over most of Florida. Image credit: NOAA/NWS Weather Prediction Center.

Track forecast for TD 9: a Florida Gulf Coast landfall
For several consecutive runs, there has been model consensus among the GFS, European, HWRF and UKMET models that TD 9 will move on a west-northwest track through Tuesday morning, slow down and turn north in the central Gulf of Mexico on Tuesday afternoon, then get caught in the steering flow of a trough of low pressure passing to the north on Wednesday. These steering currents should bring TD 9 to a landfall on the Florida coast north of Tampa on Wednesday or Thursday. There is significant spread in the timing of TD 9’s landfall in Florida, with the HWRF model predicting a Wednesday afternoon landfall as a Category 1 hurricane, and the GFS and European model predicting a Thursday morning or afternoon landfall as a 40 - 45 mph tropical storm. In their 11 am EDT Monday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from TD 9 along the Gulf Coast of Florida were 43%, 39%, and 34%, respectively, for Cedar Key, Tampa, and Apalachicola, Florida. Tropical storm-force winds may also occur on the east coast of Florida near where the storm exits the coast after crossing the state: NHC gave odds of tropical storm-force winds in excess of 30% to Orlando, The Villages, Daytona Beach, Gainesville and Jacksonville in Florida, and to King Bay in Georgia.

Intensity forecast for TD 9: more uncertain than usual
Once TD 9 pulls away from Cuba, a round of steady intensification is likely, with the system reaching tropical storm strength by Monday night. Satellite imagery late Monday morning showed that this process was already underway, with a notable increase in the storm’s organization. The SHIPS model on Monday morning predicted moderately favorable conditions for intensification, with wind shear falling to a moderate 10 - 15 knots Monday afternoon through Wednesday. SSTs will be a very warm 30 - 30.5°C (86 - 87°F), and mid-level relative humidity was predicted to be a reasonably moist 65 - 70%. However, there is a significant amount of dry air at middle and upper levels of the atmosphere that may interfere with development, and the usually reliable European and GFS models showed little development of TD 9 in their 12Z Sunday and 0Z Monday (8 pm EDT Sunday) runs because of this dry air, NHC forecaster Stacy Stewart argued in his forecast discussion on Sunday night. Our best dynamical intensity model, the HWRF model, had TD 9 intensifying into a Category 1 hurricane just before landfall, though, and our two best statistical intensity models, the DSHIPS and LGEM models, had TD 9 as a borderline tropical storm/Category 1 hurricane at landfall. NHC is going with a forecast of a 65 mph tropical storm at landfall, noting that increasing wind shear in the final day before landfall may stop the intensification process. TD 9 could be a Category 1 hurricane at landfall, as suggested by our best intensity models, and residents along the Gulf Coast of Florida should anticipate this possibility. This portion of the coast is highly vulnerable to large storm surges, due to the extensive stretch of shallow continental shelf water offshore that extend up to 90 miles from the coast. A worst-case Category 1 hurricane hitting at high tide can cause a storm surge that will inundate the Florida Gulf Coast north of Tampa to a depth to 9 - 10 feet, as seen in SLOSH model imagery available in WU’s storm surge pages.

NOAA/RAMMB has some impressive rapid scan loops of TD 9 with images taken every minute.

TD 8 headed for the Outer Banks of North Carolina
A Tropical Storm Watch is up for the Outer Banks of North Carolina as Tropical Depression Eight churns northwest at 7 mph towards the state. TD 8 has not strengthened into a tropical storm yet, confirmed an Air Force hurricane hunter aircraft on Monday morning, as the plane found top winds in the storm near 35 mph, with a central pressure holding steady at 1011 mb.


Figure 3. Latest satellite image of Tropical Depression Eight.

Satellite images on Monday morning showed TD 8 had a vigorous circulation but only a meager amount of heavy thunderstorms. The depression was not developing due to very dry air (45 - 50% relative humidity at mid-levels of the atmosphere), combined with moderate wind shear of 10 - 20 knots. Sea surface temperatures (SSTs) near TD 8’s center remained favorable for development, though, near 29°C (84°F).

Forecast for TD 8: grazing the Outer Banks of North Carolina
The computer models are in excellent agreement that TD 8 will continue on its current northwest track through Tuesday morning, then make a sharp turn to the north and northeast on Tuesday afternoon after getting caught in the steering flow of a trough of low pressure passing to the north. These steering currents should bring the center of TD 8 very close to Cape Hatteras, North Carolina on Tuesday night. A storm surge of 1 - 2 feet can be expected along the Outer Banks, along with heavy rains of 1 - 3 inches. In their 11 am EDT Monday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from TD 8 were 46% for Cape Hatteras, North Carolina. Dry air and moderate wind shear will continue to affect TD 8 through Wednesday, and it is unlikely this storm will be stronger than a 50 mph tropical storm at the time of its closest approach to the coast on Tuesday night. As TD 8 accelerates away from the coast on Wednesday and Thursday, more significant strengthening may occur.

A new tropical wave worth watching is leaving the coast of Africa
A large tropical wave with plenty of spin will emerge from the coast of Africa late on Monday, move through the Cabo Verde Islands on Tuesday, and potentially develop into a tropical depression later in the week as it heads west at 15 - 20 mph across the tropical Atlantic. The latest runs of our three top models for forecasting tropical cyclone genesis, the GFS, ECMWF and UKMET models, continue to agree that this wave will develop into a tropical depression late in the week. The wave should remain on a fairly straightforward west to west-northwest path through the week, arriving near or just north of the Lesser Antilles Islands by Sunday evening. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 0% and 50%, respectively.



Figure 4. Visible satellite image of Hurricane Gaston as of 1415Z (10:15 am EDT) Monday, August 29, 2016. Image credit: NOAA/NESDIS.

Powerful Gaston loiters over the remote Atlantic
Hurricane Gaston continues to aim its formidable power on the open Atlantic, about 600 miles east of Bermuda, rather than on people and structures. After peaking early Monday with top sustained winds of 120 mph, Gaston weakened slightly, but it remains a strong Category 2 hurricane with top winds of 110 mph in the 11 am EDT Monday NHC advisory. Gaston’s show of strength is remarkable given that it is practically parked over the remote subtropical Atlantic: it has moved less than 100 miles in the 24 hours ending at 11 am EDT. Hurricanes moving this slowly are often weakened as they churn up cooler water--but the western subtropical Atlantic is remarkably warm this summer, with SSTs of 29°C (84°F) near Gaston more than 1.5°C above average. Currently drifting north at just 2 mph, Gaston has another day or so of favorable conditions before a North Atlantic trough bumps up the currently-light wind shear and begins accelerating Gaston toward the northeast and toward cooler waters. It will likely hold its own for several days, though, perhaps remaining a hurricane till Thursday or Friday.

Gaston is on track to sweep over or near the Azores this weekend. The 5-day NHC outlook puts Gaston close to the northern Azores as a 60-mph tropical storm on Saturday, September 3. The Azores see a tropical cyclone landfall only about once a decade on average, although the nation was struck this past January by the bizarrely out-of-season Hurricane Alex. The islands have never recorded two landfalls in a single year.


Figure 5. GOES-West infrared image of Hurricanes Madeline (center) and Lester (right), both moving west toward Hawaii (left). Image credit: NASA/MSFC Earth Science Office.

Will the Fujiwhara effect keep Hurricanes Madeline and Lester away from Hawaii?
Amazingly, two hurricanes are rolling across the North Pacific, one behind the other, on tracks aiming toward the seldom-struck Hawaiian islands. Either one could make landfall in the next week--but it’s also possible that an obscure atmospheric mechanism will kick in just in time to steer one or both of them away from the 50th state.

Hurricane Madeline is the more immediate threat. As of the 11 am EDT Monday advisory from the Central Pacific Hurricane Center, Category 2 Madeline was located about 700 miles east of Hilo, HI, moving west-northwest at 10 mph with top sustained winds of 100 mph. Madeline has been getting stronger over the past day, with a broadening shield of convection and an eye intermittently visible. It’s not out of the question that Madeline will become a major hurricane by Tuesday, with wind shear very low (around 5 knots) and SSTs of 26-27°C. Somewhat drier air and increasing wind shear will begin affecting Madeline by midweek, with gradual weakening expected. The official CPHC track takes Madeline on a west-southwest arc, brushing the Big Island on Wednesday night as a Category 1 storm. At its closest, the center of Madeline is projected to be roughly 100 miles south of Hilo--which is smaller than the average three-day track error of 130 miles in this region. The upshot is that the first-ever recorded hurricane strike on the Big Island is within the realm of possibility. Even if Madeline stays south of the Big Island, very strong northeast winds rotating around the hurricanes could produce torrential rains and flooding on the east side of the island.

Hard on the heels of Madeline is even-stronger Category 3 Hurricane Lester, which on Monday become the fourth major hurricane of the 2016 East Pacific season. Packing top sustained winds of 125 mph as of the 11 am EDT Monday advisory, Lester is showing signs of evolving into an annular hurricane--the type that features a large eye and convection concentrated in a single ring around that eye, rather than in spiral bands. Annular hurricanes tend to be slow to weaken, and Lester shows no signs of weakening in the near future. Wind shear will remain less than 15 knots for the next several days, with SSTs around 26-27°C adequate to keep Lester going. NHC predicts that Lester will be about 150 miles northeast of Hilo on Saturday morning as a Category 1 hurricane, on a northwestward-angling track that could keep the storm just north of the islands. Lester is now moving west at 14 mph, bringing it gradually closer to Madeline.


Figure 6. The Fujiwhara effect causes two tropical cyclones near each other to rotate around a common midpoint. This motion is on top of the preexisting movement of each cyclone. Image credit: Hong Kong Observatory.


Ironically, the coexistence of Madeline and Lester may help keep either one from a direct landfall on Hawaii, thanks to the Fujiwhara effect, which was discovered nearly a century ago by Japanese researcher Sakuhei Fujiwhara. When two tropical cyclones get within about 800 miles of each other, the interaction tends to make the pair rotate around a common point in between, with the effect superimposed on the storm’s preexisting motions. In a case like this, the easterm storm (Lester) would angle northward and the western storm (Madeline) would angle southward. Both effects would tend to angle Madeline and Lester away from Hawaii.

Hurricanes and tropical storms are getting more common around Hawaii
The prospect of two potential Hawaiian landfalls in one week is an exceptional event, since tropical storms are so rare in the state. Only five tropical storms have struck since records began in 1949, and two of those have been in the last three years:

--Hurricane Darby made landfall along the southeast shore of Hawaii’s Big Island on July 23, 2016, as a minimal tropical storm (top sustained winds of 40 mph). Damage was minimal and there were no deaths from Darby.

--Tropical Storm Iselle, which, like Darby, made landfall along the southeast shore of the Big Island, arriving as a 60-mph tropical storm on August 8, 2014. Iselle killed one person and did $79 million in damage.

--Hurricane Iniki, which hit Kauai as a Category 4 hurricane, killing 6 and causing $1.8 billion in damage (1992 dollars.)

--Hurricane Dot, which hit Kauai as a Category 1 hurricane, causing 6 indirect deaths and $6 million in damage (1959 dollars.)

--An unnamed 1958 storm that had sustained winds of 50 mph at landfall on the Big Island. The storm killed one person and caused $0.5 million in damage.

The region around Hawaii has seen a lot of tropical activity over the past four years, including a number of near-misses. Partly this is a result of El Niño, which warmed the waters of the tropical Central and Eastern Pacific where Hawaii-heading cyclones are born. However, the uptick may also be a harbinger of things to come. See the August 2014 post, Climate Change May Increase the Number of Hawaiian Hurricanes.


Figure 7. Tracks of all tropical cyclones (tropical depressions, tropical storms, and hurricanes) to pass within 100 miles of the Hawaiian Islands, 1949 - 2016. On July 25, 2016, Tropical Storm Darby made the closest approach on record by a tropical storm to Honolulu, passing just 40 miles to the south and west of Hawaii’s capital with sustained 40 mph winds. Darby brought torrential rains in excess of ten inches to portions of Oahu. Image credit: NOAA/CSC.

We'll be back with updates as conditions warrant.

Jeff Masters and Bob Henson


Hurricane

Invest 99L Finally Develops Into Tropical Depression 9 in the Florida Straits

By: Jeff Masters , 12:11 AM GMT on August 29, 2016

After spending ten days in meteorological limbo-land frustrating forecasters as an “Invest”, 99L finally developed into Tropical Depression Nine, confirmed a NOAA hurricane hunter aircraft late Sunday afternoon. But the storm isn’t done perplexing us yet—the model predictions for the future intensity of the storm remain wildly divergent, even if we now have growing confidence that this storm will track into the coast of Florida north of Tampa on Thursday.


Figure 1. MODIS visible satellite image of Tropical Depression Nine (formerly 99L) forming in the Florida Straits on Sunday afternoon, August 28, 2016. Image credit: NASA.

Satellite images on Sunday evening showed a steady increase in the intensity and areal coverage of TD 9’s heavy thunderstorms, though Key West radar showed only a few spiral bands trying to form near the center. The depression is not likely to organize quickly, as it was dealing with wind shear that was a moderately high 15 - 20 knots. TD 9 was also struggling with dry air, as seen on water vapor satellite imagery. Sea surface temperatures (SSTs) remained favorable for development, though, near 30 - 30.5°C (86 - 87°F).


Figure 2. Total rainfall from Tropical Depression Nine as of 8 pm EDT Sunday, August 28, 2016. Rainfall amounts of 2 - 4 inches were common over Cuba.

Track forecast for TD 9
There is now model consensus among the GFS, European, HWRF and UKMET models that TD 9 will continue on its current west-northwest track through Monday, slow down and stall out in the central Gulf of Mexico on Tuesday, then get caught in the steering flow of a trough of low pressure passing to its north on Wednesday. These steering currents should bring TD 9 to a landfall on the Florida coast north of Tampa on Thursday. In their 5 pm EDT Sunday Wind Probability Forecast, the highest odds for getting tropical storm force winds of 34+ mph from TD 9 were 26%, 24%, and 22%, respectively, for Apalachicola, Panama City, and Cedar Key, Florida. Tampa was given 18% odds.

Intensity forecast for TD 9
The SHIPS model on Sunday afternoon predicted moderately favorable conditions for intensification, with wind shear falling to a moderate 10 - 15 knots, Monday through Wednesday. SSTs will be a very warm 30°C (86°F), and mid-level relative humidity was predicted to be a reasonably moist 65 - 70%. However, the usually reliable European and GFS models showed little to no development of TD 9 in their latest 12Z Sunday (8 am EDT) runs. Our best intensity model, the HWRF model, had TD 9 rapidly intensifying into a strong Category 2 hurricane just before landfall. Other intensity models like the DSHIPS and LGEM models had TD 9 as a borderline Category 1 hurricane at landfall. This storm’s history has been to under-perform, so NHC’s conservative forecast of a 50 mph tropical storm at landfall was a reasonable first guess, given the storm’s weak showing in the GFS and European models. But TD 9 is hiding its cards—still—and could easily be an intensifying hurricane at landfall. The latest 18Z Sunday forecast from the GFS model had TD 9 about 5 - 10 mph stronger than in its previous run. I support a forecast of TD 9 being a strong tropical storm near hurricane strength at landfall—70 mph winds—until the models come into better alignment.

Bob Henson had a detailed summary of the action in the rest of the tropics in his 2 pm EDT Sunday post. We’ll be back Monday morning with a fresh look at the tropics.

Jeff Masters

Hurricane

TD 8 Forms in Atlantic; Still Watching 99L; Hawaii in Sights of TS Madeline

By: Bob Henson , 5:18 PM GMT on August 28, 2016

We’re now approaching the early-September climatological peak of the Atlantic hurricane season, and this week shows how active things can get. We may see up to three newly named storms between now and Labor Day in the Atlantic. Each of these could eventually affect the United States in some form or fashion. Another named storm could sweep over Hawaii in the next week--and Japan is bracing for its third tropical cyclone in a week.

TD 8 may swing past the Outer Banks as a tropical storm
In contrast to the prolonged saga of Invest 99L (see below), Tropical Depression 8 didn’t waste any time becoming a tropical cyclone. An impressive shield of showers and thunderstorms (convection) blossomed on Saturday night, and wind data from an airborne scatterometer (see Figure 4 below) showed that the system had a closed circulation. National Hurricane Center upgraded the system to TD 8 at 11 am EDT Sunday.

TD 8 was located about 400 miles southeast of Cape Hatteras, North Carolina, moving west at 9 mph. Moderate vertical wind shear from the southeast (15-20 knots) has pushed the convection toward northwest side of the low-level center of circulation, as shown in Figure 3 below. An Air Force Hurricane Hunter flight will investigate TD 8 on Sunday afternoon. TD 8 has some kinship to Tropical Storm Fiona: NHC notes that the latter’s remnants were absorbed into a larger circulation that ended up giving birth to TD 8.


Figure 1. Visible satellite image of Tropical Depression 8 as of 1508Z (11:08 am EDT)
Sunday, August 28, 2016. The low-level circulation is evident toward the southeast end of TD 8’s shield of showers and thunderstorms.


Figure 2. Infrared image of Tropical Depression 8 as of 1645Z (12:45 pm EDT) Sunday, August 28, 2016.


Figure 3. The Advanced Scatterometer (ASCAT) aboard the European MetOp-1 satellite detected the circulation around TD 8 (bottom of image) early Sunday, August 28, 2016. Winds of 20-30 mph were evident on the depression’s north side. Image credit: NOAA/NESDIS.

The outlook for TD 8
TD 8 is tucked into the south side of a compact upper-level ridge extending from the mid-Atlantic states into the northwest Atlantic. Computer models are in close agreement on a two-phase track that would bring TD 8 northwest to within several hundred miles of the Outer Banks of North Carolina by Tuesday before the ridge weakens and a sharp right-hand turn takes TD 8 out to sea. A tropical storm watch may be issued for the Outer Banks later Sunday, according to NHC. Tropical storm conditions could affect the Outer Banks even if TD 8 does not make landfall.

NHC projects TD 8 to become Tropical Storm Hermine by late Monday but keeps it at minimal strength (40 mph) thereafter. Wind shear should remain light to moderate (10-20 knots) for the next couple of days, and sea-surface temperatures (SSTs) will remain close to 29°C (84°F), roughly 1-2°C above average. Dry air (relative humidities of only around 40-50%) will be the main impediment to TD 8’s development, with some gradual moistening predicted over the next 3-4 days. By the time it makes its sharp right turn out to sea, TD 8 will be flayed by increasingly strong wind shear.


Figure 4. WU depiction of National Hurricane Center forecast for Tropical Depression 8 as of 11 am EDT Sunday, August 28, 2016.

99L remains a threat to the eastern Gulf of Mexico
The persistent tropical wave known as Invest 99L may yet become a tropical cyclone to contend with, although you might not guess it from satellite imagery. Once again, 99L appears less impressive in real life than some well-regarded computer models had predicted several days ago. On Sunday morning, 99L was grinding its way west-northwest toward the Florida Straits, with a weakly defined center located midway between South Florida and the coast of Cuba. This center was growing more defined early Sunday afternoon, though, with some rotation of the storm's shower activity apparent on Key West radar and some spiral banding near the center beginning to appear. Satellite loops showed that 99L's heavy thunderstorms were relatively modest in areal coverage and intensity, but with some increase in organization in the past few hours. 99L will pass across the Florida Keys late Sunday, bringing some heavy thunderstorms, gusty winds, and moderately high surf (up to 4-6 feet in the Florida Straits). The disturbed weather over the Keys and South Florida should extend into Monday and perhaps Tuesday, even though the center of 99L will have moved into the southeast Gulf of Mexico by that point.

After nearly a week of attention heaped on 99L--perhaps the most intensely covered “invest” in history--it may be tempting to let this system go, but now is not the time for complacency. Throughout the past week, computer models have leaned toward the Gulf of Mexico as the most likely spot for 99L to intensify, although the GFS model has been the most consistently skeptical. Now all three of our best models for tropical cyclone genesis--the GFS, ECMWF, and UKMET--develop 99L into at least a tropical storm as it makes its way through the Gulf over the next several days. The general tendency in these models is to keep 99L rolling west-northwest into the southeast Gulf for another couple of days, then to hook it fairly sharply by midweek to the north and northeast toward Florida’s Gulf Coast. Nearly all of the 50 members of the 00Z Sunday ECMWF ensemble run follow this scenario, with only a couple of runs taking 99L as far west as the Mississippi or far southeast Louisiana coast. The GFS ensemble track forecast from 00Z Sunday is quite similar, though about a day slower. It’s too soon to know which day 99L would make landfall; assuming it does, it could be anywhere in the latter half of the coming week.


Figure 5. MODIS visible satellite image of Tropical Depression Nine (formerly 99L) forming in the Florida Straits on Sunday afternoon, August 28, 2016. Image credit: NASA.

How strong could 99L get?
The major models have also converged on their strength outlooks for 99L. About 50-60% of the ECMWF and GFS ensemble members members bring 99L up to tropical storm strength over the Gulf. Fortunately, only about 10-20% of ECMWF ensemble runs, and none of the GFS ensemble runs, make 99L a Category 1 hurricane. The HWRF model has been consistently stronger--but although the HWRF is among the best intensity models for tropical storms and hurricanes that have already formed, it has a well-known extreme high bias on systems that have not yet developed, so it’s best to ignore HWRF intensity predictions at this point.

Conditions in the Gulf of Mexico will be moderately supportive of development for 99L. Wind shear will be dropping to the light range by Tuesday (5-15 knots). The atmosphere will be fairly moist (relative humidities should hold in the 65-75% range), and 99L will be passing over very warm sea-surface temperatures (SSTs) of around 30°C or 86°F. These warm waters do not extend very far down, however, so the amount of oceanic heat along 99L’s expected path in the eastern Gulf is not particularly large. Also, because 99L is entering the Gulf as a large, disheveled system, we can expect development to be slower than it might be for a smaller, better-organized circulation. In its 8 am EDT Tropical Weather Discussion, NHC gives 99L a 40% chance of development into at least a tropical depression by early Tuesday and a 60% chance by early Friday.

If 99L does become a tropical storm (which would be named Ian), it could have as little as 24-48 hours to organize further before making landfall, which reduces the odds of a significant hurricane. Nevertheless, 99L’s size gives it the potential to become a large-scale rainmaker. We can expect 4” - 8” amounts to be widespread along the Florida Gulf Coast and far South Florida over the next several days (see Figure 6). Moreover, even if it doesn’t make hurricane strength, 99L’s large size could enhance its ability to push water onshore and produce more coastal flooding than one might expect from a typical tropical storm.


Figure 6. Projected 5-day rainfall from 12Z (8:00 am EDT) Sunday, August 28, through 12Z Friday, September 2, 2016. The forecast assumes that 99L will make its way northward through the eastern Gulf of Mexico. Image credit: NOAA/NWS Weather Prediction Center.


Figure 7. Enhanced infrared image of Hurricane Gaston as of 1615Z (12:15 pm EDT) Sunday, August 28, 2016. Image credit: NOAA/NESDIS.

Category 2 Gaston spins through open Atlantic
Though it’s a powerful storm, with top sustained winds of 105 mph, Hurricane Gaston is a well-behaved storm in terms of impact. Located about 600 miles east of Bermuda as of 11 am EDT Sunday, Gaston is creeping northwest at just 5 mph en route to a very gradual recurvature. Gaston should be able to take advantage of favorable atmospheric conditions and unusually warm SSTs to gain a bit more strength before it begins accelerating east-northeast by midweek. By Friday or Saturday, Gaston could be passing just north of the Azores as a minimal hurricane, based on the latest NHC outlook.

An African wave worth watching
A powerful tropical wave will move off Africa this week and begin a long and potentially eventful trek across the tropical Atlantic. NHC gives the wave a 60% chance of development into at least a depression between Tuesday and Friday. The GFS, ECMWF, and UKMET models agree that this wave could be a significant tropical cyclone by next weekend. The wave should remain on a fairly straightforward west to west-northwest path through the period.

At least one landfall may be in the cards for Hawaii
Unusually high SSTs associated with El Niño and long-term warming have given Hawaii more than its historical share of tropical storm action in recent years. Two tropical cyclones moving from east to west became the first since Hawaii's statehood (1959] to make landfall in the Big Island: Tropical Storm Iselle (August 2014) and Tropical Storm Darby (July 2016). In addition, an unnamed tropical storm hit the Big Island in 1958. Now we have one tropical cyclone heading toward Hawaii--and another on its heels that could make a swipe at the islands just days later.

Tropical Storm Madeline continues on a course that is making a Big Island landfall increasingly plausible. Moving northwest at about 7 mph, Madeline was located about 1000 miles east-southeast of Hilo, HI, as of the 11 am EDT Sunday advisory from the NOAA/NWS Central Pacific Hurricane Center in Honolulu. Madeline is fairly small but well-structured, with top sustained winds of 50 mph. The storm is now over waters that are 1-2°C above average, around 28°C, and wind shear will remain very light (less than 10 knots) for the next couple of days. Working against strengthening will be fairly dry air, as mid-level relative humidities around 60-70% now will drop to around 40-50% by midweek. In addition, SSTs along its path will be dropping from around 28°C on Sunday to around 26-27°C by Tuesday, though they should increase later in the week. CPHC brings Madeline to minimal hurricane strength on Tuesday, with a slow decline as the rest of the week unfolds.


Figure 7. Enhanced infrared image of Tropical Storm Madeline (left) and Hurricane Lester (right) as of 1500Z (11:00 am EDT) Sunday, August 28, 2016. Image credit: NOAA/NESDIS.

Madeline is currently angling toward a weakness in the strong ridging to its north. By midweek, the ridge will re-strengthen, and Madeline should take a significant left turn, putting it on a course bending west and then west-southwest toward Hawaii. The 11 am EDT Sunday outlook from CPHC places Madeline near the south end of the Big Island by early Thursday. The average track error in a four-day forecast in this region is 185 miles, so it is too soon to know whether Madeline will actually strike Hawaii or which island(s) it might affect. It would most likely be a tropical storm by this point. If Madeline did pass toward the south end of the Big Island, very heavy rains could result, as strong east winds rotating around the storm would slam against the island’s high mountains. The only tropical cyclone on record to affect Hawaii from the east-northeast is Hurricane Orlene, which made landfall on the Big Island as a tropical depression in September 1994.

And then there’s Lester…
Next up in this unusual queue is Hurricane Lester, now churning well to the east of Madeline (about 1000 mile west-southwest of Cabo San Lucas, Mexico) with top sustained winds of 85 mph, down from peak winds of 105 mph on Saturday night. A strong ridge to its north should keep Lester on a strikingly direct east-to-west path between latitude 18°N and 19°N for the next few days. Lester’s intensity should also hold steady in the Category 1 hurricane range for at least the next several days, with shear remaining knots (10 knots or less) and SSTs holding above 26°C.

The big question concerning Hawaii is whether Lester will begin an expected gradual recurvature in time to avoid the islands. The 00Z Sunday runs of the GFS and ECMWF model take Lester about 100-200 miles north of the islands next weekend. It’s possible that Lester and Madeline will end up within 1200 miles of each other, enough to possibly trigger Fuijiwhara interaction—the process by which two hurricanes can gain a rotational element of motion around a common center. If this were the case, it would tend to nudge Lester further to the north and reduce its potential impact on the islands. The same effect could enhance Madeline’s potential arc toward the south.


Figure 9. The VIIRS instrument aboard NASA-NOAA's Suomi NPP satellite captured this image of Typhoon Lionrock at 0405Z Friday, August 26, 2016. Image credit: NOAA/NASA Goddard Rapid Response Team.

Heavy rains the big threat to Japan from Category 4 Lionrock
Fearsome Typhoon Lionrock is clawing its way through the Northwest Pacific with top sustained winds of 130 mph, as reported by the Joint Typhoon Warning Center at 12Z Sunday. This makes Lionrock the equivalent of a Category 4 hurricane. Lionrock is moving northeast parallel to Japan, about 400 miles south of Tokyo, but by midweek it will make a dramatic left turn into a mid-latitude storm system, not unlike the track taken by Hurricane Sandy in 2012. As with Sandy, the exact timing and angle of the turn will make a big difference in the impacts on northern Japan, which has been deluged over the last few days from Tropical Storms Mindulle and Kompasu.

Lionrock may have already peaked in intensity, with some degradation on its north side evident in satellite imagery over the last few hours. JTWC predicts that Lionrock will weaken to near minimal typhoon/hurricane strength by the time it makes landfall in northern Honshu. Torrential rains can still be expected across much of northern Japan, as well as along the far southeast Russian coast.

Jeff Masters and I will be back with a full update on Monday morning. We’ll also add updated information on 99L within this post on Sunday evening.

Bob Henson


Little Change to 99L in The Bahamas; 91L Headed Towards North Carolina

By: Jeff Masters , 4:35 PM GMT on August 27, 2016

There is little new to say about the saga of tropical wave Invest 99L, which continued to chug west-northwest at 10 mph through the northwestern Bahamas on Saturday morning towards South Florida and the Florida Keys. Satellite loops late Saturday morning showed little change in the storm’s organization and heavy thunderstorms since yesterday; 99L still lacked a well-organized surface circulation center and the amount of heavy thunderstorm activity was modest at best. Wind shear was a moderately high 15 - 20 knots, and 99L was still struggling with dry air, as seen on water vapor satellite imagery. Sea surface temperatures (SSTs) remained favorable for development, though, near 30 - 30.5°C (86 - 87°F). The Hurricane Hunter missions for Saturday afternoon have been cancelled.


Figure 1. Latest satellite image of 99L.


Figure 2. View from a webcam on Great Exuma Island in The Bahamas at 11:36 am EDT August 27, 2016. Heavy rains and gusty winds from 99L were affecting the island. Image credit: Bonefish’s Webcam.

Track forecast for 99L
There is now model consensus among the GFS, European, and UKMET models that 99L will continue on its current west-northwest track for the next three days and not turn to the north along the west coast of Florida. A strong ridge of high pressure now covering much of the Southeast U.S. and northern Gulf of Mexico will remain in place through Tuesday, which should keep 99L on its general west-northwest track at 5 - 10 mph. The storm will pass by South Florida and the Florida Keys on Sunday, bringing heavy rains to South Florida and Cuba on Saturday afternoon through Tuesday afternoon. The 7-day precipitation outlook from NOAA’s Weather Prediction Center (WPC) calls for a large area of 3 - 7” rains across South Florida over the coming week.

Intensity forecast for 99L
The SHIPS model on Saturday morning predicted that wind shear would fall slightly, to 10 - 15 knots, Sunday afternoon and beyond. SSTs will increase to 30.5°C (87°F) by Wednesday, and mid-level relative humidity was predicted to decrease from 65% to 55 - 60%. The shear and dry air may be strong enough to continue to keep 99L from organizing into a tropical depression, as predicted by NHC: in their 8 am EDT Saturday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day odds of development into a tropical depression or tropical storm of 20% and 40%, respectively.

Two of our three reliable models for predicting tropical genesis, the ECMWF and GFS, continued to show no development of 99L into a tropical cyclone over the next five days in their latest 0Z Saturday (8 pm EDT Friday) runs. Our other reliable tropical cyclone genesis model, the UKMET, continued to predict that 99L would develop into a hurricane in the central Gulf of Mexico by the middle of the week. Even though the development odds for 99L have decreased markedly since the storm moved past Puerto Rico, we should not relax our guard with this storm until the UKMET falls in line with the GFS and European models. I will continue to mistrust 99L, since a strong tropical wave moving through the Gulf during the peak part of hurricane season is always a potentially dangerous situation.


Figure 3. Latest satellite image of Invest 91L off the coast of North Carolina.

91L headed towards North Carolina
An area of disturbed weather located about 500 miles east-southeast of North Carolina is moving west-northwest towards that state at about 10 mph, and was designated Invest 91L by NHC on Friday afternoon. Satellite loops on Saturday morning showed 91L had a vigorous but elongated surface circulation, and only a limited amount of heavy thunderstorms, which were removed from the center. A few showers from 91L were affecting Bermuda on Saturday morning, as seen on Bermuda radar. Conditions favorable for development include moderate wind shear near 10 knots, and sea surface temperatures (SSTs) of 29.5°C (85°F). These SSTs were about 2 - 3°F above average, and in July 2016, were the warmest July SSTs ever recorded for that area of ocean. However, 91L is embedded in area of dry air with humidities around 40 - 45% at mid levels of the atmosphere, and this dry air was stymying development.

Forecast for 91L
Steering currents favor a west-northwesterly motion for 91L at about 5 - 10 mph over the next three days. By Monday evening, 91L will begin spreading heavy rains along the coast of North Carolina, and the storm could move ashore on Tuesday before turning northeastwards, out to sea. That doesn’t give 91L a lot of time to develop, particularly since the atmosphere surrounding the storm is expected to remain quite dry. The 8 am EDT Saturday run of the SHIPS model predicted otherwise favorable conditions for development through Tuesday, with wind shear in the moderate range, 10 - 20 knots, and unusually warm SSTs near 29 - 29.5°C (84 - 85°F.)

The Saturday morning operational runs of our three reliable models for predicting tropical cyclone genesis, the European, GFS and UKMET models, did not show development of 91L, but the GFS ensemble forecast, done by taking the operational high-resolution version of the model and running it at lower resolution with slight perturbations to the initial conditions in order to generate a range of possible outcomes, had more than 50% of its twenty ensemble members predict that a tropical depression would form. Less than 20% of the 50 members of the 00Z Saturday European ensemble model forecasts showed development. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave 91L 2-day and 5-day development odds of 30%. Since the atmosphere 91L is embedded in is so dry, this storm should not be a huge rain maker for North Carolina relative to their usual tropical storms..

New tropical wave due to emerge from Africa on Tuesday may develop
The Saturday morning operational runs of our three reliable models for predicting tropical cyclone genesis, the European, GFS and UKMET models, all predicted that a strong tropical wave expected to emerge from the coast of Africa on Tuesday would develop into a tropical depression by late next week as it moves to the west at 15 - 20 mph. In their 8 am EDT Saturday Tropical Weather Outlook, NHC did not yet give 2-day and 5-day development odds for this system, but I give the wave 5-day development odds of 10%.


Figure 4. Latest satellite image of Tropical Storm Gaston.

Elsewhere in the tropics
The tropics are very busy this weekend, but that’s what we expect this time of year. In the Atlantic, we have Tropical Storm Gaston churning the waters about 750 miles east-southeast of Bermuda. Gaston is not a threat to land, but is expected to intensify into a Category 2 hurricane as it recurves to the northeast early in the week.


Figure 5. Latest satellite image of Tropical Storm Madeline.


Figure 6. Latest satellite image of Hurricane Lester.

Hawaii is keeping a close eye on Tropical Storm Madeline, which is forecast to pass through the islands on Thursday. Close behind Madeline is Hurricane Lester, which may pass near or through the islands next Saturday.


Figure 7. Latest satellite image of Typhoon Lionrock.

Japan is watching Category 3 Typhoon Lionrock, which is expected to curve to the northwest and hit Japan as a tropical storm on Wednesday.

Bob Henson will be back with a new post by early Sunday afternoon.

Jeff Masters

Hurricane

Wait-and-See Drama With 99L Will Continue For Up to Another Week

By: Jeff Masters , 9:28 PM GMT on August 26, 2016

The week-long wait-and-see drama will continue for up to another week with the large but disorganized tropical wave (Invest 99L) chugging west-northwest at 10 mph through The Bahamas. Satellite loops late Friday afternoon showed an increase in the storm’s organization and heavy thunderstorms, but there was no well-organized surface circulation center and the amount of heavy thunderstorm activity was modest at best. Wind shear dropped from a high 20 - 25 knots early Friday morning to a moderate 10 - 15 mph by Friday afternoon, which allowed the increase in 99L’s heavy thunderstorms near its center to occur. However, 99L was still struggling with dry air, as seen on water vapor satellite imagery. Arc-shaped lines of surface cumulus clouds forming along the outflow boundaries of dry thunderstorm downdrafts, as seen in visible satellite imagery (Figure 1), were more evidence that the storm was struggling with dry air. Sea surface temperatures (SSTs) remained favorable for development, though: 29.5°C (85°F). Satellite-measured surface winds at 10:17 am EDT Friday from the ASCAT instrument were as high as 35 mph in the heaviest thunderstorms to the east of 99L’s center. The Hurricane Hunters did not investigate 99L on Friday, but are scheduled to investigate again on Saturday afternoon.


Figure 1. MODIS visible satellite image of 99L taken Friday morning, August 26, 2016. A surface circulation center was trying to form over the central Bahamas. Heavy thunderstorms were building along the southeast side of the center, but were ingesting dry mid-level air. This dry air causes evaporative cooling, creating cooler, dense air and strong downdrafts within 99L’s thunderstorms, robbing them of moisture. When these downdraft hit the ocean, they spread out horizontally, creating an arc-shaped band of surface cumulus clouds showing the outflow boundary of the air from the downdraft. As 99L continues to struggle with dry air, expect to see more of these arc-shaped outflow boundaries.


Figure 2. The vertical wind shear between 200 mb and 850 mb as analyzed by the University of Wisconsin/CIMSS on Friday morning, August 26, 2016. In the space of six hours, the wind shear near the core of 99L fell from about 25 - 30 knots (yellow-green to yellow colors), to about 15 knots (dark blue-green colors.) The dashed light blue lines show where there had been a 5 knot decrease in wind shear over the previous 24 hours; the white solid lines outline areas where the wind shear had increased by 5 knots in 24 hours. The reduction in wind shear on Friday helped allow 99L to grow more organized and build some heavy thunderstorms near its center.

Track forecast for 99L
A strong ridge of high pressure now covering much of the Southeast U.S. and northern Gulf of Mexico will remain in place through Sunday, which should keep 99L on its general west-northwest track at 5 - 10 mph for the next three days. The storm will reach South Florida and the Florida Keys on Sunday morning. At that point, the models diverge, with the GFS and European model predicting that the ridge of high pressure steering the storm will weaken, allowing 99L to turn more to the north. The UKMET model keeps 99L moving west-northwest into the central Gulf of Mexico, keeping the storm in the Gulf through next Friday.

Intensity forecast for 99L
Since 99L has moved into an area of higher moisture and lower wind shear, the storm is likely to slowly but steadily organize. The SHIPS model on Friday afternoon predicted that wind shear would remain mostly in the moderate range, 10 - 15 knots, for the next five days. SSTs will increase to 30°C (86°F) by Sunday, and mid-level relative humidity was predicted to increase from 65% to 70%. However, there will still be some dry air for 99L to contend with.

Two of our three reliable models for predicting tropical genesis, the ECMWF and GFS, did not show development of 99L into a tropical cyclone over the next five days in their latest 12Z Friday (8 am EDT) runs. These models brought 99L northwards along the west coast of Florida early next week, spreading heavy rains of 3 - 5” along the west coast of Florida. Our other reliable tropical cyclone genesis model, the UKMET, continued to predict that 99L would develop into a hurricane in the central Gulf of Mexico by the middle of next week. At this point, pretty much any outcome you can imagine is still on the table--failure to develop, development into a weak but rainy tropical storm, or intensification into a hurricane over the Gulf of Mexico. In their 2 pm EDT Friday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day odds of development into a tropical depression or tropical storm of 30% and 60%, respectively.

Regardless of whether or not 99L ever becomes Tropical Storm Hermine, Florida is going to get some rain from this system. The 7-day precipitation outlook from NOAA’s Weather Prediction Center (WPC) calls for a large area of 3 - 5” rains across southern and western Florida over the coming week. If 99L takes the more westerly track predicted by the UKMET model, west Florida will be spared the heaviest rains.

I’ll continue to plug away documenting the life and times of 99L and its seemingly eternal quest to get its name on Saturday.

Jeff Masters

Hurricane

Still Too Soon to Write Off Struggling 99L

By: Bob Henson , 3:42 PM GMT on August 26, 2016

After vexing and perplexing forecasters and the public for days, the strong tropical wave dubbed Invest 99L is attempting once more to organize itself. The elongated circulation associated with 99L extends from the far southeastern Bahama Islands across eastern Cuba to just west of Jamaica. As of early Friday, 99L had failed to develop a coherent circulation, with a low-level center devoid of showers and thunderstorms (convection) spinning hundreds of miles north of intense convection over parts of Hispaniola and Jamaica. The NOAA Hurricane Hunter mission that had been scheduled for Friday morning was cancelled.

The main reason for 99L’s failure to develop has been unexpectedly strong vertical wind shear, as evident in the high-level cirrus blowing northward off the tops of thunderstorms. This wind shear has injected relatively dry air into the wave while keeping a consolidated center from forming. However, 99L is now moving into a region of much lower wind shear for the next couple of days, as indicated by Friday morning (12Z) data from the SHIPS statistical model. A new burst of convection appeared on Friday morning near the low-level circulation between far eastern Cuba and the southeast Bahama Islands (see Figure 1). If this were to persist, it could make the beginning of a long-awaited growth phase for 99L. The National Hurricane Center reduced the 2-day odds that 99L would become a tropical depression to 20% in its 8 AM Tropical Weather Update. I wouldn’t be surprised to see those odds going back up at 2 pm if the convection continues to blossom near 99L’s center. In order to develop further, 99L will need to fend off dry air that continues to influence its thunderstorm activity (see Figure 2).


Figure 1. Infrared image of Invest 99L as of 1415Z (10:15 am EDT) Friday, August 26, 2016. Image credit: NOAA/NESDIS.


Figure 2. Visible satellite image of 99L taken at 11:08 am EDT August 26, 2016. A surface circulation center was trying to form over the central Bahamas. Heavy thunderstorms were building along the southeast side of the center, but were ingesting dry mid-level air. This dry air is heavier than moist air, and creates strong downdrafts within its thunderstorms that are robbing 99L of moisture. When these downdraft hit the ocean, they spread out horizontally, creating an arc-shaped band of surface cumulus clouds showing the outflow boundary of the air from the downdraft. As 99L continues to struggle with dry air, expect to see more of these arc-shaped outflow boundaries.

A victory for the GFS--but it’s too soon to write off 99L
In the latest iteration of the “ECMWF vs. GFS” battle that has raged for years, the GFS appears to have won the latest round. Several of our most reliable models, including the ECMWF and UKMET, developed 99L into a tropical storm by today, while the GFS remained insistent that no major development would occur. Kudos to the GFS for this one!

Even the more gung-ho runs of the ECMWF and UKMET earlier this week generally intensified 99L at a fairly modest pace for the period from Wednesday through Friday, with more rapid strengthening toward this weekend. Although it is not well organized enough to take full advantage of the situation, 99L is now entering a region of light to moderate wind shear (5 - 15 knots) and very warm sea-surface temperatures (29.5 - 30°C or 85 - 86°F), which would allow a more organized system to intensify. Though the odds of any rapid development are fairly low, it would be prudent to monitor Friday’s convective blow-up and make sure that 99L has no surprises up its sleeve.

Outlook for 99L this weekend and beyond
The 00Z Friday ensemble of the GFS and ECMWF are in general agreement that 99L will continue moving west-northwest on a track that would put it somewhere in or near the Florida Keys around Sunday. None of the GFS and ECMWF ensemble members bring 99L up to tropical storm strength before that point, while the HWRF--overall the best-performing intensity model of recent years--is an outlier, calling for 99L to reach tropical storm strength by Sunday. If 99L does organize today, we'll be looking closely at the next couple of rounds of models to see if any major changes occur. In any event, this morning’s burst of convection reminds us that 99L is capable of bringing very heavy rains to southern Florida over the weekend.

There remains plenty of uncertainty over 99L’s future beyond the weekend. The operational GFS and ECMWF model runs from 0Z Friday take 99L northward through the eastern Gulf and into the upper Gulf Coast of Florida. A minority of GFS ensemble members bring 99L further west, while the four ECMWF ensemble members that make up the “high probability cluster” (those that have performed the best on 99L over the last 24 hours) keep the system moving northward, very close to Florida’s west coast, as a tropical depression or weak tropical storm. The HWRF and UKMET are more aggressive on intensifying 99L further west in the open Gulf, where there would be less interaction with land. We can expect models to get a better handle on 99L if and when it develops a coherent circulation.

The bottom line: 99L remains a system well worth monitoring as it makes its way into south Florida and the Gulf of Mexico.


Figure 3. Members of the GFS ensemble modeling system (GEFS) are in close agreement that 99L will end up in or near the Florida Straits, but there is still wide disagreement on its path beyond that point later next week. These paths were generated at 12Z (8:00 am EDT) Friday, August 26, 2016, based on data from the 06Z Friday model runs.

A pair of East Pacific systems worth watching
Next week may be active in the East and Central Pacific, as two systems--Tropical Storm Lester and Invest 98E--chug westward on trajectories that could put them in the vicinity of the Hawaiian Islands late next week or beyond. Already well-organized, Lester is packing top sustained winds of 60 mph. Wind shear is injecting some dry air into Lester, but that should abate over the weekend, allowing Lester to become a hurricane as it continues on a nearly due-west path as projected by the 5:00 am EDT outlook from NHC. Further to the west, 98E is off to a healthy start, with a large and growing shield of convection. NHC gives 98E an 80% chance of development by Sunday, and models are in broad agreement that it will become Tropical Storm Madeline in the next several days. 98E will have a northward component to its motion at first, but in the longer range it should end up heading westward a few hundred miles ahead of Lester, likely peaking below hurricane strength.


Figure 4. Enhanced infrared image of Tropical Storm Lester produced with data from NASA’s MODIS (Moderate Resolution Imaging Spectroradiometer) as of 0915Z (5:15 am EDT) Friday, August 26, 2016. Image credit: CSU/RAMMB/CIRA.

A couple of factors are making 98L and Lester worth keeping an eye on:

--Although SSTs are cooler than average near the equator (with a borderline La Niña attempting to develop) and in the subtropics around 30°N, sandwiched in between is a region of SSTs that are 0.5°C to 1.0°C above average in the latitude belt around 15-20°N, which includes Hawaii. This would allow a westward-moving system approaching Hawaii to remain over waters near or just above 26°C (79°F), a standard benchmark for supporting tropical development.

--A very strong upper-level ridge in the Northeast Pacific should keep Lester and 98E from heading very far northwest or recurving northeast in the common fashion of East Pacific tropical storms and hurricanes. Such paths typically sound the death knell for these cyclones, as they end up over the cool waters north of 20°N.

Unusually high SSTs associated with El Niño and long-term warming have given Hawaii more than its historical share of tropical storm action in recent years. Two tropical cyclones moving from east to west became the first since Hawaii's statehood (1959] to make landfall in the Big Island: Tropical Storm Iselle (August 2014) and Tropical Storm Darby (July 2016). (In addition, an unnamed tropical storm hit the Big Island in 1958.) We have plenty of time to monitor both 98E and Lester. If 98E does become Madeline, it will keep us well ahead of schedule for East Pacific named storms. For the period 1971-2009, the average formation date of the “M” storm was September 28.

Jeff Masters will be back with our next update late Friday afternoon.

Bob Henson


Figure 5. Long-range projected tracks for Invest 98E (EP98) and Tropical Storm Lester, produced by the 20 members of the GFS ensemble modeling system (GEFS) as of 00Z Friday, August 26, 2016. Any encounter with the Hawaiian Islands would be no sooner than Wednesday, August 31, and the systems would most likely be at tropical storm strength at best. Image credit: NOAA/ESRL.

Hurricane

Taking Its Time, 99L Remains a Potential Threat for Florida, Gulf

By: Jeff Masters and Bob Henson , 10:20 PM GMT on August 25, 2016

The watching and waiting continues for Invest 99L as it rolls toward The Bahamas. 99L remained a very large but very disorganized tropical wave on Thursday afternoon. Visible satellite imagery shows one circulation near the extreme southeastern portion of the Bahamas and Turks and Caicos Islands. This circulation is largely naked, with little cloudiness around it, although some filling-in was evident late Thursday afternoon. Air Force Hurricane Hunters found winds of tropical-storm force in 99L early Thursday, but since the circulation was not fully closed, it could not yet be classified as a tropical depression or tropical storm. On their second mission of the day, Air Force Hurricane Hunters found that winds to the east of this not-quite-closed circulation had decreased by early afternoon Thursday to just below tropical storm force.

Well south of the storm-free circulation center of 99L, very intense thunderstorms were developing late Thursday afternoon along the north coast of Haiti and the Dominican Republic, while decreasing on the south side of the island. Most provinces of the Dominican Republic were on flood alert Thursday evening, and rivers were already surging according to an Associated Press report.

Based on the Thursday afternoon flight data, NHC’s Tropical Weather Outlook issued at 2:00 pm EDT Thursday reduced the odds that 99L will develop into at least a tropical depression to 40% by Saturday and 70% by Tuesday. The slow pace of development thus far isn’t a total shock, as a number of computer models suggested as far back as Monday night that 99L would be no more a mid-strength tropical storm at best by this point. However, the lack of development at this point will keep 99L from growing as intense as it otherwise might have been by this weekend.


Figure 1. Visible image of Invest 99L as of 2045Z (4:45 pm EDT) Thursday, August 25, 2016. Image credit: NOAA/NESDIS.


Figure 2. Enhanced infrared image of Invest 99L as of 2045Z (4:45 pm EDT) Thursday, August 25, 2016. Image credit: NOAA/NESDIS.

99L remains a potential threat for Florida and other Gulf Coast states
It’s too soon to completely write off 99L as a threat to Florida, as it still has at least 48 hours to organize before reaching the area. The wave remains a potentially significant threat to the Gulf of Mexico coast next week. Up to now, a substantial amount of vertical wind shear has helped to displace the upper- and lower-level portions of 99L, while also driving large amounts of dry air into the wave. The University of Wisconsin-CIMSS wind shear analysis from Thursday afternoon was still showing a high 25 knots of wind shear over the core of 99L, which likely accounts for the inability of the storm to develop any heavy thunderstorm activity at its center of circulation. This state of affairs may change in the near future, though. Wind shear as analyzed by the SHIPS model is forecast to fall below 15 knots by Friday afternoon and then to remain in the low to moderate range, 5 - 15 knots, through Tuesday.

Along with the reduced wind shear on Friday and Saturday, 99L will be traveling over very warm waters (around 30°C or 86°F). Since tropical waves often develop convection overnight, we’ll have to see if 99L can form a core of showers and thunderstorms around its low-level center during this usual noctural peak. If it does, the chances of a substantial tropical storm reaching Florida will rise significantly.


Figure 3. Vertical wind shear (shown here as the difference between winds at the 200- and 850-mb heights, or between about 40,00 and 5000 feet) at 18Z (2:00 pm EDT) Thursday, August 25, 2016. Northerly shear of greater than 20 knots has prevailed over 99L, but the wave will be moving into an area of lower wind shear this weekend. Image credit: tropicaltidbits.com.

99L is likely heading toward the Gulf of Mexico
The lack of a well-defined center makes it more difficult for computer models to predict the track and intensity of 99L. If a center does consolidate, it appears likely to move in a general west-northwest direction that would bring it near the southern Florida peninsula late Saturday or Sunday. A slight bend toward the west, as suggested by the ECMWF model, could allow 99L to miss the peninsula entirely, perhaps crossing the Florida Straits. In either case, 99L appears likely to enter the eastern Gulf of Mexico by the start of next week. Conditions in the eastern Gulf will favor development, and the ECMWF, UKMET, and HWRF models have been largely consistent in showing 99L intensifying across the Gulf, perhaps well beyond hurricane strength. The GFS is stubbornly in the other camp, continuing to insist that 99L will not develop significantly.

NOAA/RAMMB has this excellent 1-minute resolution loop of 99L. You can really see how an upper level anticyclone centered to the east of 99L is bringing strong southerly upper-level winds to the south of 99L, blowing the tops of the thunderstorms from south to north over Cuba and Hispaniola.

A multi-day animation from the Navy is excellent, too.

We’ll be back with an update by midday Friday.

Bob Henson and Jeff Masters

Hurricane

Is the Wait-and-See Game With 99L Ending? Also, Tornadoes Rip Indiana and Ohio

By: Jeff Masters and Bob Henson , 3:27 PM GMT on August 25, 2016

A high-stakes game of wait-and-see is underway with a large but disorganized tropical wave (Invest 99L) centered near the southeastern Bahama Islands on Thursday morning. The storm brought heavy rains of 3 - 5” over the past 24 hours to portions of the Dominican Republic and Puerto Rico, and was generating winds near tropical storm force in the waters to the north of Hispaniola. 99L could become a tropical depression or tropical storm at any time over the next three days as it heads west-northwest through The Bahamas. If 99L develops a well-defined surface circulation and tropical storm-force winds, it will be called Tropical Storm Hermine.

Satellite loops late Thursday morning showed the possibility that the game of wait-and-see may be ending. A well-defined surface circulation was attempting to form near the extreme southeastern portion of the Bahamas and Turks and Caicos Islands. However, 99L’s heavy thunderstorms were several hundred miles to the southeast of this surface swirl, and there was very little thunderstorm activity near the center. Development of heavy thunderstorms was being inhibited by high wind shear of 15 - 25 knots and dry air, as seen on water vapor satellite imagery. Sea surface temperatures (SSTs) were favorable for development, though: 29°C (84°F). An Air Force hurricane hunter aircraft was investigating 99L late Thursday morning, and at 11 am EDT found winds just below tropical storm-force--35 mph--a few hundred miles north of Hispaniola, on the east side of 99L's surface circulation.


Figure 1. Satellite image of 99L taken at 10:39 am EDT August 25, 2016. A surface circulation center was trying to form over the southeastern Bahamas and Turks and Caicos Islands.

Track forecast for 99L
A strong upper-level ridge now covering much of the Southeast U.S. and northern Gulf of Mexico will remain in place through Saturday, which should keep 99L on its general west-northwest track for the next three days. The storm will slow down from its current 15 mph forward speed to about 8 mph on Friday and Saturday, and reach the northwest Bahamas on Saturday and South Florida or the Florida Keys on Sunday. At that point, the models predict that the ridge of high pressure steering the storm will weaken, allowing 99L to turn more to the north. The timing of this turn and how far 99L might make it into the Gulf of Mexico is uncertain, though the Florida Gulf Coast is currently the area of the Gulf Coast considered to be most at risk by the models.

Intensity forecast for 99L
Since 99L is now pulling away from the high mountains of Hispaniola and is moving into an area of higher moisture and lower wind shear, the storm is likely to consolidate around the circulation center attempting to form in the southeast Bahamas. The storm should be able to develop into a tropical depression or tropical storm on Friday or Saturday. The SHIPS model predicts that wind shear will fall to the low range, 5 - 10 knots, on Friday through Sunday, and SSTs will increase to 30°C (86°F)—conditions that are very favorable for development. However, there will still be some dry air for 99L to contend with, and the storm is large, which will slow down development. The sinking air over the Western Atlantic associated with a phase of the Madden-Julian Oscillation (MJO) that is unfavorable for tropical storm development has decreased over the past two days, and it appears that the MJO will not be a factor in the intensity forecast for 99L.

Two of our three reliable models for predicting tropical genesis, the ECMWF and UKMET, continued to show development of 99L into a tropical storm by Friday in their latest 0Z Thursday (8 pm EDT Wednesday) runs. These models brought 99L across South Florida or the Florida Keys on Sunday and into the Gulf of Mexico, with a second landfall occurring on the Florida Gulf Coast on Tuesday. Our other reliable tropical cyclone genesis model, the GFS, continued to insist that 99L would not develop through Sunday. The initial track forecast for 99L in this morning’s run of the GFS was too close to the coast of Hispaniola compared to the European model’s track, so I think the European’s model’s forecast will be superior to the GFS forecast. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day development odds of 50% and 80%, respectively. Unfortunately, we don’t have much skill forecasting rapid intensification, but the possibility exists that 99L could undergo a period of rapid intensification in the 24 hours before landfall in South Florida and arrive there as a Category 1 hurricane on Sunday. However, it is more likely that South Florida will experience a tropical storm. If the storm manages to spend an extra day of two over the warm waters of the Gulf of Mexico on Monday and Tuesday, 99L will have additional time to organize and intensify, and there is a greater chance it will be a hurricane for its second potential landfall on Tuesday.

Regardless of whether or not 99L ever becomes Tropical Storm Hermine, Florida is going to get a lot of rain from this system. The 7-day precipitation outlook from NOAA’s Weather Prediction Center (WPC) calls for a large area of 5” - 8” rains across southern and western Florida over the next week. Such rains could put stress on the 80-year-old dike that protects thousands of residents near Lake Okeechobee, as I discussed in a special post yesterday.


Figure 2. The twister that produced widespread damage in Kokomo, IN, was captured in this still image drawn from a video. Image credit: Bruce Robinson via AP.

Localized tornado outbreak pummels Indiana, Ohio
An afternoon of garden-variety Midwest thunderstorms morphed into an Wednesday-evening swarm of tornadoes that surprised residents and forecasters alike. As of Thursday morning, NOAA’s Storm Prediction Center (SPC) had tallied 35 tornado reports in Indiana and Ohio from 12Z (8:00 am EDT) Wednesday through 12Z Thursday. Most of these were associated with two powerful supercell thunderstorms that traveled from northern Indiana into northwest Ohio. Data from survey teams that were canvassing the region on Thursday will help nail down how many independent damage swaths were produced by the two apparent tornado “families”. The NWS/Indianapolis office will be posting images and information at a dedicated website.

Hardest-hit was the Kokomo area, about 60 miles north of Indianapolis, which was struck at the beginning of the outbreak just after 3:00 pm EDT Wednesday. The NWS confirmed EF3 damage in Kokomo, according to weather.com. Dozens of homes and businesses were damaged, and a Starbucks shop was flattened in dramatic fashion, as captured on the video embedded at bottom. People inside the Starbucks were not seriously hurt, as they had taken shelter within a bathroom. Amazingly, despite the widespread damage, only minor injuries had been reported in Kokomo as of Wednesday morning. Subsequent tornadoes produced far-flung but non-catastrophic damage for the next five hours, with the final twister in the mini-outbreak occurring around 8:00 pm EDT near Gallup, OH, more than 100 miles east-northeast of Kokomo.


Figure 3. A vehicle rests on another after a tornado struck in Kokomo, IN, on Wednesday, August, 24, 2016. Multiple tornadoes touched down in Indiana on Wednesday, tearing the roofs off apartment buildings, sending air conditioners falling onto parked cars, and cutting power to thousands of people. Image credit: TeeJay Crawford via AP.

What happened—and why?
Severe weather researchers are already puzzling over Wednesday’s event, which did not present itself earlier in the day as a classic set-up for tornadoes. Humid, unstable air was located over southern Indiana and Ohio, but this is a common occurrence across the Midwest in August. Cloudier, more stable conditions prevailed toward the north, and the resulting boundary appears to have served as a focal point for the day’s severe storms. NOAA/SPC had flagged northern Indiana and Ohio with only a marginal risk of severe weather earlier in the day, with less than a 2% chance of tornadoes. However, nearly all of the tornadoes were captured by SPC tornado watches, including one issued just a few minutes before Kokomo was struck, and the tornadoes themselves received ample warning from local NWS offices.

The biggest surprise is the lack of robust upper-level support for the tornadic storms. Paul Markowski (Pennsylvania State University) found only a modest amount of deep vertical wind shear in the closest upper-air soundings collected at 8:00 pm EDT Wednesday from Wilmington, OH, and Detroit, MI. Although vertical wind shear through the lowest few miles of the atmosphere is destructive to hurricanes, it helps support the asymmetric structure of tornadic supercells. However, the Wilmington and Detroit soundings did show strong low-level wind shear (between the surface and about 3000 feet). In addition, high relative humidities near the surface meant that the bases of the supercell storms were quite low, which would have enhanced rising motion in the vicinity of the low-level shear—a boon for tornado development.

Bob Henson (tornado portion) and Jeff Masters (99L portion)



Video 1. The exterior of a Starbucks shop is destroyed by tornadic winds as videotaped from an adjacent business. Patrons at the Starbucks had taken shelter in the store’s bathroom, and none were hurt seriously.

Hurricane Tornado

Huge 99L Generating 55 mph Winds, But Remains Disorganized

By: Jeff Masters , 10:13 PM GMT on August 24, 2016

A huge and powerful tropical wave (Invest 99L) is generating winds of tropical storm force near the Virgin Islands, and could become a tropical storm at any time over the next two days as it heads west-northwest at 15 mph towards The Bahamas. If 99L develops a well-defined surface circulation, it will be called Tropical Storm Hermine. An Air Force hurricane hunter aircraft was investigating 99L early Wednesday afternoon and found sustained surface winds of 50 - 55 mph just to the northeast of St. Croix in the Virgin Islands. The storm has brought widespread rainfall amounts of 1” to Puerto Rico and the Virgin islands as estimated by San Juan radar, with a one area of northwest Puerto Rico receiving over 3”. A flash flood watch continues for Puerto Rico and the U.S. Virgin Islands until midnight Wednesday.

Satellite loops late Wednesday afternoon showed a dramatic west-northwestward expansion of 99L’s heavy thunderstorm activity, and this sprawling tropical wave was spreading heavy rains across a 1000-mile wide stretch of ocean, from the southeast Bahama Islands to Barbados in the southern Lesser Antilles Islands. It will be difficult for such a massive storm to develop a well-defined surface circulation, and multiple swirls separated by hundreds of miles have been evident in the system during the day. By Wednesday afternoon, wind shear had increased by 10 knots in 24 hours over 99L’s northern flank, to a high 15 - 35 knots. This wind shear was prohibiting the storm from getting organized, as was dry air from the Saharan Air Layer (SAL), as seen on water vapor satellite imagery. Sea surface temperatures (SSTs) were favorable for development: 29°C (84°F), about 1°C above average.


Figure 1. At 4:45 pm EDT Wednesday, 99L sprawled out over a area 1,000 miles across. Two swirls in the cloud pattern were evident, and 99L did not have a single well-defined circulation center. Image credit: NOAA.


Figure 2. Tropical storm-force winds of 55 mph in 99L as seen from an altitude of 600 feet during Wednesday afternoon’s hurricane hunter flight. Image credit: ARWO 1st LT Carpenter, Air Force Hurricane Hunters. Thanks go to wunderground member Saltydogbwi1 for posting this image in the blog comments.

Forecast for 99L still uncertain
Since 99L has not yet formed a well-defined circulation center, it has been difficult for models to agree on its future track and intensity. This situation will likely continue until at least Thursday afternoon, when the SHIPS model predicts that wind shear will fall to the moderate range, 10 - 20 knots. This drop in shear will potentially allow 99L to organize into a tropical storm and give the models something more substantial to chew on.

A strong upper-level ridge now covering much of the Southeast U.S. and northern Gulf of Mexico will remain in place through the weekend, which should keep 99L on its general west-northwest track through at least Friday. Two of our three reliable models for predicting tropical genesis, the ECMWF and UKMET, continued to show development of 99L into a tropical storm by Friday in their latest 12Z Wednesday runs. These models brought 99L across or near South Florida on Sunday and into the Gulf of Mexico, with a second landfall occurring on the Florida Gulf Coast on Tuesday. Our other reliable tropical cyclone genesis model, the GFS, continued to insist that 99L would not develop through Sunday. In their 2 pm EDT Wednesday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day development odds of 60% and 80%, respectively.

We’ll be back with a full analysis on Thursday morning. The Hurricane Hunters have flights scheduled every six hours into the storm. Wunderblogger Steve Gregory offers his take on 99L in his Wednesday afternoon post.

Jeff Masters



Hurricane

99L Poised to Become Tropical Storm Hermine

By: Bob Henson and Jeff Masters , 3:57 PM GMT on August 24, 2016

Invest 99L is already bringing winds of tropical storm force to the northern Lesser Antilles Islands, and it could become a tropical storm at any time over the next day as it heads west-northwest at 15 mph towards Puerto Rico and the Virgin Islands. At Barbuda, sustained winds at minimal tropical storm-force—39 mph—were observed at 7:18 am AST, with a wind gust of 45 mph. At 10 am AST, Princess Juliana Airport in St. Maartin recorded sustained winds (below tropical storm-force) of 32 mph, gusting to 48 mph.

An Air Force hurricane hunter aircraft was investigating 99L late Wednesday morning, and found sustained surface winds of 45 - 50 mph, and sustained winds at their 500-foot flight level of 50 - 55 mph. At 11:35 am EDT, the National Hurricane Center issued a special Tropical Weather Outlook noting that the reconnaissance mission was still ongoing. As soon as 99L develops a well-defined surface circulation, it will be called Tropical Storm Hermine.

The storm was generating heavy rains over the northern Lesser Antilles Islands on Wednesday morning, as seen on radar out of Martinique. Satellite loops on Wednesday morning showed that heavy thunderstorm activity had increased in intensity over the previous 24 hours, and had grown more organized. A well-defined surface circulation had not yet formed, though several swirls in the cloud pattern suggested that the storm may not be far from establishing one. Moderate to high wind shear of 15 - 25 knots along the north side of the storm was keeping all of 99L’s heavy thunderstorms confined to the south. Dry air from the Saharan Air Layer (SAL) was still hindering development, as seen on water vapor satellite imagery. Sea surface temperatures (SSTs) were favorable for development: 29°C (84°F), about 0.5°C above average. A flash flood watch is posted for Puerto Rico and the U.S. Virgin Islands, where 2 - 4 inches of rain are expected.


Figure 1. Latest satellite image of 99L.


Figure 2. Radar image of 99L taken at 9:45 am EDT August 24, 2016. Heavy rains were observed over the northern Lesser Antilles Islands. Image credit: Meteo France.

Track forecast: Still uncertainty beyond The Bahamas
Since 99L had not yet formed a well-defined circulation center as of early Wednesday, it has been difficult for models to agree on its future track and intensity. Moreover, it appears that upper- and lower-level circulations are not yet well aligned. As a result, there remains a good bit of spread in how models are foreseeing its potential track.

A strong upper-level ridge now covering much of the Southeast U.S. and northern Gulf of Mexico will remain in place through the weekend, keeping 99L on its general west-northwest track for the time being. Among our three most reliable models for tropical genesis and track forecasting, the ECMWF has been the most consistent over the last couple of days. The ECMWF continues to bring 99L across or near South Florida on Sunday or Monday and moves it onward into the Gulf of Mexico. More than 90% of the 50 ECMWF ensemble members from 00Z Wednesday--and all four members of the “high probability cluster” (those that performed the best on 99L during the preceding 24 hours)--bring 99L into the Gulf. The system could make landfall next week anywhere from Texas to the Florida Panhandle, according to various ECMWF ensemble members.

The UKMET model has largely agreed with the ECMWF in recent runs, bringing 99L over Florida and into the Gulf. In contrast to its two counterparts, the GFS has been skeptical that 99L will develop at all, keeping it as a weak low--although the 00Z Wednesday run of the GFS does develop the system on its way out to sea off Florida’s East Coast. The GFS ensemble from 00Z Wednesday offers a wide range of track possibilities, with some members bringing 99L into the eastern Gulf and others keeping the system well offshore of the southeast U.S. Coast. It appears that the GFS has been putting more weight on the potential effect of Tropical Storm Gaston in helping to create a potential out-to-sea route for 99L well to the northeast of Florida. Meanwhile, UKMET ensemble runs are consistent in bringing 99L across Florida and into the Gulf, though at varying latitudes. See Figure 3 below for graphics depicting the GFS and UKMET ensemble run from 00Z Wednesday.

Given the overall agreement between the UKMET and ECMWF, and the latter’s typically high performance, a track somewhere through Florida or the Florida Straits and into the Gulf appears to be the most likely outcome, though we can’t yet rule out the northward GFS possibilities shown below. The Florida Keys will need to be especially vigilant, as 99L could track over or near them as soon as Sunday if the relatively speedy ECMWF projection is correct. The Keys are extremely vulnerable to hurricane impacts, and up to 84 hours are needed for a full evacuation along the only highway that runs from Key West to the mainland.


Figure 3. Ensemble output from the GFS (red) and UKMET (white) models from 00Z Wednesday, August 24, 2016, shows a wider range of track possibilities for Invest 99L (left) than for Tropical Storm Gaston (right). Image credit: NOAA/ESRL/GSD.

Intensity forecast: Watching and waiting
Models are now in close agreement that 99L will spend roughly 2 to 3 days over the very warm waters surrounding The Bahamas, where sea-surface temperatures are running around 29-30°C (84-86°F). The latest output from the SHIPS model (12Z Wednesday) indicates that wind shear will remain light to moderate over 99L, holding around 10-20 knots for the next couple of days and then dropping to the 5-10 knot range by this weekend. These factors support intensification, which would become more probable as soon as 99L forms a more consolidated center. The longer 99L takes to organize and the further south it tracks, the better the chance of interaction with the mountains of Hispaniola, which would interfere with its development. If the track remains further north, 99L will have a better shot at intensifying sooner. The 00Z Wednesday run of the HWRF--our top intensity model in recent years--keeps 99L as a weak wave or depression until Friday, then brings it to hurricane strength just east of South Florida on Sunday. The 06Z Wednesday HWRF run is similar, though less intense and even slower.

Even our best models are not very skilled at forecasting rapid intensification. If 99L makes it into the Bahamas with a well-organized center of circulation, the possibility of a burst of strengthening prior to any encounter with Florida would need to be monitored very closely, given the prime conditions that may be present. Should 99L enter the Gulf, it could have a longer period of potential intensification before any landfall.

The bottom line: it may well be another day, or several, before 99L manages to become a tropical storm or hurricane (assuming it does). If 99L does intensify by this weekend, there is still the possibility for tropical storm or even hurricane-level impacts in Florida. We also have the chance of a potentially serious hurricane in the Gulf of Mexico at some point next week.

Already a concern: Heavy rains and flood potential
Even if it’s well below hurricane strength, a tropical system that’s as large, moist, and slow-moving as 99L can produce torrential amounts of rain. The flow around 99L is likely to produce huge rainfall amounts and the potential for flash flooding over parts of the Dominican Republic and Haiti later this week. As noted above, a flash flood watch continued in effect Wednesday for the U.S. Virgin Islands and for Puerto Rico, where the eastern half of the island is at particular risk of flash flooding.

The 7-day precipitation outlook from NOAA’s Weather Prediction Center (WPC) calls for a large area of 3” - 8” rains across Florida from Lake Okeechobee southward. Such rains could put stress on the 80-year-old dike that protects thousands of residents near Lake Okeechoobee, as discussed by Jeff Masters in a special post earlier today.

The WPC outlook also projects several inches of rain for hard-hit southeast Louisiana early next week, anticipating the potential effects of 99L’s circulation in the Gulf of Mexico.


Figure 4. Enhanced infrared image of Tropical Storm Gaston at 1445Z (10:45 am EDT) Wednesday, August 24, 2016. Image credit: NOAA/NESDIS.

Gaston holding just below hurricane strength
Tropical Storm Gaston remained just below hurricane strength as of the 11 am EDT Wednesday advisory from NHC. Located in the remote central Atlantic near 16.1°N, 39.4°W, Gaston was packing top sustained winds of 70 mph as it headed northwestward at around 16 mph. Gaston has only a brief window on Wednesday to become a hurricane before strong wind shear (25-30 knots) will likely put the brakes on intensification--and perhaps even weaken the storm--on Thursday and Friday. The shear should relax over the weekend, by which point Gaston will be slowing and moving over unusually warm subtropical waters (28-29°C) over the Northwest Atlantic. Gaston may well hit its peak intensity beyond the 5-day forecast period, when it should begins to recurve between latitudes 50° and 60°W, well east of Bermuda.



Figure 5. The 5-day track forecast for Typhoon Lionrock from the Joint Typhoon Warning Center as of 12Z (8:00 am) Wednesday, August 24, 2016.

Japan keeps an eye on formidable Typhoon Lionrock
The felicitously named Typhoon Lionrock is now roaring, with top sustained winds of 100 knots (115 mph) as of 12Z (8:00 am EDT) Wednesday, according to the Joint Typhoon Warning Center. Lionrock is now a Category 3 equivalent on the Saffir-Simpson scale. Still moving on an unusual southwest path at about 8 mph, Lionrock is located nearly 1000 miles southwest of Tokyo. The typhoon will soon be making a U-turn and heading to the northeast at an accelerating clip, with conditions close to ideal for strengthening for at least the next several days (SSTs of 30-31°C and wind shear below 10 knots). The 12Z Tuesday and 00Z Wednesday run of the ECMWF bring Lionrock into Japan’s most populous island, Honshu, as an extremely powerful typhoon early next week, on a left-turn hooked path reminiscent of Hurricane/Superstorm Sandy. The 00Z and 06Z Wednesday runs of the GFS are considerably weaker, with a similar left-turn landfall but well to the north of Tokyo. For more on Lionrock, see the comments section of this week’s post by wunderground member 1900hurricane.

We’ll be back with an update late Wednesday afternoon.

Bob Henson and Jeff Masters


Figure 6. Enhanced infrared MODIS image of Typhoon Lionrock (1 km resolution). Image credit: CSU/RAMMB/CIRA.

Hurricane

Unwelcome Rains Will Put Stress on Lake Okeechobee’s Dike

By: Jeff Masters , 11:32 AM GMT on August 24, 2016

A steadily organizing Invest 99L was bringing heavy rains and strong wind gusts to the northern Lesser Antilles on Wednesday morning, and is an increasing threat to develop into a tropical storm. Even if 99L never develops into a tropical cyclone, it has the potential to dump a large amount of rain on a place that doesn’t need it—the catchment basin of Lake Okeechobee in Central Florida. The huge lake represents an important source of fresh water to South Florida, but also poses a grave danger. The 25 - 30'-tall, 143-mile long Herbert Hoover Dike surrounding the lake was built in the 1930s out of gravel, rock, limestone, sand, and shell using old engineering methods. The dike is tall enough that it is very unlikely to be overtopped by a storm surge from the waters inside the lake, but the dike is vulnerable to leaking and failure when heavy rains bring high water levels to the lake. Torrential rains of 7+ inches from a tropical storm or hurricane are capable of raising the lake level by over three feet in a few weeks; this occurred in 2008, when Tropical Storm Fay took a leisurely romp across Florida, and again in 2012, when Tropical Storm Isaac lumbered past. At a lake water elevation of 15.5’, the U.S. Army Corps of Engineers dumps water out of the lake as fast as it can in order to keep high stresses on the dike from causing a failure. The lake reached this level early in 2016, after unusually heavy winter rains. The Corps was forced to do emergency dumping for most of February, and dumping has continued into August—though at a slower rate.

Currently, the lake level stands at 14.7’; the Wednesday morning Quantitative Precipitation Forecast from the National Weather Service predicted that the Lake Okeechobee region would receive 3 - 6” of rain over the next week, which would likely be enough to raise the lake level by 1 - 2 feet and bring it near the 15.5’ level where maximum dumping must occur. If 99L develops into a hurricane whose core passes over Lake Ockeechobee, the lake could easily received 10 - 15” of rain, enough to raise the lake level to near the 18.5’ level, where failure is possible. Under ideal conditions, the Army Corps can only lower the lake at a rate of about 0.4" per day, so they will have to do a lot of emergency dumping if 99L brings heavy rains to Florida. And 99L may only be setting the stage for the next storm—Florida often experiences multiple tropical storms or hurricanes in one year, and a wet storm later in September could really cause a serious situation with the dike.


Figure 1. Water level of Florida's Lake Okeechobee between January 2015 and August 23, 2016. Heavy winter El Niño rains forced emergency dumping in February, and dumping at a slower rate has continued all year. The Army Corps tries to keep the lake level below 15.5'; the dike surrounding the lake is in danger of failure when the lake level hits 18.5'. As of August 23, 2016, the lake level was 14.7’. Lake Okeechobee reached an elevation of 18.6' and 18.5'--both 1-in-30-year events--in 1995 and 1998. Image credit: U.S> Army Corps of Engineers.

Probable failure rate: once every 14 years
The Army Corps of Engineers has spent $500 million since 2007 to upgrade to the Herbert Hoover Dike. Replacement of 32 culverts has been partially completed, and has been funded through 2021. However, an additional $800 million in unfunded repairs is needed. A 2011 risk assessment estimated the dike's probable failure rate at every fourteen years. A 2008 Army Corps of Engineers study said this about the vulnerable dike:

"There is limited potential for a dike failure with lake levels as low as 18.5 feet. The likelihood of a failure increases at higher lake levels. At a lake level of 21 feet--a 1-in-100 year flood event--a dike failure would be likely at one or more locations. In the event of a dike failure, waters from Lake Okeechobee would pass through the breach--uncontrollably--and flood adjacent land. Flooding would be severe and warning time would be limited. And with 40,000 people living in the communities protected by the Herbert Hoover Dike, the potential for human suffering and loss of life is significant. Our engineering studies indicate the southern and eastern portions of the dike system are more likely to fail than the northern and western portions of the dike. In general, we would expect a warning time of 24 to 48 hours prior to a dike failure that releases water from the lake; however, under some conditions the warning time might be longer, and under others, a dike failure could occur with no warning."

The city most at risk from a dike failure may be Belle Glade (population 18,000) on the southeast shore. Belle Glade is at 16' elevation. If Lake Okeechobee is at 20' above mean sea level when the dike fails, this implies that at least three feet of water could flood Belle Glade. If a wide section of the dike breaks and there is a Cat 3+ hurricane driving a massive storm surge at the time, then the flood could be much higher. During the 1928 hurricane, which had 130 mph winds while over the lake, the water from the storm surge reached seven feet above ground level in Belle Glade.


Figure 2. Aftermath of the 1928 Okeechobee Hurricane, showing damage to a cluster of Everglades scientific work stations in Belle Glade. The hurricane killed 2,500 people, mostly near Belle Glade. Image credit: University of Florida, via the historicpalmbeach.com.

The Great 1928 Lake Okeechobee Hurricane
The shores of Lake Okeechobee are the site of the second deadliest natural disaster in U.S. history--the 1928 Okeechobee Hurricane. This mighty hurricane caused catastrophic damage where it struck the Florida coast as a Category 4 storm near Palm Beach, and weakened only slightly to Category 3 strength with 130 mph winds when it passed over Lake Okeechobee. The powerful winds of the hurricane brought a 12' storm surge to the south end of the lake, which overwhelmed the 6' high levees protecting the farm lands to the south. The resulting flood covered an area of hundreds of square miles with water up to 20' deep, and killed at least 2,500 people--mostly black migrant farm workers. A mass grave at the Port Mayaca Cemetery east of Port Mayaca contains the bodies of 1,600 victims of the hurricane. The Herbert Hoover Dike was built in the 1930s around most of Lake Okeechobee in response to this disaster.
 

Figure 3. Tom Wippick, from the Florida Department of Environmental Protection, takes a sample for testing of the awful smelling algae near the Central Marine boat dock along the St. Lucie River on July 11, 2016 in Stuart, Florida. The algae bloom, due to polluted water from Lake Okeechobee, created angry communities, closed beaches and has had an economic impact as tourists and others are driven away by the smell and inability to enjoy some of the waterways. (Photo by Joe Raedle/Getty Images.)

Lake Okeechobee runoff contributing to toxic algae blooms
In May, a 33-square-mile algal bloom blossomed over Lake Okeechobee, due to heavy agricultural pollution. The Corps has been dumping water out of the lake all year to keep the lake below 15.5’, and most of this excess water was sent out Lake Okeechobee's western drainage canal into the Caloosahtchee Estuary, which empties into the Gulf of Mexico at Fort Myers. A lesser amount of Lake Okeechobee water has been sent eastwards into the St. Lucie River, where it drains into the Atlantic Ocean near Stuart through the Indian River Lagoon. A similar level of discharge goes down the C-51 canal in into the estuary by West Palm Beach, the Lake Worth Lagoon. The polluted Lake Okeechobee water, combined with large amounts of polluted local runoff water from heavy rains, has caused havoc in these coastal waters this summer, shutting down businesses and closing beaches during the critical summer tourism season. A state of emergency was declared last month in the coastal counties of St. Lucie, Martin and Palm Beach because of  algae blooms caused by the polluted Lake Okeechobee water.

We'll have a full update on 99L late this morning (as well as on Tropical Storm Gaston).

Jeff Masters

Hurricane

Still-Disorganized 99L Remains a Threat to Southeast U.S. Coast; Gaston Growing

By: Bob Henson , 9:54 PM GMT on August 23, 2016

A large and strong tropical wave about 250 miles east of the Lesser Antilles, Invest 99L, continues to move west-northwest at about 20 mph on a course that would bring it within striking distance of the southeast U.S. coast by late in the weekend or early next week, quite possibly at hurricane strength. Venturing into 99L on Tuesday, Air Force Hurricane Hunters found that the wave had an poorly defined circulation, elongated from north to south. Often it will take one or more days for a symmetric, circular center to emerge inside a tropical wave this large. After that point, convection (showers and thunderstorms) can consolidate and the system can grow more rapidly if conditions are favorable. A substantial growth in convection may occur overnight Tuesday, as tropical systems often experience convective bursts during the nighttime hours. Late Tuesday afternoon, convection was scattered over a broad area around 99L (see Figure 1). Increasing upper-level outflow was evident, especially on the wave’s north side.

Two of the latest Tuesday (12Z) operational runs of our three reliable models for predicting tropical cyclone genesis--the European, and UKMET models--showed development of 99L into a tropical storm over the next five days. In its 2 pm EDT Tropical Weather Outlook, the National Hurricane Center maintained 40 percent odds that 99L will develop at least to tropical depression strength by Thursday and 60 percent odds by Sunday. Given the scenarios being painted by models, and given the location and time of year, the 60% NHC odds that we will have at least a tropical depression by Sunday are on the conservative side, as noted by Jeff Masters in this morning’s post.


Figure 1. Enhanced infrared image of Invest 99 as of 1945Z (3:45 pm EDT) Tuesday, August 23, 2016. Image credit: CIRA/RAMMB/CSU.

Track forecast: 99L likely to affect The Bahamas and Southeast U.S.
One big question for 99L where a center of circulation might develop within the north-south swath of elongation. In its Tropical Weather Discussion issued at 2 pm EDT Tuesday, NHC placed the elongation between 12°N and 20°N, with a center of low pressure near 17°N, 56°W. Late Tuesday afternoon, satellite imagery suggested that a center of circulation may be trying to form around 15-16°N, roughly 70 to 140 miles further south. If this apparent trend holds, it could raise the odds of 99L passing near Puerto Rico on Wednesday and just to the north of Hispanola on Thursday, potentially slowing any rapid short-term development of 99L. Regardless of where the center consolidates, heavy rains from 99L’s large circulation could easily pose flooding problems for islands as far west as Hispaniola. A flash flood watch was already in effect Wednesday for Puerto Rico and the U.S. Virgin Islands, where widespread 2” - 4” totals and locally higher amounts are expected.

Models are in strong agreement on 99L’s general west-northwest motion around a strong upper-level ridge in the Atlantic subtropics over the next several days, which would bring 99L into The Bahamas by Friday into the weekend. There could be a slight northwestward bend around Friday due to a potential weakness in the ridge produced by the remains of now-deceased Tropical Storm Fiona. Beyond that point, models generally agree that the ridge will restrengthen and send 99L back toward the west-northwest or west. The 12Z runs of the ECMWF and UKMET models take 99L on quite similar tracks across southern Florida on Sunday or Monday, then onward into the Gulf of Mexico, which would open the possibility of a landfall somewhere along the Gulf Coast next week. The other of the three most reliable models for tropical cyclone genesis, the GFS, has failed to develop 99L for its last several runs, keeping it as an open wave. The 12Z run of the HWRF model also failed to develop 99L--a major shift from its previous few runs.

Intensity forecast for 99L: a potentially dangerous storm for the Southeast U.S. (and perhaps the Gulf)
It may take some time for 99L to develop into what could be a formidable storm. The 2 pm EDT Tuesday run of the SHIPS model continued to show moderately favorable conditions for development through Friday. Wind shear will be in the light to moderate range, 5 - 15 knots, and SSTs will increase from 28.5°C (83°F) to 30°C (86°F), accompanied by an increase in the total heat content of the ocean. Working against development of 99L will be the large size of the storm, dry air from the Saharan air layer (SAL), and large-scale sinking air over the tropical Atlantic imparted by an unfavorable phase of the Madden-Julian Oscillation (MJO).

Assuming that it consolidates and develops into a tropical cyclone, as projected by the majority of reliable models, 99L has the potential to strengthen significantly toward the weekend. The region surrounding The Bahamas is a notorious breeding ground for hurricane development, especially in late August and September, when the fairly shallow waters are at their warmest. The track forecast suggests that 99L could be moving for 2-3 days across near-record warm waters around 30°C (86°F). There will be ample amounts of oceanic heat content, and upper-level wind shear is expected to remain light. In short, we have the potential for 99L--which has a large circulation--to rapidly intensify into a large major hurricane that would strike the southeast U.S. coast on Sunday or Monday, potentially moving into the Gulf of Mexico. At present, Florida appears to be at greater risk than states further north. The uncertainties are high, though, and we will have to wait for the storm to develop into a tropical depression before we can have more confidence in what the models are saying.


Figure X. Enhanced infrared image of Tropical Storm Gaston as of 1745Z (1:45 pm EDT) Tuesday, August 23, 2016. Image credit: NOAA/NESDIS.

Gaston is on its way to hurricane status
After an impressive growth spurt earlier on Tuesday, Tropical Storm Gaston continued to pack top sustained winds of 65 mph as of the 5 pm EDT update from NHC. An eye-like feature has intermittently appeared within Gaston during the day Tuesday. Located far out in the eastern Atlantic at 14.2°N, 35.8°W, Gaston is now projected to become a strong Category 1 hurricane before it encounters several days of strong wind shear (above 20 knots). Beyond that point, it could strengthen even further as it passes over subtropical SSTs that will be 1-2°C above average for this time of year. Fortunately, Gaston’s northwestward path will keep it many hundreds of miles from any land area for at least the next several days, and it is likely to recurve long before approaching North America or even Bermuda.

We’ll have our next update on Wednesday morning.

Bob Henson

Hurricane

Threat to Southeast U.S. Growing From 99L; Gaston Forms in Eastern Atlantic

By: Jeff Masters , 3:40 PM GMT on August 23, 2016

The Hurricane Hunters are in the air, investigating Invest 99L, a steadily organizing tropical wave that was located about 300 miles east of the northern Lesser Antilles late Tuesday morning. This storm will bring heavy rains and gusty winds to the northern Lesser Antilles Islands Tuesday evening through Wednesday, and is likely to develop into a tropical storm or hurricane that will affect the Bahamas and the Southeast U.S. coast late this week or early next week.

Satellite loops on Tuesday morning showed that heavy thunderstorm activity associated with 99L had slowly increased over the the previous 24 hours. The storm was still poorly organized, though, with no surface circulation center apparent and few low-level spiral bands. Upper-level cirrus clouds streaming away from the northeast side of the storm gave evidence that an upper-level outflow channel was trying to develop, however. Dry air from the Saharan Air Layer (SAL) that has been interfering with development over the past few days had decreased, as seen on water vapor satellite imagery. Wind shear was marginally favorable for development, at 10 - 15 knots, and sea surface temperatures (SSTs) were favorable for development: 28.5 - 29°C (83 - 84°F) (about 0.5°C above average.)


Figure 1. Latest satellite image of 99L.

Track forecast: 99L likely to affect the Bahamas and Southeast U.S.
A strong ridge of high pressure will keep 99L headed west-northwest over the next few days, and the storm will traverse the northern Lesser Antilles Tuesday night through Wednesday, track close to or just north of Puerto Rico on Wednesday night, then reach the Southeastern Bahamas by Thursday. At this time, the models keep 99L far enough north of the island of Hispaniola to prevent the high mountains there from significantly disrupting 99L. However, we cannot have confidence in this forecast until 99L develops a well-defined center that the models can track. Due to its large size, 99L will be capable of bringing torrential rains and flash flooding and mudslides to the Dominican Republic and Haiti. The uncertainty about the track increases greatly on Friday and into the weekend, due to a potential weakness in the ridge of high pressure steering the storm caused by a trough of low pressure passing to the north of 99L. The storm should slow its forward motion to 5 - 10 mph Friday through Saturday, in response to this trough, and may turn to the northwest or north near the central Bahamas. Over 80% of the ensemble members from the 00Z Tuesday runs of the European and GFS models show 99L hitting the Southeast U.S. between Florida and South Carolina sometime Sunday or later; very few show the storm missing the U.S. entirely. The track and intensity of 99L may be affected by the remnants of Tropical Storm Fiona, which NHC stopped issuing advisories on at 11 am EDT Tuesday. The remains of Fiona will be a few hundred miles to the north or northeast of 99L this weekend. Hurricane Gaston will be too far from 99L to exert a steering influence on it.


Figure 2. The 00Z Tuesday morning run of the operational European model (run at high resolution, shown in red) and its ensemble members (50 runs with slightly different initial conditions done at lower resolution, shown in black) came up with a variety of solutions for the future track and intensity of 99L. The operational model run—which is usually the best forecast, since the model runs at the highest resolution with the proper initial conditions—showed a track for 99L across Florida and into the Gulf of Mexico, with the storm peaking as a Category 1 hurricane. The track and intensity forecasts of the four members of the European ensemble that have done the best job tracking the progress of 99L over the previous 24 hours (called the “high probability cluster”) are also plotted here. They show that a more northeasterly track may be likely. Note that while the European model is our best model for predicting hurricane tracks, it does a poor job forecasting intensity and is generally disregarded by NHC for making intensity forecasts. Image taken from a custom software package used by Weather Underground.

Intensity forecast for 99L: a potentially dangerous storm for the Southeast U.S.
Heavy rains from 99L will likely cause flash flooding problems in the Lesser Antilles, Virgin Islands, and Puerto Rico. Wind damage should not be an issue, since 99L will likely be, at worst, a moderate-strength tropical storm with 55 mph winds once it leaves the islands. The 8 am EDT Tuesday run of the SHIPS model showed moderately favorable conditions for development through Friday. Wind shear will be in the light to moderate range, 5 - 15 knots, and SSTs will increase from 28.5°C (83°F) to 30°C (86°F), accompanied by an increase in the total heat content of the ocean. Working against development of 99L will be the large size of the storm, dry air of the SAL, and large-scale sinking air over the tropical Atlantic imparted by an unfavorable phase of the Madden-Julian Oscillation (MJO). Two of the Tuesday morning (00Z) operational runs of our three reliable models for predicting tropical cyclone genesis--the European, and UKMET models--showed development of 99L into a tropical storm over the next five days. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day development odds of 50% and 60%, respectively. I think the 5-day odds should be higher, at 70%.

This storm has the potential to be a dangerous one for the Bahamas and the Southeast United States. The models are predicting a favorable environment for development near the Bahamas, and the storm will be moving quite slowly during its closest approach to the islands, potentially allowing for some very high rainfall totals in the Bahamas. While the 00Z Tuesday run of the GFS model did not show development of 99L into a tropical depression, the European model ensemble forecast had 1/3 of its members predicting that 99L would eventually become a hurricane. The storm will likely spend 2 - 3 days over a region of ocean with SSTs that are near record-warm: 29 - 30°C (84 - 86°F). Warm waters extend to great depth, resulting in an unusually high total ocean heat content. With some models forecasting that wind shear will be in the light to moderate range at this time, we have the potential for 99L--which has a large circulation--to rapidly intensify into a large major hurricane that will hit the Southeast U.S. coast on Sunday or Monday. The uncertainties are high, and we will have to wait for the storm to develop into a tropical depression before we can have more confidence in what the models are saying.

Gaston forms in the Eastern Atlantic
Tropical Storm Gaston, the seventh named storm of the 2016 Atlantic hurricane season, formed early Wednesday morning in the eastern Atlantic, a few hundred miles west of the Cabo Verde Islands. Gaston’s formation date of August 23 comes more than three weeks earlier than the usual September 16 formation date of the season’s seventh storm. Gaston is headed northwestwards into a area of ocean where it is highly unlikely to be a threat to any land areas.

We’ll have a new post late this afternoon.

Jeff Masters

99L a Potential Threat to Hispaniola, Bahamas, U.S. East Coast

By: Jeff Masters and Bob Henson , 4:38 PM GMT on August 22, 2016

The main Atlantic tropical weather threat to populated areas continues to be Invest 99L, a large tropical wave with an increasing amount of heavy thunderstorm activity that was located about 800 miles east of the northern Lesser Antilles Islands late Monday morning. This disturbance was moving west to west-northwest at 15 - 20 mph, and will bring heavy rains and gusty winds to these islands beginning on Tuesday evening. Conditions for development will steadily improve in the coming days, and the storm could be trouble for the Bahama Islands late this week--and is a threat to make landfall along the U.S. East Coast early next week. Satellite loops on Monday morning showed that 99L had finally managed to fire up a respectable amount of heavy thunderstorm activity near its core, in defiance of the dry air from the Saharan Air Layer (SAL) that had been interfering with development over the past few days. Water vapor satellite imagery showed that there continued to be a fair amount of dry air around the storm, though the amount of dry air had decreased since Sunday. Other conditions were generally favorable for development, with wind shear a light 5 - 10 knots and sea surface temperatures (SSTs) near 27.5°C (82°F), which was close to average.


Figure 1. Latest satellite image of 99L.

Track forecast: 99L a potential threat to Hispaniola, the Bahamas, and the U.S.
A strong ridge of high pressure will keep 99L headed north of due west over the next few days, and the storm should pass through the northern Lesser Antilles Tuesday night through Wednesday, track close to Puerto Rico on Wednesday night, and affect Hispaniola and the Southeastern Bahamas by Thursday. The uncertainty about the track increases greatly thereafter, due to a potential weakness in the ridge of high pressure steering the storm caused by a trough of low pressure passing to the north of 99L. The storm should slow its forward motion to 5 - 10 mph, in response to this trough, and may turn to the north near the central Bahamas. The track of 99L may also be affected by the remnants of Tropical Storm Fiona, which could be a few hundred miles to the north or northeast. At this time, it appears that 90L near the Cabo Verde Islands off the coast of Africa, which is expected to become Tropical Storm or Hurricane Gaston late this week, will be too far from 99L next week to exert a steering influence on it. The steering situation is too complex next week to say how great a threat the storm may pose to the U.S., but 99L is a legitimate threat to make landfall along the East Coast.


Figure 2. The dry air of the Saharan Air Layer (SAL) as analyzed by satellite at 8 am EDT Sunday, August 21, 2016 (top) and Monday, August 22, 2016 (bottom). The amount of dry air from the SAL interfering with 99L has decreased since Sunday. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.

Intensity forecast for 99L: commentary by Jeff Masters
Heavy rains from 99L will be capable of causing flash flooding problems in the Lesser Antilles, Virgin Islands, and Puerto Rico, but wind damage should not be an issue, since 99L will likely be, at worst, a moderate-strength tropical storm with 55 mph winds once it leaves the islands. The 8 am EDT Monday run of the SHIPS model showed moderately favorable conditions for development through Thursday. Wind shear will be in the light to moderate range, 5 - 15 knots, and SSTs will increase from 27.5°C (82°F) to 29°C (84°F), accompanied by an increase in the total heat content of the ocean. Working against development of 99L will be the large size of the storm, dry air of the SAL, potential interaction with the land areas of Puerto Rico and Hispaniola, and large-scale sinking air over the tropical Atlantic imparted by an unfavorable phase of the Madden-Julian Oscillation (MJO). None of the Sunday morning (00Z) operational runs of our three reliable models for predicting tropical cyclone genesis--the European, GFS and UKMET models--showed development of 99L into a tropical depression or tropical storm over the next four days, though the UKMET model predicted it could be a tropical depression in the Bahamas in five days. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day development odds of 20% and 50%, respectively. I think these odds are too low, and should be 30% and 60%, respectively. The Hurricane Hunters are scheduled to investigate 99L on Tuesday afternoon.


Figure 3. Total oceanic heat content (called the Tropical Cyclone Heat Potential, or TCHP) in kilojoules per square centimeter (kJ/cm^2), for August 21, 2016. TCHP was at near-record or record values in the waters surrounding the Bahamas. TCHP in excess of 90 kJ/cm^2 (orange colors) is commonly associated with rapid intensification of hurricanes. Image credit: NOAA/AOML.

I am concerned about the storm’s potential impacts on the Bahamas and the U.S. beyond five days, when the storm will likely be near or just north of the central Bahamas. The models are predicting a more favorable environment for development then, and the storm will be moving quite slowly, potentially allowing for some very high rainfall totals in the Bahamas, and to a lesser extent, Haiti and the Dominican Republic. While the 00Z Monday runs of the GFS and European model ensemble forecast had fewer than 10% of their members predicting that 99L would eventually become a hurricane, the storm will likely spend at least three days over a region of ocean with SSTs that are near record-warm: 29 - 29.5°C (84 - 85°F). Warm waters extend to great depth, resulting in an unusually high total ocean heat content. With some models forecasting that wind shear will be in the light to moderate range early next week, we have the potential for 99L--which does have a large circulation--to rapidly intensify into a large hurricane. Hopefully, the unfavorable phase of the MJO will help to put the brakes on such a possibility.


Figure 4. MODIS visible satellite image of 90L to the southwest of the Cabo Verde Islands, taken on Monday morning, August 22, 2016. Image credit: NASA.


90L in the Eastern Atlantic near tropical depression status
A large tropical wave a few hundred miles southwest of the Cabo Verde Islands off the coast of Africa (Invest 90L) is near tropical depression status. Satellite images on Monday morning showed a well-organized system with plenty of spin, low-level spiral bands, and an increasing amount of heavy thunderstorms. Wind shear was moderate, 10 - 15 knots, and SSTs were warm enough for development, 28°C (82°F). The Monday morning operational runs of the European, GFS and UKMET models all showed development of 90L into a tropical storm in 1 - 2 days. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 100%. The wave will head west-northwest at 15 - 20 mph through Tuesday, skirting the Cabo Verde Islands, then turn more to the northwest on a path similar to Fiona’s. The storm will be moving into a region of ocean where very few tropical cyclones ever make the long trek westwards to hit the United States or Bermuda. The next name on the list of Atlantic storms is Gaston.


Figure 5. Enhanced infrared image of Tropical Storm Fiona at 1515Z (11:15 am EDT) Monday, August 22, 2016. Image credit: NOAA/NESDIS.

Fiona hangs in for now
Weak but resilient, Tropical Depression Fiona continues to chug through the central Atlantic. Fiona’s convection has been pulsing and decaying every few hours, but it continues to be skewed toward the east side of the low-level center, prohibiting any further development. Fiona was downgraded to depression status by NHC at 11:00 pm EDT Sunday, but the change only involved a drop of 5 mph in Fiona’s top sustained winds. In its 11:00 am EDT update, NHC kept Fiona as a depression with top winds still at 35 mph. Strong vertical wind shear of about 30 knots is still plaguing Fiona, and dry Saharan air surrounds the circulation, with mid-level relative humidity at only around 40%.

Fiona continues to move west-northwest around the west end of the large upper-level ridge that’s also helping to steer 99L and 90L. Over the next day or so, shear will decrease, humidity will increase, and SSTs will remain in the very warm 29-30°C range along Fiona’s track. Most dynamical models project that Fiona will continue to slowly weaken over the next several days, and NHC predicts Fiona to be a remnant low by Tuesday evening. If Fiona manages to make it through Tuesday, a life extension isn’t out of the question, although shear will again increase by late in the week. A weakness in the ridge would allow Fiona to angle more northwestward later in the week, after which the track forecast would become much more complex, especially if there were any interaction with 99L. If Fiona becomes a remnant low, its moisture could well be entrained by 99L’s circulation toward the end of the week.

If Fiona loses its identity as a tropical cyclone before any landfall, as seems probable, it will break the Atlantic’s remarkable string of five consecutive landfalls from the first five named storms of 2016.


Figure 6. Infrared satellite image of Invest 98E as of 1500Z (11:00 am EDT) Monday, August 22, 2016. Image credit: NOAA/NESDIS.

East Pacific: Kay on its last legs as 98E begins cranking up
As expected, cooler waters along the path of Tropical Storm Kay are taking a toll. Kay is now a minimal tropical storm, with top sustained winds of 40 mph. Kay’s west-northwest path will soon take it over waters cooler than the 26°C (79°F) benchmark for tropical development. In its 11:00 am EDT Monday advisory, NHC projects Kay to be a depression by tonight and a remnant low by late Tuesday.

Even further offshore, Invest 98E--about 850 miles south of Cabo San Lucas, Mexico--has a decent shot at eventual development. 98E is now a large, poorly organized wave, but statistical and dynamical models agree on significant strengthening, especially toward the end of the week. The NHC gives 98E a 10% chance of development by Wednesday and a 50% chance by Saturday. On its consistent west-northwest track, 98E will remain far away from any land areas.


Figure 7. Visible satellite image of Tropical Storm Mindulle at 0009Z Monday, August 22, 2016 (9:09 pm Sunday EDT), several hours before Mindulle swept northward into Japan near Tokyo. Image credit: CIRA/RAMMB/CSU, courtesy wunderground member 1900hurricane.

West Pacific: Mindulle hammers Japan while Lionrock lurks to the south
At least one person has died and 29 have been injured by winds and rain associated with Tropical Storm Mindulle, according to the Japan Times. Mindulle made landfall on Japan’s Honshu island near Tateyama, about 50 miles south of Tokyo, around 12:30 pm Monday local time (11:30 pm Sunday EDT). After being briefly upgraded by the Japan Meteorological Agency to a minimal Category 1 typhoon, Mindulle passed just east of the Tokyo city center, with millions affected by cancelled or delayed flights and trains across the region. At 12Z Monday (8:00 am EDT), Mindulle was passing across northernmost Honshu, its top winds down to 40 mph. As it transitions to an extratropical cyclone, Mindulle will make its next landfall on eastern Hokkaido island, which was drenched over the weekend by torrential rain associated with Tropical Storm Kompasu. All of Hokkaido and northern Honshu islands were under warnings Monday for heavy rain and potential landslides.


Figure 8. Rainfall rates from 6:00 pm to midnight JST Monday night, August 22, 2016, averaged 30-50 mm/hr (1.2” to 2”/hr) over parts of northern Honshu island, Japan, in bands around the center of Tropical Storm Mindulle. Image credit: Japan Meteorological Agency.

The burst of tropical cyclones across the Northwest Pacific in recent days resulted in some odd interactive behavior, including the multil-day southwestward motion of Tropical Storm Lionrock. Located several hundred miles south of Honshu late Monday local time, Lionrock was drifting south-southeast at just 3 mph. Lionrock’s top sustained winds are just 40 mph, and little strengthening is expected over the next few days as the storm embarks on a leisurely clockwise loop. Further down the line, a weakness in the ridge blocking Lionrock will open up north of Japan, which should induce a northward or northeastward acceleration. Conditions by that point will favor strengthening: the GFS and ECMWF agree that Lionrock could be a significant typhoon by this weekend--possibly threatening Japan and/or the Korean peninsula, --but with a great deal of uncertainty on its eventual track. Wunderground member 1900hurricane posted a detailed analysis on Sunday of the West Pacific situation, including Lionrock’s potential future.

Baton Rouge has its wettest month in 174 years of recordkeeping
The astounding rains that led to catastrophic flooding in Louisiana led to a phenomenal monthly rainfall total in Baton Rouge, where formal weather observations began quite early, in 1843. As of Sunday, the August rainfall total in Baton Rouge was 26.97”, which crushes the previous record of 23.73” (May 1907), according to WU weather historian Christopher Burt. This monthly total includes 0.76” on Sunday and a total of 6.08” since August 14, when the rains directly associated with the flood-making upper low had already ceased. For the summer since June 1, Baton Rouge has picked up an amazing 40.95”--more rain in three months than downtown Los Angeles has recorded over the last five years (38.79”)!

Jeff Masters and Bob Henson

Hurricane Extreme Weather

99L and 90L May Soon Develop in Atlantic; Fiona Clings to Life

By: Jeff Masters and Bob Henson , 4:28 PM GMT on August 21, 2016

A large but nearly thunderstorm-free tropical wave (Invest 99L) was located in the tropical Atlantic about 1100 miles east of the Lesser Antilles Islands late Sunday morning, and was headed west at 15 - 20 mph. This disturbance is not likely to develop into a tropical depression or tropical storm before moving into the Lesser Antilles Islands on Tuesday night, but will bring some heavy rains and gusty winds to the islands on Tuesday and Wednesday. The storm could be a long-range threat to the U.S. East Coast from Florida northwards in about 7 - 10 days.

The most impressive thing about 99L when viewing satellite loops is its very large size and excellent spin. A large region of the atmosphere has been put in motion by this disturbance, which is both good news and bad news: good news because such large disturbances typically take a long time to spin up into a tropical cyclone, but bad news because once they do, they affect a large area and will resist rapid weakening. The other notable feature of the storm on Sunday morning was the lack of heavy thunderstorm activity, due to dry air from the Saharan Air Layer (SAL), as seen in water vapor satellite imagery. However, this dry air was only moderately dry, with humidities at mid-levels of the atmosphere between 500 - 700 mb running about 60%, and the circulation of 99L was getting more defined late Sunday morning, potentially signaling that the storm might be about to build some heavy thunderstorms near its center. Other conditions were generally favorable for development, with wind shear a moderate 10 - 15 knots and sea surface temperatures (SSTs) near 27.5°C (82°F), which was close to average. 


Figure 1. Latest satellite image of 99L.

Track forecast: 99L a potential threat to Hispaniola, the Bahamas, and the U.S. coast from Florida northwards
A strong ridge of high pressure will keep 99L headed north of due west over the next few days, and the storm should pass through the northern Lesser Antilles Tuesday night through Wednesday, track close to Puerto Rico on Wednesday night, and affect Hispaniola and the Southeastern Bahamas by Thursday. The uncertainty about the track increases greatly thereafter, as a weak trough of low pressure passing to the north of 99L late this week may be strong enough to turn the storm to the north before it can reach the U.S. East Coast. The track of 99L may also be affected late this week by tropical wave 90L (see below), which could grow into a hurricane that comes close enough to exert a steering influence.


Figure 2. The dry air of the Saharan Air Layer (SAL) as analyzed by satellite at 8 am EDT Sunday, August 21, 2016. The SAL was interfering with both Fiona and 99L, but was not as concentrated as we saw early in August. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.

Intensity forecast for 99L
99L has a lot of hurdles to overcome to become a named storm. The 8 am EDT Sunday run of the SHIPS model showed moderately favorable conditions for development through Thursday, with wind shear in the moderate range, 10 - 15 knots, a relatively moist atmosphere, and SSTs near 28°C (83°F.) The total heat content of the ocean will steadily increase as 99L moves westwards, as well. But working against development of 99L will be the large size of the storm, dry air of the SAL, potential interaction with the land areas of Puerto Rico and Hispaniola, and large scale sinking air over the tropical Atlantic imparted by an unfavorable phase of the Madden-Julian Oscillation (MJO). The active portion of the MJO is currently located in the Western Pacific, which is leading to increased tropical cyclone activity there--three named storms were active there on Sunday morning. This positioning of the MJO typically leads to compensating sinking air and surface high pressure over the tropical Atlantic, with reduced chances of tropical cyclone development there.

None of the Sunday morning (00Z) operational runs of our three reliable models for predicting tropical cyclone genesis--the European, GFS and UKMET models--showed development of 99L into a tropical depression or tropical storm over the next five days. However, beyond five days, when the storm will likely be near or just north of the central Bahamas, the models are predicting a more favorable environment for development. The 00Z Sunday runs of the GFS and European model ensembles had 5% and 12% of their members predicting that 99L would eventually become a hurricane, after seven days. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day development odds of 20% and 50%, respectively.


Figure 3. MODIS visible satellite image of 90L off the coast of Africa, taken on Sunday morning, August 21, 2016. Image credit: NASA.

90L off the coast of Africa likely to develop
A large tropical wave with plenty of spin (Invest 90L) emerged from the coast of Africa on Saturday night, and appears poised to become a tropical depression by Tuesday as it heads west-northwest at about 10 mph. Satellite images on Sunday morning showed that 90L was well-organized, with low-level spiral bands and an increasing amount of heavy thunderstorms. Wind shear was light, 5 -10 knots, and SSTs were warm enough for development, 27°C (81°F).

The Sunday morning operational runs of the European, GFS and UKMET models all showed development of this wave into a tropical depression in 1 - 3 days. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 70% and 90%, respectively. The wave will head west-northwest through Tuesday, skirting the Cabo Verde Islands to the south, then turn more to the northwest on a path similar to Fiona’s. The storm will be moving into a region of ocean where very few tropical cyclones ever make the long trek westwards to hit the United States--though 90L could be a long-range threat to Bermuda. The next name on the list of Atlantic storms is Gaston, and we can expect 90L to be called Hurricane Gaston by this weekend.


Figure 4. With its shell of convection displaced far to its east by strong vertical wind shear, Tropical Storm Fiona’s low-level center was left completely exposed (left) at 0745Z (3:45 am EDT) Sunday, August 21, 2016. Less than seven hours later, at 1415Z (10:15 am EDT), a new burst of convection had developed near Fiona’s center (right). Image credit: NOAA/NESDIS.

Fiona the fighter lives to see another day
The epitaph was almost written Saturday night on Tropical Storm Fiona. The storm had intensified Saturday afternoon, with a substantial core of convection helping the storm’s top sustained winds to increase to 50 mph as measured by scatterometer. Overnight, though, strong wind shear (around 30 knots) ripped away the convection and left Fiona’s low-level circulation completely open. Early Sunday morning, yet another burst of convection erupted near Fiona’s center. Fiona’s small size and the typical overnight maximum in tropical convection are helping to facilitate these ups and downs in intensity. As of 11 am EDT Sunday, Fiona’s top winds were 40 mph, barely keeping it a tropical storm. Working its way across the open Atlantic, at 22.9°N, 53.3°W--roughly 700 miles northeast of the Leeward Islands--Fiona was continuing west-northwest at 16 mph.

The main question in Fiona’s future is whether the storm will survive the 20-30 knot wind shear projected for the next 1-2 days. After that point, wind shear will drop below 20 knots, and the storm will be traveling over very warm SSTs of around 30°C (86°F), around 1-2°C above average for this time of year. The contrast between these warm waters and cold upper-level temperatures in the subtropics would allow for strong convection, and Fiona’s small size could allow the storm to intensify more quickly than a larger storm might. Recent runs of the European model weakens Fiona into an open trough, while the GFS, UKMET, and HWRF maintain it as a weak low through the 5-day forecast period. The official NHC outlook downgrades Fiona to depression strength by Monday and makes Fiona a remnant low by Tuesday. If Fiona does survive, it could become a respectably strong tropical storm late in the week, potentially passing near Bermuda on a recurving path north and northeast.

East Pacific: Tropical Storm Kay on the downswing
About 300 miles southwest of Cabo San Lucas, Tropical Storm Kay peaked at 50 mph sustained winds on Saturday. Although it’s a well-structured storm, Kay will continue weakening as it passes over progressively colder waters on its slow northwest track, most likely becoming a remnant low by Tuesday. There are no other systems of interest in the East Pacific for the next several days.


FIgure 5. Infrared image of Tropical Storm Kay as of 1500Z (11:00 am EDT) Sunday, August 21, 2016. Image credit: CSU/RAMMB/CIRA.


Figure 6. Satellite image early Sunday EDT of Tropical Storm Mindule (center of image), which was approaching the Tokyo area on Sunday. Tropical Storm Lionrock can be seen at lower left, while Tropical Depression Kompasu is off the image to the northeast.

Back-to-back tropical systems hitting Japan
The northern West Pacific has been bubbling with activity this weekend (see embedded video below), though none of the systems were at typhoon strength on Sunday. Tropical Depression Kompasu brushed eastern Japan on Sunday before dissipating. Kompasu produced heavy rains across Hokkaido, where flood and landslide alerts have been issued. On the island’s north coast, at least 3000 residents have been evacuated from the city of Kitami due to flooding, with 5” - 7” of rain reported across the area. Hard on the heels of Kompasu is Tropical Storm Mindule, with current sustained winds of 50 mph. Mindule is projected to make landfall near Tokyo on Monday before racing northeastward on a path similar to Kompasu’s, which will likely exacerbate flood conditions on Hokkaido.

The most offbeat of the bunch is Tropical Storm Lionrock, which has been tracking toward the southwest over the last three days, paralleling the coast of Japan about 200 miles offshore. Now located about 200 miles southeast of Kyushu, with top sustained winds of just 40 mph, Lionrock will continue drifting southwest for the next several days.

We’ll be back with our next update on Monday.

Jeff Masters and Bob Henson



Tropical

Disorganized 99L May Develop Before Reaching the Caribbean; Fiona No Threat

By: Jeff Masters and Bob Henson , 4:38 PM GMT on August 20, 2016

A large but disorganized tropical wave (Invest 99L), located in the tropical Atlantic midway between Africa and the Lesser Antilles Islands on Saturday morning, was headed west at 15 - 20 mph. Residents of the islands in the eastern Caribbean should closely monitor this disturbance, as it has the potential to develop into a tropical depression or tropical storm before moving into the Lesser Antilles Islands on Tuesday night. The most impressive thing about 99L when viewing satellite loops is its very large size and impressive amount of spin. A large region of the atmosphere has been put in motion by this disturbance, which is both good news and bad news: good news because such large disturbances typically take a long time to spin up into a tropical cyclone, but bad news because once they do, they affect a large area and will resist rapid weakening. The other notable feature of the storm Saturday morning was the relative lack of heavy thunderstorm activity, due to dry air from the Saharan Air Layer (SAL), as seen in water vapor satellite imagery. However, this dry air was only moderately dry, with humidities at mid-levels of the atmosphere between 500 - 700 mb running 65 - 70%. Other conditions were generally favorable for development, with wind shear a moderate 10 -15 knots and sea surface temperatures (SSTs) 27.5 - 28°C (82 - 83°F), which was about 1°C (1.8°F) above average.


Figure 1. MODIS visible satellite image of 99L and a new tropical wave about to emerge from the coast of Africa, taken on Saturday morning, August 20, 2016. The next name on the list of “Invests” is 90L. Image credit: NASA.

Track forecast: 99L a potential long-range threat to Hispaniola, the Bahamas, Cuba, and the U.S.
A strong ridge of high pressure will keep 99L headed slightly north of due west over the next few days, and the storm should enter the Lesser Antilles Islands by Tuesday night, spreading heavy rains and gusty winds. By Wednesday, the storm will likely respond to a weakness in the ridge of high pressure steering it, taking a more west-northwesterly track close to Puerto Rico, reaching Hispaniola or the Southeastern Bahamas by Thursday. The uncertainty about the track increases greatly thereafter, and the entire U.S. coast from Texas to Maine could potentially be a target for 99L in the 7 -10 day time frame.


Figure 2. The dry air of the Saharan Air Layer (SAL) as analyzed by satellite at 8 am EDT Saturday, August 20, 2016. The SAL was interfering with both Fiona and 99L, but was not as concentrated as we saw early in August. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.

Intensity forecast for 99L: NBD or OMG?
As usual, the intensity forecast is a challenging one. The 8 am EDT Saturday run of the SHIPS model showed moderately favorable conditions for development through Wednesday, with wind shear in the moderate range, 10 - 15 knots, a relatively moist atmosphere, and SSTs near 28°C (83°F.) The total heat content of the ocean will steadily increase as 99L moves westwards, as well. Working against development of 99L will be the dry air of the SAL to its north, plus large scale sinking air over the tropical Atlantic imparted by the Madden-Julian Oscillation (MJO). The active portion of the MJO is currently located in the Western Pacific, which is leading to increased tropical cyclone activity there—three named storms were active there on Saturday morning. This positioning of the MJO typically leads to compensating sinking air and surface high pressure over the tropical Atlantic, with reduced chances of tropical cyclone development there. However, by Wednesday, forecasts from the GFS model predict that the MJO will weaken, allowing large scale rising air to set up over the eastern Caribbean—potentially increasing the chances for 99L to develop.

Some of the runs of the GFS model on Friday showed development of 99L into a tropical storm or hurricane in the eastern Caribbean, but the Saturday morning (00Z and 06Z) forecasts from the model have backed off on that idea. Given 99L’s large size and current state of disorganization, the earliest it is likely to become a tropical depression is Monday, so the Lesser Antilles Islands are not going to see a hurricane from this storm. The Saturday morning (00Z) operational runs of our three reliable models for predicting tropical cyclone genesis—the European, GFS and UKMET models—had only one of the three--the UKMET model—showing development of 99L into a tropical storm over the next five days. However, at least five of the twenty members of the 00Z Saturday GFS ensemble showed 99L eventually becoming a hurricane after seven days, as did eight of the fifty members of the European model ensemble. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day development odds of 10% and 50%, respectively. The next name on the Atlantic list of tropical storm names is Gaston—but it is possible that the tropical wave emerging off the coast of Africa on Saturday night could become a named storm before 99L develops, in which case 99L would grab the name Hermine.

A new tropical wave emerging from the coast of Africa may develop
A large new tropical wave with plenty of spin will emerge from the coast of Africa on Saturday night, and the Saturday morning operational runs of the European, GFS and UKMET models all showed development of this wave into a tropical depression in 3 - 5 days. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 10% and 50%, respectively. The wave will head west-northwest to northwest over the next five days on a path similar to Fiona’s, into a region of ocean where very few tropical cyclones ever make the long trek westwards to hit the U.S.


Figure 3. Infrared image of Tropical Storm Fiona as of 1515Z (11:15 am EDT) Saturday, August 20, 2016. Image credit: NOAA/NESDIS.

Tropical Storm Fiona is having a rough weekend
Tenacious Tropical Storm Fiona is fighting its way across the central Atlantic in the midst of moderate to strong vertical wind shear (20-30 knots) that is forcing dry Saharan air into the storm. As of the 11 am EDT Saturday advisory, Fiona was centered near 20.3°N, 47.8°W and moving west-northwest at 14 mph. Fiona is now a minimal tropical storm, with top sustained winds estimated of just 40 mph (at best). The storm’s convection had largely dissipated until early Saturday morning, when a new burst of showers and thunderstorms developed on the northeast side of the low-level center of circulation. Although the HWRF model (currently the most reliable at short-term intensity forecasts) strengthens Fiona briefly over the weekend, the strong consensus among models, including HWRF, is that Fiona will weaken below tropical storm strength by early next week. It could maintain its identity as a tropical depression for at least another day or two after that.

Fiona’s west-northwest motion should continue till around Tuesday, when we can expect it to begin arcing northward between around 60°W and 65°W, toward a weakness in the broad ridging that covers most of the Atlantic subtropics. By this point, wind shear may relax and SSTs beneath Fiona will have risen to 29-30°C, which could help keep Fiona clinging to life if it survives till then. The stronger Fiona is at this point, the more northward a track it would take. Fiona could end up passing near Bermuda around the middle of next week, although it seems unlikely there would be much more than gales and squalls (if that) associated with it.


Figure 4. Residents flee flooded Baton Rouge as waters rose on Saturday, August 13, 2016. (Phin Percy)

Louisiana disaster survivors with disabilities need your support after historic flooding
The Portlight.org disaster relief charity, founded and staffed by members of the wunderground community, is responding to this week's devastating floods in Louisiana. The disaster is particularly troublesome for a state that is still in recovery from major flooding just last March, and many resources are completely depleted because of the March flooding. That storm left more than 5,000 homes damaged or destroyed and cost $1.5 billion across a three-state area. With at least 40,000 homes damaged or destroyed, this week's flooding will cost at least $2 billion, and perhaps much more, according to insurance broker Aon Benfield. There is an urgent need for durable and consumable medical supplies as well as housing. Portlight will be working with the American Red Cross, local stakeholder organizations, and federal partners to respond to this historic flooding event. Your support is needed to make this happen! Please consider making a donation to Portlight's disaster relief fund at the portlight.org website to further their reach and response in the state of Louisiana. Thank you for any support you can offer!

Attention Bay Area readers: Tell us what you think
WU is holding two focus group sessions in San Francisco aimed at gathering input on our products and services. The sessions will take place on Wednesday, August 31, and Thursday, September 1, from 7:45 to 9:45 pm PDT. If you are interested in joining either of these, please fill out this survey. In addition, we always welcome your input via our Customer Feedback website.

Jeff Masters and Bob Henson

Hurricane

Fiona May Fizzle; 99L is Set to Strengthen

By: Bob Henson , 4:35 PM GMT on August 19, 2016

A less-than-impressive Tropical Storm Fiona continues to work its way across the central tropical Atlantic, while another system--Invest 99L—is drawing more interest from tropical weather watchers. It appears 99L could make a break for the Caribbean early next week, perhaps as a significant tropical cyclone (see below).

As of 11 am EDT Friday, Fiona was located at 17.8°N, 43.5°SW, or about 1300 miles west of the Cape Verdes, with top sustained winds holding at 45 mph. Fiona is moving west-northwest at about 10 mph and poses no threat to land. Never a massive storm, Fiona was looking rather spindly on satellite imagery Friday morning, with several fragmented bands feeding into a somewhat disheveled core of convection (showers and thunderstorms). Sea surface temperatures are more than adequate for development, at around 27°C (81°F)—about 0.5°C above the seasonal average—and the storm will be passing over progressively warmer waters. However, Fiona’s overall convective pattern has weakened over the last few hours, and it appears the decline will continue for the next day or two, as vertical wind shear increases to moderate levels (20-25 knots) and dry Saharan air works its way into the circulation (see Figure 2 below).


Figure 1. Latest visible satellite image of Tropical Storm Fiona.


Figure 2. A large mass of dry air from the Sahara Desert (yellows and reds) was encircling Tropical Storm Fiona, located near 45°W, as of 1200Z (8:00 am EDT) Friday, August 19, 2016. Invest 99L, centered to the southeast around 35°W, is expected to track well south of the Saharan air layer for at least the next couple of days. Image credit: University of Wisconsin-Madison/CIMMS/NOAA

The outlook for Fiona
Models are in near-unanimous agreement that Fiona will continue on its west-northwest track for the next 3 to 4 days, heading toward a weakness in the sprawling ridge that extends across the subtropical Atlantic. Uncertainty increases somewhat beyond that point, as there are signs that ridging will begin to fill in the weakness, slowing Fiona’s progress. Both dynamical and statistical models agree that Fiona should remain below hurricane strength throughout the next five days, and some members of the ECMWF and GFS ensembles bring Fiona back to depression status by this weekend or early next week. The National Hurricane Center expects Fiona to weaken into a tropical depression by Sunday.

Assuming that Fiona holds together as a tropical cyclone, it could end up linger around the latitude of 30°N for at least a day or two next week, perhaps longer. A belt of unusually warm SSTs is straddling that latitude, but if Fiona tarries too long in one area, it may churn up cooler waters that could have a weakening effect. Climatology would suggest that eventual recurvature toward the north and northeast remains the most likely destiny for Fiona, if it survives that long.

Keeping an eye on 99L
Located about 500 miles southwest of the Cape Verde Islands, 99L is a large but disorganized tropical wave, with a broad zone of scattered convection. The wave is located close to 10°-15°N and 30°W and is heading westward. 99L’s low latitude, and the lack of northward component to its motion, means that it is well positioned to head toward the Caribbean. An upper-level ridge to the northwest of 99L should keep it on a low-latitude path for the next several days, and models are in strong agreement that 99L will continue on its general westward trek, which could put it in the vicinity of the Lesser Antilles by around the middle of next week. The track forecast becomes more uncertain by that point, and it’s too soon to tell which parts of the island chain might be affected.

Wind shear is expected to remain fairly light along 99L’s path (10-15 knots), and SSTs should remain in the 28-29°C range over the next several days. Both of these factors favor development of 99L. In its 8 am EDT tropical weather discussion, NHC gave 99L only a 10% chance of development by Sunday, but a 50% chance by Wednesday. The discussion noted another wave coming off Africa over the weekend that has a 30% chance of development by Wednesday.


Figure 3. Infrared image of Invest 99L from 1145Z (7:45 am EDT) Friday, August 19, 2016. Image credit: NOAA/NESDIS.


Figure 4. Steering flow at the 200-mb level, about 40,000 feet above the surface, will help keep Invest 99L moving generally westward for the next several days. Wind speeds are colored in knots; multiply by 1.15 to obtain wind speed in mph. 99L and Fiona are indicated as low-pressure centers (“L”) around 30°W and 45°W, respectively. Image credit: tropicaltdibits.com.

Nearly all of the ECMWF and GFS ensemble members keep 99L below tropical storm strength for at least the next several days, with the GFS ensemble suggesting that 99L could intensify in the 4- to 5-day period. A word of caution, however: although the ECMWF and GFS are among the top-performing models for tropical cyclone track forecasts, they are much less skilled at predicting intensity, especially for periods less than 5 days. The best-performing model for intensity over the last several years has been the HWRF, as we discussed in our May post on hurricane intensity prediction. Both the 00Z and 06Z Friday runs of the HWRF model intensify 99L to tropical storm strength over the weekend and to hurricane strength as soon as Tuesday or Wednesday. The IVCN, a blend of several high-performing models, also projects that 99L could reach hurricane strength by early next week.

Climatology favors the development of long-track hurricanes in the deep tropical Atlantic during late August. Given the favorable conditions at hand, we will need to watch 99L very closely.


Figure 5. The latest WU hurricane tracking map shows a plethora of systems across the Northern Hemisphere tropics.

Elsewhere in the (busy) tropics
It’s looking a lot like late August across the northern tropics, with no fewer than seven systems showing up on WU’s hurricane tracking map on Friday morning (see Figure 5 above). Fortunately, none of the current systems were at hurricane/typhoon strength. In the East Pacific, Tropical Storm Kay, christened on Thursday, is paralleling the coast of Mexico several hundred miles south of Baja California. Only a minimal tropical storm, with 40-mph sustained winds, Kay has only a short window of potential modest strengthening before it moves over progressively cooler SSTs. The storm is expected to remain offshore as it gradually weakens.

Tropical Storm Dianmu made landfall on the far north coast of Vietnam around 06Z (2 am EDT) Friday morning. A giant burst of convection is dumping torrential rains on the mountains of northern Vietnam and Laos, and the heavy rains will spread into northern Thailand on Friday and Saturday. Rainfall amounts of up to 16 inches (400 mm) have been reported in parts of northern Vietnam, according to vietnamnet.vn. Further east, Tropical Storm Mindulle is about to accelerate northward while strengthening, which could bring it onto the coast of Japan’s Honshu island near Tokyo as a minimal hurricane by Monday.

Jeff Masters and I will be monitoring the tropics throughout the weekend. We’ll have our next update on Saturday.

Bob Henson


Figure 6. Infrared image of Tropical Storm Kay off the southwest coast of Mexico as of 1500Z (11 am EDT) Friday, August 19, 2016. Image credit: RAMMB/CIRA/CSU.

Hurricane

Fiona Forms in Tropical Atlantic; Tens of Thousands Flee California Wildfire

By: Bob Henson , 4:47 PM GMT on August 18, 2016

A tropical depression in the remote eastern tropical Atlantic became Tropical Storm Fiona on Wednesday afternoon. At 11:00 am EDT Thursday, Fiona was located at 16.4°N, 40.5°W, more than 1000 miles west of the Cape Verde Islands, with top sustained winds of 45 mph. Fiona is a small storm, with tropical-storm force winds extending just 35 miles from its center. On Wednesday evening, Fiona was almost devoid of deep convection (showers and thunderstorms), but tropical cyclones typically experience an uptick in convective activity overnight--a phenomenon called the nocturnal convective maximum. This helped Fiona to develop a compact core of deep convection early Thursday, although some decrease was already evident by late morning.


Figure 1. Latest visible satellite image of Tropical Storm Fiona.

Outlook for Fiona
There are both pros and cons in Fiona’s future as the storm heads west-northwest at a leisurely 8 mph, far from any land areas. Sea-surface temperatures beneath Fiona are around 27°C (81°F) and vertical wind shear is fairly light, less than 10 knots. Wind shear should remain light to moderate for the next day or two, and Fiona’s path will take it over increasingly warmer water of 28-29°C associated with a tongue of unusually warm SSTs (1-2°C above average) extending across the subtropical North Atlantic. While these factors favor development, Fiona is also half-surrounded by a large mass of dry air from the Sahara Desert lying to its north (see Figure 3 below). The storm is likely to ingest some of this dry air, weakening its ability to consolidate convection. Wind shear may also become more of a problem in 2 or 3 days.

The track forecast for Fiona is relatively straightforward, as the storm heads toward a weakness in the upper-level ridge sprawling across the subtropical North Atlantic. Both the GFS and ECMWF--our best models for predicting tropical cyclone tracks--keep Fiona on a west-northwest to northwest heading throughout the next few days, bringing it to around 50-55°W by early next week. The main variable in Fiona’s track is its intensity. Should Fiona remain weak, it is more likely to be steered by low-level winds and continue moving on its present track. The more Fiona manages to intensify, the more likely it would be to arc toward the north, as upper-level winds become more important in steering the storm. The 00Z and 06Z Thursday runs of the GFDL model brings Fiona to hurricane strength this weekend on a north-northwest track east of 50°W. However, other dynamical models do not strengthen Fiona significantly in the next five days, including the most recent operational runs of the GFS (06Z Thursday) and ECMWF (00Z Thursday). Several members of the 00Z Thursday ECMWF ensemble weaken Fiona below the tropical storm threshold (40 mph sustained winds) by the weekend. We have plenty of time to monitor Fiona as the storm gradually moves toward warmer waters. Climatology strongly favors the idea that Fiona will eventually recurve well east of North America, as evident in Figure 2 below.


Figure 2. The tracks and intensities of all tropical storms and hurricanes in August (1851 - 2015) that passed within 2° latitude and longitude of Fiona’s position as of Thursday morning, August 18, 2016.


Figure 3. A large mass of dry air from the Sahara Desert (yellows and reds) was encircling Tropical Storm Fiona, located near 40°W, as of 1500Z (11:00 am EDT) Thursday, August 18, 2016. Another tropical wave was located to the south of the Saharan air layer, centered around 30°W. Image credit: University of Wisconsin-Madison/CIMMS/NOAA

The African wave train continues
As we dive into the heart of the Cape Verde season, tropical waves continue to shuttle off the coast of Africa into the tropical Atlantic. One sizable wave, with an increasing amount of convection on Thursday morning, is located around 30°W, while another wave now over far West Africa will enter the tropical Atlantic during the weekend. In its Thursday morning tropical weather outlook, NHC assigned 10% odds of this second wave developing by next Tuesday, August 23. Longer-range models have been vacillating on the future of these two waves, but they both bear watching, as one or both of them will likely be moving at fairly low latitudes across the tropical Atlantic toward the Caribbean over the next week or so.


Figure 3. Infrared image of Invest 97E off the southwest coast of Mexico as of 1530Z (11:30 am EDT) Thursday, August 18, 2016. Image credit: NOAA/NESDIS.

97E slowly organizing
In the Eastern Pacific, Invest 97E may become a tropical depression or tropical storm by this weekend. Now with sustained winds of around 30 mph, 97E was located about 350 miles west-southwest of Manzanillo, Mexico, moving toward the northwest. Waters along 97E’s path are quite warm, around 29°C (84°F), and wind shear is expected to remain light to moderate (10-20 knots) for the next several days. Although it is not yet well organized, 97E experienced a healthy burst of convection on Wednesday night. Most computer models are keeping 97E below the tropical storm threshold for the next 3-5 days, but given the favorable factors above, it would not be a shock to see 97E become Tropical Storm Kay by early next week. In its Thursday morning outlook, NHC gave 97E a 70% chance of development to at least depression status by Saturday. 97E could eventually take a path toward Baja California, especially if it intensifies.


Figure 4. An infrared image of Tropical Depression Dianmu heading into northern Vietnam as of 1530Z (11:30 am EDT) Thursday, August 18, 2016. Dianmu's showers and thunderstorms span more than 600 miles from east to west. Image credit: RAMMB/Colorado State University

Elsewhere in the tropics
An active monsoon trough in the Northwest Pacific is keeping the region hopping with tropical cyclones. The biggest concern at present is Tropical Depression Dianmu, which is bringing heavy rains to northern Vietnam and Laos. Dianmu’s large envelope of convection could trigger flash floods and landslides as it slogs into the region’s mountainous terrain on Thursday and Friday. Rainfall amounts of 8” - 12” will be widespread, with localized totals on the order of 20”, according to Vietnam’s National Center for Hydro-Meteorological Forecasting.

Meanwhile, Tropical Storm 12 will swing eastward several hundred miles south of Japan over the next couple of days as it remains well below typhoon strength, perhaps intensifying early next week. At the same time, Tropical Depression Ten will be arcing north toward Japan by the weekend, perhaps interacting with TS 12 as it does so.


Figure 5. Embers from the Blue Cut fire smolder along Lytle Creek Road near Keenbrook, CA, on Wednesday, August 17, 2016. Image credit: AP Photo/Noah Berger.

A high-impact wildfire continues to rage east of Los Angeles
More than 82,000 Southern Californians were under mandatory evacuation orders on Thursday as the Blue Cut fire continued burning just north of of the city of San Bernardino, about 60 miles east of Los Angeles. The fire, which now spans more than 31,000 acres, exploded into a huge inferno in a matter of hours on Tuesday afternoon. It’s not yet clear how many structures have already been affected, but officials and locals are bracing themselves for an “immense tally of devastation,” according to the Los Angeles Times. The evacuation warnings include an estimated 34,500 homes, according to InciWeb.

The Blue Cut fire was fed by hot, dry southwest winds heading into the Cajon Pass, which lies between the San Gabriel and San Bernardino mountain ranges. The rugged topography and tinder-dry vegetation has led to challenging conditions for more than 1500 firefighters. Near the center of the fire, Interstate 15, which connects Los Angeles and Las Vegas via the Cajon Pass, was completely shut down on Tuesday, and the southbound lanes remained closed on Thursday morning. As of Thursday morning, the blaze was only 4% contained. A red flag warning remained in effect on Thursday and could be extended into Friday, with relative humidities expected to fall below 10 percent and winds gusting as high as 30-40 mph. The weather will improve only slightly over the next couple of days, with somewhat lower temperatures and higher humidities, but gusty winds are expected to continue.

Largely bypassed by the El Niño rains of 2015-16, Southern California will be extremely vulnerable to major wildland fire over the next few weeks until winter rains (hopefully) arrive. Five years of drought have left a landscape packed with dead or dying trees and brush, and September and October often bring some of the hottest, driest weather of the year. Other parts of the U.S. West will need to stay on alert as well. For Thursday, the NOAA Storm Prediction Center outlook warned of critical fire weather conditions over the Okanogan Valley of eastern Washington, with several other regions of elevated risk (including interior parts of Southern California east of Los Angeles and San Diego).

Bob Henson


Figure 6. The burned-out hulk of a 1960s Ford Falcon is seen on Wednesday, August 17, 2016, after flames from the Blue Cut wildfire overnight swept through a rural area near Phelan, CA. Image credit: AP Photo/Christine Armario.

Hurricane Fire

Warmest July--and Warmest Month--On Record for the Globe

By: Jeff Masters and Bob Henson , 8:11 PM GMT on August 17, 2016

July 2016 was Earth's warmest July since record keeping began in 1880, said NOAA's National Centers for Environmental Information (NCEI) on Wednesday. In the NOAA database, July 2016 came in 0.87°C (1.15°F) warmer than the 20th-century average for July, beating the previous record for July, set in 2015, by 0.06°C. NASA also reported the warmest July in its database.

Even more impressive, July 2016 was also Earth’s warmest month in recorded history in absolute terms. This is because July is the planet’s hottest month of the year overall. Land areas heat and cool more readily than the ocean surface, and most of Earth’s land area is in the Northern Hemisphere, where summer arrives in the middle of the year. The global average surface temperature is normally about 3-4°C (5-7°F) warmer in July than in January. In relative terms, February 2016 was Earth’s warmest month on record, according to NASA, since it came in at 1.32°C (2.38°F) warmer than the 20th-century average for that month. NOAA rated March 2016 as the month with the warmest anomaly on record, at 1.22°C (2.20°F). Both of these effects--relative and absolute--can be seen in Figure 1 below.


Figure 1. The departure from average (compared to temperatures from 1980 - 2015) of Earth’s surface temperature from 1880 to 2016, with the seasonal cycle left in. July 2016 was Earth’s hottest month on record in absolute terms, while February 2016 had the largest departure from average (in relative terms) from average of any month in the historical record. Image credit: Gavin Schmidt, director of NASA’s Goddard Institute of Space Studies.


Figure 2. Departure of temperature from average for July 2016, the warmest July for the globe since record keeping began in 1880. Pockets of record warmth were observed across every major ocean basin and over a few land areas. Image credit: National Centers for Environmental Information (NCEI).

A year-plus streak of global records
uly 2016 marked the 15th consecutive month that NOAA’s global monthly temperature record was broken, which is the longest such streak since global temperature records began in 1880. The record-warm July extended to both global ocean and global land temperatures in the NOAA database. For the lowest 8 km of the atmosphere, global satellite-measured temperatures in July 2016 were the 2nd warmest for any July in the 38-year record, according to the University of Alabama in Huntsville (UAH).

With the powerful 2015-16 El Niño event now over, the impressive global warmth in recent months can mostly be attributed to the steady build-up of heat-trapping greenhouse gases due to human activities. NOAA’s global surface temperature for the year so far (January-July 2016) is 1.03°C (1.85°F) above the 20th-century average and a remarkable 0.19°C (0.34°F) warmer than the previous January-to-July record, set in 2015 (see Figure 3 below). Following the 1997-98 “super” El Niño, monthly global temperature records were set through August 1998. The departure of the equally strong 2015-16 El Niño and the possible arrival of La Niña late this year should allow temperatures to drop slightly, perhaps breaking our string of record-warm months sometime in the near future. However, temperatures would have to plummet between now and December in order to keep 2016 from becoming the warmest year in global record keeping. According to Gavin Schmidt, director of NASA’s Goddard Institute of Space Studies, there is a 99 percent chance that 2016 will end up as Earth’s third consecutive hottest year on record.


Figure 3. Departure from average for the global January-through-July temperature for the years 1880 - 2016. This year has seen by far the warmest temperatures on record for the year-to-date period. Image credit: NOAA/National Centers for Environmental Information (NCEI).

Weak La Niña favored for this fall
El Niño dissipated in May 2016, giving way to El Niño-Southern Oscillation (ENSO)-neutral conditions as sea surface temperatures declined across the tropical Pacific Ocean. According to NOAA's August ENSO forecast from the Climate Prediction Center, a weak La Niña is favored to develop during the August - October peak of hurricane season and last through the Northern Hemisphere winter. The La Niña odds in the August outlook were 55 - 60%, which were lower than the 75% odds given in their June forecast.

Arctic sea ice hits its third lowest July extent on record
The rate of July sea ice loss was below average last month, due to cool and stormy conditions in the Arctic. As a result, sea ice extent in July 2016 was just the third lowest in the 38-year satellite record, according to the National Snow and Ice Data Center (NSIDC). So far, March and July have been the only months in 2016 that has not set a new record low for Arctic-wide sea ice extent (March 2016 was second lowest, just above 2015). A new record low September ice extent "now appears to be unlikely", NSIDC said in their August update. However, the “Great Arctic Cyclone of 2016” is currently spinning over the Arctic, as Bob Henson detailed in a Tuesday afternoon post. This massive storm could break up the ice, leading to lower ice extent in September than would otherwise have been observed.

Three billion-dollar weather disasters for July 2016: Typhoon Nepartak, China floods, Netherlands severe weather
According to the July 2016 Catastrophe Report from insurance broker Aon Benfield, two billion-dollar weather-related disaster hit the planet in July: $1.5 billion in damage from Typhoon Nepartak's impact on the Philippines, Taiwan, and China; and flooding in northeast China on July 16 - 24 that cost $5 billion. An additional $6 billion in flood damage occurred along the Yangtze River basin, bringing the total flood damage in that region from monsoon rains that began in May to a staggering $28 billion dollars--the third most expensive non-U.S. weather related disaster in recorded history. Additionally, a severe weather outbreak in the Netherlands during June accumulated enough damage claims to be rated a billion-dollar disaster by the end of July. Between January - July 2016, there were 21 billion-dollar weather disasters--one fewer than occurred during January - July 2013, the year that ended up with the most billion-dollar weather disasters on record: 41. Here is the tally of billion-dollar weather disasters for January - July 2016:

1) Flooding, Yangtze Basin, China, 5/1 - 8/1, $28.0 billion, 475 killed
2) Flooding, Germany, France, Austria, Poland, 5/26 - 6/6, $5.5 billion, 17 killed
3) Drought, India, 1/1 - 6/30, $5.0 billion, 0 killed
4) Flooding, Northeast China 7/16 - 7/24, $5.0 billion, 289 killed
5) Wildfire, Fort McMurray, Canada, 5/2- 6/1, $5.0 billion, 0 killed
6) Severe Weather, Plains-Southeast U.S., 4/10 - 4/13, $3.75 billion, 1 killed
7) Flooding, China, 6/18 - 6/23, $2.3 billion, 68 killed
8) Severe Weather, Rockies-Plains-Southeast-Midwest U.S., 3/22 - 3/25, $2.2 billion, 0 killed
9) Winter Weather, East Asia, 1/20 - 1/26, $2.0 billion, 116 killed
10) Tropical Cyclone Roanu, Sri Lanka, India, Bangladesh, Myanmar, China, 5/14 - 5/21, $1.7 billion, 135 killed
11) Severe Weather, Plains-Midwest U.S., 4/29 - 5/3, $1.6 billion, 6 killed
12) Drought, Zimbabwe, 1/1 - 3/1, $1.6 billion, 0 killed
3) Typhoon Nepartak, Philippines, Taiwan, China, 7/8 - 7/9, $1.5 billion, 111 killed
13) Severe Weather, Plains-Southeast U.S., 3/17 - 3/18, $1.3 billion, 0 killed
14) Flooding, Argentina and Uruguay, 4/4 - 4/10, $1.3 billion, 0 killed
15) Severe Weather, Plains-Midwest-Southeast-Northeast U.S., 3/4 - 3/12, $1.25 billion, 6 killed
16) Severe Weather, Plains-Midwest-Southeast-Northeast U.S., 2/22 - 2/25, $1.2 billion, 10 killed
17) Severe Weather, Plains-Midwest U.S., 5/21 - 5/28, $1.1 billion, 1 killed
18) Severe Weather, Netherlands, 6/23 - 6/24, $1.1 billion, 0 killed
19) Flooding, Plains-Rockies U.S., 4/15 - 4/19, $1.0 billion, 9 killed
20) Tropical Cyclone Winston, Fiji, 2/16 - 2/22, $1.0 billion, 44 killed
21) Winter Weather, Eastern U.S., 1/21 - 1/24, $1.0 billion, 58 killed


Figure 3. This summer's Yangtze River basin floods in China are the third most expensive weather-related natural disaster on record outside of the U.S., according to the International Disaster database, EM-DAT.

And here are the three disasters from July 2016 in more detail:


Disaster 1. Severe thunderstorms swept through the Netherlands, causing hail, wind and isolated flash flood damage in South Holland and Utrecht provinces on June 23 - 24. In this image, we an arcus cloud from a severe thunderstorm over the A2 between Utrecht and Amsterdam on 23 June 2016. Image credit: ROBIN VAN LONKHUIJSEN/AFP/Getty Images.


Disaster 2. Super Typhoon Nepartak hit Taiwan on July 7 as a Category 4 storm with 150 mph winds, killing 3 and causing $21.1 million in damage. After weakening to a tropical storm, Nepartak made landfall in mainland China, where it killed at least 83 people and caused $1.51 billion in damage. Here, we see a radar image of Super Typhoon Nepartak taken at 11:30 am EDT July 7, 2016 (11:30 pm local time in Taiwan), when Nepartak was a Category 5 storm with 160 mph winds. Image credit: Taiwan CWB.


Disaster 3. Torrential rains fell in northern sections of China from July 16-24, leaving at least 289 people dead or missing, and causing $5 billion in damage. The hardest-hit provinces included Beijing, Hebei, Henan, Shanxi, Tianjin, and Shandong. More than 300,000 homes were damaged or destroyed and as many as 1.2 million hectares (3.0 million acres) of cropland was submerged. This photo taken on July 21, 2016 shows people making their way through a flooded area in Changping District in Beijing. Image credit: STR/AFP/Getty Images.

Bountiful but deadly monsoon rains continue in India
India, whose $5 billion drought was Earth's second most expensive weather-related natural disaster of the first half of 2016, is finally getting a good monsoon after two straight years of poor rains. According to the India Meteorological Department, monsoon rains during the period July 1 - August 16, 2016 were right at average. However, July monsoon floods left more than 230 people dead or missing in India, Nepal, Pakistan, Indonesia and Afghanistan as tens of thousands of homes were destroyed.

Notable global heat and cold marks set in July 2016
Hottest temperature in the Northern Hemisphere: 54.0°C (129.2°F) at Mitribah, Kuwait, 21 July*
Coldest temperature in the Northern Hemisphere: -30.5°C (-22.9°F) at Geo Summit, Greenland, 31 July**
Hottest temperature in the Southern Hemisphere: 41.1°C (106.0°F) at Matupa, Brazil, 11 July
Coldest temperature in the Southern Hemisphere: -82.4°C (-116.3°F) at Concordia, Antarctica, 8 July
(Courtesy of Maximiliano Herrera.)

** Ties lowest temperature ever recorded in July in the entire Northern Hemisphere.

Major weather stations that set (not tied) new all-time heat or cold records in July 2016 (Courtesy of Maximiliano Herrera)
Beru (Kiribati) max. 35.2°C, 2 July
Owase (Japan) max. 38.6 °C,3 July
Ruoqiang (China) max. 43.9°C, 3 July
Shahdad (Iran) max. 51.7°C, 4 July
Indio Hatuey (Cuba) max 38.2°C, 5 July
Ulan Ude (Russia) max. 40.6°C, 8 July
Krasnyj Chikoj (Russia) max. 39.3°C, 8 July
Iquique (Chile) max. 33.4°C, 8 July
La Paz (Mexico) max. 44.0°C, 10 July
Xi Ujimqin Qi (China) max. 39.7°C, 10 July
Deadhorse Airport (Alaska,USA) max. 29.4°C, 13 July
Kuparuk (Alaska,USA) max. 29.4°C, 13 July
San Jon (New Mexico,USA) max. 43.9°C, 13 July
Tindouf (Algeria) max. 48.3°C, 15 July
Ploumanach (France) max. 36.2°C, 19 July
Cap de la Hague (France) max. 32.3°C, 19 July
Salahaddin (Iraq) max. 41.5°C, 20 July
Basra (Iraq) max. 53.4°C, 21 July: New national record high for Iraq; increased to 53.9°C on 22 July
Kut al Yai (Iraq) max. 52.0°C, 21 July
Amarah (Iraq) max. 52.2°C, 21 July
Kanaqin (Iraq) max. 52.6°C, 21 July
Mitribah (Kuwait) max. 54.0°C, 21 July: New national record high for Kuwait *
Al Salmi (Kuwait) max. 50.4°C, 21 July; increased to 50.9°C on 22 July
Dunhuang (China) max. 43.1°C, 30 June
Mazong Shan (China) max. 36.2°C, 30 June

Notes from Maximiliano Herrera:
* 54.0°C at Mitribah is a new record of highest temperature for Asia (being the same value reached at Tirats Tsvi in 1942 highly unreliable- in my opinion around 4-5°C overestimated). It also ties the 54.0°C or 129.2°F (although officially 129°F) set at Death Valley, California on 30 June 2013 as the world record of the highest reliable temperature ever recorded. (The 134°F official record in Greenland Ranch are in my opinion around 12°F to 15°F overestimated). Therefore, It is also a new world record for July.

On 9 July Hong Kong tied its record of highest temperature ever with 37.9°C at Happy Valley. The same value was recorded at the same location in 2015.

On 22 July, Delhoran, Iran recorded 53.0°C, tying the Iranian highest reliable temperature on record which was set at the same station and at Gotvand.

Four all-time national heat records set or tied in July 2016
Four nations or territories--Kuwait, Iraq, Iran, and Hong Kong--set or tied records in July 2016 for their all-time hottest temperature on record. From January through August 16, 2016, a total of sixteen nations or territories tied or set all-time records for their hottest temperature in recorded history. Only 2010 has had more all-time heat records set--seventeen. Also, one all-time cold temperature record has been set so far in 2016 (in Hong Kong.) "All-time" record here refers to the warmest or coldest temperature ever reliably reported in a nation or territory. The period of record varies from country to country and station to station, but it is typically a few decades to a century or more. Most nations do not maintain official databases of extreme temperature records, so the national temperature records reported here are in many cases not official. Our data source is international weather records researcher Maximiliano Herrera, one of the world's top climatologists, who maintains a comprehensive list of extreme temperature records for every nation in the world on his website. If you reproduce this list of extremes, please cite Maximiliano Herrera as the primary source of the weather records. Here are 2016's all-time heat and cold records as of August 16:

Iraq set its all-time hottest record on July 22, 2016, when the mercury hit 53.9°C (129.0°F) at Basrah.

Iran tied its all-time hottest record on July 22, 2016, when the mercury hit 53.0°C (127.4°F) at Delhoran.

Kuwait set its all-time hottest record on July 21, 2016, when the mercury hit 54.0°C (129.2°F) at Mitribah.

Hong Kong Territory (China) tied its all-time hottest record on July 9, 2016, when the mercury hit 37.9°C (100.2°F) at Happy Valley.

Niger set its all-time hottest record on June 8, 2016, when the mercury hit 49.0°C (120.2°F) at Bilma.

Palau tied its all-time hottest record on June 8, 2016, when the mercury hit 34.4°C (93.9°F) at Koror AWS.

India set its all-time hottest record on May 19, 2016, when the mercury hit 51.0°C (123.8°F) at Phalodi.

Maldives set its all-time hottest record on April 30, 2016, when the mercury hit 35.0°C (95.0°F) at Hanimaadhoo.

Thailand set its all-time hottest record on April 28, 2016, when the mercury hit 44.6°C (112.3°F) at Mae Hong Son.

Cambodia set its all-time hottest record on April 15, 2016, when the mercury hit 42.6°C (108.7°F) at Preah Vihea.

Burkina Faso set its all-time hottest record on April 13, 2016, when the mercury hit 47.5°C (117.5°F) at Dori.

Laos set its all-time hottest record on April 12, 2016, when the mercury hit 42.3°C (108.1°F) at Seno.

Vanuatu in the South Pacific set its all-time hottest record on February 8, 2016, when the mercury hit 36.2°C (97.2°F) at Lamap Malekula.

Tonga set its all-time hottest record on February 1, 2016, when the mercury hit 35.5°C (95.9°F) at Niuafoou.

Wallis and Futuna Territory (France) set a new territorial heat record with 35.8°C (96.4°F) on January 10, 2016 at Futuna Airport. This is the second year in a row that Wallis and Futuna has beaten its all-time heat mark; the previous record was a 35.5°C (95.9°F) reading on January 19, 2015 at the Futuna Airport.

Botswana set its all-time hottest record on January 7, 2016, when the mercury hit 43.8°C (110.8°F) at Maun.

Hong Kong Territory (China) set its all-time coldest mark on January 24, 2016, when the mercury dipped to -6.0°C (21.2°F) at Tai Mo Shan.

TD 6 likely to become Tropical Storm Fiona
Tropical Depression 6 in the remote eastern Atlantic is likely to become Tropical Storm Fiona by later Wednesday, according to the National Hurricane Center. See our post from Wednesday morning for more on TD 6's status and outlook. We'll be back Thursday with our next update.

Jeff Masters and Bob Henson

Climate Summaries

TD 6 Continues to Develop; Where Were News Media during Louisiana Flood?

By: Bob Henson and Jeff Masters , 3:52 PM GMT on August 17, 2016

Tropical Depression Six formed on Tuesday evening in the remote waters of the Central Atlantic about 700 miles west of the Cabo Verde Islands. Satellite loops on Wednesday morning showed that TD 6 had a modest area of heavy thunderstorms that were not particularly vigorous, but the storm was well organized, with good low-level spiral banding. Wind shear was light, 5 - 10 knots, and sea surface temperatures (SSTs) were warm enough for development: 27°C (81°F). Water vapor satellite imagery showed that TD 6 was in a moist environment on its southern flank, but dry air from the Saharan Air Layer (SAL) was along the northern side of the storm, interfering with development. Overall, these conditions are favorable for some modest development.


Figure 1. Latest satellite image of TD 6.

Forecast for TD 6
Steering currents favor a northwesterly motion at about 10 - 12 mph for TD 6 the remainder of the week. This track will likely take the system too far to the north for it to be a long-range threat to the Lesser Antilles Islands, though we can’t rule out a threat to Bermuda yet. The 8 am EDT Wednesday run of the SHIPS model showed moderately favorable conditions for development through Friday, with wind shear light to moderate range, 5 - 15 knots, a moist atmosphere, and SSTs near 27°C (81°F.) In its 11:00 am EDT update on TD 6, the National Hurricane Center projected that the depression would become Tropical Storm Fiona later on Wednesday. However, beginning on Friday, the storm is expected to encounter a very dry air mass and high wind shear near 20 knots, which should cause weakening. The European model predicts that TD 6 will dissipate this weekend, which is certainly a possibility.

More African waves coming
A series of tropical waves will emerge from the coast of Africa during the next week, and we will have to watch these for development as they track westward to west-northwestward across the tropical Atlantic. The models have been inconsistent in their handling of the track and potential development of these waves over the past few days, though the Wednesday morning runs of the GFS and European models agreed that a tropical wave due to come off of Africa on Saturday might develop by early next week.


Figure 2. David McNeely (left) and Jason Schexnayder walk through a flooded street in Sorrento, LA, as an early-morning fog blankets the area on Wednesday, August 17, 2016. Image credit: Joe Raedle/Getty Images.

Slow recovery and big questions in Louisiana as flood waters recede
As the immediate emergency subsides in southeast Louisiana, residents are dealing with a massive clean-up effort and wondering how the past weekend’s flooding turned out to be so disastrous. An estimated 40,000 homes have been damaged or destroyed by flooding, with 20,000 people rescued from high water and 10,000 in shelters, in what is likely to be a billion-dollar-plus disaster. Enormous rainfall was the obvious trigger, as detailed in our posts of the last several days. Even in a state as wet and flood-prone as Louisiana, some places are hit more regularly than others. Many of the areas flooded in this event had not been under water in living memory, which added to the shock and pain of this event.

A long-planned diversion project designed to channel water a few miles westward from the Comite River to the Mississippi River just north of Baton Rouge might have kept thousands of homes in the Baton Rouge area from flooding, as reported Tuesday in the Baton Rouge Advocate. The canal would have diverted water from the Comite before that river’s record crest had a chance to pass through northeast parts of the Baton Rouge area. In addition, since the Comite joins with the Amite River near hard-hit Denham Springs, the canal would have reduced major flooding there and downstream as well. Taxpayers in three parishes approved a property tax more than a decade ago to fund the canal, and some progress has been made, but state and federal funding to date has been insufficient to complete the project, according to the Advocate article. The U.S. Army Corps of Engineers has estimated that the canal would cost just over $200 million--which could end up on par with the component of damage that resulted from the canal’s absence. As was the case with Hurricane Katrina in 2005, delays in flood control ended up exacerbating the toll from flooding in the suffering of a state dealing with multiple other co-factors, including rising sea levels, a coastline plagued with subsidence, and a climate more prone to intense rainfall.


Figure 3. Historic flooding in Port Vincent, along the Amite River southeast of Baton Rouge, was captured on Sunday, August 14, in an aerial photography mission carried out by NOAA’s Remote Sensing Division. A useful NOAA website allows you to zoom in on a map of the hard-hit region and view additional photos. Mark Schleifstein, the award-winning environmental reporter from the New Orleans Times-Picayune, has compiled several science-oriented sources of flood imagery. Image credit: Courtesy NASA Earth Observatory.

Why did the flooding get so little national attention for days?
As floodwaters were spreading across southeast Louisiana, the emerging disaster got surprisingly little notice in U.S. news media. Although the Louisiana governor declared a state of emergency on Friday, the New York Times posted no staff-written articles on the crisis until Sunday evening, and it took until Tuesday for a Times reporter to reach the scene from New Orleans. “Many readers have expressed disappointment in the coverage,” said Times public editor Liz Spayd in a mea-culpa essay on Tuesday entitled “On Gulf Coast Flooding, The Times Is Late to the Scene.” In a Wednesday report titled “National media fiddle as Louisiana drowns,” Mike Scott from the New Orleans Times-Picayune highlighted the contrast between the huge amount of information and photos posted to social media during the flood (albeit much of it unvetted) with the lack of traditional coverage.

A whole host of factors led to a perfect storm of media inattention. Among the other events jostling for news coverage just this past weekend:
--the competition in Rio de Janeiro
--protests in Milwaukee
--the ongoing U.S. presidential race

Weekends are an increasingly challenging time to cover breaking news, especially during the peak vacation season of August. Newspaper and TV journalists across the U.S. have been hit with major job cuts over the last few years, leaving many newsrooms understaffed, as readers drop print subscriptions or cut the cable. On top of all this, the slow-moving low that triggered the Louisiana rains was not officially designated as a tropical cyclone and thus went unnamed. “The American public is somewhat conditioned to perceive a named or higher-category storm as more of a threat,” said Marshall Shephard (University of Georgia) in a thoughtful Forbes op-ed on Tuesday. Shepherd also notes the possible role of “flood fatigue” in the wake of many high-profile flash and river floods across the nation in recent months and years. Over the last 18 months, according to weather.com’s Jon Erdman, there have been 18 major flood events across the four-state region of Arkansas, Oklahoma, Louisiana, and Texas. “The number, extremity, and widespread nature of flood events has been incredible in this region,” said Erdman.

Heavy rains shift to Texas
Residual moisture from the soggy, slow-moving upper low continues to produce heavy rain across parts of south Texas, where flash flood watches are in effect. The downpours have been fairly scattered and disorganized, reducing any potential for widespread flooding. CoCoRaHS maps show that several pockets of 3” - 6” rains developed near San Antonio and north of Austin in the 24-hour period ending at 7 am CDT Wednesday. Toward the coming weekend, the preexisting moisture will combine with a seasonally strong cool front to produce several days of heavy rain in a belt from southwest Texas into the Dallas-Fort Worth area.


Figure 4. Precipitation totals projected for the 5-day period from 12Z (8:00 am EDT) Wednesday, August 17, 2016, to 12Z Monday, August 22. Image credit: NOAA/NWS Weather Prediction Center.


Figure 5. Danielle Blount kisses her 3-month-old baby Ember as she feeds her while they wait to be evacuated by members of the Louisiana Army National Guard near Walker, LA, on Sunday, August 14, 2016, after heavy rains inundated the region. (AP Photo/Max Becherer)

Louisiana disaster survivors with disabilities need your support after historic flooding
The Portlight.org disaster relief charity, founded and staffed by members of the wunderground community, continues to respond to this past week's devastating floods in Louisiana. The disaster is particularly troublesome for a state that is still in recovery from major flooding just last March, and many resources are completely depleted because of the March flooding. That storm left more than 5,000 homes damaged or destroyed and cost $1.5 billion across a three-state area. With at least 40,000 homes damaged or destroyed, this week's flooding may end up being even more costly, according to insurance broker Aon Benfield. There is an urgent need for durable and consumable medical supplies as well as housing. Portlight will be working with the American Red Cross, local stakeholder organizations, and federal partners to respond to this historic flooding event. Your support is needed to make this happen! Please consider making a donation to Portlight's disaster relief fund at the portlight.org website to further their reach and response in the state of Louisiana. Thank you for any support you can offer!

We’ll have a post this afternoon on the July climate summary for Earth.

Bob Henson and Jeff Masters

Flood Hurricane

The Great Arctic Cyclone of 2016: After Four Years, a Summer Sequel

By: Bob Henson , 6:29 PM GMT on August 16, 2016

As of Tuesday, the deepest cyclone in the Northern Hemisphere wasn’t anywhere near the tropics--it was spinning in the central Arctic Ocean. A surface low located near 83°N, about 500 miles from the North Pole and about 1000 miles north of Barrow, Alaska, deepened to a central pressure of 968 millibars (mb) at 2 am EDT Tuesday morning, August 16. This is on par with the central pressure you might find in a moderately-sized Category 2 hurricane. Such lows are a common feature of Arctic climate, but they rarely gain such intensity in the middle of summer. The only deeper Arctic cyclone on record in August is the Great Arctic Cyclone (GAC) of 2012. According to a 2012 study by Ian Simmonds and Irina Rudeva (University of Melbourne), this low bottomed out at 966 mb on August 6, yielding the lowest pressure analyzed across more than 1600 August cyclones in the Arctic since 1979. The cyclone's minimum pressure was even lower--963 mb--in the real-time analyses produced by Environment Canada while the storm was raging.


Figure 1. The Arctic cyclone was analyzed with a central pressure of 968 mb at 06Z (2:00 am EDT) Tuesday, August 16, 2016. The central pressure had risen to 971 mb by 12Z (8:00 am EDT). Image credit: tropicaltidbits.com.


Figure 2. Surface analyses over the Arctic Ocean show the Great Arctic Cyclone of 2012 at its deepest (left, at 06Z August 6, 2012) juxtaposed with the current cyclone (right, as of 00Z Tuesday, August 16, 2016). The initial Image credit: Environment Canada.

Wind, waves, and ice
The GAC of 2012 churned across the Arctic for ten days while its central pressure was below 1000 mb. The cyclone had major effects on the distribution of regional ice and appears to have played at least some role in that summer’s record depletion of Arctic sea ice. Normally, low pressure near the North Pole causes ice to spread out (as surface waters and sea ice move to the right of the surface wind). Yet the intensity and duration of the 2012 cyclone’s winds and waves appears to have more than compensated for that effect, leading to an overall loss of ice extent. The extent plummeted in August 2012 en route to a record-low extent in September.

A study in 2013 led by Jinlun Zhang (University of Washington) found that the GAC quadrupled the melting of sea ice from below by pushing warm surface water against the bottom of wind- and wave-tossed ice floes. However, because much of the Arctic ice was already thin and compromised, much of the extent loss that would occur in August and September was already baked into the system when the cyclone came along. Zhang and colleagues estimated from a model simulation that the record September minimum was only about 4% lower as a result of the GAC of 2012.

The kind of ice-breaker we don’t need
It’s too soon to know exactly how this year’s storm--let’s call it the Great Arctic Cyclone of 2016 for now--will affect the Arctic. However, according to polar researcher James Screen (University of Exeter), “This certainly has the potential to be an interesting event and possibly have a big influence on whether or not we see a new record sea ice minimum next month.” As many reports have noted in the last few years, the Arctic’s summer ice pack is in the midst of a dramatic long-term decrease due to global and regional warming. However, there remains a good bit of year-to-year variation in the ice extent. Each summer’s ice pack has a different character in terms of the area it covers, its thickness, the extent and location of surface melt ponds, and so on. This means the impacts of a strong August cyclone in the high Arctic could be quite different from one year to the next. The current cyclone is located near a zone that separates relatively thick, dense ice to its east (north of the Canadian Archipelago) from thinner, more dispersed ice extending from the eastern coast of Siberia all the way up to near the North Pole (see Figure 4 below).


Figure 3. Near-surface circulation around the low in the central Arctic as of early Tuesday, August 16, 2016. Sustained winds of at least 30 mph appear to be affecting a large area. Image credit: earth.nullschool.net.


Figure 4. Sea ice concentration across the Arctic as of August 15, 2016. The approximate location of the strong Arctic cyclone as of Tuesday, August 16, is shown by the large L. Image credit: University of Bremen.


Figure 5. Largely due to incredible winter warmth, temperatures averaged north of the Arctic Circle for the period January through July were far higher this year than in any year since records began in 1948. Image credit: Zach Labe, UC-Irvine, @ZLabe.


Potential impacts
As of early August, the Arctic’s sea ice extent was among the four lowest on record since satellite monitoring began in 1979. Temperatures across the Arctic for the year thus far have been far above record levels (see Figure 5), so there is concern that the ice pack may be weaker than satellite measurements and models imply. We can expect some dramatic changes over the next few days, as winds and waves break up ice and churn up relatively warm water from below. Much will depend on the exact track of this Arctic cyclone and how long it persists as an intense low.

Remarkably, the most recent runs of the ECMWF and GFS keep the current low and/or subsequent lows spinning across the Arctic Ocean for at least the next week--perhaps at pressures below 990 mb for much or most of the time. The models even flag the possibility of another unusually intense cyclone at some point next week. One caution from polar scientist Steven Cavallo (University of Oklahoma): “There is not really much skill in the forecast models accurately predicting the strength of an Arctic cyclone more than 3 days ahead of time.”  However, if the models' overall message of unusually persistent and strong low pressure in the central Arctic verifies, there could be very significant impacts to the sea ice pack extending through the rest of the melt season.

Will climate change lead to more Great Arctic Cyclones?
Given the amount of change occurring to the state of summer sea ice in the Arctic, it’s natural to wonder if there might be a change in the upward-pointing influence on atmospheric circulation. “An interesting question is whether the long-term loss of ice (and increased heat and moisture fluxes) is making Arctic cyclones more intense,” Screen noted. “The jury is still out on that one, but if that was the case, we could expect GAC-type events to occur more frequently.” A 2009 modeling study led by Yvan Orsolini (Norwegian Institute for Air Research) estimated that the number and strength of summertime Arctic cyclones would increase slightly through the 21st century. Over the long term, “we do expect a lowering of the mean sea level pressure over the Arctic with sea ice loss," says climate modeler Clara Deser (National Center for Atmospheric Research]. "To to the extent that this signal reflects cyclone behavior, I would concur that under climate change, it might be the case that cyclones would intensify. However, attribution of the extent to which the Great Arctic Cyclone of 2016 is a result of the loss of sea ice is more challenging, since internal variability may also contribute."

For deep coverage of the deep Arctic cyclone, check out the dedicated post at the Arctic Sea Ice Blog, as well as recent entries in “The 2016 melting season” at the Arctic Sea Ice Forum.

Elsewhere on the blog
Jeff Masters filed an update earlier Tuesday on the devastating floods in southeast Louisiana, which have now inundated some 40,000 homes, and on the current state of Invest 98L, which is likely to become a tropical depression in the next day or two. We’ll be back with our next post by Wednesday.

Bob Henson


Figure 6. Sea ice across the Arctic experienced a major decline in extent and concentration during the first few days of August 2012, as the Great Arctic Cyclone of 2012 churned across the area. Image credit: National Snow and Ice Data Center.

Arctic Climate Change

Louisiana Floods Overtop Levee, Inundating 15,000 Homes; 98L May Develop

By: Jeff Masters , 3:48 PM GMT on August 16, 2016

The highest flood crest ever observed on Louisiana’s Amite River has overtopped the Laurel Ridge Levee in Ascension Parish, about 20 miles southeast of the capital of Baton Rouge, resulting in the flooding of at least 15,000 homes—one third of the parish’s homes, reported the Baton Rouge Advocate on Tuesday morning. The river was not forecast to fall below its previous record height (set in 1983) until Wednesday. At least nine people have died in flooding in Louisiana that began last Friday, with at least 20,000 people rescued from flooded homes and vehicles; 10,000 people are in shelters due to the disaster. The federal government has declared the event a major disaster in four parishes: Tangipahoa, St. Helena, East Baton Rouge and Livingston. The governor of Louisiana, John Bel Edwards, said on CNN that he expects nearly half of all of the state’s parishes—30 out of 64—to be declared disaster areas. Update: Some 40,000 homes in all have now been affected by the Louisiana flooding, according to a Tuesday morning press conference from Governor Edwards.


Figure 1. This aerial image shows flooded areas on and near the campus of Louisiana State University (LSU), Saturday, Aug. 13, 2016, in Baton Rouge, La. (Patrick Dennis/The Advocate via AP)


Figure 2. The Amite River at Port Vincent, Louisiana crested at least 2.8 feet above its previous record level on Monday. The river was forecast to fall below its previous record height (set in 1983) by Wednesday. Image credit: NWS/AHPS.

A tropical depression-like storm with massive amounts of moisture
Multi-day rainfall amounts of 20” to 30”, produced by a slow-moving low pressure center similar to a tropical depression, hit a swath of south-central and southeast Louisiana from Friday through Sunday. Some parts of Louisiana recorded more than 20" of rain in 48 hours, which qualifies as a 1-in-1,000 year rainfall event (having a 0.1 percent chance of occurring at a particular location in any given year), according to the NWS Lower Mississippi River Forecast Center. The highest rainfall total from the storm was 31.39” in Watson, Louisiana. The storm system carried near-record amounts of atmospheric moisture, drawn from the Gulf of Mexico and northwest Atlantic, where sea-surface temperatures (SSTs) were at near-record levels. SSTs over the northern Gulf of Mexico have since cooled, due to the week-long period of cloud cover and strong winds from the storm.


Figure 3. MODIS visible satellite image of 98L, located about 500 miles west-southwest of the Cabo Verde Islands, taken on Tuesday morning, August 16, 2016. The brownish colors to the northeast of the disturbance are due to a large region of African dust. Image credit: NASA.

African tropical wave 98L may develop later this week
A strong tropical wave located about 500 miles west-southwest of the Cabo Verde Islands on Tuesday morning was headed west-northwest at 15 mph, and has the potential to develop into a tropical depression this week. This disturbance was designated Invest 98L on Sunday evening by NHC. Satellite loops on Tuesday morning showed 98L had a modest area of heavy thunderstorms which had acquired a respectable amount of spin. Wind shear was light to moderate, 5 - 15 knots, and sea surface temperatures (SSTs) were warm enough for development—28°C (83°F), which was about 1°C (1.8°F) above average. Water vapor satellite imagery showed that 98L was in a moist environment on its southern flank, but dry air from the Saharan Air Layer (SAL) was along the northern side of the disturbance, interfering with development. These conditions are marginal for development.

Forecast for 98L
Steering currents favor a west-northwesterly to northwesterly motion at about 10 - 15 mph for 98L on Tuesday, slowing to 5 - 10 mph during the remainder of the week. This track will likely take the system too far to the north for it to be a long-range threat to the Lesser Antilles Islands, though we can’t rule out a threat to Bermuda yet. The 8 am EDT Tuesday run of the SHIPS model showed moderately favorable conditions for development through Friday, with wind shear in the moderate range, 10 - 15 knots, a moist atmosphere, and SSTs near 27 - 28°C (81 - 83°F.) Working against development of 98L will be the dry air of the SAL to its north, plus large scale sinking air over the tropical Atlantic imparted by the Madden-Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days. The active portion of the MJO is currently located in the Western Pacific, which leads to increased typhoon activity in the Northwest Pacific, but compensating sinking air and surface high pressure over the tropical Atlantic, with reduced chances of tropical cyclone development there.

The Tuesday morning operational runs of our three reliable models for predicting tropical cyclone genesis, the European, GFS and UKMET models, had two of the three--the European and GFS--showing development of 98L into a tropical storm late this week. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave 98L 2-day and 5-day development odds of 70% and 80%, respectively. 98L appears to be struggling with dry air today, and I think these development odds should be reduced to 50% and 70%, respectively. Should 98L become a tropical storm, the next name on the Atlantic list is Fiona.

More African waves coming
A series of tropical waves will emerge from the coast of Africa during the next week, and we will have to watch these for development as they track westwards to west-northwestwards across the tropical Atlantic. The models have been inconsistent in their handling of the track and potential development of these waves over the past few days; the Tuesday morning runs of the GFS and European models showed that the only new tropical wave that might develop in the coming week is one due to come off of Africa on Saturday.


Figure 4. In this Sunday, Aug. 14, 2016 photo, people wait for members of the Louisiana Army National Guard to rescue them from the side of the road near Walker, La., after heavy rains inundated the region. (AP Photo/Max Becherer) 

Louisiana disaster survivors with disabilities need your support after historic flooding
The Portlight.org disaster relief charity, founded and staffed by members of the wunderground community, is responding to this week's devastating floods in Louisiana. The disaster is particularly troublesome for a state that is still in recovery from major flooding just last March, and many resources are completely depleted because of the March flooding. That storm left more than 5,000 homes damaged or destroyed and cost $1.5 billion across a three-state area. This past week's floods have affected at least three times as many homes, and insurance broker Aon Benfield anticipates that the floods may end up being even more costly than the March disaster. There is an urgent need for durable and consumable medical supplies as well as housing. Portlight will be working with the American Red Cross, local stakeholder organizations, and federal partners to respond to this historic flooding event. Your support is needed to make this happen! Please consider making a donation to Portlight's disaster relief fund at the portlight.org website to further their reach and response in the state of Louisiana. Thank you for any support you can offer!

Why have so many people been unaware of this huge disaster? Marshall Shepard has a great essay in Forbes, 5 Reasons Some Were Unaware Of One Of The Biggest Weather Disasters Since Sandy.

Bob Henson will have a post this afternoon on the Great Arctic Cyclone (2016 version) that is currently churning through the Arctic.

Jeff Masters

Flood Hurricane

Billion-Dollar Flood Has Louisiana Reeling; 98L May Become a Tropical Depression

By: Bob Henson and Jeff Masters , 4:19 PM GMT on August 15, 2016

Floodwaters have finally crested across most of southern Louisiana after a harrowing weekend of record-high water that left at least six people dead, pushed at least 10,000 people into shelters, and prompted the rescues of more than 20,000 people. The federal government has declared the event a major disaster in four parishes: Tangipahoa, St. Helena, East Baton Rouge and Livingston. According to the insurance broker Aon Benfield, “The major flood and thunderstorm event that impacted parts of Louisiana, Texas, and Arkansas during March 2016 caused roughly USD1.5 billion in economic damage. It is currently anticipated that the August 2016 event will approach and possibly exceed this cost once all damage incurred to homes, businesses, public facilities, vehicles, infrastructure, and agriculture is taken into account.”


Figure 1. In this aerial photo over Hammond, La., flooded homes are seen off of LA-1064 after heavy rains inundated the region, Saturday, Aug. 13, 2016. (AP Photo/Max Becherer)

Multi-day rainfall of 10” to 20”, produced by a very slow-moving low pressure center, covered a large swath of south-central and southeast Louisiana (see Figure 2 below). Bands of heavy rain also extended northeastward as far as Ohio along a preexisting frontal boundary, as moisture was funneled northward from the Gulf Coast low. (Parts of the St. Louis area received 4” to 6” of rain on Sunday night.) The storm system carried near-record amounts of atmospheric moisture, drawn from the Gulf of Mexico and northwest Atlantic, where sea-surface temperatures are well above average. Climate change has already been shown to increase the amounts of rain falling in the most intense events across many parts of the world, and extreme rainfall events like this week's Louisiana storm are expected to grow increasingly common in the coming years.


Figure 2. Rainfall amounts analyzed by data from multiple sensors for the week extending from 12Z (8:00 am) Monday, August 8, 2016, to 12Z Monday, August 15. Most of the rain in Louisiana has fallen since Thursday, August 11. Image credit: NOAA/NWS Advanced Hydrologic Prediction Service.


Figure 3. Some parts of Louisiana recorded more than 20" of rain in 48 hours, which qualifies as a 1-in-1,000 year rainfall event. In other words, an event of this magnitude has a 0.1 percent chance of occurring at a particular location in any given year. Image credit: NWS Lower Mississippi River Forecast Center.

The numbers
Some local rainfall totals during this event were truly astounding, as catalogued in a NOAA/NWS precipitation summary issued on Monday morning. The top multiday amounts observed in each state through 10 am EDT Monday, August 15, include:

Watson, LA: 31.39”
Gloster, MS: 22.84”
Panama City Beach, FL: 14.43”
Ellsinore, MO: 12.10”
Gloster, AL: 9.94”
Tomball, TX: 8.82”
Makanda, IL: 8.05”
Vincennes, IN: 5.52”
Pocahontas, AR: 5.44”

Flooding over the weekend was most intense across the southeast corner of Louisiana, especially the region east of Baton Rouge and north of Lake Ponchartrain. Among the all-time record crests observed on Sunday:

Amite River at Denham Springs: 46.2’ (old record 41.5’ on April 8, 1983)
Comite River at Comite Joor Rd.: 34.22’ (old record 30.99’ on June 9, 2001)
Tickfaw River at Holden: 22.16’ (old record 21.04’ on April 7, 1983)
Tangipahoa River at Robert: 27.33’ (old record 27.10’ on March 14, 1921)

The previous record crests shown above are a telltale sign that most of the biggest flood events in southeastern Louisiana occur following large-scale winter and spring rainfall events rather than landfalling tropical cyclones. The latter typically (but not always) come and go more quickly, dumping heavy rain but not persisting long enough to cause widespread river flooding.

An odd-duck storm with tropical-cyclone-like impacts
The low pressure center that generated the past week of torrential rain along the Gulf Coast was a strange one indeed. Because surface winds were light and the surface low stayed generally onshore (see embedded video at bottom), the system was never declared a tropical cyclone by the National Hurricane Center. At the same time, for much of its life the storm was a symmetric warm-core low, the same type of structure associated with tropical cyclones. Regardless of its classification, the storm behaved much like other tropical depressions, tropical storms, and hurricanes that have taken their time moving through the Gulf Coast region. Two analogs that come to mind for the past week’s events are:

Figure 4. Analysis of total rainfall amounts (in inches) produced in Louisiana from the August 1940 hurricane. The hurricane’s path is indicated by the arrow. Image credit: David Roth, “Louisiana Hurricane History,” NOAA.


Hurricane 2 (unnamed), August 1940: Also called the 1940 Louisiana hurricane, this compact cyclone (hurricane-force winds extended just ten miles from its center) stayed just off the central Gulf Coast for most of its lifespan, intensifying as it nudged toward the coast of southwest Louisiana and coming ashore at Sabine Pass, TX, with top sustained winds of 100 mph. The hurricane’s slow motion both offshore and onshore resulted in mammoth, widespread rains, making it the wettest tropical cyclone in Louisiana history. The town of Crowley reported a storm total of 33.71”, including 19.76” in 24 hours. Almost 2 million acres of land reportedly went underwater by at least a foot. Six lives were lost, and damages totaled $9 million (1940 dollars), according to a NOAA report.

Tropical Storm Allison, June 2001: Allison moved onto the Texas coast as a tropical storm, then made a languid loop through the eastern part of the state over several days, depositing colossal amounts of rain. Houston was the hardest-hit metro area, as more than 70,000 homes and hundreds of businesses were flooded, including much of the Texas Medical Center, leaving the city virutally paralyzed for days. The port of Houston received a storm total of 36.99” of rain, with many other reports of 20” to 38” across Harris County. Allison caused 41 direct and 14 indirect fatalities, with damages totalling $9 billion (2001 dollars).


Figure 5. MODIS visible satellite image of 98L south of the Cabo Verde Islands taken on Monday morning, August 15, 2016. The brownish colors to the north of the disturbance are due to a large region of African dust. Image credit: NASA.

African tropical wave may develop late this week
A strong tropical wave located a few hundred miles south of the Cabo Verde Islands is headed west-northwest at 15 mph, and has the potential to develop into a tropical depression late this week. This disturbance was designated Invest 98L on Sunday evening by NHC. Satellite loops on Monday morning showed 98L had a modest area of heavy thunderstorms which had acquired a moderate amount of spin. Wind shear was light to moderate, 5 - 15 knots, and sea surface temperatures (SSTs) were warm enough for development--27.5°C (82°F), which was about 1°C (1.8°F) above average. Water vapor satellite imagery showed that 98L was mostly in a moist environment, with the dry air from the Saharan Air Layer (SAL) along the northern side of the disturbance. These conditions are generally favorable for development.

Forecast for 98L
Steering currents favor a west-northwesterly to northwesterly motion at about 15 mph for 98L this week, which will likely take the system too far to the north for it to be a long-range threat to the Lesser Antilles Islands. The 8 am EDT Monday run of the SHIPS model showed moderately favorable conditions for development through Thursday, with wind shear in the light to moderate range, 5 - 15 knots, a moist atmosphere, and SSTs near 26.5 - 27.5°C (80 - 82°F.) Working against development of 98L will be the Madden-Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days. The active portion of the MJO is currently located in the Western Pacific, which leads to increased typhoon activity in the Northwest Pacific, but compensating sinking air and surface high pressure over the tropical Atlantic, with reduced chances of tropical cyclone development there.

The Monday morning operational runs of our three reliable models for predicting tropical cyclone genesis, the European, GFS and UKMET models, had two of the three--the European and GFS--showing development of 98L. The 00Z Monday runs of the GFS and European model ensemble forecasts, done by taking the operational high-resolution version of the model and running it at lower resolution with slight perturbations to the initial conditions in order to generate a range of possible outcomes, had more than 50% of their ensemble members predict that 98L would become a tropical depression. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave 98L 2-day and 5-day development odds of 20% and 30%, respectively. Should 98L become a tropical storm, the next name on the Atlantic list is Fiona.

What's next?
We’re not particularly concerned about 98L's potential to cause trouble, since atmospheric steering currents are currently expected to take the storm far enough to the north that it will have difficulty making the long crossing to North America without recurvature. However, the next wave to come off the coast of Africa--due to emerge on Thursday--is likely to experience steering currents that will keep it farther to the south, on a course that could potentially bring it into the Caribbean by the middle of next week. We are now entering the peak part of hurricane season, and tropical waves like this one can become dangerous hurricanes that do not recurve harmlessly out to sea if atmospheric conditions come into alignment.


Figure 6. Danielle Blount kisses her 3-month-old baby Ember as she feeds her while they wait to be evacuated by members of the Louisiana Army National Guard near Walker, La., after heavy rains inundating the region, Sunday, Aug. 14, 2016. (AP Photo/Max Becherer)

Louisiana disaster survivors with disabilities need your support after historic flooding
The Portlight.org disaster relief charity, founded and staffed by members of the wunderground community, is responding to this week's devastating floods in Louisiana. The disaster is particularly troublesome for a state that is still in recovery from major flooding just last March, and many resources are completely depleted because of the March flooding. That storm left more than 5,000 homes damaged or destroyed and cost $1.5 billion across a three-state area. This week's flooding may end up being even more costly, according to insurance broker Aon Benfield. There is an urgent need for durable and consumable medical supplies as well as housing. Portlight will be working with the American Red Cross, local stakeholder organizations, and federal partners to respond to this historic flooding event. Your support is needed to make this happen! Please consider making a donation to Portlight's disaster relief fund at the portlight.org website to further their reach and response in the state of Louisiana. Thank you for any support you can offer!

Bob Henson and Jeff Masters





Hurricane Flood

Historic Flood Event in Louisiana From 20-30 Inches of Rain

By: Jeff Masters and Bob Henson , 5:31 PM GMT on August 14, 2016

A historic flooding event continues over southern Louisiana, where widespread rainfall amounts in excess of twenty inches since Friday have brought all ten river gauges on the Amite, Tickfaw, and Comite Rivers to record flood crests, flooded thousands of homes, and caused over 1,000 water rescues. The most extreme floods have occurred on the Amite River, which flows along the east side of the Baton Rouge metropolitan area. Flood waters stranded hundreds of cars on Interstate 12 just east of Baton Rouge for more than 24 hours, Saturday through Sunday. About 25 miles east-southeast of Baton Rouge, the flood crest on the Amite River appears likely to overtop the levee system built at Port Vincent after the destructive floods of April 1983 (at the tail end of the 1982-83 “super” El Niño). The flood control system was designed to handle a recurrence of the 14.6-foot crest observed in that record event. However, the Amite at Port Vincent had already reached 14.91 feet as of 9:15 am CDT Sunday, and it is projected to hit a crest of 16.5 feet early Monday, remaining above the previous record until Tuesday. Major flooding can be expected to the south of Port Vincent in southern parts of Ascension Parish, where voluntary evacuations are already in effect. “If you can get out, get out now,” said parish president Kenny Matassa on Sunday morning.


Figure 1. Major flooding in Prarieville, Louisiana on Friday, August 12, 2016. (@presleygroupmk/twitter.com) 


Figure 2. The Amite River at Denham Springs, just east of Baton Rouge, was at 46.2 feet on Sunday morning, August 14, 2016, nearly five feet above its previous record crest of 41.5 feet on March 8, 1983. Records there date back to at least 1921, making this an impressive feat. Today's crest is the only one of the river’s top 80 historic crests to occur during August, since the worst floods in the region are more commonly associated with winter and spring rainfall than with landfalling tropical cyclones. The Amite crested at 58.56 feet in Magnolia, Louisiana, topping the old record at that location by more than six feet set on April 23, 1977. Image credit: NOAA/NWS Advanced Hydrologic Prediction Service.


Figure 3. Aerial view of flooding in Hammond, Louisiana on August 13, 2016. AP Photo/Max Becherer.

Some of the 24-hour rains that fell on Friday in Louisiana (ending at 11AM CDT/16UTC) had a recurrence interval at over 500 years, according to Metstat. Topping the list of phenomenal rainfall amounts catalogued by the NWS Weather Prediction Center for the period 6:00 am CDT Tuesday, August 9, 2016, through 9:00 am CDT Sunday was 31.39” near Watson, Louisiana. Other impressive amounts:

27.47”  Brownfields, LA
22.84”  Gloster, MS
14.43”  Panama City Beach, FL
8.97”  Fairhope, AL
8.11”  Williamsville, MO
7.90”  Cobden, IL


Figure 4. In a dramatic rescue on Saturday near Baton Rouge, a woman and her dog were pulled to safety just as their car went under water. See the powerful video here. Image credit: WAFB.

A tropical depression-like storm with tropical depression-like impacts
The storm system responsible for the record rains formed a distinct surface low just inland along the Alabama coast on August 11, with a central pressure of 1013 mb. By August 13, the low had drifted over northwest Louisiana, and intensified to a central pressure of 1007 mb. Like a tropical depression, the low had a warm core, and the counter-clockwise flow of air around the storm brought huge amounts of tropical moisture from the near record-warm waters of the Gulf of Mexico and northwest Atlantic northwards over land. The amount of moisture in the atmosphere over the Gulf Coast region over the past week has been nothing short of phenomenal. Over multiple days, soundings of the atmosphere collected by weather balloon from locations such as New Orleans have measured record or near-record amounts of precipitable water (the amount of moisture in the atmosphere over a given point), often in the 2.5” to 2.75” range; sounding data extends back to 1948 in most cases. Sunday morning’s precipitable water of 2.61” in Lake Charles, LA, was among the top-ten values on record for that station.


Figure 5. Three-day precipitation totals ending at 10 EDT Sunday, August 14, 2016 showed several areas of 20+ inches had fallen over portions of Louisiana. Image credit: NOAA/NWS.


Figure 6. Projected rainfall from 12Z (8:00 am EDT) Sunday, August 14, 2016, through 12Z Wednesday. Image credit: NOAA/NWS/WPC.

The northern flank of this historic flood event
Separate from the heavy rain along the central Gulf Coast, a band of sometimes-torrential rain has pulsated over the last several days along a pre-existing frontal zone along and north of the Ohio River, stretching roughly from Arkansas to Ohio. This pattern bears some of the fingerprints of a PRE—a “predecessor rain event.” As we noted in a post last October, PREs tend to develop along preexisting frontal boundaries a few hundred miles north of landfalling tropical cyclones, as prevailing winds funnel huge amounts of moisture northward from the cyclone and concentrate it along the frontal zone. One challenge with such events is nailing down the location of the frontal zone, which can oscillate north or south as a multi-day PRE unfolds. Late Sunday into Monday, the Gulf Coast low itself will begin migrating northward along the frontal zone, further raising the possibility of flooding rains. Flash flood watches extended on Sunday morning along a belt from northern Arkansas to extreme northwest Pennsylvania.

On Friday, torrential rains put a damper on the Illinois State Fair at the state capital, Springfield, where an all-time calendar-day rainfall record was set with 5.59” (beating 5.44” from September 8, 1926). Of that total, 3.44” fell in just one hour. Although some events had to be cancelled or rescheduled, no injuries to people or livestock were reported at the fair. Parts of the southwestern Chicago suburbs received 4-5” of rain on Friday.


Figure 7. MODIS image of a strong tropical wave off the coast of Africa south of Cabo Verde as seen on Sunday morning, August 14, 2014. A large region of African dust is visible from the coast of Africa extending over Cabo Verde. Image credit: NASA.

African tropical wave may develop late this week
NHC was not highlighting any Atlantic tropical weather threat areas in their 5-day Tropical Weather Outlook on Sunday morning. However, two of our reliable models for predicting tropical cyclone genesis--the European and GFS models--showed that a strong tropical wave that moved off the coast of Africa on Saturday night does have the potential to develop into a tropical depression late this week as it moves west to west-northwest at 15 mph into the central Atlantic. In their 00Z Sunday runs, about 30 - 50% of the members of the European and GFS model ensemble forecasts predicted development of this system into a tropical depression late this week.

Working against development of this wave will be the fact that the Madden-Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days, is currently located in the Western Pacific. When the MJO is located there, we can expect to see increased typhoon activity in the Northwest Pacific, but compensating sinking air and surface high pressure over the tropical Atlantic, with reduced chances of tropical cyclone development there. Dry air from the Saharan Air Layer (SAL) will also likely interfere with Atlantic development this week, though the SAL is currently less prominent over the tropical Atlantic than it was early in August.



Video 1. The Amite River Basin between Watson and Central, as videotaped by the U.S. Coast Guard on Saturday. Image credit: Louisiana GOHSEP, via NWS New Orleans.

Jeff Masters and Bob Henson

Flood

Record Flooding in Southeast Louisiana May Get Worse

By: Bob Henson , 8:20 PM GMT on August 12, 2016

A devastating flood event was unfolding over southeast Louisiana on Friday, and conditions may get worse yet, as an extremely slow-moving center of low pressure is dumping colossal amounts of rain on the region. This sprawling, “stacked” low is carrying more water vapor than many tropical cyclones, and its slow motion is leading to persistent rains that could add up to all-time record totals in some places.

Multi-sensor analyses indicate that several areas in southeast Louisiana and southermost Mississippi racked up more than 6” of rain from 7:00 am CDT Thursday, August 11, to 7:00 am Friday (see Figure 1). More than 10” of rain was analyzed just northeast of Baton Rouge, the hardest-hit area thus far. In the 24 hours from 2:00 pm CDT Thursday to 2:00 pm Friday, Baton Rouge Metropolitan Airport recorded a preliminary total of 8.49” of rain. Since records began in 1892, the city’s largest calendar day total is 11.99” (set on April 14, 1967), and the largest two-day calendar total is 14.03” (June 6-7, 2001). Given the very slow motion of the stacked low, these all-time records are conceivably within reach. A cooperative observer in Livingston, LA, reported 17.09” of rain from midnight to 3:00 pm CDT Friday. The state’s official 24-hour record is 22 inches, reported near Hackberry on August 28-29, 1962.


Figure 1. Multi-sensor rainfall analysis for the period from 7:00 am CDT Thursday, August 11, to 7:00 am Friday shows a gyre-like pattern of torrential rains spinning around a low in southern Mississippi. Image credit: NOAA/NWS Advanced Hydrologic Prediction Service.


As the low edges westward over the next 24-48 hours, the zone of heaviest rain potential will shift toward west Louisiana and east Texas, but southeast Louisiana will remain under the gun for more downpours at least into early Saturday. The short-range HRRR model produces another 2”-6” of widespread rain over southeast Louisiana through Saturday morning, with localized totals of 8-12” not out of the question.


Figure 2. Enhanced infrared satellite image for the central Gulf Coast reveals the vast scope of the area of low pressure generating torrential rains in southeast Louisiana. Image credit: NOAA/NESDIS.

Severe flood threat for Baton Rouge area
Both flash flooding and river flooding threats are looming large for southeast Louisiana, where flash flood warnings were in place on Friday afternoon. Major flooding has already occurred throughout the day Friday, and a flash flood emergency (the most urgent type of flash flood warning) was in effect Friday afternoon for parts of Feliciana, West Feliciana, St. Helena, and East Baton Rouge parishes, which extend roughly from Baton Rouge northward. Water rescues and evacuations were under way in this region, according to the NWS. Even if the rains ease during the weekend, the area faces a major flood threat. The Tickfaw River at Montpelier, LA, hit a record crest of 22.75 feet at 1:30 pm CDT Friday, with several more feet expected this weekend. A number of other rivers across southeast Louisiana are projected to reach all-time crests, including the Amite River, where record levels of flooding can be expected to inundate many homes and roadways on the eastern side of the Baton Rouge metro area for an extended period.


Figure 3. Forecasts issued on Friday morning, August 12, 2016, were calling for an all-time record flood crest of 42.5 feet late Sunday on the Amite River at Denham Springs, just east of Baton Rouge, LA. The forecast keeps waters above the previous record of 41.5 feet (April 8, 1983) for a full 24 hours. These projections could be boosted further in light of the heavy rains persisting in the area on Friday. The last major crest in this region was 36.09 feet on March 13, 2016. Image credit: NOAA/NWS Advanced Hydrologic Prediction Service.

Tropical cyclone or not? Does it matter?
Although this system does not qualify as a tropical cyclone--its center has remained just inland--the point is moot in terms of impact, as the torrential rains and flooding from this low could end up ranking among some of the more damaging tropical depressions and tropical storms on record. The low’s rainmaking power is a combination of its extremely slow motion and the astoundingly moist air mass feeding into it. The upper air sounding launched from Slidell, LA, at 12Z Friday (7:00 am CDT) showed that the atmosphere was carrying 2.85” of precipitable water (the amount of water in a column of air over a given point). This is the second-highest amount of water measured in any sounding since records began in the New Orleans area in 1948, and just 0.03” below the record of 2.88”. In Jackson, MS, only two other dates have seen more precipitable water than the 2.74” measured on Friday morning, with the record being just 0.02” higher (2.76”). These values may seem puzzlingly low compared to the amounts of rain occurring. This is because showers and thunderstorms can concentrate the amount of moisture present in the atmosphere throughout a region, so they can produce much higher local totals than the precipitable water values would suggest.

The New Orleans Times-Picayune is providing live updates on the situation in southeast Louisiana. Governor John Bel Edwards has declared a state of emergency for the entire state through at least Saturday.

Bob Henson


Figure 4. An inundated boat launch ramp in New Iberia, LA, on Friday, August 12, 2016. Image credit: wunderphotographer kaiju76.




Flood

Warm Nights Tip the Scales as Summer Grinds On

By: Bob Henson , 5:11 PM GMT on August 11, 2016

Data for July 2016 make it clear that this summer is worming its way into the nation’s warmest batch on record, thanks in large part to consistently sultry nights in many areas. Meteorological summer so far--June plus July--has been the fifth warmest for the contiguous U.S. in 122 years of recordkeeping, according to the July climate report released on Thursday by NOAA’s National Centers for Environmental Information (NCEI). Ahead of 2016 at this point are the scorching Dust Bowl summers of 1934 (#4) and 1936 (#1) along with the recent 2006 (#3) and 2012 (#2). The toasty summer so far is the result of the nation’s warmest June on record followed by its 14th warmest July. Last month’s warmth was focused across the nation’s southern and eastern halves (see Figure 1), with New Mexico and Florida each recording their hottest July on record. Fourteen other states made it into their top ten warmest for July.


Figure 1. Statewide rankings for average temperature during July 2016, as compared to each July since 1895. Darker shades of orange indicate higher rankings for warmth, with 1 denoting the coldest month on record and 122 the warmest. Image credit: NOAA/NCEI.

Warmest nights on record for June-July
It’s the muggy nights that are imprinting themselves on the psyche of millions of Americans this summer. The average daily minimum for the contiguous U.S. was the warmest on record for June and July combined: 60.57°F, beating out 2015, 2010, 2002, and 2006. (See Figure 2.) Warmer nights are a hallmark of a climate being heated by added greenhouse gases, and it’s long been recognized that nights should generally warm more than days, and winters more than summers, as climate change proceeds. Of the eight June-July periods with the warmest average daily temperatures (including both highs and lows), four are from Dust Bowl years. However, the eight years with the warmest average daily minimum temperatures are all from the 21st century. Urban heat islands are no doubt helping to increase overnight lows in large metropolitan areas; however, the nationwide extent of the trend toward warm nights goes well beyond this effect.

For the year to date through July, the U.S. has seen 15,061 daily record highs and just 2709 record daily lows, according to data compiled by meteorologist Guy Walton. In an email, Walton told me that “2016 is in a race with 2012 for that dubious distinction of having the highest ratio of record daily highs to record daily lows since 1920.”


Figure 2. The average daily minimum temperature for the contiguous U.S. is at record-warm values for the summer of 2016 thus far (June plus July). Image credit: NOAA/NCEI.

Wilting in Washington, D.C.
Several U.S. locations have been setting records for the most consecutive nights above unpleasant thresholds of warmth. Reagan Washington National Airport made it five weeks--July 5 through August 8--without once getting below 70°F. In records dating back to 1872, that stretch of 35 days above 70°F beats the previous record of 32 days notched from July 15 to August 15, 1980. This summer, the city has set only one record high--100°F on July 25--but there were three record-warm lows in a row: 81°F on July 25, 80°F on July 26, and 81°F on July 27. The next several days will test the patience of D.C. residents, as lows are expected to hang in the upper 70s and afternoons warm into the mid to upper 90s.

The warm nights extend well beyond the contiguous 48 states. As of Wednesday, Anchorage, Alaska, had gone 59 days without dipping below 50°F, topping the record string of 53 days set from June 23 to August 14, 2013. The city also saw a run of 18 consecutive days above 55°F this summer (July 13-30), which doubles the old record of 9 days (July 16-24, 1984). In Fairbanks, the stretch of 41 nights above 50°F this summer (June 24 to August 3) smashed the old record of 32 nights (June 25 to July 26, 1975). These are just a few examples of the exceptional mildness bathing Alaska for months now. Eric Holthaus (Slate) points out an ominous milestone: Alaska’s average for the year to date of 33.9°F is the first time in records going back to 1925 that the January-to-July period has topped 32°F. The freezing mark carries extra physical and psychological significance in a state like Alaska, where entire ways of life are based on the reliable presence of ice for most of the year.


Figure 3. Statewide rankings for average precipitation during July 2016, as compared to each July since 1895. Darker shades of green indicate higher rankings for moisture, with 1 denoting the driest month on record and 122 the wettest. Image credit: NOAA/NCEI.

Wet to the north, dry to the south
After an unusually dry June across the contiguous U.S., July produced generous rains across much of the nation's northern tier, while leaving most of the Northeast and the nation's southern half on the dry side. It was the second-driest July on record for Georgia and the third-driest for Florida, with Wyoming and New Mexico also coming in among their top-ten driest. July was among the ten wettest on record in five states: Illinois, Kentucky, Minnesota, Missouri, and North Dakota. The weekly U.S. Drought Monitor issued on Thursday morning shows that more than 40% of California remains in extreme or exceptional drought, a situation that is unlikely to change before the 2016-17 wet season arrives (if then). Much of the state remains extremely vulnerable to fast-growing wildfire, especially over the next several months. Wildland fire potential is also expected to increase across the South this autumn, according to the latest outlooks from the National Interagency Coordination Center. (Torrential rains scattered over parts of the upper Gulf Coast this week will help tamp down the immediate drought and fire risk in those areas.)

The wet-north/dry-south tendency evident in July may be a foreshadowing of La Niña influence to come (see Figure 4 below). The strong El Niño of 2015-16 is giving way to borderline La Niña conditions, with the weekly index of sea-surface temperatures in the Niño3.4 region hanging near the -0.5°C threshold of La Niña over the past month. NOAA maintained a La Niña Watch in its monthly ENSO discussion issued Thursday, although the event is projected to be relatively weak if it does take shape. NOAA is giving a 55-60% chance of La Niña being present this fall and winter and a negligible chance of El Niño (below 10%). Because La Niña typically leads to a more consolidated jet stream, it often leaves southern parts of the 48 states on the dry side of upper-level flow as storm systems whip across the nation’s heartland, keeping northern areas more moist.


Figure 4. Typical effects produced by La Niña (the cold phase of the El Niño–Southern Oscillation) during northern winter, December through February. The most common effects are largely but not totally the opposite of what typically occurs during El Niño. Image credit: NOAA/NWS/CPC.

NOAA ups its Atlantic hurricane outlook for 2016
In an update released Thursday, NOAA increased the totals that it had projected in May for the 2016 Atlantic hurricane season. The agency is now calling for a 70% chance of 12-17 named storms, 5-8 hurricanes, and 2-4 major hurricanes. These numbers compare to the May outlook of 10-16 named storms, 4-8 hurricanes, and 1-4 major hurricanes.

“We’ve raised the numbers because some conditions now in place are indicative of a more active hurricane season, such as El Niño ending, weaker vertical wind shear and weaker trade winds over the central tropical Atlantic, and a stronger west African monsoon,” said Dr. Gerry Bell, lead seasonal hurricane forecaster at NOAA’s Climate Prediction Center. “However, less conducive ocean temperature patterns in both the Atlantic and eastern subtropical North Pacific, combined with stronger wind shear and sinking motion in the atmosphere over the Caribbean Sea, are expected to prevent the season from becoming extremely active.”

The NOAA update lines up closely with the most recent forecast from Colorado State University, issued on August 5. Several other forecast groups have gone bigger for this season, as you can see from the forecast compilation graphics now being produced by CSU.

For the time being, the global tropics remain quiet. No areas of interest were highlighted on Thursday morning by the National Hurricane Center for the Atlantic. In the East Pacific, NHC is giving Invest 94E, located a few hundred miles southwest of Baja California, a 20% chance of development between two and five days from now as it moves west over open water. The only active tropical cyclone on Earth was Tropical Storm Conson, spinning harmlessly in the Northwest Pacific about 1000 miles west-northwest of Wake Island. Conson should remain a tropical storm as it gradually accelerates north-northwestward toward Japan and Korea, perhaps crossing the Kuril Islands. Longer-range models suggest that the Northwest Pacific could spring to life in the next few days, with several simultaneous tropical cyclones possible. Models also suggest the potential for a tropical cyclone in the North Bay of Bengal at some point next week. Meanwhile, as Jeff Masters pointed out yesterday, the Madden-Julian Oscillation favors an active couple of weeks ahead over the Western Pacific and relatively quiet conditions over the East Pacific and Atlantic.

We’ll have a new post by Friday afternoon.

Bob Henson

Extreme Weather Climate Summaries Hurricane

Mexico's 2nd Highest Death Toll From an Atlantic Storm Since 1988: 45 Killed in Earl

By: Jeff Masters , 9:20 PM GMT on August 10, 2016

Hurricane Earl, reinvigorated to a strong tropical storm with 60 mph winds as it passed over the southernmost portion of Mexico's Bay of Campeche on Friday, August 5, dropped torrential rains in excess of twelve inches over the coastal mountains of Mexico east of Mexico City over the weekend, unleashing flash floods and mudslides that are being blamed for 45 deaths in Mexico. This is an unusually high death toll for Mexico, which prides itself on its excellent civil defense efforts that usually keep hurricane death tolls quite low. According to EM-DAT, the international disaster database, the last Atlantic hurricane to exact a higher death toll than Earl in Mexico was Hurricane Gilbert of 1988, which hit Cozumel and the Yucatan Peninsula as a Category 5 storm, killing 240 people in the nation. The National Hurricane Center, though, gives HUrricane Stan of 2005 a death toll of 80 in Mexico. There have been three Pacific hurricanes since 1988 to have higher death tolls in Mexico--Hurricane Manuel of 2013 (169 killed), Hurricane Pauline of 1997 (220 killed), and Hurricane Ismael of 1995 (105 killed.) Earl made landfall near Belize City, Belize at 2 am EDT Thursday, August 4, 2016 as a Category 1 storm with 80 mph winds, causing over $100 million in damage to agriculture in Belize. Infrastructure in the capital, Belize City, experienced additional heavy damage from Earl's storm surge and winds. Earl also killed thirteen people in the Dominican Republic when the storm was still classified as a tropical wave. Given the high death toll in Mexico and extensive damage in Belize, it is quite possible the name Earl will be retired from the active list of hurricane names next year.


Figure 1. MODIS image of Tropical Storm Earl approaching a second landfall along the Bay of Campeche coast of Mexico at 12:50 pm EDT August 5, 2016. After losing strength and becoming a tropical depression during a long traverse of Mexico's Yucatan Peninsula, Earl became re-invigorated to a tropical storm with 60 mph winds in this image when the storm's center moved over the extreme southern portion of the Gulf of Mexico. Image credit: NASA.


Figure 2. Residents Enriqueta Diaz (R) and Juana Lechuga work with shovels amid the damage caused by a landslide ensuing the passage of Tropical Storm Earl in the community of Xaltepec, Puebla state, eastern Mexico on August 8, 2016. A total of 29 people died in the communities of Xaltepec, Tlaola and Huauchinango in the Mexican state of Puebla after their homes were buried by landslides following heavy rains from Earl, which reached Mexican territory on Thursday as a tropical storm and Saturday was only a remnant low on Saturday. Image credit: ALFREDO ESTRELLA/AFP/Getty Images.


Figure 3. Total rainfall amounts for the period August 6 - 8, 2016 over Mexico. The town of Huauchinango, where 25 people died in mudslides (marked by a black diamond), received 315.2 mm (12.41") of rain. Image credit: Conagua, the Mexican weather service.


Quiet in the Atlantic
NHC is not highlighting any Atlantic tropical weather threat areas in their Tropical Weather Outlook, and none of the reliable models for forecasting tropical cyclone genesis are showing anything developing for the next five days. The Madden-Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days, is currently located in the Western Pacific, where we can expect to see increased typhoon activity over the next week or two. Long-range model runs show that the MJO and an associated area of rising air and surface low pressure in the Western Pacific will continue for the next two weeks, and the models predict that the Northwest Pacific's Tropical Storm Conson will be joined by at least one other named storm next week in the waters a few hundred miles northeast of the Philippines. Compensating sinking air and surface high pressure are expected over the tropical Atlantic, which should result in a relatively quiet period for tropical cyclones in the Atlantic.


Figure 4. Departure from average (at an altitude of 200 mb) of the velocity potential, a good measure of large-scale rising or sinking motions in the atmosphere. Where large scale rising motion occurs (green colors), surface low pressure and storminess typically result. Where large-scale sinking motion occurs (orange colors), surface high pressure and fair weather are favored. This 10-day forecast made at 12Z (8 am EDT) August 10, 2016 valid on August 20, 2016, shows that rising air and storminess associated with the active phase of the Madden-Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days, is expected over the Western Pacific. This will favor increased typhoon activity there. A compensating area of sinking air and surface high pressure is expected over the Atlantic and Eastern Pacific, which should suppress hurricane activity there. The forecast was made by the GEFS model, also known as the GFS ensemble model, a collection of 21 runs of the GFS model made using slightly different initial conditions in order to generate an ensemble of possible outcomes. Image credit: Tropical Tidbits.

Our next post will be Friday afternoon at the latest.

Jeff Masters

Hurricane

Researching the Wet, Wild World of Atmospheric Rivers

By: Bob Henson , 2:51 PM GMT on August 09, 2016

When a molecule of water vapor heads toward the poles, it may well be hitching a ride on an atmospheric river (AR). A growing amount of research is zeroing in on these narrow but powerful channels of airborne moisture, which are far more widespread and influential than scientists once thought. Garden-variety ARs tend to have a beneficial influence overall, but the biggest and baddest ARs can produce colossal rainfall and snowfall and major destruction. And Godzilla notwithstanding, major AR events are actually no more likely to strike California during El Niño than during La Niña.

I’m getting up to speed on ARs this week at the 2016 International Atmospheric Rivers Conference. Close to 100 forecasters and researchers are gathered here at a global epicenter of AR science: the Scripps Institution of Oceanography at the University of California, San Diego. The conference kicked off with an opening talk by Martin (Marty) Ralph, a long-time NOAA scientist who joined Scripps in 2013. Ralph is one of the leaders of the burgeoning science of ARs and the founder of the Scripps-based Center for Weather Weather and Water Extremes (CW3E). A growing amount of research at CW3E and elsewhere is aimed at producing useful AR guidance for weather forecasters and water resource managers. Many of the scientists at this meeting are also working on a book on ARs to be published in 2017.


Figure 1. This graphic shows an atmospheric river interacting with U.S. West Coast mountains and a midlatitude cyclone over the northeast Pacific on 5 February 2015. Labeled are the approximate locations of tropical moisture entering the atmospheric river and a warm conveyor belt (WCB) transporting warm, moist air just ahead of a cold front. Image credit: Adapted from NOAA/ESRL/PSD; Source: EOS Meeting Report.


What exactly is an AR?
The very definition of an atmospheric river is something of a work in progress. Although heavy rain and snow is the main concern with ARs, the events are best classified by the amount of water vapor being carrried through the air. In general, an AR is a narrow corridor transporting large amounts of water vapor. ARs typically have a fairly strong meridional component (movement along a north-south axis), although they can be oriented zonally (east-west) as well. They are closely associated with the preexisting concept of low-level jet streams, and of course they’re not literally channeled in banks like an earthbound river.

The AR label was coined by Yong Zhu and Reginald Newell in a 1998 paper published in Monthly Weather Review. Zhu and Newell estimated that about 95% of the water vapor moving across midlatitudes toward the poles was being carried in plumes that spanned just 10% of Earth’s circumference at those latitudes. (At least some of this moisture may be generated directly at midlatitudes rather than being transported there, an idea put forth by Helen Dacre [University of Reading] in a 2015 paper in the Bulletin of the American Meteorological Society.)

ARs are traditionally defined as being at least 2000 kilometers long and no wider than 1000 km, although a growing practice is simply to required that the length be at least twice the width. In midlatitudes, ARs typically have a peak value of vertically integrated water vapor transport (IVT) of at least 250 kilograms per meter per second. IVT denotes the amount of water vapor being carried across a line drawn at Earth’s surface, going up through the full depth of the atmosphere.

As with tropical cyclones, satellites are critical for detecting ARs over the ocean. At first, satellites could only provide limited detail on how much atmospheric moisture was present at each level of the atmosphere. The advent of the Special Sensor Microwave Imager (SSM/I) in 1987 allowed for the detection of integrated water vapor--the total amount above a point at Earth’s surface. This was a huge advance, opening the door to eventual real-time mapping of water vapor across the oceans as well as tracking of atmospheric rivers as they headed toward shore. Other tools such as GPS water vapor sensors are employed over land, where the SSM/I product doesn’t work as well. We now have automated routines that can infer the presence of ARs over land and sea in both historical and current datasets, and project their development days into the future. One product at CW3E projects AR development based on GFS model forecasts going out up to 180 hours.



Figure 2. Forecast of integrated water vapor transport (IVT, in kilograms per meter per second) derived from the GFS model run at 06Z (2:00 am EDT) Tuesday, August 9, 2016, and valid at 06Z (2:00 am EDT) Thursday, August 11. High values of IVT correspond to large amounts of water vapor being transported. Flash flooding is possible across parts of Minnesota late Wednesday (see below) as an AR-type moisture channel interacts with a strong upper-level trough and an associated cold front. Image credit: Center for Western Weather and Water Extremes, Scripps/UCSD.


Figure 3. Two runners watch as waves crash against the rocks near San Francisco’s Golden Gate Bridge on December 28, 2005. A series of wet winter storms associated with an atmospheric river event struck California in late December 2005 and early January 2006, bringing more than 20” of precipitation to the Sierra Nevada and widespread 24-hour rainfall totals of more than 5” on New Year’s Eve. Image credit: Justin Sullivan/Getty Images.

More than California
The iconic Pineapple Express--the southwesterly current that gained fame in the 1990s as a prolific rain and snow producer for the U.S. West Coast--is just one type of atmospheric river. As much as half of precipitation along the U.S. West Coast has been found to be AR-related. However, ARs can be found across the globe, especially if you broaden the definition so that the required IVT amounts are a certain percentage above the local norm rather than a fixed threshold worldwide. Based on this type of location-adjusted classification, ARs extend from the tropics all the way to Greenland and Antarctica, where they can account for a surprising percentage of precipitation. In a 2014 paper for Geophysical Research Letters, Irina Gorodestskaya (University of Aveiro, Portugal) reported that major spikes in snowfall across parts of East Antarctica in 2009 and 2011 were related to clusters of several AR events in each year.


Figure 4. The percentage of total annual precipitation that falls during atmospheric river events, based on an algorithm that detects AR events in climatological analyses. Across much of California as well as Tennessee and Kentucky, the percentage tops 30%. Image credit: Courtesy Bin Guan, UCLA and NASA/JPL.


Figure 5. California’s Lake Mendocino in December 2006. Image credit: Kglavin/Wikimedia Commons.

How AR forecasts might help save water ahead of drought
One of the perversities of California’s water storage system is the requirement that some large reservoirs release water in midwinter to help protect against the potential for late-season flooding. The system works beautifully as a flood prevention tool, but it’s based mainly on decades-old, by-the-book rules that take into account only the water that’s fallen, the amount being stored, and the time of year, not the long-range weather forecast or the seasonal climate outlook. California’s Lake Mendocino, built in 1958 on the Russian River north of San Francisco, has never gone over its spillway, thanks to careful management by the U.S. Army Corps of Engineers (USACE). As an AR-type series of storms dumped more than 20 inches of rain on the region in December 2013, some 25,000 acre-feet of water--more than half a typical winter’s storage--was released from the lake, even though it was far from full at that point. The rest of the winter produced less than 10 inches of rain, and the region’s drought intensified over the next year. A more flexible storage system based on weather and climate guidance might have allowed more of that much-needed water to be kept in the lake.

Lake Mendocino is now the focus of a proof-of-concept study called FIRO (Forecast-Informed Reservoir Operations) that could provide a major boost to California’s drought readiness. The goal is to apply weather and climate guidance and determine if the USACE could safely make adjustments to its mandated water-release levels, thus allowing it to keep more of the water from big AR events. “Lake Mendocino is our guinea pig,” said USACE engineer Cary Talbot. He pointed out that Congress mandates the Corps to reduce flood risk but not to protect water supply. This means that any water-saving measures must be rigorously evaluated and shown not to affect flood risk. FIRO’s goal is a big one, with a hefty forecasting challenge that’s brought in a raft of collaborating institutions. Nobody here in drought-tormented California needs to be told that the stakes are high. As Jeanine Jones (California Department of Water Resources) put it, “These days we expect our water to work a lot harder….What was good enough in the past [isn’t] good enough now.”

I’ll have more news from the AR conference in an upcoming post. For more background on ARs, see:

--this handy 2015 overview in Forbes by Marshall Shepherd (University of Georgia)
--a more detailed FAQ from NOAA’s Earth System Research Laboratory
--an even more comprehensive mini-review by Luis Gimeno (University of Vigo, Spain) and colleagues, published in 2014 in the open-access journal Frontiers in Earth Science


Figure 6. Total rainfall projected by NOAA’s Weather Prediction Center for the 7-day period from 8:00 am EDT Tuesday, August 9, 2016, to 8:00 am EDT Tuesday, August 16. Some localized amounts could exceed these projections by a considerable margin. Image credit: NOAA/NWS/WPC.

Big rains still on tap for Southwest, Gulf Coast
As we reported on Monday, this week holds the potential for major deluges along the central Gulf Coast and the southern Arizona desert. The NWS cautioned on Monday afternoon that significant river flooding would be possible later this week along parts of the Gulf Coast. Parts of the region have already notched 6” to 10” over the last several days, and additional rainfall amounts in some spots could exceed 10” by the weekend, especially near the coastline from New Orleans, LA, to Apalachicola, FL. A flood watch was in effect from Tuesday morning through Wednesday evening for much of the northern and western Gulf Coast of Florida.

Flash flood watches are in place over most of Arizona and parts of western New Mexico, with heavy rains already occurring south of Tucson on Tuesday morning. An unusually strong upper-level trough for early August will be interacting with a slug of moisture circulating into the area along the east side of Tropical Depression Javier, which is weakening as it grinds northwestward along the west coast of Baja California. Showers and thunderstorms should dump at least an inch of rain over widespread areas, with much higher amounts possible locally as a result of small-scale features impossible to predict in advance. Nighttime storms are possible with this unusually potent set-up, which would exacerbate the risk for any motorists attempting to drive through high water.

In addition, as shown in Figure 2 above, large amounts of atmospheric moisture will be converging on the Upper Midwest by late Wednesday ahead of the strong-for-August upper-level trough. The NWS has tagged parts of north central Minnesota with a moderate risk of flash flooding by Wednesday afternoon and evening.

Apart from Javier, there are no tropical cyclones or areas of immediate interest in the Atlantic or Eastern Pacific. In the Northwest Pacific, Tropical Storm Conson is gradually strengthening over open waters, while Tropical Storm Omais continues on the decline east of Japan.

We’ll have our next post by Thursday at the latest.

Bob Henson


Atmospheric Phenomena Flood

Wet Week Ahead for Gulf Coast, Arizona Deserts

By: Bob Henson , 8:02 PM GMT on August 08, 2016

With moist air predominant over much of the United States, several distinct areas of heavy rain will take aim on roadways, drainage systems, and people’s nerves over the coming week. The deluge has already begun across the central Gulf Coast, especially along the Florida coast from Tallahassee to Tampa, where totals of 3-6” were widespread from Sunday into Monday. These showers and thunderstorms are being fed by very high amounts of atmospheric moisture (more than 2” of precipitable water, the amount of water vapor in a column of air) across the southeastern U.S. The main trigger for development is a broad, weak mid-level low centered near the Florida Big Bend, together with an upper-level low off the east coast of Florida and a surface low over southern Alabama. This complex will trudge westward during the week, perhaps reaching Louisiana by around Friday.

The tilted structure of the low and its location near the coast will work against tropical development, but the slow movement will lead to prodigious rainfall totals. A broad swath of rains may total 7” to 15” by the end of the week along the central Gulf Coast (see Figure 1). Within this area, we’re bound to see localized weather features producing higher amounts--perhaps quite a bit higher--though the locations will be near-impossible to predict before the thunderstorms actually develop. We do know that the central Gulf Coast can get amazing amounts of rain in a short time when conditions are favorable. Historic flash floods were produced on April 29-30, 2014, as 10-15” of rain fell in less than 24 hours at both Mobile, AL, and Pensacola, FL. Pensacola racked up 20.47” in just 48 hours.


Figure 1. Total rainfall projected by NOAA’s Weather Prediction Center for the 7-day period from 8:00 am EDT Monday, August 8, 2016, to 8:00 am EDT Monday, August 15. Some localized amounts could exceed these projections by a considerable margin. Image credit: NOAA/NWS/WPC.

Big surge in the Southwest monsoon
Showers and thunderstorms typically increase across the U.S. Southwest during late July and August with the peak of the North American Monsoon, as streams of moisture head toward the region. Typically the low-level moisture arrives via the Gulf of California and the tropical East Pacific, with higher-level moisture imported from the Gulf of Mexico. As is often the case, this year’s monsoon has been sporadic and spotty, with pulses of heavy rain around the start of July followed by an extended monsoon “break” of more than two weeks. The action has picked up over the last few days, though. On Friday, August 5, the Phoenix area was socked by heavy rains and flash flooding. Close to 3” fell in Scottsdale in just three hours.


Figure 2. A burst of heavy rain falls across the Phoenix metro area on Friday, August 5, 2016. Photo credit: Bruce Haffner, courtesy NWS/Phoenix.

This week will bring additional potential for heavy Southwest rains, thanks in large part to a slug of moisture flowing up the Gulf of California around the east side of Tropical Storm Javier (see below). Precipitable water values may approach 1.75” - 2.00” in the Phoenix and Tucson areas, which would be among the highest levels on record for August. An upper-level trough extending unusually far south for August will also boost the potential for strong to severe thunderstorms packing high winds, especially on Wednesday. The juxtaposition of upper-level forcing with such rich moisture is quite unusual, suggesting that localized rainfall amounts--not everywhere, but in scattered locations--could be truly impressive. Scatered heavy rains may extend into parts of Utah, Colorado, and New Mexico by Thursday. Residents should take extra caution in areas of heavy rain and high water. The phrase “turn around, don’t drown” is especially apt in the U.S. Southwest, where dry creekbeds can quickly turn into raging torrents.

More soakings ahead for Upper Midwest
It’s been a soggy summer over much of the Midwest. Initial data from the forthcoming U.S. monthly climate summary (we’ll cover the full report on Thursday) show that last month ranked among the ten wettest Julys on record for Kentucky, Illinois, Missouri, Minnesota, and North Dakota. NOAA/WPC is projecting that showers and storms will drop widespread 2” - 4” rainfall amounts this week from eastern North Dakota to northern Minnesota and Wisconsin, as the potent-for-August upper-level trough mentioned above drags its way across the region.


Figure 3. Enhanced infrared satellite image of Tropical Storm Javier at 1930Z (2:30 pm EDT) Monday, August 8, 2016. Image credit: NOAA/NESDIS.

From Earl to Javier
There are no areas of potential development being highlighted by NOAA’s National Hurricane Center today, but traces of last week’s Hurricane Earl have made it into the East Pacific. After Earl dissipated over the mountains of southern Mexico on Saturday, its westward-moving remnants fed into an area of disturbed weather south of Mexico. The result was Tropical Storm Javier, which sprang to life on Sunday. As of the 2:00 pm EDT Monday update from NHC, Javier was located about 55 miles southeast of Cabo San Lucas, Mexico, with top sustained winds of 50 mph. Javier appeared to be holding steady in strength based on data from an Air Force Hurricane Hunter flight on Monday morning. Hurricane warnings have been dropped for parts of the Baja California coast, although tropical storm warnings remain in effect for both side of the peninsula south of about 25°N, with a tropical storm watch further north along the Pacific coast to Puerto San Andresito.

Javier is loosely organized, with its core of showers and thunderstorms pulsing and fading, and its northwestward path lies close enough to Baja California for some interference with its development. Javier does have some potential to strengthen over the next couple of days, as wind shear will be light (less than 10 knots) and the storm will be traveling over unusually warm waters (sea-surface temperatures of 26-28°C, about 1-2°C above average). From Wednesday onward, Javier will move over cooler waters (24-26°C). Together with land interaction, this should bring Javier below tropical storm strength. Models are in some disagreement on Javier’s path by later this week, but NHC projects Javier to continue northwestward, dissipating by Thursday. Javier’s remnants may flow into the Southwest U.S. by the weekend; before then, as noted above, the storm will be channeling moisture into the region.

Elsewhere in the tropics
Tropical Storm Ivette is on its last legs in the Central Pacific, where it is projected to dissipate amid high wind shear by Tuesday. In the Northwest Pacific, Tropical Depression Eight will gradually strengthen, perhaps becoming a typhoon late this week, while Tropical Depression Omais spins down east of Japan.

Bob Henson

Hurricane Flood

CSU’s Latest Hurricane Outlook: Steady As She Goes

By: Bob Henson , 7:13 PM GMT on August 05, 2016

There were no major adjustments in the August update to the 2016 CSU Atlantic hurricane outlook, issued on Thursday by Colorado State University. These outlooks are produced by Dr. Phil Klotzbach, who worked closely with CSU’s Dr. William Gray for many years prior to Dr. Gray’s passing in April. In the updated outlook, CSU projects a total of 15 named storms this year, including the four that developed before August 1 (Hurricane Alex and Tropical Storms Bonnie, Colin, and Danielle) as well as this week’s Hurricane Earl. The outlook also calls for a seasonal total of 6 hurricanes (including Alex and Earl) and 2 major hurricanes. These numbers are slightly above the long-term annual average, although CSU expects the rest of the season to produce near-normal activity. The predicted numbers of hurricane days and major hurricane days for 2016 as a whole were nudged upward slightly (from 21 to 23 and from 4 to 5, respectively).

In addition to these single-number forecasts, the CSU outlook includes uncertainty brackets similar to those used in NOAA’s seasonal hurricane outlooks. For example, CSU assigns two-thirds odds that the number of named storms developing after August 1, including Earl, will range betwen 8.7 and 13.3, or one standard deviation within the single-number prediction of 11.


Figure 1. Forecasts/hindcasts for the amount of accumulated cyclone energy (ACE) ocurring after August 1 (magenta line) compared to the actual number (blue line). The statistical scheme used by CSU for its August outlooks was updated in 2012, so the pre-2012 forecasts shown here are actually “hindcasts,” generated as if the current statistical tools had been in place then. The year-to-year ups and downs in ACE values are well predicted by this model, resulting in a strong correlation between observed and predicted values of 0.86. However, the ACE values for the last decade are generally overpredicted. According to Dr. Phil Klotzbach, this may be related to an increasing trend in July SSTs over the northeast subtropical Atlantic since the late 1970s. July SSTs in this region is one of the predictors used in creating the August outlook. Warmer SSTs here in July tend to be correlated with increased Atlantic hurricane activity, but a long-term warming trend may affect the predictive value, according to Klotzbach. Image credit: Courtesy Phil Klotzbach, CSU.

Mixed signals remain
There is still considerable uncertainty shrouding how the rest of the season will unfold, according to CSU. The variables in play include the potential development of La Niña (still a question mark in its timing and strength), as well as the unorthodox North Atlantic pattern now in place. Sea surface temperatures have been extremely warm across the western Atlantic, coupled with cooler-than-average SSTs in the eastern subtropical Atlantic and the far North Atlantic. This is roughly consistent with the negative phase of the Atlantic Multidecadal Oscillation as tracked by an AMO index developed by Klotzbach and Gray. This index has been in negative territory since late 2014, although it crept close to the zero line in July 2016, noted Klotzbach in a tweet on Wednesday. Other signals are also in some conflict: for example, upper-level wind shear has been fairly low, which favors tropical cyclone development, but the atmosphere across the tropical Atlantic has been more stable than usual over the last several months, which works against development.

CSU found that the closest analog years for hurricane-related conditions observed this past June and July are 1958, 1959, 1966, 1978, 1992, and 1998. These years produced anywhere from 7 to 14 named storms, 4 to 10 hurricanes, and 1 to 3 major hurricanes, reflecting the wide range of outcomes still possible.


Figure 2. Departures from seasonally averaged sea surface temperature (SST, shown in degrees C) in the Atlantic Ocean for the last week of July 2016. SSTs were well above average over large parts of the western Atlantic adjoining North and South America, as well as across the deep tropics of the North Atlantic that serve as the main development region for Atlantic hurricanes. Below-average SSTs prevailed over most of the northeast Atlantic subtropics and midlatitudes. Image credit: NOAA/NHC.

Are we ready for a major hurricane?
CSU is giving 51 percent odds that a major hurricane will strike somewhere along the U.S. Gulf or Atlantic coast--which is a quite unsettling prospect, given that a major landfall (Category 3 or stronger on the Saffir-Simpson scale) has not occurred in the U.S. since Hurricane Wilma in 2005. Moreover, the Gulf of Mexico has not seen a hurricane outside of the Bay of Campeche since Ingrid in 2013, and Florida has not experienced a landfalling hurricane of any strength since Wilma. All three of these “droughts” are unprecedented in length, a point whose consequences are analyzed in an excellent roundup on Thursday by Capital Weather Gang.

See our posts from May 27 and June 1 for more background on the outlooks issued this spring by CSU and other groups. NOAA will be updating its seasonal hurricane outlook on Thursday, August 11, and CSU will be issuing 14-day outlooks every two weeks from now through October. Note that the CSU outlooks are now linked from a new website, tropical.colostate.edu.



Figure 3. Number of named storms in the Atlantic basin predicted for the 2016 season by various forecast groups. Bottom axis shows the name of each group and the time frame when its forecast was issued (or updated). The zone between the orange and red horizontal lines indicates the number of named storms one might expect in a typical year. Image credit: Barcelona Supercomputing Center and Colorado State University.

A new tool for comparing and contrasting seasonal outlooks
At last, we have a one-stop virtual shop for evaluating how the seasonal hurricane outlooks issued by an increasing number of entities stack up against each other. CSU’s Phil Klotzbach teamed up with Louis-Philippe Caron at the Barcelona Supercomputing Center in Spain to produce the new site, which was designed by Iskiam Jara and supported by the Ireland-based insurance firm XL Catlin. The site formally debuted on Thursday with a CSU press release.

Colorful graphics depict each forecast and whether it was issued only as a single number or includes a probabilistic range. The site allows you to toggle between the outlooks for the number of named storms, hurricanes, and major hurricanes. At the bottom of the main forecast page, you’ll find blurbs on each of the 17 organizations that have released seasonal hurricane outlooks, including The Weather Company. Klotzbach is hoping to compile forecasts issued in past years by the various groups, which would make the site a true treasure trove for researchers and weather enthusiasts.

For a full update on current tropical activity, including the potential for multi-day heavy rains along the eastern Gulf Coast associated with a near-shore disturbance, see this morning’s post from Jeff Masters. Have a great weekend, everyone!

Bob Henson

Hurricane

Earl Lingering Over Mexico; New Disturbance Will Bring Heavy Rains to Southeast U.S.

By: Jeff Masters , 3:04 PM GMT on August 05, 2016

Tropical Storm Earl was clinging to tropical storm status on Friday morning, as its center skirted the southern coast of the Gulf of Mexico's Bay of Campeche. Earl is still a major rainfall threat, but so far the torrential destructive rains that were feared from the storm have not materialized. Twenty-four hour rainfall amounts in Mexico's Yucatan Peninsula ending at 8 am EDT August 4 were less than three inches, according to Conagua, the Mexican weather service. Heavy rains of up to seven inches were measured in Chiapas in the 24 hours ending at 8 am EDT Friday.

Heavy damage to buildings and infrastructure is being reported in Belize, primarily from Earl's winds and storm surge. Earl made landfall near Belize City, Belize as a Category 1 storm with 80 mph winds near 2 am EDT Thursday, August 4, 2016. Most of the weather stations in Earl's path stopped transmitting before the storm's peak winds arrived, but Half Moon Caye, which received a battering from a portion of Earl's northern eyewall, recorded a wind gust of 89 mph at 10:00 pm local time Wednesday.


Figure 1. Floodwaters from Earl's heavy rains collapsed a bridge near the Belize/Guatemala border in Melchor de Mencos, Petén, Guatemala. Photo: CONRED (Guatemala civil defense agency), via Norman Avila.


Figure 2. VIIRS image of Tropical Storm Earl approaching landfall in Belize on Wednesday afternoon, August 3, 2016. Image credit: NOAA Satellites on Twitter.

Earl's rains continue
Satellite loops and radar imagery from Mexico on Friday morning showed Earl was still generating heavy thunderstorms over portions of southern Mexico. Earl will continue moving on a mostly westward track at 10 - 15 mph over the next few days, and will likely dissipate by Saturday. Earl's remnants have the potential to merge with an area of disturbed weather and re-organize into a tropical depression over the waters off the Pacific coast of Mexico southwest of Puerto Vallarta in the Monday - Wednesday time frame, though. In their 8 am EDT Friday Eastern Pacific Tropical Weather Outlook, NHC gave the remnants of Earl 2-day and 5-day development odds of 40% and 70%, respectively. The 00Z Friday run of the European model predicted that Earl's remnants would regenerate into a tropical storm that would bring heavy rains to the southern tip of Mexico's Baja Peninsula on Tuesday and Wednesday.


Figure 3. Predicted precipitation for the 7-day period ending Friday, August 12, 2016. Rainfall amounts in excess of five inches (bright orange colors) are expected along a stretch of the Gulf Coast from New Orleans, Louisiana to Tampa, Florida. Image credit: National Weather Service.

Elsewhere in the Atlantic
A broad surface low pressure system is forecast to develop near the coast of Alabama by Sunday, and the counter-clockwise flow of air around this low will bring a moist flow of air over the coast from Alabama to Tampa, resulting in heavy rains. In their 00Z Friday runs, about 40 - 60% of the members of the European and GFS model ensemble forecasts predicted that this low could develop into a tropical depression sometime Monday through Wednesday, and drift slowly northwards or northeastwards--inland over the Southeast U.S. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave this future disturbance 2-day and 5-day development odds of 0% and 20%, respectively. Regardless of development, coastal regions from New Orleans to Tampa can expect heavy rains exceeding five inches during the coming week, as highlighted in the latest precipitation forecast from NOAA (Figure 3.) Water temperatures in the northeast Gulf of Mexico are at near-record warm levels, and the evaporation from these warm waters will provide plenty of moisture to fuel heavy rains.

Tropical storms Ivette and Omais active in the Pacific
The Eastern Pacific continues to be active, with Tropical Storm Ivette in the Pacific waters southwest of the Mexican coast. Ivette will be moving away from the Mexican coast on a west to west-northwest track, and is expected to find less favorable atmospheric conditions over the weekend, dying out well before reaching Hawaii.

In the Northwest Pacific, Tropical Storm Omais formed on Thursday, and is expected to intensify into a Category 1 typhoon this weekend as it moves northwards a few hundred miles east of Japan. Omais is not expected to affect any land areas as a tropical cyclone.

Bob Henson will be back this afternoon with a post on the long-range Atlantic hurricane season outlook.

Jeff Masters

Hurricane

Hurricane Earl--First Caribbean Hurricane Since Sandy of 2012--Hits Belize

By: Jeff Masters , 2:02 PM GMT on August 04, 2016

Hurricane Earl--the first hurricane in the Caribbean since Hurricane Sandy of 2012--made landfall near Belize City, Belize as a Category 1 storm with 80 mph winds near 2 am EDT Thursday, August 4, 2016. Earl was the strongest hurricane to hit Belize since Hurricane Richard on October 23, 2010, which made landfall as a Category 2 hurricane with 100 mph winds. Earl's strongest winds observed at the Belize Airport were sustained at 34 mph, gusting to 58 mph, between 10 pm and midnight local time on Wednesday evening. However, data stopped transmitting at 3 am when the center of the storm reached the airport. A personal weather station on Ambergris Caye, which received a battering from a portion of Earl's northern eyewall, recorded sustained winds of 46 mph, with a wind gust of 69 mph at 10:45 pm local time Wednesday. The station lost power shortly thereafter. Damage is likely to be significant in Belize City.


Figure 1. Storm chaser Josh Morgerman of icyclone.com rode out Earl from Belize City, and took this photo of the storm surge inundating the city at 2 am local time on August 4, 2016. "Definitely the most-epic Cat 1 I've chased. A hurricane of consequence. Will be remembered by Belizeans," he commented on his Twitter feed.


Figure 2. Belize radar as seen at 11:15 pm EDT August 3, 2016, shortly before Hurricane Earl made landfall near Belize City. Check out this extended radar loop of Earl saved by Brian McNoldy.

Dangerous rains coming from Earl
Satellite loops and Belize radar on Thursday morning showed Earl was steadily weakening as it moved inland over the Yucatan Peninsula, with the heavy thunderstorm activity shrinking in areal coverage and intensity. Earl will continue moving on a mostly westward track at 10 - 15 mph over the next few days, and will likely dissipate by Sunday. Earl is not likely to emerge over the Gulf of Mexico far enough to undergo any significant regeneration.

Earl grew into a large hurricane on Wednesday, and was able to tap into the Eastern Pacific as an additional source of moisture. When hurricanes are able to tap into both the Atlantic and Pacific as moisture sources, very dangerous heavy rains that affect a large portion of Mexico and Central America usually result, and Earl's heavy rains are going to be a major concern for the region. With rainfall amounts in excess of 8" expected over a swath of northern Honduras, northern Guatemala, most of Belize, and a big chunk of southern Mexico, expect to see many reports of life-threatening flash floods and landslides.


Figure 3. Lightning lights up the interior of Tropical Storm Earl as seen by the Air Force Hurricane Hunters on August 2, 2016. Image credit: 1st Lt. Leesa Froelich, 53rd Weather Reconnaissance Squadron, Air Force Hurricane Hunters Facebook page.


Figure 4. Predicted precipitation for the 7-day period ending Thursday, August 11, 2016. Rainfall amounts in excess of three inches (orange colors) are expected along a stretch of the Gulf Coast from New Orleans, Louisiana to Tampa, Florida. Image credit: National Weather Service.

Elsewhere in the Atlantic
There are no other tropical cyclone threat areas in the Atlantic to discuss today, and none of the reliable models for tropical cyclone formation is predicting development during the coming five days. A broad surface low pressure system is forecast to develop near the coast of Alabama early next week, and the counter-clockwise flow of air around this low will bring a moist flow of air over the coast from Alabama to Tampa, resulting in heavy rains. In their 00Z Thursday runs, about 20% of the members of both the European and GFS model ensemble forecasts predicted that this low could develop into a tropical depression sometime Monday through Wednesday. Regardless of development, coastal regions from New Orleans to Tampa can expect heavy rains exceeding three inches next week, as highlighted in the latest precipitation forecast from NOAA (Figure 4.)

Ivette active in the Eastern Pacific
The Eastern Pacific continues to be active, with Tropical Storm Ivette gathering strength in the Pacific waters south of the Mexican coast. Ivette will be moving away from the Mexican coast on a west to west-northwest track, and is expected to peak as a Category 1 hurricane on Sunday before cooler waters and less favorable atmospheric conditions result in weakening. Ivette will likely die out well before reaching Hawaii.

There is one more area of concern in the Eastern Pacific: the possible arrival of the remnants of Hurricane Earl early next week. Earl will cease to exist as a named storm during its long traverse of Mexico during the coming weekend, but if Earl's remnants manage to cross over Mexico and arrive over the waters off the coast of Puerto Vallarta with some spin still intact, regeneration into a tropical storm is possible. In their 8 am EDT Thursday Eastern Pacific Tropical Weather Outlook, NHC gave the remnants of Earl 2-day and 5-day development odds of 0% and 60%, respectively. The 00Z Thursday run of the European model predicted that Earl's remnants would regenerate into a tropical storm that would bring heavy rains to the southern tip of Mexico's Baja Peninsula on Tuesday and Wednesday.

Jeff Masters

Hurricane

Hurricane Warnings in Belize, Honduras, and Mexico for Earl

By: Jeff Masters , 3:29 PM GMT on August 03, 2016

Hurricane warnings are flying for the coast of Belize, the southern portion of Mexico's Yucatan Peninsula, and the islands off the north coast of Honduras, as a strengthening Tropical Storm Earl speeds westwards at 14 mph. The Hurricane Hunters did not find hurricane-force winds in Earl in a mission that departed from the storm around 8 am EDT Wednesday, but a new airplane arrived in Earl around 11 am, and will likely find that Earl is a hurricane by mid-afternoon Wednesday. Satellite loops on Wednesday morning showed Earl was steadily gaining in organization, with an increase in symmetry, low-level spiral bands and heavy thunderstorm activity. No eye was apparent in the visible satellite imagery, but we should see one appear before sunset on Wednesday. The outer bands of Earl were just beginning to appear on Belize radar late Wednesday morning.


Figure 1. Tropical Storm Earl as seen at 3:30 pm EDT August 3, 2016, when the storm had top sustained winds of 70 mph. Image credit: NHC Facebook page.

Forecast for Earl
The forecast for Earl appears straightforward. Earl is trapped to the south of a strong area of high pressure that will keep the storm moving on a track slightly north of due west at 10 - 14 mph over the next four days. This motion will bring the center of the storm within twenty miles of Guanaja Island off the coast of Honduras near 4 pm EDT Wednesday, then to the coast of central Belize around 4 am EDT Thursday. Earl has favorable atmospheric and oceanic conditions for intensification: light to moderate wind shear of 5 - 15 knots, and very warm ocean waters near 30°C (86°F). These warm waters extend to great depth, providing plenty of fuel to power intensification of the storm. Typically, storms that approach landfall begin to undergo interaction with land that causes a slowdown in intensification or weakening. However, storms in the Western Caribbean often undergo intensification right up until landfall, due to the extremely warm waters with high heat content that lie along the coast. The topography of the coast in the right-angle bend between Belize, Guatemala, and Honduras may also act to aid intensification by giving storms more spin, as air gets deflected into a counter-clockwise motion by the high terrain ringing the ocean. This effect has been shown to exist in modeling studies of some storms in the Gulf of Mexico's Bay of Campeche, but has not been studied (to my knowledge) for the region along the coasts of Honduras, Guatemala, and Belize, though.

The main concern from Earl is its heavy rains. With rainfall amounts in excess of 8" expected over a swath of northern Honduras, northern Guatemala, most of Belize, and a chunk of Mexico, expect life-threatening flash floods and landslides. The storm's 4 - 6' storm surge will cause additional flooding along the coast near and to the right of where the center hits in Belize. Strong winds will also be a major concern. In their 11 am EDT Wednesday Wind Probability forecast, NHC gave Guanaja Island a 97% and 30% chance of experiencing tropical storm-force winds of 39+ mph and hurricane-force winds of 74+ mph, respectively. For Belize City, these odds were 94% and 17%, respectively. Earl is already a killer: high winds in the Dominican Republic associated with the tropical wave that became Earl brought power lines down and sparked a fire aboard a bus, killing 6 and injuring 12 people, according to weather.com. Three others were killed after a tour boat overturned, although that incident had not yet been confirmed to be weather-related.


Figure 2. Tracks of the approximately 80 tropical depressions, tropical storms, and hurricanes that have hit Belize since 1851. Image credit: NOAA/CSC.

Belize hurricane history
Belize is often struck by tropical storms and hurricanes. Approximately 80 tropical depressions, tropical storms, and hurricanes have hit Belize since 1851, but it has been five years since the last landfall by a named storm--Tropical Storm Harvey, which hit on August 20, 2011, with 65 mph sustained winds. Harvey's flooding rains killed five people in Mexico, but did little damage in Belize. The last hurricane to hit Belize was Hurricane Richard on October 23, 2010, which made landfall about 20 miles south of Belize's largest city, Belize City (population approximately 100,000--1/3 of Belize's population.) Richard hit as a Category 2 hurricane with 100 mph winds, but was a small hurricane, with hurricane-force winds affecting a region of coast of no more than 20 - 30 miles wide. The hurricane killed one and did about $80 million in damage. The last major hurricane to hit Belize was Hurricane Iris on October 9, 2001, which made landfall in southern Belize as a Category 4 storm with 145 mph winds, killing 35 and doing $250 million in damage.

Links for Earl
Belize radar
National Emergency Management Organization of Belize

Webcam links posted by WU members in the comments:
Belize webcams
Ambergris Caye, Belize
Roatan, Honduras

Elsewhere in the Atlantic
There are no other tropical cyclone threat areas in the Atlantic to discuss today, and none of the reliable models for tropical cyclone formation is predicting development during the coming five days. Despite the relative lack of dry air and dust from the Sahara Desert over the tropical Atlantic, the strong tropical wave near 9°N, 38°W, midway between Africa and the Lesser Antilles, is showing no signs of development, and the 50 members of the European model ensemble and 20 members of the GFS model ensemble are giving little support to development of this wave over the coming week.

A few recent runs of the European model have shown that a weak trough of low pressure will move over the Southeast U.S. early next week and move over the northeast Gulf of Mexico, off the Florida Panhandle coast. About 10% of the 50 members of the European model ensemble forecast have been highlighting this area for possible formation of a tropical depression in 5 - 7 days.

Howard and Ivette active in the Eastern Pacific
The Eastern Pacific continues to be active, with two named storms, Howard and Ivette. Neither storm is a threat to land. Tropical Storm Howard hit its peak with 60 mph winds on Tuesday, and is steadily deteriorating as it runs into cooler waters and a more stable atmosphere. Howard should be a remnant low by Thursday night, but could bring some squally weather and high surf to portions of Hawaii on Sunday. Next up is Tropical Storm Ivette, which is gathering strength in the Pacific waters south of the Mexican coast. Ivette will be moving away from the Mexican coast on a west to west-northwest track, and is expected to peak as a Category 1 hurricane on Saturday before cooler waters and less favorable atmospheric conditions result in weakening as the storm approaches Hawaii early next week.

There is one more area of concern in the Eastern Pacific: the possible arrival of the remnants of Hurricane Earl early next week. Earl will cease to exist as a named storm during its long traverse of Mexico during the coming weekend, but if Earl's remnants manage to cross over Mexico and arrive over the waters off the coast of Acapulco, Mexico with some spin still intact, regeneration into a tropical storm is possible. In their 8 am EDT Wednesday Eastern Pacific Tropical Weather Outlook, NHC gave the remnants of Earl 2-day and 5-day development odds of 0% and 30%, respectively. If Earl's circulation has become unidentifiable the time it crosses into the Pacific, the new storm would be named Javier.

Jeff Masters

Hurricane

Tropical Storm Earl Forms in Caribbean

By: Bob Henson , 5:11 PM GMT on August 02, 2016

After racing across the eastern Caribbean as an strong tropical wave, Invest 97L has finally been dubbed Tropical Storm Earl. Late Tuesday morning, an Air Force hurricane-hunter mission found that Earl had developed a closed circulation center with a minimum central pressure of 1001 millibars. Flight-level winds reached 52 knots (57 mph) just after noon EDT Tuesday. In an special update issued at noon EDT Tuesday, the National Hurricane Center placed the center of newly christened Earl about 535 miles east of Belize City, Belize, with top sustained surface winds of 45 mph. Carrying a large though somewhat disorganized assortment of showers and thunderstorms (convection), Earl was moving westward at 22 mph, a pace expected to slow over the next 24-48 hours as Earl approaches Belize and the Yucatan Peninsula. Even before Earl’s designation as a tropical storm, high winds in the Dominican Republic brought power lines down and sparked a fire aboard a bus, killing 6 and injuring 12 people, according to weather.com. Three others were killed after a tour boat overturned, although that incident had not yet been confirmed to be weather-related. The Meteorological Service of Jamaica issued a tropical storm warning for 97L on Monday night, and on Tuesday the Cayman Islands National Weather Service was cautioning small craft to exercise caution in open waters. Surface winds at Kingston, Jamaica, peaked at 29 mph early Tuesday morning, with only light rain observed, although showers and squalls have affected other parts of Jamaica.


Figure 1. Flight-level wind data gathered by an Air Force hurricane-hunter flight through 1604Z (12:04 pm EDT) Tuesday, August 2, 2016. The light westerly winds to the west of Earl’s center (the dot with the “1002”-mb label) showed that a closed circulation was evident at flight level. Image credit: tropicaltidbits.com.


Figure 2. Infrared satellite image of Tropical Storm Earl at 1615Z (12:15 pm EDT) Tuesday, August 2, 2016. Image credit: NOAA/NESDIS.

What took Earl so long?
During its time as 97L, Earl puzzled tropical weather watchers and forecasters with its dramatic appearance on satellite coupled with its inability to qualify as a tropical depression or tropical storm. In part, this is because Earl has been more organized aloft than at the surface. During the classic nocturnal peaks of convection (shower and thunderstorm activity) on both Sunday and Monday night, 97L developed a large mass of convection near its center, with fairly symmetric upper-level outflow evident in all directions on satellite imagery. Because 97L’s rapid westward motion of 25-30 mph was roughly in line with upper-level winds, there was little vertical wind shear affecting the system, thus helping the convection to remain symmetric and well structured. The crucial ingredient missing at the surface was a closed center of circulation around which surface winds were rotating. In order to qualify as a tropical depression or tropical storm, a closed surface wind circulation is required by definitions employed by NHC and the World Meteorological Organization for the North Atlantic and East Pacific. Surface winds measured on Monday by the Advanced Scatterometer (ASCAT) aboard Europe’s MetOp-B satellite showed easterly winds of 30-40 mph on the north side of 97L, but southerly rather than westerly winds on its south side. An Air Force reconnaissance flight into 97L was aborted due to mechanical difficulties on Monday afternoon, so Tuesday morning’s flight was the first to gather data from within the storm.


Figure 3. WU depiction of the NHC track forecast for Tropical Storm Earl issued at noon EDT Tuesday, August 2, 2016.

The outlook for 97L
Although Earl kept forecasters guessing as to exactly when it would become a tropical storm, forecast models have come into increasing solidarity on Earl’s prognosis. A sprawling area of high pressure to the north of Earl will keep the storm moving on a general westerly path. Even though it will slow down a bit over the next day or two, Earl still has less than two days to gather strength before it makes landfall late Wednesday, most likely in Belize but possibly on the far southeast Mexican coast of the Yucatan peninsula. The official NHC outlook keeps Earl just below hurricane strength at landfall late Wednesday night. Wind shear is predicted by the SHIPS model to remain at a light to moderate 10 - 20 mph, although parts of the circulation are being affected by stronger shear to the north associated with a slow-moving upper-level trough over the northwest Atlantic. We will have to keep a close eye on Earl tonight and Wednesday, as sea-surface temperatures are more than 1°C above average over the northwestern Caribbean, and Earl’s path will put it on the south edge of an area of of extremely high oceanic heat content that covers most of the northwest Caribbean. These waters would support rapid intensification if other conditions were favorable, although Earl may not be well enough organized to take full advantage of this oceanic rocket fuel.

The 00Z Tuesday ensemble runs of the ECMWF model were unanimous in keeping Earl as a weak tropical storm, while most members of the 06Z Tuesday GFS ensemble brought Earl up to strong tropical storm strength. The 12Z Tuesday run of the SHIPS statistical model gives Earl a 21% chance of a 24-hour increase of 25 knots in sustained winds, which would bring Earl to the threshold of hurricane status. Although the prospect is unlikely, I wouldn’t be shocked to see Earl become a minimal hurricane just before landfall, as predicted by the 06Z Tuesday run of the HWRF model. We’re very lucky that Earl is moving across the Caribbean so quickly, or else it could have become a much more formidable storm. If Earl remains far enough north on its trek across the Yucatan, it may have a brief window to reorganize across the southern Bay of Campeche, where the short-lived Tropical Storm Danielle formed in late June. It’s worth noting that Earl will be the fifth of all five named systems in the Atlantic this year to make landfall somewhere.

Elsewhere in the tropics
The short life of Tropical Storm Howard may have already hit its high point, with Howard’s sustained winds holding at 60 mph at 9:00 am EDT Tuesday. Located about 1250 miles west of the southern tip of Baja California, Howard was chugging west-northwest at 15 mph. Models are consistent in keeping Howard on this track well away from any land areas, with a gradual arc toward the west by late in the week. Although Howard will be moving over waters cooler than 26°C by Wednesday, wind shear will remain light to moderate for several days, so Howard will likely take a while to spin down, potentially remaining a tropical storm throughout the week. By this weekend, Howard’s path is expected to arc gradually leftward toward the northern Hawaiian Islands. It is unlikely that Howard would strike any of the islands as a tropical storm, although Howard or its remnants could bring some squally weather and high surf by Saturday or Sunday. Southeast of Howard, the next in the conga line of East Pacific storms may soon develop from Invest 92E, which was classified on Monday. As with its predecessors, 92E will be moving away from the Mexican coast on a straightforward west-northwest track. Models suggest that 92E is likely to become Tropical Storm Ivette by midweek—which would make it the ninth named storm for the East Pacific in just five weeks!—but there is little sign of major intensification beyond that point, as 92E will be moving across cooler waters.


Figure 4. WU depiction of the NHC track forecast for Tropical Storm Howard issued at 11:00 am EDT Tuesday, August 2, 2016.


Figure 5. A MODIS image of Typhoon Nida taken from aboard NASA’s Aqua satellite as Nida it approached the China coast on August 1, 2016. Image credit: NASA Earth Observatory.

Nida reaches Hong Kong
The southern coast of China came away relatively unscathed from the arrival of Typhoon Nida, which swept across the northernmost part of the Hong Kong area at Category 1 strength around 1800Z (2:00 pm EDT) Monday, August 1, or just after midnight Tuesday local time, moving on to pass directly over the sprawling inland city of Ghaungzhou. There was major disruption across the region on Tuesday, including hundreds of flight cancellations and delays, but little damage and no injuries have been reported. Wind gusts peaked at around 64 mph at Hong Kong’s Chek Lap Kok airport. Nida was continuing to dump very heavy rains in coastal provinces to the west of Hong Kong as it pushed inland late Tuesday local time.


”My friends”: Remembering the irreverent, irreplaceable Dave Schwartz
The world of weathercasting lost one of its most colorful and beloved figures on Saturday with the passing of the Weather Channel’s Dave Schwartz on Saturday (see this Weather Channel tribute). Dave was a fixture on TWC’s Weather Underground series (#WUTV) from the show’s inception in 2015, with his blackboard-based “WOW Factor” analyses and his quirky yet accessible style helping to solidify the show's role as a home for unapologetic, full-on weather geekery. Dave was also forthright about his longtime struggle with cancer. Early this year, on World Cancer Day, Dave told viewers about his extended battle (the full story can be found at this Weather Channel blog post). Back in the mid-2000s, Dave learned he had stage 2 pancreatic cancer. The five-year survival rate for all forms of pancreatic cancer is only 6%, yet Dave survived not only the initial cancer but a recurrence a year later. After almost ten years of remission, Dave was diagnosed last year with stomach cancer, but he remained on the air well into 2016. “In a sense, having cancer and the impact it has on my life has really enriched my life tremendously through strengthening my relationships with people,” he said.

Dave had been part of the Weather Channel for most of its 34-year existence, starting in 1985 as a newsroom assistant and becoming an on-camera meteorologist in 1992. A native of Philadelphia and a weather geek from childhood, Dave earned his bachelor’s degree in psychology from Temple University, later earning a certificate in meteorology studies from Mississippi State University. I believe this hybrid background helped shape Dave’s unique, people-centered approach to weathercasting, which was exemplified in his love of language, his eye for captivating weather tidbits, and his perennial reference to viewers as “my friends.” I greatly enjoyed Dave’s weathercasts from the start and was fortunate enough to meet him on a TWC visit years ago, though I wish I’d had the chance to know him better. WUTV’s Mike Bettes and Alex Wilson paid tribute to Dave on Monday (see embedded video below), including a collection of tributes from coworkers placed on his beloved chalkboard. “His chalkboard was his weather canvas that wowed us every day,” noted Bettes. From Wilson: “He taught me more than I could have learned from years of school, and I’ll never forget his zest for life.”

Bob Henson



Video 1. Mike Bettes and Alex Wilson pay tribute to Dave Schwartz on the Weather Underground series on Monday, August 1, 2016.


Hurricane

Tropical Storm Likely to Form in Caribbean; Flash Flood Devastates Ellicott City, MD

By: Bob Henson , 2:00 PM GMT on August 01, 2016

The tropical wave in the eastern Caribbean dubbed Invest 97L continued to organize on Sunday night, and it will likely become a tropical depression or tropical storm on Monday and move into the western Caribbean as an intensifying tropical storm over the next day or so. Infrared satellite imagery on Sunday night revealed that shower and thunderstorm activity (convection) had become more intense and far more symmetric around 97L’s core, which was located early Monday morning about 350 miles east-southeast of Kingston, Jamaica. As of early Monday morning, 97L had not yet consolidated a closed low-level center of circulation, as seen by the lack of westerly winds in the ASCAT scatterometer image in Figure 2 below. However, surface winds were close to tropical storm strength on the north side of the system. As of early Monday morning, a NOAA Hurricane Hunter aircraft was tentatively scheduled to carry out a reconnaissance mission into 97L on Tuesday afternoon.

97L continues to chug westward at 20 to 25 mph, a speed that typically limits the ability of a tropical wave to intensify. In this case, however, upper- and lower-level winds are close enough in speed and direction to reduce the amount of wind shear that would otherwise affect fast-moving 97L. Long-range models agree that wind shear will remain light (around 10 knots or less) for at least the next three to four days, perhaps longer, along 97L’s path. In its 8:00 am EDT Monday tropical weather discussion, the National Hurricane Center gave 97L an 80% chance of development over the next two days. (On occasion, the NHC will bypass tropical depression status and upgrade a strong wave directly to tropical storm status.) 97L will be sweeping just south of Jamaica on Tuesday, perhaps as a moderately strong tropical storm (which would be named Earl], before approaching the Yucatan Peninsula later in the week (see below).


Figure 1. Enhanced infrared image of Invest 97L as of 1245Z (8:45 am EDT) Monday, August 1, 2016. Image credit: CIRA/RAMMB/CSU.


Figure 2. Low-level winds (in knots) detected by the Advanced Scatterometer (ASCAT) aboard the MetOp-A satellite as of 1302Z (9:02 am EDT] Monday, August 1, 2016. Image credit: NOAA/NESDIS/STAR.

The outlook for 97L
It’s now clear that 97L will survive its trek through the eastern Caribbean, which has long been known as the “hurricane graveyard” thanks to the climatological minimum in tropical cyclone formation over the area. According to a 2010 study in the Bulletin of the American Meteorological Society led by Owen Shieh (University of Oklahoma), this phenomenon is largely because of a predominant low-level southerly jet in the central Caribbean and an accompanying tendency toward low-level divergence over the eastern Caribbean. This pattern is most prevalent early in the season, peaking in July.

With a sprawling area of upper-level high pressure to its north, 97L should continue on a fairly straightforward west to west-northwest path for at least the next couple of days. There is strong agreement among our best longer-range track models, the ECMWF and GFS, that 97L will move south of Jamaica on Tuesday and approach the Yucatan Peninsula around Thursday. There are no signs of any major changes to the upper-level pattern that would divert 97L from this longer-term path, which could eventually bring it across the Yucatan Peninsula and into the Bay of Campeche. We will have to watch for any signs of a northward bend in 97L’s path later this week, but no members of the most recent GFS and ECMWF ensemble runs are indicating a track that would bring 97L into the heart of the western Gulf. Residents of Nicaragua and Honduras, and especially Belize, Guatemala, and eastern Mexico, will need to keep a close watch on 97L. Toward the end of the week, 97L could pose a threat to the western coast of the Bay of Campeche if it crosses the Yucatan Peninsula and survives the trek.


Figure 3. Oceanic heat content over the Caribbean on July 30, 2016, in kilojoules per square centimeter. The value is produced by integrating the vertical temperature from the ocean surface to the depth of the 26°C contour. 97L will be traveling over a large area of heat content greater than 100 kJ/cm2 on Tuesday and Wednesday. Image credit: NOAA/AOML.

Extremely warm water lies ahead of 97L
As it sweeps toward the western Caribbean, 97L will encounter a more favorable atmospheric regime as well as ominously warm water both at the surface and below. Sea-surface temperatures (SST) along 97L’s expected path from the area around Jamaica westward are around 29-30°C (84-86°F), which is roughly 1°C above average for this time of year. These warm waters extend to great depths, with large amounts of oceanic heat in the uppermost 200 meters (660 feet) of the northwest Caribbean (see Figure 3). Across the Caribbean and adjacent waters, the breadth and depth of oceanic heat content has been at near-record levels in recent weeks, as we discussed in a post on July 18. High levels of oceanic heat content are a major boon to tropical storm and hurricane intensification, because the storms do not churn up as much cold water as they otherwise would as they intensify. Rapid intensification is often associated with regions of high oceanic heat content, assuming that other conditions are favorable as well.


Figure 4. Infrared satellite image of Tropical Storm Howard at 1300Z (9:00 am EDT) Monday, August 2, 2016. Image credit: NOAA/NESDIS.

Howard becomes the Eastern Pacific’s eighth tropical storm of the season
Tropical Storm Howard was christened at 5:00 am EDT Monday, August 1, just a few hours too late to become a record-setting eighth tropical storm in the Eastern Pacific during the month of July. When it became a tropical depression on Sunday morning, Howard managed to tie the record, set in July 1985, for the eighth tropical cyclone of at least depression strength to form in the basin. The basin’s record of seven named storms for July, also set in 1985, was tied on July 22 with the formation of Tropical Storm Georgette (which went on to become the basin’s second Category 4 hurricane of the year).

Apart from extending the Eastern Pacific’s remarkable month-long string of tropical cyclones, Howard is unlikely to make much of a splash. At 5:00 am EDT Monday, Howard was located about 1000 miles west-southwest of the southern tip of Baja California. As it moves along a west-northwest path well offshore during the next 24 to 36 hours, Howard will have a window of light to moderate wind shear (10-15 knots) and SSTs of around 27°C, but it is not exceptionally well-organized and will have trouble strengthening much beyond mid-range tropical storm intensity. After Tuesday, Howard will encounter much cooler waters and greater shear, and the NHC outlook diminishes Howard to a post-cyclone remnant low by Thursday. The remains of Howard should arc leftward and may pass near or north of Hawaii by next weekend.


Figure 5. VIIRS visible satellite image of Typhoon Nida at 0509Z (1:09 am EDT) Monday, August 1, 2016. The Hong Kong area is outlined in a box along the yellow border denoting the southeast China coast. Image credit: RAMMB/CIRA/CSU.

Nida heads toward Hong Kong
Typhoon Nida continues to chug toward the southeast China coast, where it could make landfall very near Hong Kong. Nida dumped rains of more than 11 inches over parts of northern Luzon as it swept through the northern Philippines over the weekend. Fortunately, Nida has failed to intensify very much, with sustained winds of 70 knots (80 mph) as of the 5:00 am EDT bulletin from the Joint Typhoon Warning Center. Nida is unlikely to strengthen much further before making landfall on Tuesday night local time (midday Tuesday EDT). Even as a minimal typhoon, Nida may cause a fair bit of disruption for the seven million residents of the Hong Kong area. A number of destructive typhoons have struck in or near Hong Kong, including Wanda (1962), which produced wind gusts of 161 mph in Hong Kong Harbor.



Figure 6. Workers gather by street damage in Ellicott City, Md., on Sunday, July 31, 2016, after Saturday night's flooding. Historic, low-lying Ellicott City, Maryland, was ravaged by floodwaters that killed at least two people and caused devastating damage to homes and businesses, officials said. Image credit: Kevin Rector/The Baltimore Sun via AP.

Two killed as historic Maryland city ravaged by flash flooding
High water poured through the streets of historic Ellicott City, Maryland, on Saturday night in a dramatic flash flood (see YouTube embedded video at bottom) that killed at least two people and left a trail of damage and debris through the historic downtown area. Destruction in the city was reported to be severe, on par with the damage reported in the floods of 1868 and 1972 (the latter associated with Hurricane Agnes). While heavy rains were observed across much of the Washington/Baltimore area on Saturday evening (see Figure 7), the worst was focused near Ellicott City (Figure 8), where a rain gauge maintained by Howard County recorded the following amounts:

DURATION AMOUNT TIMEFRAME
----------------------------------------
1 minute ..0.20” - from 7:51pm-7:52pm
5 minutes ..0.80” - from 7:50pm-7:55pm
10 minutes..1.44” - from 7:50pm-8:00pm
15 minutes..2.04” - from 7:46pm-8:01pm
20 minutes..2.48” - from 7:44pm-8:04pm
30 minutes..3.16” - from 7:36pm-8:06pm
60 minutes..4.56” - from 7:30pm-8:30pm
90 minutes..5.52” - from 7:00pm-8:30pm
2 hours.....5.92” - from 6:45pm-8:45pm

Based on data from NOAA’s Atlas 14, which estimates recurrence intervals for heavy rains, the rainfall amounts within this two-hour interval have a less than one-in-a-thousand chance of occurring in any given year. Across many parts of the nation and globe, the heaviest precipitation events have become increasingly intense in recent years, with more water vapor available in an atmosphere warmed by increasing greenhouse gases paving the way for the potential for heavier rains when conditions are otherwise favorable. A Climate Central analysis shows that the Baltimore area has seen a particularly sharp increase in the heaviest downpours since the 1990s. See my post from July 30 for background on the Big Thompson Flood of July 31, 1976, a disaster that influenced our current warning and safety process for flash flooding.

I’ll be back with an update by Tuesday morning at the latest.

Bob Henson


Figure 7. 24-hour rainfall amounts through 8 am EDT Sunday, July 31, 2016, in the Washington-Baltimore area. Image credit: NWS DC/Baltimore.


Figure 8. Preliminary analysis of 3-hour rainfall totals between 6 and 9 pm EDT on Saturday, July 30, 2016, in the vicinity of Ellicott City, MD. Image credit: Greg Carbin, NOAA/WPC.


Video 1. Astounding video of the Elliott City, MD, flash flood of July 30, 2016. The videographer is looking down from a restaurant as cars float through the road! The video is a powerful reminder of the need to avoid being in vehicles during high water. Two feet of flowing water is enough to float most vehicles (even SUVs). Video credit: Nezacant.

Hurricane Flood


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Category 6™

About

Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather