Category 6™

Rare Hurricane Pounds Cape Verde Islands

By: Jeff Masters and Bob Henson , 4:23 PM GMT on August 31, 2015

For the first time since 1892, a full-fledged hurricane is pounding the Cape Verde islands, as Hurricane Fred heads northwest at 12 mph through the islands in the far eastern North Atlantic. The eye of Fred passed just southwest of Boa Vista Island in the Republic of Cabo Verde (formerly called the Cape Verde Islands) near 8 am EDT Monday, with the northeastern eyewall likely hitting the island. The center of Fred is expected to pass over or very close to the northwestern Cape Verde islands of Sao Nicolou, Santa Luzuia, Sao Vicente, and Sao Antao by Monday night. All three reporting stations in the islands went off-line early Monday morning, so we have no observations to report. Despite their name (which translates to “green cape” in English), the Cape Verde islands have a semi-desert climate, with an average annual rainfall of only around 10 inches. The torrential rains of 4 - 6" predicted from Fred, with isolated totals of up to 10”, are likely to cause unprecedented flood damage on the islands. Fred may well turn out to be the Republic of Cabo Verde's most expensive natural disaster in history.


Figure 1. MODIS image of Hurricane Fred from NASA's Terra satellite taken at approximately 11:15 am EDT Monday August 31, 2015. At the time, Fred had top sustained winds of 85 mph. Image credit: NASA.


Figure 2. MODIS image of Hurricane Fred from NASA's Aqua satellite taken at approximately 8:15 am EDT Monday, August 31, 2015. At the time, Fred had top sustained winds of 80 mph. Image credit: NASA.

Most easterly hurricane formation location ever observed
Fred was able to form and intensify at an unusually easterly location, due to a pocket of anomalously warm waters (1-2°C above average, or about 27-28°C) that surround the Cape Verde islands. Since ocean temperatures are often just marginally warm enough to support tropical cyclones near the islands, it is rare to see a tropical storm or hurricane affect them. When Fred became a hurricane at 2 am EDT Monday at 22.5°W longitude, this was the easternmost formation location for any hurricane in the historical record; the previous record was held by Hurricane Three of 1900, which became a hurricane at 23°W, south of the Cape Verde islands. There have been six other hurricanes in the historical records that existed at a more easterly longitude, but all were recurving storms that formed much farther to the west than Fred. (The easternmost hurricane ever observed was Hurricane Faith of 1966, which maintained hurricane status to a position north of the British Isles, at 2.9°W.)


Figure 3. Tracks of all tropical cyclones in the vicinity of the Cape Verde islands from the NOAA historical database, which extends back to 1851 (although reports were scanty from the far eastern Atlantic until the satellite era began in the 1960s). Only a handful of tropical depressions (blue lines) and tropical storms (green lines) have affected the islands, and no direct hurricane landfalls (yellow lines) have been recorded. The two yellow tracks labeled above are an unnamed 1892 hurricane and 1998’s Hurricane Jeanne. When the National Hurricane Center named Fred at 5:00 am EDT Sunday, it was located at 18.9°W longitude; only three other named storms have existed at a more easterly longitude since hurricane records began in 1851. Ginger of 1967 at 18.1°W; Storm 3 of 1900 at 18.5°W; and Storm 6 of 1988 at 18.5°W. Image credit: NOAA Historical Hurricane Tracks.

A historic hurricane for the Cape Verdes?
The Atlantic's most terrifying and destructive hurricanes typically start as tropical waves that move off the coast of Africa and pass near the Cape Verde islands. This class of storms is referred to as "Cape Verde hurricanes", in reference to their origin. Despite the fact that the Atlantic's most feared type of hurricanes are named after the Cape Verde islands, the islands themselves rarely receive significant impacts from one of their namesake storms. This is because tropical waves coming off the coast of Africa have very little time to organize into tropical storms before arriving at the Cape Verde islands, which lie just 350 miles west of the African coast. There is no reliable record of any bona fide hurricane having made landfall on the Cape Verde islands (see Figure 2). The closest analogue for Fred is an 1892 storm that bisected the islands, moving between the northern cluster (Ilhas do Barlavento, or windward islands) and the southern cluster (Ilhas do Sotavento, or leeward islands). This 1892 storm reportedly intensified to hurricane strength while passing south of the northwestern Cape Verde islands. Another close approach came from 1998’s Hurricane Jeanne, which reached hurricane strength while passing about 100 miles south of the southern islands. Decaying tropical cyclones in the open Atlantic have occasionally circled southeastward to take a swipe at the Cape Verdes as extratropical storms, but none have reached the island at hurricane strength.

According to EM-DAT, the International Disaster Database, there have been only two deadly tropical cyclones in Cape Verde history. Like Jeanne, they both passed south of the Ilhas do Sotavento. The deadliest was Tropical Storm Fran of 1984, which brushed the southernmost islands on September 16 as a tropical storm with 50-mph winds. Fran brought sustained winds of 35 mph and torrential rains to the islands. The rains triggered flash flooding that killed more than two dozen people and caused damages of almost $3 million (1984 dollars.) The other deadly named storm was Tropical Storm Beryl of 1982, which passed about 30 miles south of the southwestern islands on August 29, with 45-mph winds. The storm's heavy rains killed three people on Brava Island, injured 122, and caused $3 million in damage.

The most recent named storm to affect the islands was Hurricane Humberto of 2013, which passed the islands to the south as a tropical storm. Humberto brought wind gusts of up to 35 mph and heavy rain squalls to the islands, triggering flooding that washed out roads and damaged homes. Hurricane Julia of 2010, the easternmost Category 4 hurricane ever recorded in the Atlantic, passed about 50 miles south of Sao Filipe, on the island of Fogo in the southern Cape Verde islands, as a tropical storm with 45 mph winds, bringing wind gusts of 30 mph and some minor flooding.


Figure 4. Track of Tropical Storm Fran of 1984, which brushed the southwestern Cape Verde islands on September 16 as a tropical storm with 50 mph winds. Torrential rains from Fran killed at least 29 people in the Cape Verde islands, making it the deadliest storm in their history.

Hurricane historian Mike Chenoweth, who has spent innumerable hours poring over dusty old ship logs, has published a number of histories of old hurricanes. Here are his comments on the history of hurricanes in the Cape Verde islands:

"There is very little data from land stations in the islands even for storms we know about that pass over or near the islands, particularly before the mid-20th century. The only hurricane in the HURDAT record (1892) is based on a single press account which does not specify any particular island that received the damage (it took a month for the news to get from the islands to Lisbon).

"It is very likely that the storm history for the islands is under-estimated given the paucity of available land and ship data in existing data bases. For example, storm 2 of 1927 has a tropical storm passing between the islands (without a direct island landfall as in 1892) but there are no accounts of any effects the storm may have had on the islands. That such effects probably occurred but remain unrecovered is supported by the report of a hurricane near the Cape Verde islands reported by the "E.R. Sterling" which sailed from Port Adeliade, Australia on 16 April 1927 for London. The ship was damaged in a storm north of the Falkland Islands on 4 July and partially dismasted but continued northward. It then encountered a hurricane near the Cape Verde islands on 4 September and lost her foremast and the Chief Officer was killed. The ship was forced to put into St. Thomas, Virgin Islands on 15 October. So the storm was apparently stronger than indicated in HURDAT and may have been stronger in the Cape Verde islands as well. Source: http://www.dragonsearch.asn.au/nletters/MLSSA_NL_370_October_2009.htm.

"In 1901, a storm was estimated to have passed south of the northern islands of the Cape Verde islands based on press accounts that I provided to the Best Track Change Committee. At the time there were no land reports from the islands in the map series or from press accounts. Today, I located a press item from the local Cape Verde press that indicates that on 29 August on Santo Antao two vessels were lost, much of the coffee and sugar cane was blown down and washed away and that mighty winds swept houses away and killed livestock and people. Most of the other islands reported at least torrential rains and flood damage. Source: "O Ultramarino" (Cidade de Praia), 17 de Outburo de 1901, nº 64, ano3, p. 2.

"In 1880, the Bremen brig "Asante" encountered a tropical storm in 15.5N 20.2W on 17 August, lowest pressure 752.3mm (1003mb) maximum winds encountered force 11 from the south. (Deutsche Seewarte, Segelhandbuch für den Atlantischen Ozean, 1899, p.187). A brig drove off from her anchorage on the night of 17-18 August on the Island of Sal in the northeast Cape Verde islands during a "severe hurricane" and was lost on the island and the crew saved (Lloyd's List, 8 September 1880).

"The most severe storm on record in the Cape Verde Republic's history pre-dates the official North Atlantic record. This was a hurricane and was felt on 2 September 1850. The first account of it was in the Boston Atlas of 3 December 1850 which reported a hurricane had caused great destruction of property; on the island of St. Antonio alone more than 600 houses were destroyed by the wind and rain combined; several American vessels were wrecked or damaged at the islands of Sal and Boa Vista. The London Times of 1 February 1851 had additional information. It stated that the storm on the island of San Nicolas was a "fearful hurricane" it began early in the morning before daylight and although it continued until the next morning, all of the damage was done in the first 3 or 4 hours, the wind blowing with such terrific violence during that short period that nearly all the crops and 600 houses were completely destroyed...the whole of the sattara root had been torn up and destroyed by the hurricane.

"In short, the official records are incomplete and although hurricane impacts are rare in the Cape Verde Islands they are not quite the unicorns we have thought to date."


Figure 3. The view from inside the eye of Hurricane Ignacio on Sunday, August 30, 2015, from an Air Force hurricane hunter aircraft. At the time, Ignacio was a Category 4 storm with 140 mph winds. Image credit: Air Force Reserve 403rd Wing. There is also an impressive video from inside the eye on their Twitter page.

Ignacio no longer a threat to Hawaii; Pacific storm show goes on
In the Central Pacific, all watches and warnings have been dropped for Hawaii due to Hurricane Ignacio, which has weakened to Category 2 strength and is expected to skirt the islands over 300 miles to their northeast. Ignacio, will, however, bring dangerous high surf to the islands, and a High Surf Warning for waves of 12 - 20 feet along east shores of the Big Island has been posted. Impressive Category 4 Hurricane Jimena, and Category 4 Hurricane Kilo continue to put on a show over the open waters of the Eastern Pacific, but neither of these storms are a threat to land. All three hurricanes reached Category 4 strength on Saturday and remained there on Sunday morning, the first time since the satellite era began in the 1960s that three simultaneous Category 4 hurricanes had existed in the waters of the Eastern Pacific, east of the Date Line.

Links
Storm surge expert Hal Needham has a Monday morning post on the storm surge potential for Fred in the Cape Verde islands.

Live webcam from Sal Island in Cabo Verde.

Live blog on Fred (in Portugese) from the islands.

We’ll be back with another update on Tuesday.

Jeff Masters and Bob Henson

Hurricane

Fred Heading for Cape Verde Islands; Ignacio Skirting Hawaii

By: Bob Henson and Jeff Masters , 7:00 PM GMT on August 30, 2015

Residents of the Cape Verde islands are going through a rare experience today--a hurricane warning--as Tropical Storm Fred intensifies in the far eastern North Atlantic. As of 2:00 pm EDT, Fred was located near 14.1°N, 20.7°W, or about 195 miles east of the Cape Verde capital city of Praia. Outer rainbands are already beginning to reach the islands. The National Hurricane Center upgraded the storm from Invest 99L to Tropical Storm Fred in its 5:00 am EDT advisory, when it was located at 18.9°W longitude. This made Fred one of just a handful of systems in the last 60 years of satellite monitoring to become tropical storms east of 20°W, as reflected in NOAA’s HURDAT database. Others include eastward-moving, high-latitude Vince, 2005 (subtropical storm at 20.6°W, tropical storm at 19.3°W); Jeanne, 1998 (depression at 17.4°W, tropical storm at 19.4°W); and cyclonically arcing Ginger, 1967 (depression 18.3°W, tropical storm 18.1°W). Several unnamed storms in the HURDAT database are believed to have attained tropical storm strength east of 20.0°W, including:

Storm 3, 1900: 18.5°W
Storm 2, 1927: 19.3°W
Storm 6, 1948: 19.7°W
Storm 5, 1982: 19.5°W
Storm 6, 1988: 18.5°W


Figure 1. MODIS image of Tropical Storm Fred from NASA's Aqua satellite taken at approximately 9:00 am EDT Sunday, August 30, 2015. At the time, Fred had top sustained winds of 50 mph. The Cape Verde islands are outlined in the upper left corner. Image credit: NASA.

Fred is moving into a well-defined pocket of unusually warm sea-surface temperatures (1-2°C above average, or about 27-28°C) that surrounds the Cape Verdes. Since SSTs are often just marginally warm enough to support tropical cyclones near the islands, this warm pocket is a important piece of Fred’s future. Computer models differ enormously in where Fred will be by Thursday; the 120-hour positions from the early-cycle guidance produced at 1200 GMT Sunday vary by more than 500 miles. However, the models agree much more closely on track for the upcoming 48 hours, as Fred is projected to move steadily northwest through the heart of the northern Cape Verde islands. Neither of the high-resolution HWRF nor GFDL models bring Fred to hurricane strength, but statistical models push Fred just beyond that threshold, and NHC is citing these as well as the GFS and ECMWF models in predicting Fred to become a minimal hurricane by midday Monday local time. A hurricane warning is in effect for the Republic of Cabo Verde (the official name for the nation since 2013). To our knowledge, these are the first hurricane warnings on record for the islands; tropical storm warnings were posted for Humberto in 2013 and Julia in 2010 (see below).


Figure 2. Tracks of all tropical cyclones in the vicinity of the Cape Verde islands from the NOAA historical database, which extends back to 1851 (although reports were scanty from the far eastern Atlantic until the satellite era began in the 1960s). Only a handful of tropical depressions (blue lines) and tropical storms (green lines) have affected the islands, and no direct hurricane landfalls (yellow lines) have been recorded. The two yellow tracks labeled above are an unnamed 1892 hurricane and 1998’s Hurricane Jeanne. Image credit: NOAA Historical Hurricane Tracks.

A historic hurricane for the Cape Verdes?
The Atlantic's most terrifying and destructive hurricanes typically start as tropical waves that move off the coast of Africa and pass near the Cape Verde islands. This class of storms is referred to as "Cape Verde hurricanes", in reference to their origin. Despite the fact that the Atlantic's most feared type of hurricanes are named after the Cape Verde islands, the islands themselves rarely receive significant impacts from one of their namesake storms. This is because tropical waves coming off the coast of Africa have very little time to organize into tropical storms before arriving at the Cape Verde islands, which lie just 350 miles west of the African coast. There is no reliable record of any bona fide hurricane having made landfall on the Cape Verde islands (see Figure 2). The closest analogue for Fred is an 1892 storm that bisected the islands, moving between the northern cluster (Ilhas do Barlavento, or windward islands) and the southern cluster (Ilhas do Sotavento, or leeward islands). This 1892 storm reportedly intensified to hurricane strength while passing south of the northwestern Cape Verde islands. Another close approach came from 1998’s Hurricane Jeanne, which reached hurricane strength while passing about 100 miles south of the southern islands. Decaying tropical cyclones in the open Atlantic have occasionally circled southeastward to take a swipe at the Cape Verdes as extratropical storms, but none have reached the island at hurricane strength.

According to EM-DAT, the International Disaster Database, there have been only two deadly tropical cyclones in Cape Verde history. Like Jeanne, they both passed south of the Ilhas do Sotavento. The deadliest was Tropical Storm Fran of 1984, which brushed the southermost islands on September 16 as a tropical storm with 50-mph winds. Fran brought sustained winds of 35 mph and torrential rains to the islands. The rains triggered flash flooding that killed more than two dozen people and caused damages of almost $3 million (1984 dollars.) The other deadly named storm was Tropical Storm Beryl of 1982, which passed about 30 miles south of the southwestern islands on August 29, with 45-mph winds. The storm's heavy rains killed three people on Brava Island, injured 122, and caused $3 million in damage.


Figure 3. Track of Tropical Storm Fran of 1984, which brushed the southwestern Cape Verde islands on September 16 as a tropical storm with 50 mph winds. Torrential rains from Fran killed more than two dozen people in the Cape Verde islands, making it the deadliest storm in their history.

The most recent named storm to affect the islands was Hurricane Humberto of 2013, which passed the islands to the south as a tropical storm. Humberto brought wind gusts of up to 35 mph and heavy rain squalls to the islands, triggering flooding that washed out roads and damaged homes. Hurricane Julia of 2010, the easternmost Category 4 hurricane ever recorded in the Atlantic, passed about 50 miles south of Sao Filipe, on the island of Fogo in the southern Cape Verde islands, as a tropical storm with 45 mph winds, bringing wind gusts of 30 mph and some minor flooding.



Figure 4. Map of the Cape Verde islands (officially the Republic of Cabo Verde). Image credit: Wikimedia Commons/Oona Räisänen (Mysid).


Figure 5. Projected track of Tropical Storm Fred from the 11:00 am Sunday advisory issued by the U.S. National Hurricane Center. Image created with the WU Storm app.

Although all of the Cape Verde islands should prepare for a potential hurricane, Fred’s predicted track would bring the worst impacts across the northern islands (Ilhas do Barlavento), which are even less experienced than the southern islands at dealing with the high winds and heavy rain of tropical cyclones. Fred could make a direct landfall on more than one of the northern islands, as its track will be roughly parallel to this chain. Among the islands in line to feel Fred’s impacts first are the heavily touristed islands of Boa Vista and Sal, which lie on the stronger (right-hand) side of the projected path. Despite their name (which translates to “green cape” in English), the Cape Verde islands have a semi-desert climate, with an average annual rainfall of only around 10 inches, so the torrential rains of a tropical cyclone could have a big impact. Rains of 3-5” are predicted from Fred, with isolated totals of up to 8”. A direct landfall on the northwestern islands could produce not only heavy rain but high winds that would be extremely unusual, if not unprecedented.

Erika’s remnants are pouring on Florida, Cuba
Western Cuba and South Florida are getting a welcome dousing from the remnants of Tropical Storm Erika, which remain disorganized in the far southeast Gulf of Mexico. NHC gives the remnants only a 10 percent chance of regenerating into a tropical cyclone (which would again be named Erika) over the next five days as they slide northward along the west coast of Florida. The heavy rain is not so appreciated along the western half of the peninsula, where the last month has already brought 10-20” of rain and widespread flooding. Tampa needs less than an inch of rain through midnight Monday night to score its wettest July-August since records began in 1890 (old record 28.31” in 1960; total through noon EDT Sunday, 27.48”). Flash flood watches now cover all of central and southern Florida. “Our aquifers are full. There's no more areas for the water to percolate to," Ed Caum, a spokesman for Pasco County's emergency operations center, told the Associated Press on Saturday. Even as a leftover tropical cyclone, Erika may still cause significant damage in Florida over the next day or two. WU contributor Lee Grenci has some early thoughts on why Erika may have left forecast models, and forecasters, so perplexed.


Figure 6. Predicted total rainfall from 1200 GMT (8:00 am EDT) Sunday, August 30, through Friday, September 4. Image credit: NOAA Weather Prediction Center.

Three Category 4 storms lace the Pacific
It’s not every day you see three well-formed Category 4 hurricanes in a row. That’s been the case for the last 24 hours over the North Pacific, where Hurricane Jimena, Hurricane Ignacio, and Hurricane Kilo have made a most impressive trio. All three reached Category 4 strength on Saturday and remained there on Sunday morning, a rare feat.


Figure . Infrared satellite imagery from GOES-West reveals the crisp eyes of Category 4 hurricanes Kilo, Ignacio, and Jimena (left to right) at 1000 GMT (6:00 am EDT) Sunday, August 30. Image credit: NOAA National Hurricane Center.


A Tropical Storm Watch remains in effect for the eastern Hawaiian islands of Maui, Molokai, Lanai, Kahoolawe, and Hawaii (the Big Island), with Ignacio located about 420 miles east of Hilo as of 8:00 am HST (2:00 pm EDT) Sunday. Ignacio is packing winds of 130 mph, but its steady northwest track will take it well north of the islands. Tropical storm force winds extend out to 125 miles (mainly on the north side), so gusty conditions may occur, especially at higher elevations. The Central Pacific Hurricane Center is warning that rainfall amounts of 2-4”, with isolated totals up to 6” at higher terrain, are still possible. There’s much higher confidence that huge waves will be impacting the islands: the CPHC warns of potential life-threatening surf, especially on the Big Island. There’s no record in the modern database of a hurricane this strong tracking north of the islands, so even well-experienced surfers could find themselves in unexpectedly treacherous conditions. Meanwhile, Jimena and Kilo are raging far away from any populated land areas; Jimena is expected to slowly weaken, while the Joint Typhoon Warning Center projects that long-lived Kilo will maintain at least Category 3 strength for the next five days as it undergoes a gradual westward turn through the subtropical North Pacific. Kilo may cross the International Date Line around Tuesday, at which point it would be dubbed Typhoon Kilo. This was the case for Genevieve in 2014 and Ioke in 2006.

We’ll be back with another update on Monday. Enjoy the rest of your weekend!

Bob Henson and Jeff Masters


Hurricane

Erika Dissipates

By: JeffMasters, 2:34 PM GMT on August 29, 2015

Tropical Storm Erika charged into the teeth of Hispaniola's high mountains on Friday night, and emerged from the encounter shattered, without a closed circulation, and is no longer a tropical storm. Measurements on Saturday morning from an Air Force hurricane hunter aircraft failed to find any tropical storm-force winds associated with Erika, and the plane did not find any westerly winds, showing the the storm had degenerated to a tropical wave.


Figure 1. Erika as seen from the International Space Station on Saturday morning, August 29, 2015. Image credit: Scott Kelly.

Erika belted the Dominican Republic and Haiti with torrential rains overnight; a Personal Weather Station (PWS) in Barahona in the Dominican Republic recorded 24.26" in rain between 1 pm Friday and 7 am Saturday. Other nearby locations showed rainfall amounts of 4" or below, so widespread flooding may not have occurred in the Dominican Republic. Erika's worst flooding was on the island of Dominica, where at least 20 people are confirmed dead. Canefield Airport on Dominica recorded 12.62" (320.6 mm) of rain in twelve hours on Wednesday night and Thursday morning from Erika.


Figure 2. Radar-estimated precipitation from Tropical Storm Erika from the San Juan, Puerto Rico radar. Erika's rains were a disappointment in eastern Puerto Rico, where severe to extreme drought conditions have led to drastic water rationing. The capital of San Juan received just 0.22" of rain on Friday, leaving them ten inches below the 33" average rainfall for this time of year.

Forecast for Erika's remains
The remnants of Erika will likely produce total rainfall accumulations of 3 - 6" across portions of the Dominican Republic, Haiti and eastern and central Cuba through Sunday, with 1 - 3" across the Turks and Caicos Islands and southeastern and central Bahamas. Rainfall amounts of 3 to 5 inches, with locally heavier amounts, are possible across southern and central Florida beginning on Sunday. Eastern Cuba, which is suffering its worst drought since at least 1901, is hoping that Erika's rains prove bounteous. Reuters reported Friday that Cuba will begin a two-month cloud-seeding campaign in September over the eastern part of the island in hopes of easing the drought.

The 00Z Saturday (8 pm EDT Friday) runs of our top three models for predicting tropical cyclone genesis had one model--the UKMET--showing regeneration of Erika into a tropical depression along the west coast of Florida on Tuesday. The other two models--the European and GFS models--showed no regeneration. The 8 am EDT Saturday run of the SHIPS model showed wind shear would drop to a moderately high 15 - 20 knots off the west coast of Florida by Tuesday; sea surface temperatures will be a very warm 30°C (86°F). These conditions support potential slow regeneration of Erika.

New tropical wave moving off the coast of Africa
A strong tropical wave (Invest 99L) with plenty of spin and heavy thunderstorm activity moved off the coast of Africa on Saturday, and has the potential to become a tropical depression early in the week as it moves west-northwest or northwest near or over the Cape Verde Islands at 10 - 15 mph. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave the wave 2-day and 5-day odds of development of 50% and 70%, respectively. This disturbance will likely move too far to the northwest to be a threat to the Caribbean islands.


Figure 3. Double feature: Hurricanes Ignacio and Jimena spin across the Pacific on Friday afternoon, August 28, 2015 in this true-color VIIRS image. Image credit: NOAA.

Hurricane Ignacio headed towards a brush with Hawaii
Category 3 Hurricane Ignacio continues to slowly intensify as it heads northwest towards Hawaii. Satellite loops on Saturday morning showed a well-organized storm with plenty of heavy thunderstorms surrounding a prominent eye. Ignacio is over warm waters with light to moderate wind shear, conditions that favor continued intensification. An Air Force hurricane hunter aircraft will investigate Ignacio on Saturday afternoon. Our top models for predicting hurricane tracks continue to show that Ignacio will pass about 300 miles of Hawaii on Tuesday--close enough to bring heavy surf, but not close enough to bring tropical-storm force winds or flooding rains. The 06Z (2 am EDT) Saturday run of the HWRF model showed Ignacio's main rain swath missing Hawaii, with some scattered areas of 2 - 4" of rain affecting portions of the islands. Hawaii probably need not worry about impressive Hurricane Jimena, which intensified rapidly to a Category 4 storm with 150 mph winds overnight. Long-range projections from the GFS and European models continue to show Jimena recurving late in the week well before reaching Hawaii.

There will be a new post on Sunday.

Jeff Masters

Hurricane

Erika’s Path Shifts West; Hawaii Still Watching Ignacio

By: Bob Henson , 5:24 AM GMT on August 29, 2015

Thumbing its nose at some of the world’s most skilled computer models and forecasters, Tropical Storm Erika cruised relentlessly almost due west through the northern Caribbean on Friday, failing to make a long-predicted northwestward turn toward the Bahamas. The National Hurricane Center placed Erika's ill-defined center at 11:00 pm EDT Friday at 18.5°N, 72.9°W, or about 40 miles west of Haiti’s capital, Port-au-Prince. Erika’s top sustained winds were set at 45 mph. Hurricane-hunter flights on Friday had found flight-level winds of as high as 55 knots (more than 60 mph) on the north side of Erika.

Erika has been a troubled-looking system, with thunderstorms mostly straggling behind and south of the center due to upper-level northwesterlies producing vertical wind shear (the difference between upper- and lower-level winds) of about 30 mph. Despite the shear, Erika’s large circulation maintained a broad north-to-south oriented region of intense convection through most of Friday before thunderstorms consolidated toward its north end on Friday evening. Most of the core convection passed just south of Puerto Rico, so by and large, the island missed out on the rain that it so desperately needs. San Juan’s Luis Munoz Marin International Airport reported just 0.25” on Thursday and 0.22” on Friday. Heavy rains swept through the Dominican Republic late Friday: a personal weather station in Barahona reported 23.76" of rain between 1 pm Friday and 2 am Saturday, including 8.80" in one hour from 8 pm to 9 pm Friday. Late Friday night, a very intense cluster of thunderstorms was moving slowly across southwestern Haiti, including Port-au-Prince. Deforestation across much of Haiti makes its hillsides especially vulnerable to flash flooding and mudslides, so it appears a difficult night is unfolding for this troubled nation, which has still not fully recovered from the devastating earthquake that killed more than 100,000 people in 2010. According to the Miami Herald, roughly 65,000 Haitians are still housed in tents, with thousands more in precarious housing near flood- and mudslide-prone areas.


Figure 1. Enhanced infrared image of Tropical Storm Erika from the GOES-East floater satellite, collected at 0415 GMT Saturday, August 27 (12:15 am EDT). Image credit: NOAA/NESDIS.


What next for Erika?
It’s tempting to avoid any speculation on what Erika might do, given its behavior in the last 24 hours. Weaker systems often bear to the left, as lower-level winds envelop a larger part of their circulation, so this could be part of why Erika’s path stayed south of nearly all model predictions from just a day earlier. Erika’s motion has resulted in several westward shifts to the NHC-produced cone of probabilities since Thursday. The forecast issued at 11 pm EDT Friday (see Figure 3 below) brings Erika into the extreme eastern Gulf of Mexico as a minimal tropical storm, paralleling Florida’s west coast on Monday and Tuesday. This is roughly consistent with the 0000 GMT Saturday early-cycle update of Friday’s 1800 GMT dynamical models, which takes Erika northward on an inland track through Florida (see Figure 2 below). Not surprisingly, several statistics-based models, which rely heavily on climatology and extrapolation of recent trends, are now taking Erika on a more westward recurvature, through the heart of the Gulf of Mexico. If Erika were to take such an extreme westward track, some rebound in strength would be plausible, since the storm would largely avoid passing over the landmass of Cuba, and deep-layer shear is expected to relax somewhat over the weekend. However, the dynamical models agree that Erika should begin arcing more northwestward across the spine of Cuba on Saturday, with a slower motion to boot. Such a scenario would quickly sound the death knell for Erika as a tropical cyclone, and this is the most probable solution, although a more westward-curving track can’t be entirely ruled out.



Figure 2. Early-cycle track guidance for 0000 GMT Friday, August 28 (left) and Saturday, August 29 (right), show the leftward shift of most dynamical and statistical models. Early-cycle guidance adjusts the model runs carried out in the previous six hours to account for more recent storm behavior. The GFDL model was a noteworthy outlier to the right at 0000 GMT Friday, while several statistical models were leftward outliers at 0000 GMT Saturday. The National Hurricane Center has an online guide to the models abbreviated on these graphics. Image credit: NCAR/RAL Tropical Cyclone Guidance Project.


Regardless of how it’s classified, Erika can be expected to produce large amounts of rain across Cuba this weekend, and the storm or its remnants will likely drench Florida early next week. Some parts of the state could use the rain, but many other parts--especially the western peninsula, including the Tampa area--are already waterlogged after an very wet few weeks (see Figure 4 below). The potential for a weak but large tropical depression to cause widespread flooding should not be underestimated.



Figure 3. The NHC outlook for Erika issued at 11:00 pm EDT on Friday, August 28.


Figure 4. Rainfall across Florida over the 30-day period ending at 8:00 am EDT on Friday, August 28. Image credit: NOAA Advanced Hydrologic Prediction Service.


Figure 5. Hurricanes Ignacio (left) and Jimena (right), in an enhanced infrared image captured by the GOES-West satellite at 0330 GMT Saturday, August 27 (11:30 pm EDT Friday).

Hawaii may dodge a bullet with Ignacio
Hurricane Ignacio continues to plow across the open waters of the Northeast Pacific, still packing winds of 90 mph as of 11:00 pm EDT Friday. Ignacio is located about 720 miles east-southeast of Hilo, Hawaii. Most of the major dynamical models have been consistent in tracking Ignacio about 100 to 200 miles north of the Hawaiian islands, along a west-northwest track roughly paralleling the islands. For a state inexperienced with hurricanes--the Big Island has never recorded one--this is a nerve-rackingly close forecast, but the consistency across models and across time, and Ignacio’s relatively steady track, lends confidence to the forecast. Ignacio is predicted to pass the islands on Monday and Tuesday as a Category 1 hurricane after intensifying to Category 2 strength over the weekend. Sea-surface temperatures are about 2°C (3.6°F) above average, which will help give Ignacio an injection of energy that’s unusual for this part of the Northeast Pacific.

Category 3 Jimena aims for the top
Hurricane Jimena put on a major show of strength Friday, vaulting to a high-end Category 3 rating. Jimena’s top sustained winds were estimated at 125 mph in the NHC advisory issued at 11:00 pm EDT Friday, which makes it the fourth major hurricane of the season in the Northeast Pacific. Jimena’s mostly westward motion is keeping it at low latitudes, with plenty of warm water at the ready, and upper-level conditions are favorable for even more strengthening. NHC is predicting Jimena to continue rapidly intensifying into Saturday, and it may reach Category 5 strength. Last year’s Hurricane Marie was the most recent Category 5 storm in the Northeast Pacific; the strongest on record was 1997’s Hurricane Linda, which packed sustained winds of 185 mph. Jimena’s track will avoid land areas for at least the next five days, and it appears likely to be picked up by a long-range trough before it would approach Hawaii. Longer-range models continue to suggest that Jimena could maintain tropical characteristics up to an unusually high latitude in the Northeast Pacific more than a week from now.

Jeff Masters will be back with our next update on Saturday.

Bob Henson



Hurricane

Will Erika Survive its Traverse of Hispaniola?

By: Jeff Masters , 3:45 PM GMT on August 28, 2015

Tropical Storm Erika is headed into the teeth of Hispaniola's 10,000-foot high mountains, as the storm marches west-northwest at 17 mph, spreading torrential rains and sustained winds of 50 mph along its path. The biggest danger of the storm to the islands is heavy rainfall; according to the Antigua Met Service, Canefield Airport on Dominica recorded 12.62" (320.6 mm) of rain in twelve hours on Wednesday night and Thursday morning from Erika, and the resulting heavy flooding has killed at least twelve people.


Video 1. Floodwaters rage through a street on Dominica island in the Caribbean on Thursday, August 27, 2015, after Tropical Storm Erika dumped 12+" of rain on the island.

An Air Force hurricane hunter aircraft was in the storm Friday morning, and found Erika continued to have a large area of tropical-storm force winds up to 50 mph to the southeast of the center. A Personal Weather Station (PWS) in southwest Puerto Rico at Barrio Hoconuco at higher elevation recorded wind gusts up to 56 mph Friday morning. Punta Cana, on the eastern tip of the Dominican Republic, had a wind gust of 40 mph at both 10 am and 11 am AST. Rainfall as of 11 am EDT Friday over Puerto Rico had mostly been below 2", according to estimates from the San Juan radar.


Figure 1. MODIS image of Erika from NASA's Aqua satellite taken at approximately 11 am EDT Friday August 28, 2015. At the time, Erika had top sustained winds of 50 mph. Image credit: NASA.

Satellite loops on Friday morning showed that Erika continued to be disorganized in the face of dry air and wind shear. There is not much heavy thunderstorm activity on the storm's north side, where there was dry air from the Saharan Air Layer, though there was a vigorous area of heavy thunderstorms on its southeast side. These thunderstorms did not change much in intensity or areal coverage on Friday morning. Wind shear due to upper-level winds out of the west was a high 20 - 25 knots, and this shear was driving dry air on the northwest side of Erika into its core, disrupting the storm.


Figure 2. Latest long-range radar image of Tropical Storm Erika from the San Juan, Puerto Rico radar.


Figure 3. Predicted rainfall for Erika for the 126-hour period ending at 8 am EDT September 2, 2015, from the 06Z (2 am EDT) Friday August 28, 2015 run of the HWRF model. This rainfall swath is likely displaced too far to the east, and will probably be centered directly over Florida. Rainfall amounts of 4 - 8" can be expected in many areas along Erika's path, with a few areas of 8+" (bright yellow colors.) Image credit: NOAA.

Will Erika survive Hispaniola?
Erika's battle against dry air and high wind shear has caused the center of the storm to reform several times to the south of its original position, closer to the storm's heaviest thunderstorms. These southward shifts mean that Erika is now poised to track directly over mountainous Hispaniola island, whose highest peak exceeds 10,000 feet in height. This encounter will not go well for Erika, particularly since wind shear will remain a high 15 - 25 knots during the traverse, and dry air will continue to wrap into Erika's circulation during the crossing. These combined factors could lead to Erika's dissipation by Saturday morning. The traverse of the island may also cause the center to reform to the west of the island, which would then mean that Erika would encounter some of the high terrain of eastern Cuba. If Erika survives into Saturday morning, which I give a 50% chance of occurring, the storm may have time to intensify into a strong tropical storm with 60 mph winds before hitting South Florida. If Erika dissipates over Hispaniola Friday night, the storm could still reorganize into a minimal-strength tropical storm with 40 mph winds before encountering South Florida. The upper low over Cuba that is bringing high wind shear to Erika today is forecast to weaken on Sunday, which should cause wind shear to drop to the moderate range, 10 - 15 knots, which would allow some modest strengthening of Erika. A trough of low pressure will turn Erika to the north on Monday, and it is possible this turn will occur just west of the Florida, bringing Erika northwards along the west coast of Florida and into the Florida Panhandle by Tuesday--as suggested by the 00Z Friday (8 pm EDT Thursday) runs of the European and UKMET models. Regardless, much of Florida can expect heavy flooding rains from Erika Sunday through Tuesday.


Figure 4. Drought conditions in the Caribbean during July 2015 as estimated via satellite. Drought is indexed here using the 6-month Standard Precipitation Index (SPI), a measure of how much rainfall has occurred in the previous six months compared to average. Drought is common in the Caribbean during El Niño years, due to an atmospheric circulation that brings plenty of dry, sinking air and high pressure to the region. The last major drought in the Caribbean was in 2010, which also had an El Niño event. Image credit: NOAA Global Drought Portal.

Erika's rains to help alleviate record Caribbean heat and drought
Record heat and drought has been widespread over the Caribbean this summer, with the worst drought conditions occurring over Haiti, Eastern Cuba, Nicaragua, Panama, Colombia, Venezuela, Honduras, and Costa Rica. Reuters reported today that Cuba will begin a two-month cloud-seeding campaign over the eastern part of the Caribbean island in hopes of easing its worst drought since at least 1901. The dry conditions and associated atmospheric circulation that has brought warm, sinking air and high pressure to the region has led to many Caribbean cities recording their all-time highest temperatures on record. According to weather records researcher Maximiliano Herrera, on Thursday, the Observatory in Santo Domingo, the capital of the Dominican Republic, hit 99°F (37.2°C), the hottest temperature ever recorded in the city (previous record: 37.0°C on August 17, 1947.) This record high was aided by the fact that dry, sinking air due to the outflow from Tropical Storm Erika was over the Dominican Republic. Another big factor in the yesterday's record high, and the record highs all across the Caribbean this year, is the fact that the year-to-date period of 2015 has been the warmest on record for the globe as a whole. Here is Mr. Herrera's list of cities in countries bordering the Caribbean that have set all-time heat records this year:

Cuba
Cienfuegos (Cuba) max. 37.0°C July 6
Jucaro (Cuba) max. 36.8°C July 10
Jucaro (Cuba) max. 37.0°C July 28
Contramaestre (Cuba) max. 38.2°C July 29
Isabel Rubio Airport (Cuba) max. 36.3°C July 29
Indio Hatuey (Cuba) max. 38.1°C July 30
Havana (Cuba), max. 37.0°C, April 26
Holguin (Cuba), max. 38.7°C, April 26
Guaro (Cuba), max. 38.0°C, April 26
Contramaestre (Cuba), max. 37.7°C, April 27
Velasco (Cuba), max. 38.6°C, April 28
Ciego de Avila (Cuba), max. 38.0°C, April 28
Puerto Padre (Cuba), max. 38.4°C, April 29
Punta Lucrecia (Cuba), max. 37.3°C, April 29
Nuevitas (Cuba), max. 38.5°C, April 30

Colombia
Riohacha (Colombia) max. 40.6°C July 13
Cartagena, Colombia, max. 40.4°C,  June 24
Santa Marta, Colombia, max, 38.6°C, June 24
Arjona, Colombia, max, 40°C, June 24
Urumitia, Colombia, max, 42.0°C, June 27
Riohacha, Colombia, max, 40.0°C, June 29

Mexico
Merida (Mexico), max. 43.6°C, April 26

Honduras
Tela (Honduras), max. 40.6°C, April 28

Venezuela
Coro (Venezuela), max. 43.6°C, April 29 (New all-time national record high for Venezuela)

Dominican Republic
Santo Domingo (Dominican Republic), 37.2°C, August 27

U.S. Virgin Islands
Charlotte Amalie (U.S. VI), 35°C (95°F), August 1 (all time high for the station and the U.S. Virgin Islands)

New tropical wave moving off the coast of Africa
A strong tropical wave will move off the coast of Africa on Saturday, and has the potential to become a tropical depression next week as it moves west-northwest near or over the Cape Verde Islands at 10 mph. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave the wave 5-day odds of development of 30%. This wave will likely move too far to the northwest to be a threat to the Caribbean islands.

Hurricane Ignacio a threat to Hawaii
Hurricane Ignacio continues to slowly intensify in the waters to the east-southeast of Hawaii, and could pass within 200 miles of Hawaii on Tuesday. Satellite loops on Friday morning showed that Ignacio had an impressive area of heavy thunderstorms, but no eye was apparent. Ignacio is over warm waters with light to moderate wind shear, conditions that favor continued intensification. Hawaii should also keep an eye on Hurricane Jimena, which is intensifying rapidly in the waters over 1500 miles east southeast of the islands. Jimena is expected to top out at Category 4 strength on Sunday, but eventually recurve to the north well before reaching Hawaii. In about ten days, Jimena has the potential to traverse ocean areas off the coast of Northern California where no tropical storm has ever been observed.

Bob Henson or myself will have another Erika post later today.

Jeff Masters

Hurricane

Erika Nears Puerto Rico with High Wind, Heavy Rain

By: Bob Henson , 3:19 AM GMT on August 28, 2015

Still poorly organized--but already deadly, and growing in size--Tropical Storm Erika is likely to cause problems in Puerto Rico on Thursday night into Friday. At least four people were killed by mudslides in the wake of Erika’s passage over Dominica. The capital of Roseau, on the island’s southwest coast, was hard-hit with major river and street flooding. According to weather.com, Canefield Airport on Dominica received 12.64” of rain between 2:00 am and 2:00 pm EDT Thursday, as the island was struck by an intense blow-up of thunderstorms (convection) on the south edge of Erika’s ill-defined circulation. In a similar fashion, the typical nighttime intensification of convection could bring torrential rain to Puerto Rico and the Virgin Islands late Thursday night and Friday. In a local statement issued at 5:32 pm AST (4:32 pm EDT) on Thursday, the National Weather Service in San Juan called for widespread 4-8” totals across Puerto Rico and nearby islands, with up to 12” possible. Although the islands are in desperate need of rain to assuage an intense drought, Erika may deliver too much of a good thing.


Figure 1. NWS radar at San Juan, Puerto Rico, showed heavy rain from Erika just southeast of the island at 10:25 pm EDT Thursday, August 27. Overlaid is the location of Erika’s poorly defined center, as located by the National Hurricane Center at 8:00 pm EDT, together with previous locations (right) and the NHC-projected path (left). Image from the tropical tracking feature on WU’s Storm app.


As expected, persistent northwesterly shear kept Erika from intensifying on Thursday, with most of the storm’s convection shunted south and east of Erika’s center of circulation. At one point, a center of circulation was clearly visible beyond the northwest edge of the canopy of upper-level clouds associated with the convection--not a sign of a healthy tropical storm. Convection was redeveloping near this center on Thursday evening, though, and Erika’s overall circulation continued to expand in size, with outflow evident in most directions away from the northwesterly shear.


Figure 2. Enhanced infrared image of Tropical Storm Erika from the GOES-East floater satellite, collected at 0115 GMT Friday, August 27 (9:15 pm EDT Thursday). Image credit: NOAA/NESDIS.

At 11:00 pm EDT, the National Hurricane Center placed Erika’s center (perhaps a large center with several smaller swirls) at 16.6°N, 65.3°W, or about 135 miles south-southeast of San Juan, Puerto Rico. Erika was moving west at about 17 mph, a bit to the left of what had been predicted. If Erika takes the west-northwest tack still predicted for later tonight and tomorrow by NHC, its poorly defined center will nick the southwest corner of Puerto Rico early Friday and graze the northeast coast of Hispaniola later Friday. Such a track might result in only minor disruption to Erika’s intensity: the storm is not too tightly organized to begin with, and its large structure could help it recover fairly quickly once back over open water. However, if Erika continues on a more westward trajectory, it could spend much more time over the higher terrain of Haiti and the Dominican Republic, which could cause it to weaken into a tropical depression or open wave. Such a track could also produce devastating rainfall over these two nations.

Erika’s future
If Erika makes it into the Bahamas relatively intact--and that remains a very big "if"--we could be dealing with it for days to come. Conditions over the Bahamas will be quite favorable for strengthening, with sea-surface temperatures above 30°C (86°F) toward the northwest Bahamas. Wind shear is also projected to drop to the 10 - 20 mph range over the weekend. Erika’s fairly large size will tend to reduce its ability to spike or plummet in intensity within hours, the way Hurricane Danny did last week. However, depending on its eventual track, Erika may have at least a couple of days to gather strength over toasty waters with relatively light wind shear.


Figure 3. The NHC outlook for Erika issued at 11:00 pm EDT on Thursday, August 27.

Erika’s strength and track are increasingly difficult to predict beyond this weekend. If anything, computer models on Thursday offered an even broader long-term palette of possibilities for Erika than they did on Wednesday. The official NHC forecast from 11:00 pm EDT Thursday is consistent with the model blend from 1800 GMT Thursday, which was toward a position near the Florida coast (either just offshore or just inland) by Monday, with a general north-northwest motion thereafter. A weaker Erika would be steered more by lower-level easterly flow, perhaps moving over or near Hispaniola and on toward South Florida and up the peninsula as little more than a tropical storm. A stronger system would be more inclined to stay offshore, perhaps heading north through the Bahamas and toward the Carolinas as a Category 1 or stronger hurricane. The “early cycle” model guidance from 0000 GMT Friday, which adjusts model forecasts that were issued six hours earlier to account for more recent storm behavior, leans more toward the weaker, more westerly solutions. The 0000 GMT Friday run of the GFS model will incorporate extensive data on the steering flow around Erika collected on Thursday by the NOAA Gulfstream IV surveilliance aircraft.

It appears unlikely Erika will recurve sharply northeast, as upper-level ridging will become more pronounced across the eastern United States and the western North Atlantic as we move into early next week (see Figure 4 below). This means Erika could be a slow-moving, rain-dumping system wherever it ends up beyond the five-day forecast window. It’s important to keep in mind that roughly a third of all tropical cyclones tracked by NHC lie outside the five-day position shown in the forecast “cone”, which is based on average errors over time. Models may swing to the west or east and back again with every six-hour set of runs over the next couple of days, but the official NHC outlooks do an excellent job of smoothing out the inevitable swings from model to model and run to run. Tracking these outlooks over time will give you a very solid sense of how the model forecasts as a whole are trending. The bottom line for now: residents from Florida to North Carolina should be ready for the potential of tropical storm or hurricane impacts at some point during the next 5 to 7 days.


Figure 4. Evolution of the flow at 500 millibars, or about 19,000 feet (black contours) and that flow’s departure from the seasonal average (blue and orange tints) from the ECWMF model’s ensemble average produced at 1200 GMT on Thursday, August 27. Left panel is the starting point (1200 GMT Thursday); right panel is the 120-hour forecast for 1200 GMT on Tuesday, September 1, by which point the trough now aligned along the U.S. East Coast (blue tint in left panel) has been replaced by a sprawling ridge (orange tint in right panel) that would block any rapid northward motion of Tropical Storm Erika or its remnants. Image credit: Levi Cowan, www.tropicaltidbits.com.


Figure 5. A multispectral (RGB) satellite image of Hurricane Storm Ignacio, collected by the GOES-West floater satellite at 0130 GMT on Friday, August 28. Image credit: NOAA/NESDIS.


Figure 6. The NHC outlook for Ignacio issued at 11:00 pm EDT on Thursday, August 27.

Ignacio may threaten Hawaii early next week
Residents of Hawaii need to keep close tabs on Hurricane Ignacio, which was packing top sustained winds of 90 mph as of 5:00 pm HST (11:00 pm EDT) Thursday. Now located just under 1000 miles east-southeast of Hilo, Ignacio is on a fairly straightforward west-northwest course that should continue over the next several days. Simply extrapolating Ignacio’s track would bring the hurricane very close to Hawaii by Monday or Tuesday. Hurricanes on such a course typically weaken before they reach the islands, traveling over surface waters near the threshold for tropical development of around 26°C (79°F). In this case, Ignacio is getting a boost from favorable upper-level conditions as well as unusually warm sea-surface temperatures, roughly 2°C (3.8°F) above average for this time of year. NHC intensifies Ignacio to just short of Category 3 strength by this weekend, then gradually weakens it starting on Monday. The official forecast calls for Hurricane Ignacio to pass about 100 miles north of Hilo on Tuesday as a Category 1 storm with sustained winds of 80 - 90 mph. No hurricane has ever been officially recorded on the Big Island of Hawaii--the strongest event on record was Tropical Storm Iselle in 2014--so today’s forecast is noteworthy in itself. Beyond the five-day period, models are tending to bend Ignacio’s track slightly leftward, as upper-level ridging strengths to the north. The predicted steering currents and the very warm SSTs make Ignacio a force to be reckoned with, and all of the Hawaiian islands should take this hurricane seriously. Large swells will become a near-certainty over the next few days.


Figure 7. An infrared satellite image of Tropical Storm Jimena, collected by the GOES-West floater satellite at 0130 GMT on Friday, August 28. Image credit: NOAA/NESDIS

Jimena may become a Category 4 powerhouse
On the heels of Ignacio, Tropical Storm Jimena is quickly strengthening, with sustained winds up to 70 mph as of the 11:00 pm EDT advisory on Thursday. Located almost 1000 miles west of Cabo San Lucas, Jimena should be a hurricane by Friday morning as it moves steadily westward. Conditions are even more favorable for Jimena than for Ignacio, and satellite images already show a well-structured storm, with stout, far-reaching spiral bands and a tight core of intense convection already in place. The SHIPS Rapid Intensification Index, which quantifies the odds that a hurricane will grow quickly, shows a 50-50 chance that Jimena’s top winds will increase by at least 30 knots (35 mph) over the next 24 hours. The official NHC forecast rolls with this scenario, making Jimena a Category 4 hurricane by Sunday. It’s too soon to know whether Jimena will pose a threat to Hawaii, although it will remain well east of the islands for at least the next five days.

Jeff Masters will have our next update on Friday morning. See also Steve Gregory’s excellent overview of Erika, posted earlier this afternoon. Another WU contributor, Weather Channel hurricane specialist Bryan Norcross, weighed in today on why it’s crucial not to take any particular forecast for Erika as gospel at this point.

Bob Henson

Hurricane

Erika Floods Dominica; Major Uncertainties on Potential U.S. Impact

By: Jeff Masters , 2:23 PM GMT on August 27, 2015

Heavy rains and strong gusty winds are sweeping through much of the northern Lesser Antilles Islands today as Tropical Storm Erika heads west at 16 mph. An Air Force hurricane hunter aircraft was in the storm Thursday morning, and at 8 am EDT found Erika's top surface winds were 50 mph, with a central pressure nearly unchanged at 1004 mb. The aircraft also found the center had jogged to the south by about 20 miles between 5 am and 8 am, a position which put the center of circulation closer to Erika's heaviest thunderstorms. According to the Antigua Met Service, Canefield Airport on Dominica recorded 8.86" (225 mm) of rain Wednesday night from Erika, and heavy flooding has been observed on the island. The Guadaloupe Airport has recorded 1.18" of rain today as of 9 am EDT, with a peak wind gust of 36 mph. A Personal Weather Station (PWS) on St. Thomas in the Virgin Islands at higher elevation recorded a wind gust of 47 mph Thursday morning. Erika's tropical storm-force winds were on the east side of the storm, so the strongest winds of the storm will not occur in the Virgin Islands until Thursday afternoon. Satellite loops on Thursday morning showed that Erika continued to be disorganized in the face of dry air and wind shear. There is not much heavy thunderstorm activity on the storm's north side, where there was dry air from the Saharan Air Layer, and Erika had only a modest area of heavy thunderstorms on its east side. These thunderstorms did not change much in intensity or areal coverage on Thursday morning. Wind shear due to upper-level winds out of the west was a high 20 knots, and this shear was driving dry air on the northwest side of Erika into its core, disrupting the storm. Sea surface temperatures (SSTs) were favorable for development, though—near 28.5°C (83°F).


Figure 1. Heavy flooding on Dominica on Thursday morning, August 27, 2015 from Erika. Image posted via Twitter to The Weather Channel.


Figure 2. Radar image of Tropical Storm Erika at 9:45 am EDT August 27, 2015, from the Guadaloupe/Martinique radar. Image credit: Meteo France.

Erika's impact on the Caribbean islands
Erika's expected rainfall amounts of 3 - 5" in the islands may cause some isolated flash flooding and mudslides, but should help alleviate severe to extreme drought conditions some of the islands are experiencing. Puerto Rico, for example, desperately needs the rain--water restrictions are in place in the capital of San Juan, where hundreds of thousands of residents receive water only two days per week.


Figure 3. MODIS image of Erika from NASA's Aqua satellite taken at approximately 10 am EDT Thursday August 27, 2015. At the time, Erika had top sustained winds of 50 mph. Image credit: NASA.

Erika's potential impact on the Bahamas and U.S. East Coast
The main words to describe the forecast for Erika for its impact on the Bahamas and the U.S. East Coast: much higher uncertainty than usual. Our main models for predicting hurricane track and intensity have been showing large differences run-to-run, and with each other. Erika could dissipate before reaching the U.S., could hit as a hurricane, or could miss the coast entirely, depending upon which model you believe. In the short term, we are pretty sure Erika will struggle, though. The 8 am EDT Thursday run of the SHIPS model predicted that wind shear would increase to a high 25 - 30 knots Thursday night though Friday afternoon, due to an upper-level low that is expected to remain near eastern Cuba through Friday. At that time, Erika will be passing just north of Puerto Rico and the Dominican Republic, and it is possible that the increasing shear and interaction with the high terrain of Hispaniola and Puerto Rico will cause Erika to dissipate. This morning's southwards jog by 20 miles may turn out to bring the center of Erika close enough to Hispaniola on Friday to significantly weaken the storm. However, dissipation of Erika is looking less likely than it did when I wrote yesterday's post, since Erika has managed to generate enough heavy thunderstorm activity to moisten its environment and help protect it from dry air and high wind shear. If Erika survives into Saturday morning, Erika could intensify into a hurricane that might affect the Bahamas and U.S. East Coast. Sea surface temperatures (SSTs) warm to near 29°C (84°F) in the Southeast Bahamas, and are 30°C (86°F) in the Northwest Bahamas, which would potentially provide plenty of extra fuel for intensification. The upper low over Cuba is forecast to weaken on Saturday, which should cause wind shear to drop to the moderate range, 10 - 15 knots, as Erika traverses the Bahamas, which should allow the storm to grow more organized. A weak trough of low pressure capable of turning Erika to the north will set up shop along the U.S. East Coast late this week, and it is possible that Erika will be strong enough to get picked up by this trough and turn to the north just before reaching the Florida coast on Sunday evening or Monday morning. If Erika stays weak, the storm is more likely to plow into Florida. To further complicate matters, steering currents may collapse next week, allowing Erika to wander offshore the Southeast U.S. coast for many days. I give a 20% chance that Erika will end up being a landfalling hurricane for the U.S. East Coast, a 20% chance storm will dissipate by Saturday, a 30% chance the storm will be too weak and disorganized to have time to organize into a hurricane before hitting the U.S. East Coast, and a 30% chance that Erika will miss the U.S. East Coast entirely.


Figure 4. The 00Z Thursday (8 pm EDT Wednesday) runs of the European and GFS models had similar predictions for the intensity and track of Erika for 5 pm Sunday August 30, 2015, showing a weak tropical storm off the coast of Florida. Image taken from our wundermap with the “Model Data” layer turned on.


Figure 5. Forecasts of the track of Tropical Storm Erika from the 00Z Thursday (8 pm EDT Wednesday) run of the GFS model from the twenty members of the GFS Ensemble model. The GFS Ensemble takes the operational version of the GFS model and makes twenty different runs of it at lower resolution with slightly different initial conditions to generate an ensemble of possible forecasts. Compared to Wednesday's runs of the GFS Ensemble, today's run has many more members showing recurvature out to sea with no impact on the U.S. The operational high-res version of the GFS is the white line.

Hurricane Ignacio a threat to Hawaii
Hawaii needs to closely watch Hurricane Ignacio, which is gathering strength in the waters about 1200 miles east-southeast of the islands. Satellite loops on Thursday morning showed that Ignacio had an impressive area of heavy thunderstorms that were growing in intensity and areal extent, and Ignacio is over warm waters with light to moderate wind shear, conditions that favor possible rapid intensification. The 00Z Thursday (8 pm EDT Wednesday) run of one of our top models for forecasting hurricane tracks, the European model, showed Ignacio passing within 100 miles of Hawaii on Tuesday. Hawaii should also keep an eye on Tropical Storm Jimena, which is gathering strength in the waters over 1500 miles east southeast of the islands.

Bob Henson will have another Erika post later today.

Jeff Masters

Hurricane

Erika Hangs on, With Big Uncertainties Ahead

By: Bob Henson , 1:36 AM GMT on August 27, 2015

After going through a rather sickly phase during the day on Wednesday, Tropical Storm Erika began to rally after sunset, a sign that it may yet survive--and perhaps eventually thrive--en route to a possible U.S. East Coast landfall. As of 8:00 pm EDT Tuesday, Erika was located at 16.7°N, 59.5°W, or about 150 miles east of Antigua. Erika remains a relatively weak tropical storm, with top sustained winds of around 45 mph. An Air Force hurricane hunter measured peak sustained winds at flight level off41 knots (47 mph) at 2339 GMT (7:30 pm EDT). Tropical storm watches and warnings are in effect throughout the Leeward Islands, but the main effect of Erika--heavy rains--is largely a blessing for this drought-parched region.


Figure 1. Infrared imagery from NOAA’s GOES-East floater satellite shows the intensification of Erika’s core showers and thunderstorms between 1815 GMT Wednesday, August 26, and 0045 GMT Thursday, August 27. Convection over tropical cyclones often intensifies during the night. Image credit: NOAA/NESDIS.

Northwesterly wind shear and pockets of mid-level dry air kept Erika from intensifying on Wednesday. The storm’s main low-level center of circulation was displaced well northwest of the showers and thunderstorms (convection), a major hindrance to any strengthening. The typical nighttime development of convection will provide Erika another chance to either refocus convection over its main center of circulation or to develop a new center. Either option could give Erika an infusion of strength and help it to survive another day’s battle with wind shear on Thursday night. On its current track, Erika should pass just northeast of Puerto Rico on Thursday night. Wednesday’s models edged northward with Erika’s track, so it looks increasingly unlikely that Erika’s future strength will be dented too much by interaction with the mountains of Hispaniola or Cuba. However, the unrelenting wind shear will actually intensify from Thursday into Friday, according to several models. Erika has a fairly large, well-structured circulation that should give it a fighting chance, although it’s still possible that the storm will degenerate into an open wave by the weekend.


Figure 2. The NHC forecast for Erika as of 2100 GMT (5:00 pm EDT) on Wednesday, August 26.

Erika and the East Coast: A big question mark
The next big question is how much strength Erika will gain over the Bahamas, assuming it survives to that point. Sea-surface temperatures are about 1°C (1.8°F) warmer than average from the Bahamas to South Florida, running in the 86°F to 90°F range, and Erika should encounter weaker wind shear by this point. The official NHC forecast from 5:00 pm EDT Wednesday (Figure 2) brings Erika into eastern Florida as a Category 1 hurricane on Monday. This is a classic case where the intensity prediction (75 mph) may not be the most likely outcome in itself, but rather a split-the-difference compromise. As NHC notes in its forecast discussion, this outlook incorporates the chance that Erika could arrive at the Bahamas too disrupted to strengthen (a real possibility) or could intensify more than predicted (another real possibility). One significant change on Wednesday was that the most reliable long-range models are now consistently calling for Erika to approach U.S. shores as a hurricane. Both the 1200 GMT ECMWF run and the 1800 GMT GFS run bring Erika north and northeast just off the Florida and Carolina coastlines as an intensifying hurricane. The now-bullish GFS has joined the chorus of the HWRF and GFDL models, which have insisted for more than a day that Erika could intensify dramatically while over the Bahamas. We need to keep in mind that if Erika were to become a north- or northeastward-moving hurricane along the East Coast, only a slight change in track angle could have a major influence on coastal impacts. It also remains possible that the upper-level trough existing the East Coast this weekend will not be strong enough to pull Erika with it, which would leave behind a slower-moving, more erratic storm that could stall offshore or cause multiple days of trouble inland (see Figure 3 below). And this all presumes that Erika will even make it to the Bahamas in solid enough shape to strengthen.

We shouldn’t put too much stock in any individual model run, as there is plenty of variety in the outcome even within the ECMWF and GFS ensembles (which simulate the weather a number of times, with slight variations in the starting point to account for observational uncertainty). The overarching message at this point is that residents from Florida to North Carolina should be aware of the possibility for a significant hurricane during the early to middle part of next week, and plan and prepare accordingly.

Jeff Masters will be back with a full update on Thursday. See also Steve Gregory’s update from Wednesday afternoon.

Bob Henson



Figure 3. Upper-atmospheric flow at the 500-mb level (about 18,000 feet) projected by the GFS model run from 1800 GMT on Wednesday, August 26, for Tuesday afternoon, September 1. In this scenario, a strengthening upper-level ridge over the eastern U.S. and a weak upper low center over Louisiana (a left-behind remnant of the upper-level trough that now extends from the northeast U.S. to the Gulf states) would put Erika in a weak steering pattern just off the southeast U.S. coast.

Hurricane

Tropical Storm Erika a Potential Threat to the Bahamas and U.S. East Coast

By: Jeff Masters , 3:33 PM GMT on August 26, 2015

Tropical storm warnings are flying for Puerto Rico, the Virgin Islands, and much of the northern Lesser Antilles Islands as Tropical Storm Erika speeds westwards at 17 mph. An Air Force hurricane hunter aircraft was in the storm Wednesday morning, and found Erika's winds had increased slightly, with top surface winds up to 45 mph from their previous 40 mph. Erika's tropical storm-force winds were all on the east and northeast sides of the storm. Satellite loops on Wednesday morning showed that Erika continued to be disorganized in the face of dry air and wind shear. The low-level center was partially exposed to view on the north side, the location of plenty of dry air from the Saharan Air Layer. Erika had only a modest area of heavy thunderstorms on its east side, and these thunderstorms did not change much in intensity or areal coverage on Wednesday morning. Wind shear due to upper-level winds out of the west was a moderate 10 - 20 knots, and this shear was driving dry air on the northwest side of Erika into its core, disrupting the storm. Sea surface temperatures (SSTs) were favorable for development, though—near 28°C (82°F).


Figure 1. Latest satellite image of Tropical Storm Erika.


Figure 2. NOAA Hurricane Hunter aircraft N43RF flew into Tropical Storm Erika on Tuesday evening, and the crew captured this rainbow SE of the center. Image credit: @NOAA_HurrHunter.

Erika's impact on the Caribbean islands
Erika's expected rainfall amounts of 3 -5" in the islands may cause some isolated flash flooding and mudslides, but should help alleviate severe to extreme drought conditions some of the islands are experiencing. Puerto Rico, for example, desperately needs the rain--the remnants of Danny brought 0.48" of rain to the capital of San Juan on Tuesday, but the city is still 10" below the normal 33" of rain it should have received by this point in the year. Water restrictions are in place in the city, whereby hundreds of thousands of residents receive water only two days per week.

Erika's potential impact on the Bahamas and U.S. East Coast
The 8 am EDT Wednesday run of the SHIPS model predicted that wind shear would remain in the moderate range through Thursday, then increase to a high 25 - 30 knots Thursday night though Friday afternoon, due to an upper-level low that is expected to remain near eastern Cuba through Friday. At that time, Erika will be passing just north of Puerto Rico and the Dominican Republic, and it is quite possible that the increasing shear and interaction with the high terrain of Hispaniola and Puerto Rico will cause Erika to dissipate, as predicted by the Wednesday morning runs of the GFS model. If Erika survives into Friday afternoon, the potential for a dangerous storm that will affect the Bahamas and U.S. East Coast increases substantially. Sea surface temperatures (SSTs) warm to near 29°C (84°F) in the Southeast Bahamas, and 30°C (86°F) in the Northwest Bahamas. This will provide plenty of extra fuel for intensification. The upper low over Cuba is forecast to weaken on Saturday, which should cause wind shear to drop to the moderate range, 10 - 15 knots, as Erika traverses the Bahamas, allowing the storm to take advantage of the warmer ocean temperatures are grow more organized. A weak trough of low pressure capable of turning Erika to the north will set up shop along the U.S. East Coast late this week, and it is possible that Erika will be strong enough to get picked up by this trough and turn to the north just before reaching the Florida coast on Sunday evening or Monday morning, as suggested by the Wednesday morning runs of two of our top models for predicting hurricane tracks, the UKMET and HWRF models. If Erika stays weak, the storm is more likely to plow into Florida, as predicted by the Wednesday morning run of the European model. I give a 20% chance that Erika will end up being a landfalling hurricane for the U.S. East Coast, a 40% chance storm will dissipate by Saturday, and a 30% chance the storm will be too weak and disorganized to have time to organize into a hurricane before hitting the U.S. East Coast. There is also a small chance (10%) that Erika could miss the U.S. East Coast--a situation that would most likely arise if Erika quickly organizes into a hurricane by Saturday, and thus "feels" the steering influence of winds higher up in the atmosphere, forcing the storm to recurve out to sea.


Figure 3. The 00Z Wednesday (8 pm EDT Tuesday) runs of the European and GFS models had two very different predictions of the intensity of Erika for 5 pm Sunday August 30, 2015. The European model showed Erika as a strong tropical storm just off the coast of Florida (purple colors = winds of at least 58 mph), while the GFS model showed Erika merely as a strong tropical wave with no closed circulation. Image taken from our wundermap with the “Model Data” layer turned on.


Figure 4. Forecasts of the track of Tropical Storm Erika from the 00Z Wednesday (8 pm EDT Tuesday) run of the GFS model from the twenty members of the GFS Ensemble model. The GFS Ensemble takes the operational version of the GFS model and makes twenty different runs of it at lower resolution with slightly different initial conditions to generate an ensemble of possible forecasts. As we can see, there are a wide variety of possible solutions. The operational high-res version of the GFS (white line) shows Erika moving over South Florida. Most of the GFS ensemble members keep Erika weak, resulting in a more southwards track for the storm than our other top models are showing.

Tropical Storm Ignacio a threat to Hawaii
Hawaii needs to closely watch Tropical Storm Ignacio, which is gathering strength in the waters about 1400 miles east-southeast of the islands. Satellite loops on Wednesday morning showed that Ignacio had an impressive area of heavy thunderstorms that were growing in intensity and areal extent, and Ignacio is over warm waters with moderate wind shear, conditions that favor possible rapid intensification. Our two top models for forecasting hurricane tracks, the European and GFS models, both showed Ignacio passing within 200 miles of Hawaii on Tuesday. Hawaii should also watch Invest 96E, which is close to tropical depression status over 1500 miles east-southeast of the islands. 96E will track towards the Hawaiian Islands over the next seven days, and could be a long-range threat late next week.

Bob Henson will have another Erika post later today. Hope you caught his retrospective look back at Hurricane Katrina on #WUTV last night!

Jeff Masters

Hurricane

Slowly Organizing Tropical Storm Erika Prompts Caribbean Watches, Warnings

By: Bob Henson , 6:01 AM GMT on August 26, 2015

Tropical Storm Erika is not looking especially fearsome tonight, although it remains a longer-term threat for parts of the southeast U.S. coast and a more immediate concern for the Caribbean. At 11:00 pm EDT Tuesday, the National Hurricane Center placed Erika at 16.0°N, 54.4°W, or about 500 miles east of Antigua. WIth top sustained winds still at minimum tropical-storm strength (40 mph), Erika was moving just north of due west at around 18 mph. That healthy clip has prompted tropical storm warnings for Anguilla, Saba, St. Eustatius, and St. Maarten, with tropical storm watches now in place for many other nearby islands, including Puerto Rico and the U.S. Virgin Islands.

Northerly wind shear on Tuesday pushed Erika’s showers and thunderstorms largely to the south of the partially exposed low-level center of circulation. Over the last several hours, a new cluster of storms has popped up closer to the low-level center, perhaps a sign of better organization to come. Erika has a large circulation, evident in the pockets of convection located far away from its center. One large cluster developed more than 500 miles west of Erika’s core on Tuesday afternoon, far enough west to make it visible on Barbados radar (thanks go to the Weather Channel's Stu Ostro for that tidbit). Together with that wide reach, Erika has a broader pool of relatively moist air to draw from than its compact predecessor, Hurricane Danny. The Sarahan layer of dry, dusty air that enveloped Danny is much less pronounced in the vicinity of Erika.


Figure 1. Infrared image from the GOES-East floater satellite, taken at 0445 GMT (12:45 am EDT) on Wednesday, August 26. Image credit: NOAA/NESDIS.

Erika’s large size will make it slower to organize and intensify than Danny, while also helping to protect the storm from any rapid decay down the line. Track models are fairly consistent in bringing Erika to the Bahamas by this weekend, but there remain big questions in how much and how quickly Erika will intensify during that time. Model guidance has continued to strengthen Erika only very gradually over the next 2 to 4 days. The most reliable longer-range dynamical models have been pessismistic on Erika’s future, with the 1200 GMT Tuesday runs of the GFS and ECMWF (as well as more recent GFS runs) weakening the storm to an open wave by this weekend. Statistical guidance, which tends to perform the best at intensity beyond about 3 days, suggested at 0000 GMT Wednesday that Erika might be only a strong tropical storm by Day 5 (late Sunday, August 30). Meanwhile, the last several runs of the two top high-resolution models (GFDL and HWRF) have sent Erika into fairly rapid intensification mode by days 4 and 5, when the storm should be over the very warm waters of the Bahamas (more than 30°C or 86°F). Before then, Erika will need to barrel through a ribbon of high-level, shear-producing westerly winds, perhaps a key reason why most models are putting little stock in Erika’s shorter-term future. Interaction with Puerto Rico and/or Hispaniola may also be an issue, as Erika is generally predicted to track near or just north of those islands.


Figure 2. The official NHC forecast for Erika as of 11:00 pm EDT on Tuesday, August 25.

Should Erika make it to the Bahamas as a well-organized tropical storm, the picture could change dramatically. This is a very favored spot climatologically for hurricane development, especially in late August and early September. The GFS and ECMWF models from 1200 GMT Tuesday, and the GFS model from 1800 GMT Tuesday, agreed on moving Erika or its remnants toward Florida, as an upper-level ridge builds over and to the northeast of Erika. The 11:00 pm EDT Tuesday forecast from NHC brings Erika to within a half-day of the Florida coast as a Category 1 hurricane by Sunday night, August 30 (see Figure 2). Unlike many systems in recent years, Erika could approach Florida with a weak upper-level trough located well to the west of the state (see Figure 3 below), which would make an immediate recurvature less likely. Putting aside the very valid question of Erika’s durability over the next several days, the overall pattern is the most favorable I’ve seen in a long time for a potential Florida landfall. It has been nearly 10 years since a hurricane has reached Florida’s coastline, the last being Hurricane Wilma (also the nation’s most recent hurricane that was rated Category 3 at landfall on the Saffir-Simpson Hurricane WInd Scale). Since records began in 1851, this is by far the state’s longest “hurricane drought,” beating out the five hurricane-free seasons from 1980 to 1984. If nothing else, any development of Erika would serve to dislodge any misplaced confidence that the state’s hurricane risk has somehow withered over the last decade.

Jeff Masters will have our next complete tropical roundup on Wednesday.

Bob Henson



Figure X Projected steering flow at the 200-millibar level (about 40,000 feet) from the 1800 GMT Tuesday run of the GFS model, valid at 1800 GMT (2:00 pm EDT) on Sunday, August 30. Image taken from our wundermap with the “Model Data” layer turned on.

Hurricane

In the Eye of Katrina: A Personal Retrospective

By: Bob Henson , 7:53 PM GMT on August 25, 2015


There’s a lot to monitor in the tropics right now, including Tropical Storm Erika, now heading toward the Lesser Antilles (see the Jeff Masters post from this morning, and watch for our update later tonight). We're also commemorating the onset of one of the biggest weather stories in U.S. history. Ten years ago today, a storm named Katrina swept into the Miami-Fort Lauderdale area, just a few hours after it attained hurricane status. Even if it hadn’t gone on to cause colossal agony and destruction on the Gulf Coast, Hurricane Katrina would be a storm worth remembering. It knocked out power to more than a million people across South Florida, inflicted more than $500 million in damage--mostly to agriculture--and caused 12 deaths. Carving out a cyclonic loop across the southern tip of the state, Katrina held its own as it passed over the swampy terrain, with few ill effects on its structure or intensity from the six-hour trek over land. That left Katrina in a strong position to grow into the Category 5 monster it became over the Gulf of Mexico.

Figure 1. The NHC forecast for Katrina issued at 1700 GMT (11:00 am EDT) on Wednesday, August 24. Image credit: NHC.
 


I had the once-in-a-lifetime experience on August 25, 2005, of experiencing the passage of a hurricane at the National Hurricane Center (NHC) in Miami. For years, as a writer for the National Center for Atmospheric Research, I’d been interested in seeing how the center grapples with forecast challenges and deals with media during a U.S. hurricane landfall. Katrina’s development gave me a chance to fulfill this long-held interest while finding out what it was like to experience a minimal hurricane from a safe vantage point. I arrived in Miami on Tuesday, August 23. My host was long-time friend and colleague Hugh Cobb, then a forecaster at NHC and now head of its Tropical Analysis & Forecast Branch.


At 11:00 am EDT the next morning--Wednesday, August 24--Tropical Depression 12 became Tropical Storm Katrina. NHC correctly predicted that Katrina would strike near Fort Lauderdale as a minimal hurricane the next night, although its subsequent tenacity while over land was underestimated and its Gulf Coast landfall remained almost a week away. Heading to NHC with Hugh that afternoon, I found a top-notch team of forecasters and analysts demonstrating remarkable calm and focus as they dealt with a growing hurricane threat in their own backyard. Although many of the models and computer-based analysis tools now used were already place by 2005, hand-drawn analysis remained--and still remains today--an useful means of gaining detailed perspective on storm structure.
 
One of the greatest pleasures of my visit was meeting NHC’s director at the time, Max Mayfield. Now retired from NOAA, Max files reports during hurricane season for Miami’s Channel 10 (WPLG). To me, Max epitomizes the friendly yet sober demeanor and the grace-under-pressure mindset that any NHC director needs to master.
 

Figure 2. Max Mayfield and me at the NHC. Not only did Max and I discover that we were both alumni of the University of Oklahoma, but Max grew up just a couple of miles away from me in Oklahoma City, and we went to the same high school (more than a decade apart, though). Go Classen Comets!




Media interest in Katrina grew through the day on Wednesday as confidence in a South Florida landfall near hurricane strength increased. That evening I watched Max and forecaster Lixion Avila conduct a “pooled” interview, where national networks and local TV stations around the country can upload and/or broadcast the same sound bites at the same time. By this point, it was apparent that Katrina would likely circle the southwest side of an upper-level high and arrive near the Gulf Coast in about 5-6 days. How strong Katrina would be by that time was anybody’s guess, as skill at intensity prediction in 2005 was substantially less than it is today, especially beyond 2 or 3 days. (See Figures 1 and 2 in my recent post on progress in tropical cyclone modeling and forecasting).
 
The 11 pm EDT outlook on Wednesday night called for Katrina to strike the Gulf Coast as a Category 1 hurricane around Monday night, August 28. In Wednesday night’s NHC discussion, Lixion wrote: “The intensity forecast follows the SHIPS model but Katrina could intensify a little more than anticipated.”



Figure 3. NHC forecaster Lixion Avila (left) and director Max Mayfield conduct a pooled media interview on Thursday night, August 25, while Katrina was over the Miami area. Image credit: Bob Henson.



The drama escalated on multiple levels as Katrina approached Miami on Thursday, August 25. Hugh and I drove to NHC that afternoon for his evening shift, slated to begin at 4:00 pm, as palm trees were tossed in gale-force winds and pulses of heavy rain arrived. Forecasters zeroed in on the short-term impacts that afternoon while keeping an eye on Katrina’s longer-term future. As the storm crescendoed that evening, I didn’t worry about my own safety: the NHC building, with concrete walls and reinforced windows, was built to withstand the rigors of a Category 5 storm, after the previous NHC location in Coral Gables was hit hard by 1992’s Hurricane Andrew. (The radome of Miami’s pre-Doppler WSR-57 radar was blown off the center’s roof in wind gusts just north of the eywall that reached 164 mph.) Even with the sense of security provided by this state-of-the-art building, it was unsettling to watch sheets of rain fall in an increasingly horizontal orientation just outside the door.
 

Figure 4
. NHC forecaster Rob Handel (now with the NOAA/NWS National Centers for Environmental Prediction) monitors the progress of Katrina on Thursday evening, August 25. Image credit: Bob Henson
 

Figure 5. Radar returns and surface observations just before 2300 GMT (7:00 pm EDT) on August 25, 2005, as Katrina was moving ashore just north of Miami. Image credit: National Weather Service.


Figure 6. The wind-speed trace from an anemometer atop the NHC building shows conditions going virtually calm around 8:30 pm, followed by a gust of more than 70 knots (81 mph) less than an hour later. Peak sustained winds at NHC during Katrina at NHC were 69 mph, with a top gust to 87 mph.


Between 8:30 and 9:15 pm, the eye of Katrina passed directly over NHC. A few of us stepped outside for a moment. It was Max’s first-ever time in the eye of a hurricane, as well as mine. The eye was mostly cloud-filled, illuminated by the lights of the Miami area, with occasional flashes of lightning along the horizon. I watched as the U.S. flag on the NHC grounds waved listlessly in a half-hearted breeze, then finally went limp. We stood quietly for a few minutes, taking in the surreal scene. Then Max, very calmly, said something to the effect of, “Time to get back to work.” Less than an hour later, the powerful eastern eyewall of Katrina was atop NHC, and winds were gusting to more than 70 mph.
 
The next morning, Hugh and I drove back to his home in Miami’s Upper East Side through a tattered landscape. Countless palm fronds littered the street, with pools of water everywhere. Rainfall totals during Katrina were as high as 16.33” in Perrine, about 20 miles southwest of Miami. After cleaning up Hugh’s yard, we went for lunch at a nearby restaurant, where it seemed all eyes were on the TV screen as a weathercaster showed the latest projected path for Katrina.
 
Figure 7. Damage was non-catastrophic but widespread across the Miami area in the wake of Katrina. Image credit: Bob Henson.
 
 
The 11:00 am EDT advisory on this morning (Friday, August 26) showed Katrina striking the Gulf Coast as a major hurricane. Looking at the TV screen, I had a surprisingly unsettling feeling, knowing that a New Orleans landfall was a possibility. I’d read many articles pointing out the city’s huge risk of catastrophe from a major hurricane strike. ”Washing Away,” a brilliant five-part series by Mark Schleifstein and colleagues published in 2002 by the New Orleans Times-Picayune, had made an especially strong impression on me. Sadly, many of the dire predictions in “Washing Away” and other articles and analyses would soon come to tragic life. (I later discovered it took Schleifstein that four years to convince his editors to let him write that series.)

Jeff Masters, who launched this blog in April 2005--only a few months before Katrina--will take a look back at this life-changing storm later this week. The posts Jeff made during Katrina, which are available in this reverse-chronological compilation of posts from August 2005, make for compelling reading even a decade later. Meanwhile, I’ll discuss my own Katrina experience, plus what’s happening with Erika, in today’s installment of the Weather Underground TV show (#WUTV on Twitter) on The Weather Channel. The show airs between 6:00 and 8:00 pm EDT; right now the Katrina segment is scheduled for the latter part of tonight’s first hour. Hope you can join us! If you don’t have access to TWC on cable, you can still access selected clips from each episode, live streaming of online-only content, and a WUTV chatroom, all on the Weather Underground WUTV website. Also of interest: Peter Neilley, senior vice president of global forecast services for The Weather Company, weighs in on how forecasts and messaging have evolved in the 10 years since Katrina struck.

We’ll post an update on Erika later tonight.

Bob Henson

Hurricane

Tropical Storm Erika Headed for the Lesser Antilles

By: Jeff Masters , 3:13 PM GMT on August 25, 2015

Tropical storm watches are up for much of the northern Lesser Antilles Islands as Tropical Storm Erika speeds westwards at 20 mph. Erika formed in the waters a few hundred miles east of the northern Lesser Antilles Islands overnight, and is likely to take a west to west-northwest course over the next few days, a path that should concern residents of the Bahamas and the U.S. East Coast. However, Erika’s survival over the next few days is not a sure thing. Satellite loops on Tuesday morning showed that Erika was struggling, with the low-level center exposed to view and only a small and decreasing area of heavy thunderstorms, limited to the storm’s south side. Wind shear due to upper-level winds out of the northeast was moderate, 10 - 15 knots, and this shear was driving plenty of dry air on the north side of Erika into its core, disrupting the storm. Sea surface temperatures (SSTs) are favorable for development, though—near 27.5°C—and will warm to 28.5°C by Thursday.


Figure 1. Latest satellite image of Tropical Storm Erika.

Forecast for Erika
The 8 am EDT Tuesday run of the SHIPS model predicted that wind shear would remain in the moderate range through the remainder of the week. Steering currents for Erika are very similar to what Danny experienced, and the 8 pm EDT Monday (00Z Tuesday) run of the GFS and European models showed Erika taking a track into or just north of the northern Lesser Antilles Islands on Wednesday night, and into the Southeast Bahamas by Saturday. A trough of low pressure capable of turning Erika to the north will set up shop along the U.S. East Coast late this week, but it is uncertain at this time whether or not Erika will be strong enough to get picked up by this trough. If Erika manages to fight off the dry air and wind shear besetting it and grow into a strong tropical storm by Thursday, it would likely take a more northerly course and be a long-range threat to the U.S. East Coast, Bermuda, or Canada early next week. If Erika stays weak, it will track more to the south, and take a path close to Hispaniola and into the Bahamas by this weekend. A path close to Hispaniola would potentially disrupt a weak Erika and cause it to dissipate; this was the solution of both the 00Z and 06Z Tuesday morning runs of the GFS model. Two of our other top three models from last year for predicting hurricane tracks--the HWRF and UKMET models--showed Erika staying stronger and taking a more northerly track with their 00Z Tuesday runs; the European model was in between, and showed Erika coming very close to the Florida coast by Monday (Figure 2). I'm not ready to ring the alarm bells on this storm yet, as a bet against significant strengthening is usually called for during a strong El Niño season. Still, El Niño years do get major hurricanes--the big hurricane of the last strong El Niño year of 1997 was also named Erika, and peaked at 946 mb /125 mph winds (thanks go to wunderground member BaltimoreBrian for this reminder.) The models should do a better job with Erika in their Tuesday night runs, since they will have data from the Hurricane Hunters to chew on. The NOAA jet is in the air today releasing dropsondes in the vicinity of Erika to characterize the upper-air steering currents, and there will also be low-level penetrations of Erika this afternoon from the Air Force Hurricane Hunters.


Figure 2. The 00Z Tuesday (8 pm EDT Monday) runs of the European and GFS models had two very different predictions of the intensity of Erika for 8 am Monday August 31, 2015. The European model showed Erika as a strong tropical storm just off the coast of Florida (purple colors = winds of at least 58 mph), while the GFS model showed Erika merely as a strong tropical wave with no closed circulation. Image taken from our wundermap with the “Model Data” layer turned on.


Figure 3. Forecasts of the track of Tropical Storm Erika from the 00Z Tuesday (8 pm EDT Monday) run of the GFS model from the twenty members of the GFS Ensemble model. The GFS Ensemble takes the operational version of the GFS model and makes twenty different runs of it at lower resolution with slightly different initial conditions to generate an ensemble of possible forecasts. As we can see, there are a wide variety of possible solutions. The operational high-res version of the GFS (white line) shows Erika dissipating near Hispaniola.

Two long-range threats for Hawaii
While Tropical Depression Kilo is no longer a threat to Hawaii, the islands should keep an eye on two tropical systems to its east, in the waters of the Eastern Pacific to the southwest of Mexico: Tropical Depression Twelve-E, and Invest 96E. According to the 00Z Tuesday run of the GFS model, both systems could eventually pass within 500 miles of the Hawaiian Islands.

WUTV Takes Over The Weather Channel again at 6 pm EDT Tuesday
I had a great time last night discussing the tropics and the history of Weather Underground on the inaugural episode of the Weather Underground live cable TV on The Weather Channel (TWC). Hosted by TWC meteorologist Mike Bettes, the show will air weekdays from 6 - 8 pm Eastern time. Today’s will discuss Erika and the rest of the action in the tropics, and will feature a look at ten years ago today, when Hurricane Katrina hit Miami. Bob Henson will be on the show tonight to offer his view of that day. Check out #WUTV for updates.  

Wunderblogger Dr. Phil Klotzbach of Colorado State has an excellent post today on the The Record-Setting Northern Hemisphere Tropical Cyclone Season - Part I

There will be at least one more post today on Erika in this blog.

Jeff Masters

Hurricane

Danny Dies; Erika Coming?

By: Jeff Masters , 4:43 PM GMT on August 24, 2015

Tropical Storm Danny has met its demise at the hands of dry air and high wind shear, a victim of an El Niño-year atmosphere over the Caribbean that has been very hostile to hurricanes. The Hurricane Hunters were unable to find a closed circulation in Danny on Monday morning, and satellite loops and radar loops show that Danny's heavy thunderstorms activity has been steadily diminishing. Danny’s remnants will continue to head west at about 15 mph over the next few days, bringing heavy rains.


Figure 1. Radar image of Tropical Storm Danny's remnants from the Guadaloupe/Martinique radar, taken at 12:15 pm EDT Monday, August 24, 2015. Image credit: Meteo France.


Figure 2. Drought conditions in Puerto Rico as of the most recent issuance of NOAA’s Drought Monitor. Severe to Extreme drought covered all of eastern Puerto Rico, including the capital of San Juan. Image credit: National Drought Mitigation Center.

A drought helper, but not a drought buster for Puerto Rico
Danny’s remnants are expected to dump 2 - 4” of rain across portions of the drought-parched northeast Caribbean islands, which may cause localized flash flooding and mudslides Monday and Tuesday. However, these rains will not be enough to break the El Niño-driven drought that has gripped the region since this spring. Since January 1, Puerto Rico’s capital, San Juan, has received about ten inches of rain less than their usual 33” of rain. As a result, severe to extreme drought conditions have hit the region, with reservoir levels hitting their lowest levels in decades. San Juan and much of the northern coast of Puerto Rico are under water restrictions, where hundreds of thousands of households receive water only two days per week. The strong El Niño event underway in the Eastern Pacific has created an atmospheric circulation that has brought dry, sinking air to the Caribbean all summer, squelching thunderstorm activity during the traditional peak of the Caribbean rainy season. Central America is also suffering; in Guatemala, one million people are starving due to the drought, and in Honduras, ten municipalities are now officially experiencing famine.


Figure 3. MODIS image of 98L from NASA's Aqua satellite taken at approximately 9 am EDT Monday, August 24, 2015. Image credit: NASA.

98L in Central Atlantic could become Erika
In the Central Atlantic about 1200 miles east of the Lesser Antilles Islands, Invest 98L appears poised to become the next tropical depression of the 2015 Atlantic hurricane season, as it steams westward at a rapid 20 mph. Satellite loops on Monday morning showed that 98L had a well-developed spin and some low level spiral bands, but heavy thunderstorm activity was limited, due to dry air. The Saharan Air Layer (SAL) analysis offered by the University of Wisconsin shows plenty of dry air to the north and west of 98L, and this dry air will potentially impede development of 98L throughout the week. Sea surface temperatures (SSTs) are favorable for development, near 27°C, and will warm to 28°C by Wednesday. The 8 am EDT Monday run of the SHIPS model diagnosed moderate wind shear of 10 - 15 knots over 98L, and predicted the shear would remain in the moderate range through at least Wednesday. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave 2-day and 5-day odds of development of 90% to 98L. Steering currents for 98L are very similar to what Danny experienced, and the 8 pm EDT (0Z) Monday run of the GFS and European models showed 98L taking a track into the northern Lesser Antilles Islands on Wednesday night, and into the Southeast Bahamas by Saturday. Wind shear will rise to a high 15 - 25 knots by Thursday as 98L brushes the Northeast Caribbean, which should slow development or cause weakening. A trough of low pressure capable of turning 98L to the north will set up shop along the U.S. East Coast late this week, but it is uncertain at this time whether or not 98L will be strong enough to get picked up by this trough. If so, 98L could represent a long-range threat to Bermuda or Canada next week. If not, then the Caribbean, Bahamas, and U.S. East Coast might be a target; it’s too early to narrow down the possibilities.

New tropical wave off of Africa little threat to develop
A strong tropical wave emerged from the coast of Africa on Sunday, and is headed west at 15 - 20 mph. The wave has warm ocean waters and moderate wind shear, but plenty of dry air from the Saharan Air Layer (SAL) is interfering with development. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave the system 2-day and 5-day odds of development of 10%.

Tropical Depression Kilo not expected to threaten Hawaii
Tropical Depression Kilo continues to mill around in the waters well west of the Hawaiian Islands, and is no longer expected to be a threat to Hawaii when the storm turns westwards away from Hawaii late this week. Outflow from Kilo brought plenty of dry, sinking air over Hawaii over the weekend, bringing sunny skies and record warm temperatures. This resulted in one of the highest temperatures ever recorded in Hawaii: a 97°F reading at Kahului on Maui Island on Saturday, August 22. According to wunderground weather historian Christopher C. Burt, this ties the all-time (for any month) high for Kahului measured on Aug. 31, 1994. According to weather records researcher Maximiliano Herrera, no other higher reliable temperatures have been measured in Hawaii (higher readings from an old station in Puunene, Maui in the 1950s are unreliable, and the Puunene station was closed in 1959.)

Hawaii should keep an eye on two tropical disturbances to its east, in the waters of the Eastern Pacific to the southwest of Mexico. Both of these disturbances (which were given 5-day development odds of 80% by NHC in their 8 am Monday Tropical Weather Outlook) could pose a long-range threat to Hawaii, according to the 6Z Monday run of the GFS model.

Typhoon Goni headed towards Japan
Category 3 Typhoon Goni is about to make landfall on the southern Japanese Island of Kyushu, with landfall as a Category 2 storm expected by the Joint Typhoon Warning Center.

Wunderblogger Steve Gregory has a post up today on 98L, and plans a more in-depth one late Monday afternoon.

WUTV Takes Over The Weather Channel Beginning at 6 pm EDT Monday
The Weather Underground hits live TV today, when the inaugural episode of the Weather Underground live cable TV show airs from 6-8 p.m. EDT Monday, August 24 on The Weather Channel (TWC). I’m in Atlanta today to help launch this unique effort, which aims appeal to everyone’s inner weather geek by focusing on the science behind weather and forecasting. Hosted by Emmy-award winning TWC meteorologist Mike Bettes, the show will air Monday through Friday from 6 - 8 pm Eastern time. I expect today we’ll be discussing Invest 98L and the rest of the action in the tropics. Check out #WUTV for updates.  

Jeff Masters

Hurricane

WUTV Takes Over The Weather Channel Beginning 6 pm EDT Today

By: Jeff Masters , 12:40 PM GMT on August 24, 2015

The Weather Underground hits live TV today, when the inaugural episode of the Weather Underground live cable TV show airs from 6-8 p.m. EDT Monday, August 24 on The Weather Channel (TWC). I’m in Atlanta today to help launch this unique effort, which aims appeal to everyone’s inner weather geek by focusing on the science behind weather and forecasting. TWC is really making an impressive effort on this, complete with a custom set built to look like an underground bar, an original theme song of Seattle-grunge type music, and state-of-the art 3D visual presentations. Hosted by Emmy-award winning TWC meteorologist Mike Bettes, the show will air weekdays from 6 - 8 pm Eastern time. Both Bob Henson and I will be featured twice this week. Bob is scheduled to go Tuesday and Thursday from our San Francisco office; I will appear today (in the Atlanta studio) and Friday (via Skype from my Michigan office.) San Francisco-based Weather Underground meteorologists Jim Menard, Shaun Tanner, and John Celenza are also scheduled to appear during the first two weeks of the show. There will periodically be guest appearances from other severe weather experts and WU community members (personal weather station owners and bloggers.) Each show will aim to promote a unique feature of the Weather Underground web site, like the latest blog post by Bob or myself, WunderPhotos, or personal weather stations. Breaking weather events will be the main focus, but climate change stories will also occasionally be featured.

WU’s unique community of personal weather station owners and fans will play an integral role in the show’s live, interactive experience--fans will be able to submit questions via #WUTV across social media, report current conditions in their area, contribute to the creation of show segments, access behind-the-scenes live-stream video via wunderground.com and participate in weather roundtables live on air.  


Video 1. Take a look at our cool new set for the WUTV show in this promo video.

The entire show will only be available on The Weather Channel cable TV show, but some live streaming content will be available throughout on http://bit.ly/wutvshow.

You can follow Weather Underground on  Twitter and Facebook for exclusive show content.

I gave Mike Bettes a tough grilling during his interview for host of the new Weather Underground TV show; you can see the humorous five-part interview here.

I'll have an update on the tropics by noon EDT today. My post from Sunday afternoon on Danny and the rest of the tropics is here.

Jeff Masters

Wunderground News

Tropical Storm Danny Weakening; TD Kilo Less of a Threat to Hawaii

By: JeffMasters, 7:19 PM GMT on August 23, 2015

Tropical Storm warnings are flying for the northern Lesser Antilles islands of Antigua, Barbuda, Montserrat, St. Kitts, Nevis, and Anguilla, as a weakening Tropical Storm Danny heads west at 15 mph. Danny passed very close to Buoy 41300 on Sunday afternoon, which measured top winds of 42 mph at 11 am EDT. High wind shear and dry air continue to take their toll on Danny, which had weakened to top winds of 50 mph as of 2:00 pm EDT Sunday. Danny remains a very small tropical storm, with tropical storm-force winds that extend out about 60 miles to the northeast of the center and 20 miles to the southwest. Satellite loops on Sunday afternoon showed that Danny's low-level circulation had become exposed to view, with only a small clump of heavy thunderstorms clustered on the storm's north and east sides.


Figure 1. A visible image of Tropical Storm Danny at 2 pm EDT on Sunday, August 23, showed that the low-level center had become exposed to view, with all of Danny's heavy thunderstorms clustered on the north and east sides of the center.

Forecast for Danny
Given Danny's small size and the continued hostile wind shear and dry air affecting the storm, further weakening is likely. Our two top models for predicting hurricane tracks, the GFS and European models, both dissipate Danny by Tuesday. Interaction with high terrain of Puerto Rico and Hispaniola could well finish Danny off, and I doubt the storm will be around on Wednesday. Rainfall from Danny could end up being more boon than bane for the islands, which are suffering one of their worst droughts in decades.


Figure 2. MODIS image of 98L from NASA's Terra satellite taken at approximately 10 am EDT Sunday, August 23, 2015. Image credit: NASA.

98L in Eastern Atlantic well-organized
In the Eastern Atlantic about 800 miles west of the Cape Verde Islands, Invest 98L appears poised to become the next tropical depression of the 2015 Atlantic hurricane season as it steams westward at a rapid 20 mph. Satellite loops on Sunday afternoon showed that 98L had a well-developed spin and some low level spiral bands beginning to form, but heavy thunderstorm activity was limited, due to the dry air. The Saharan Air Layer (SAL) analysis offered by the University of Wisconsin shows plenty of dry air to the north and west of 98L, and this dry air will potentially impede development of 98L throughout the week. Sea surface temperatures (SSTs) are favorable for development, near 27°C. The 8 am EDT Sunday run of the SHIPS model diagnosed moderate wind shear of 10 - 15 knots over 98L, and predicted the shear would remain in the moderate range through at least Wednesday. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave 2-day and 5-day odds of development of 70% and 80%, respectively, to 98L. Steering currents for 98L are very similar to what Danny experienced, and the 8 am EDT (12Z) Sunday run of the GFS model showed 98L taking a track into the northern Lesser Antilles Islands on Wednesday, passing near Puerto Rico on Thursday, then over the Dominican Republic by Friday. The European model was about 12 hours slower with 98L, bringing the system into the Southeast Bahamas by Saturday. Wind shear will rise at 98L approaches the islands, which should force the system to weaken

New tropical wave coming off of Africa a marginal threat to develop
A strong tropical wave emerged from the coast of Africa on Sunday, and may well be classified as Invest 99L by NHC by Monday. This wave is headed west at 15 - 20 mph, and will encounter similar conditions as Danny and 98L did--warm ocean waters, moderate wind shear, but plenty of dry air from the Saharan Air Layer (SAL) to contend with. These conditions favor some modest development, and in their 2 pm EDT Sunday Tropical Weather Outlook, NHC gave the system 2-day and 5-day odds of development of 0% and 20%, respectively.

97L north of Bermuda no threat
In the Northwest Atlantic, Invest 97L, a cluster of storms associated with a weakening upper low and frontal zone a few hundred miles north of Bermuda, is accelerating to the north and is about to be absorbed by a non-tropical low pressure system. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave 2-day and 5-day odds of development of 0% to 97L.

Threat to Hawaii from Kilo diminishes
Residents of Hawaii should monitor the progress of Tropical Depression Kilo, which is expected to mill around in the waters well west of the islands the next few days. Briefly a tropical storm, Kilo was downgraded on Friday night as its large area of westward-moving showers and thunderstorms (convection) outran the low-level circulation. Kilo will still likely organize into a hurricane, but the latest forecasts from the GFS and European model keep the storm well to the west of Hawaii for at least the next five days.

Hawaii should keep an eye on three tropical disturbances to its east, in the waters of the Eastern Pacific to the southwest of Mexico. The easternmost of these disturbances (which was given 5-day development odds of 70% by NHC in their 2 pm Sunday Tropical Weather Outlook) could pose a long-range threat to Hawaii next week, according to the 12Z Sunday run of the GFS model, but it is too early to be confident of this.

Elsewhere in the Pacific, there is plenty of activity, with Category 3 Typhoon Goni pounding Japan's southern islands. The highest reported gust from the typhoon was 71.0 m/s (159 mph) at Ishigakijima. Category 1 Typhoon Atsani was heading northeast out to sea, and Tropical Storm Loke was headed northwards towards Midway Island. I don't have time for a full update, as I have to run to catch a plane to Atlanta for Monday's 6 pm EDT premiere on The Weather Channel of the new Weather Underground TV show!

I'll have a new update on Monday.

Jeff Masters

Hurricane

Hurricane Danny Weakens; Tropical Depression Kilo Struggles

By: Bob Henson , 9:43 PM GMT on August 22, 2015

It’s been a challenging 24 hours for the two named tropical cyclones closest to North America, one in the Atlantic and one in the Central Pacific. After surging to Category 3 strength on Friday, Hurricane Danny has weakened almost as quickly, as wind shear and dry air chip away at its integrity. As of 5:00 pm EDT, Danny was located near 15.8°N, 53.3°W, or about 570 miles east of the northern Leeward Islands. Danny remains a very small hurricane: hurricane-force winds extend up to 10 miles and tropical-storm-force winds up to 60 miles from Danny’s center, according to the National Hurricane Center (NHC).

WIth top sustained winds now down to 75 mph (minimal Category 1 strength), Danny was continuing its steady west-northwest track at a slightly speedier pace of about 14 mph. Danny’s current bearing will take it into the northernmost Antilles by Monday, and tropical storm watches have been hoisted for Antigua, Barbuda, Montserrat, St. Kitts, Nevis, Anguilla, Saba, St. Eustatius, St. Maarten, Guadeloupe, St. Barthelemy, and St. Martin. Although no watches are yet in effect, Puerto Rico could experience tropical storm conditions by Tuesday. (A watch means that tropical storm conditions are possibe within the next 48 hours.)


Figure 1. A visible image of Hurricane Danny from NOAA’s GOES-Floater satellite, collected at 2045 GMT (4:45 pm EDT) on Saturday, August 22. Image credit: NOAA/NESDIS.


Figure 2. Vertical wind shear over Danny is now around 20 knots (23 mph), and even larger value above 30 knots lie ahead in Danny’s path over the next day or two. Image credit: NOAA/NESDIS


Danny has encountered increasing vertical wind shear and dry, dusty air since Friday, and these factors are taking an obvious toll on the hurricane. The well-defined eye and eyewall that Danny boasted for a time on Friday are long gone. Spiral banding has largely dissipated around Danny, and the central core of convection is much more fragmented and asymmetric, as upper-level westerlies push showers and thunderstorms east from Danny’s center. Data from an Air Force hurricane-hunter aircraft that surveyed Danny on Saturday afternoon provided further evidence that Danny is going downhill, with maximum flight-level winds peaking at only 61 knots (70 mph), just below hurricane strength, at 1821 GMT (2:21 pm EDT). Danny’s decline is echoed in the 5:00 pm EDT outlook from the NHC (see Figure 3), which brings Danny down to tropical-storm strength on Saturday night. Both upward and downward swings in intensity can be especially large and rapid with small hurricanes like Danny.



Figure 3. The NHC’s outlook for Hurricane Danny as of 5:00 pm EDT Saturday.

Could Danny get a new lease on life?
Danny’s weakening is a very confident forecast for the next couple of days: both statistical and dynamical models agree that Danny should be a mid-range tropical storm at best by the time it approaches Puerto Rico on Tuesday. The 1200 GMT run of the HWRF model, one of the best for short-term intensity change, weakens Danny to a tropical depression by Monday. Given Danny’s small size, it’s entirely possible Danny could be little more than a tropical wave by then.

The longer-range outlook for Danny has a bit more uncertainty, as long-range outlooks so often do. Track models continue to bring Danny or its remnants very close to Puerto Rico and Hispanola, though perhaps staying just north of the islands. Rains from Danny would be more than welcome over these drought-stricken islands, but only a small difference in track angle at this point could make a big difference in the outcome. Given Danny’s small size, it might be able to avoid the detrimental effect of the islands’ mountainous terrain if its track is on the north side of the model solutions, consistent with the 1200 GMT run of the GFDL model. There still might not be much left of Danny at that point, but if a reasonable circulation does persist, large-scale conditions could support some restrengthening. Sea-surface temperatures will be very warm (around 29°C or 84°F), and an upper-level ridge is forecast to strengthen over the western North Atlantic, keeping Danny or its remnants rolling westward with the potential for relatively low wind shear. Further down the line, any revitalizated version of Danny would face an upper-level trough predicted to dig into the southeast United States. If the models are correct, the resulting southwesterly flow across Florida would most likely force a recurvature of Danny around the middle of next week. Given the very warm waters and deep oceanic heat content over the Bahamas, any potential tropical cyclone bears watching, but at present it does not appear that Danny will pose a major threat to the U.S. East Coast.


Figure 4. The 1200 GMT run of the GFS model on Saturday, August 22, valid at 1200 GMT Thursday, August 27, depicts an upper-level trough across the southeast United States at the 200-mb level (about 40,000 feet high). If this trough intensifies and shifts east as forecast, it would likely steer Danny or its remnants away from the U.S. East Coast. Image credit: Levi Cowan, tropicaltidbits.com.


Figure 5. Large zones of dry, dusty Saharan air (yellow and orange areas) ontinue to prevail across the Atlantic subtropics, although Hurricane Danny and a large tropical wave to its east have made inroads into the dry air. Image credit: University of Wisconsin/CIMMS and NOAA Hurricane Research Division.

Elsewhere in the Atlantic
The Cape Verde wave train--which normally peaks from late August to late September--is now in high gear, with multiple large easterly waves developing over Africa and rolling into the eastern tropical Atlantic. One such wave, called out in the 4:00 pm EDT tropical discussion from NHC, is located near 18°N, 28°W, moving west at about 20 mph. Convection blossomed around this wave on Friday night, then subsided on Friday, a typical sequence for developing tropical cyclones. The wave is embedded in a large swath of moisture, boding well for its development, although SSTs of around 26-27°C (79-81°F) are only modestly supportive. A strengthening upper-level ridge across the subtropical Atlantic should keep this wave scooting westward for several days, at which point models suggest a break in the ridge could allow it to move northward. As of 2:00 pm EDT Saturday, NHC was giving the wave 30% odds of developing into a depression by Monday and 60% odds by Thursday. Another healthy wave is now approaching the African west coast at a somewhat lower latitude, which may give it a better chance of traversing the deep Atlantic tropics. NHC gives this wave 10% odds of development by Monday and 30% odds by Thursday.

In the Northwest Atlantic, Invest 97L, a cluster of storms associated with a weakening upper low and frontal zone, is nearly devoid of convection, and models show little inclination to develop 97L into a tropical or subtropical storm over the next several days.

Slow going for Kilo, but Hawaii still a possible target
Residents of Hawaii should keep their guard up despite the laggard development of Tropical Depression Kilo. Briefly a tropical storm, Kilo was downgraded on Friday night as its large area of westward-moving showers and thunderstorms (convection) outran the low-level circulation. This outcome is somewhat surprisingly, since vertical wind shear has been quite low near Kilo (10 knots or less). Infrared satellite loops over the last few hours show Kilo developing a large core of strong convection, although still strongly sheared from the east, suggesting that more than one low-level center of circulation may be present. An Air Force reconnaissance flight found maximum surface winds of only 26 knots (30 mph) at around 1800 GMT Saturday.


Figure 6. Infrared image of Tropical Depression Kilo, collected from the GOES-Floater satellite at 2000 GMT (4:00 pm EDT) on Saturday, August 22. Image credit: NOAA/NESDIS.


Figure 7. The outlook for Kilo from the Central Pacific Hurricane Center as of 11:00 am HST (5:00 pm EDT) Saturday, August 22.


Forecast models agree that Kilo will eventually organize and begin a northwestward trek. An upper-level ridge is forecast to strengthen over Kilo about 2-3 days from now, which will slow its movement and support its intensification (assuming it organizes as expected). Thereafter, the upper-level steering currents become somewhat chaotic, although the most likely outcome is that westerlies north of Hawaii would steer Kilo in the general direction of the western Hawaiian islands for at least a period of time. The high-resolution HWRF and GFDL models have consistently projected a powerful hurricane within striking distance of Kauai around the middle of next week, although the timeline has been shunted back due to the predicted weak steering currents in the 3-to-5-day period. Longer-range global models are generally pulling Kilo back to the northwest before it might reach the islands, with the exception of the ECMWF model, which is closer to the HWRF and GFDL tracks. Statistical model guidance, which is generally the best with intensity in the 3- to 5-day period, is less aggressive on strengthening Kilo than the HWRF and GFDL models. The 11:00 am EDT outlook from the Central Pacific Hurricane Center (CPHC) calls for Kilo to be an intensifying Category 1 hurricane well southwest of Kauai by Thursday. With the upper-level prognosis cloudy and Kilo itself less organized than expected at this point, the outcome is highly uncertain, but the high potential risk to Hawaii still calls for vigilance in monitoring Kilo.

West of Kilo, Tropical Depression Loke became the fifth named storm in the Central Pacific on Friday. Eric Blake (NHC) noted on Twitter that this is the largest number of tropical storms assigned names in the Central Pacific since modern records began, beating the record of four (Akona, Hana, Ema, and Iwa) set in 1982 (which, like 2015, was the year of the onset of a major El Niño event). These numbers do not reflect the total number of tropical cyclones affecting the basin, since other named storms move into the Central Pacific from the Northeast Pacific. This brings another record to the fore: Loke is the seventh named storm to either form or pass through the Central Pacific this year, well above the previous record of five set in 1982, 1994, and 2013, as tweeted by Phil Klotbach (Colorado State University). Loke should have little impact outside the record books: as it moves slowly northward over progressively cooler waters, it is projected to regain only minimal tropical-storm strength over the next five days.


Goni heading toward Japan, leaving 10 dead in Philippines
After giving the northernmost Phillipines a stronger sideswipe than expected, Typhoon Goni is now heading north-northeast toward Japan. Goni embarked on a very sharp but very slow recurvature on Friday, taking a 90-degree turn from west to north just north of the Philippine island of Luzon. The island was on the weaker left-hand side of Goni, but the typhoon’s large shield of heavy rain extended over the northern reaches of the island for several days, triggering mudslides and flooding. At least 10 deaths were reported, and some locations received more than a foot of rain, according to weather.com’s Nick Wiltgen. More than 5,000 people were reportedly evacuated during the storm.

Now heading north-northeast, Goni is down to Category 1 strength, with top sustained 1-minute winds of 90 mph as of 1800 GMT Saturday. Wind shear will be light for at least the next couple of days over Goni as the typhoon passes over the warm Kuroshio Current (the Pacific’s analogue to the Gulf Stream). The Joint Typhoon Warning Center projects that Goni will reintensify to Category 3 strength by early Monday local time as it passes near Japan’s small southernmost islands. Though weakening as it moves further north, Goni could still strike the western part of Kyushu as a minimal typhoon. Further to the northeast, Category 1 Typhoon Atsani--also down to 90-mph sustained winds--will continue to weaken as it moves northeast over open water.

Jeff Masters will be back with our next update on Sunday.

Bob Henson

Hurricane

Danny Vaults to Category 3 Status; Tropical Storm Kilo Aims for Hawaii

By: Bob Henson , 5:16 PM GMT on August 21, 2015

Going against the grain of a hurricane-snuffing El Niño event, a tiny tropical cyclone has become the strongest hurricane in years over the deep Atlantic tropics. Hurricane Danny intensified dramatically on Thursday night and Friday morning, strengthening to Category 3 status on the Saffir-Simpson Hurricane Wind Scale. At 11:00 am EDT, Danny was located near 14.0°N, 48.2°W, about 930 miles east of the Leeward Islands, and still moving west-northwest at a modest clip of around 10 mph. At 11:00 am EDT, Danny’s top sustained winds were estimated at close to 105 mph. Indicative of Danny’s unusually compact size, hurricane-force winds extended only 15 miles from the storm’s center, and tropical-storm-force winds extended out up to 70 miles. It can be difficult for satellite-based instruments to estimate the intensity of very small hurricanes like Danny due to limited sensor resolution, but it’s clear that Danny is a surprisingly well-organized hurricane. A NOAA P-3 hurricane-hunter aircraft originally scheduled to sample the air west of Danny was instead approaching the storm early Friday afternoon, so we may soon have a stronger estimate of Danny’s actual intensity. Regular NOAA and Air Force reconnaissance flights into Danny are slated to begin on Saturday afternoon. Update: Based on reconnaissance data from the NOAA flight mentioned above, the National Hurricane Center has upgraded Danny to Category 3 status as of 2:00 pm EDT Friday, with top sustained winds at 115 mph. Danny is the first major Atlantic hurricane since Hurricane Gonzalo in 2014.


Figure 1. Visible imagery of Danny at 1-km resolution from the GOES-East satellite. Image credit: CIMMS/University of Wisconsin.


Figure 2. A comparison of imagery collected aboard polar-orbiting satellite by the NASA Visible Imaging Infrared Radiometer Suite (VIIRS) at around the same time (close to 0400 GMT Friday, August 20) for Typhoon Atsani (left) and Hurricane (Danny). The two images are at the same scale, revealing how compact Danny is, although Danny has enlarged somewhat since this image was taken. Image credit: Dan Lindsey, RAMMB/CIRA/University of Colorado.

In satellite imagery, Danny appears almost like a scale model of a well-developed hurricane, with quite symmetric structure, a clearly defined eye, and a minuscule but dense central core of strong thunderstorms (convection). Spiral banding is focused toward the east and south sides of Danny, where the atmosphere is comparatively moist. Danny is the strongest hurricane observed in the open North Atlantic tropics between the Lesser Antilles and Africa since Hurricane Julia in 2010, which became a Category 4 further east (longitude 32°W) than any other Atlantic hurricane since regular satellite observations began in the 1970s.

Danny is also one of the smaller hurricanes on record in the Atlantic. The smallest tropical cyclone in Atlantic history is 2008’s Tropical Storm Marco, whose brief life played out in the southern Bay of Campeche. Marco’s central core of convection was only about 10 miles in diameter--smaller than many supercell thunderstorms! The Pacific basin tends to produce a wider variety of tropical cyclone sizes, from tiny typhoons to the world’s largest and strongest on record: Typhoon Tip, which boasted a global record low for sea-level barometric pressure (870 mililbars, or 25.69”) and gale-force winds that at one point spanned some 1,380 miles in diameter.

The outlook for Danny: becoming more complex
Track models remain in fairly good agreement on a continued west-northwest track for Danny. The NHC forecast track (see Figure 3) now brings Danny to the vicinity of Puerto Rico by Tuesday and Hispanola by Wednesday. Assuming the west-northwest bearing remains solid, only a slight deviation could play a big role in Danny’s future, as interaction with the mountainous terrain of these islands could quickly disrupt weaken a storm as small as Danny. If the model trend further toward the north continues, Danny has a better chance of escaping landfall on the islands; in this case, its small size could actually result in less disruption from the islands than for a larger hurricane.


Figure 3. The outlook for Hurricane Danny issued at 11:00 am EDT on Friday, August 21.

Danny’s recent surge in strength has drawn on seasonably warm warm sea-surface temperatures (SSTs) of around 27-28°C, together with light to moderate wind shear (now running between 10 and 20 knots). Danny will be encountering higher wind shear (20 – 40 knots) over the next couple of days, as it nears a belt of upper-level westerlies extending across the Caribbean into the North Atlantic. Even though the shear within this zone has been lessening over the last couple of days, it will remain a formidable impediment to Danny. The same west-to-east belt also features a large zone of dry, dusty air with roots in the Saharan Desert. Thus far, Danny has managed to wall off a central core of convection intense enough to keep dry-air intrusions at bay, but this will become an increasing challenge for Danny over the next several days. Both dynamical and statistical models are in strong agreement that Danny will begin weakening by Saturday, as it encounters the increasing shear and dry air, and Danny will most likely be a strong tropical storm or weak Category 1 hurricane by Monday as it approaches Puerto Rico. An important caveat: tropical cyclones like Danny can strengthen or weaken very quickly, so there is more uncertainty than usual in this intensity forecast, especially at longer range.

If Danny manages to track well north of Puerto Rico and Hispanola with its structure relatively intact, it could encounter a more favorable environment for some potential restrengthening in the 5-to-6-day window, as suggested by the 0600 and 1200 GMT run of the GFDL model. Should such a scenario occur, the fate of Danny would then hinge on the state of an upper-level low that will scoot across the Midwest and Northeast over the next few days. An extension of that low is forecast to settle into the southeast United States as a weak upper-level trough, and the steering flow on the east side of that low would determine how soon Danny would recurve, assuming that it moves north of the Caribbean early next week. The most reliable long-range track models (GFS and ECMWF) suggest that this weak upper trough may be far enough east to keep Danny or its remnants offshore, but it is still far too soon to know with confidence how the upper trough will evolve.



Figure 4. A NOAA GOES-West infrared satellite image of Tropical Storm Kilo (center), gathering strength south of Hawaii, at 1600Z (noon EDT) on Friday, August 21. Image credit: NOAA National Hurricane Center.

Tropical Storm Kilo gaining strength, threatening Hawaii
The risk of a rare Hawaiian hurricane is growing as Tropical Storm Kilo (pronounced KEE-lo) slowly organizes in the Central Pacific. (“Kilo” is a Hawaiian term meaning meaning “to observe carefully”; the U.S. Navy maintains a research vessel at the University of Hawaii dubbed the R/V Kilo Moana, or “observing the ocean carefully”.)

At 5:00 am HST (11:00 am EDT) Friday, Kilo was located near 12.7°N, 151.7°W, or about 720 miles southeast of Honolulu, according to the Central Pacific Hurricane Center (CPHC). Kilo is moving west-northwest at about 16 mph, and that general track should continue through Sunday before upper-level westerlies force Kilo to undergo a sharp recurvature toward the north and northeast. Tropical cyclones approaching Hawaii from the south can generally maintain their strength more easily than those approaching from the east, as they spend less time over marginally warm water and may experience less wind shear. The two most intense hurricanes to strike Hawaii in modern times--Iniki (1992) and Dot (1959)--both arrived at Kauai from the south (see Figure 6 at bottom).


Figure 5. The outlook for Tropical Storm Kilo issued at 11:00 am EDT on Friday, August 21.

Kilo has a large, consolidated mass of convection within a moist overall environment, and there is a good chance that Kilo will strengthen significantly, with SSTs of 28-29°C (82-84°F) along its path, about 2°C above the seasonal average. Vertical wind shear is quite low over Kilo (less than 10 knots) and it has relaxed significantly over the last several days over Hawaii, further increasing the risk that Kilo will intensify. Although the amount of heat in the upper layer of the ocean is not particularly high by the standards of the Caribbean or tropical Northwest Pacific, Kilo’s brisk motion should minimize any negative effects from upwelled cooler water.

The timing and sharpness of Kilo’s recurvature will be critical to any potential impacts on Hawaii. The 1200 GMT GFDL model portrays a potent hurricane of at least Category 2 strength in the vicinity of Kauai by Monday, while the 1200 GMT Friday run of the HWRF projects a slower approach that could put other Hawaiian islands at risk in the Tuesday/Wednesday time frame. The most recent official outlook from the Central Pacific Hurricane Center places Kilo west-southwest of Kauai as an intensifying Category 2 storm. People throughout Hawaii need to keep very close tabs on Kilo, which could become the most significant hurricane to approach Hawaii since the devastating Hurricane Iniki of 1992. An Air Force C-130 hurricane-hunter aircraft is scheduled to fly into Kilo late Friday night.

Elsewhere in the tropics
Invest 97 could become a subtropical storm south of Bermuda late this weekend or early next week, most likely moving north or northeastward away from the United States. Although Typhoon Goni and Typhoon Atsani continue to rage over the Northwest Pacific, both have embarked on recurvature, and neither of them pose major landfall threats over the next day or so, although Goni will pass near Japan’s southern islands over the weekend and could strike Kyushu as a weakening typhoon early next week. In the Central Pacific, Tropical Depression 4-C could become a named storm but will remain far at sea.

I’ll have a full update by midday Saturday at the latest.

Bob Henson


Figure 6. Tracks of all tropical cyclones (tropical depressions, tropical storms, and hurricanes) to pass within 100 miles of the Hawaiian Islands, 1949 - 2014. Hurricanes approaching from the east typically fall apart before they reach Hawaii due to the cool waters and dry air that lie to the east of the islands. Only two named storms approaching from the east have hit the islands since 1949, an unnamed 1958 tropical storm and Tropical Storm Iselle of 2014, which hit the Big Island. Hurricanes approaching from the south represent the biggest danger to the islands, due to the warmer waters and more unstable air present to the south. The only two major hurricanes to have affected the islands since 1949, Hurricane Iniki of 1992 and Hurricane Dot of 1959, both came from the south. Image credit: NOAA/CSC.

Hurricane

July 2015: Warmest Month on Record Globally

By: Jeff Masters and Bob Henson , 9:56 PM GMT on August 20, 2015

The Atlantic and Pacific tropics were buzzing with activity on Thursday (see bottom of this post for a very brief update), but Thursday brought other big news as well: July 2015 was the warmest single month in 1627 months of global records that go back to January 1880, said NOAA's National Centers for Environmental Information (NCEI). The globally averaged temperature above both land and ocean surfaces was 1.46°F (0.81°C) ahead of the 20th-century average. This trumps the record for any month that was set in July 1998, surpassing that value by 0.08°F (0.14°C). On average, July is the warmest month of the year globally, tpyically driven by midsummer conditions across the Northern Hemisphere’s extensive land areas. However, according to NOAA, record warmth across much of the Pacific and Indian oceans played a major role in July’s new global record. NASA also rated July 2015 as the warmest July on record. July 2015's warmth makes the year-to-date period (January - July) the warmest such period on record, according to both NOAA and NASA. A potent El Niño event in the Eastern Pacific that crossed the threshold into the "strong" category in early July continues to intensify, and strong El Niño events release a large amount of heat to the atmosphere, typically boosting global temperatures by at least 0.1°C. This extra bump in temperature, when combined with the long-term warming of the planet due to human-caused emissions of heat-trapping gases like carbon dioxide, makes it extremely likely that 2015 will be Earth's second consecutive warmest year on record.


Figure 1. Departure of temperature from average for July 2015, the warmest single month for the globe since record keeping began in 1880. Large areas of record warmth were analyzed across many parts of the Indian and Pacific oceans, as well as in northern South America, southeast Africa, and parts of southern Europe. Image credit: National Centers for Environmental Information (NCEI) .

Global satellite-measured temperatures in July 2015 for the lowest 8 km of the atmosphere were the 10th warmest in the 37-year record, according to the University of Alabama Huntsville (UAH). The lowest 8 km of the atmosphere heats up dramatically in response to moderate to strong El Niño events, with a time lag of several months--as occurred during the El Niño events of 1998 and 2010. Thus, we should see Earth's lower atmosphere temperature hit record levels late this year and/or early in 2016.

Deadliest weather disaster of July 2015: monsoon floods in Asia
The deadliest weather-related disaster of July 2015 was flooding in Asia due to the annual monsoon, which claimed over 200 lives in Pakistan, Myanmar, Afghanistan, Bangladesh, Vietnam, India, and China. Severe flooding in these countries continued during the first ten days of August, bringing the total monsoon death toll to over 400, as reported by Bob Henson in his August 11 post.


Figure 2. Navigating a flooded area of Peshawar, Pakistan, on July 26, 2015. Torrential rains and floods in Pakistan left 36 dead and affected more than 250,000 people, disaster management officials said July 25, with swollen rivers and water channels damaging hundreds of villages. Photo credit: A Majeed/AFP/Getty Images.



Two billion-dollar weather disasters in July 2015 in China
Two billion-dollar weather-related disasters hit the Earth last month, both in China, according to the July 2015 Catastrophe Report from insurance broker Aon Benfield: Typhoon Chan-hom ($1.6 billion in damage) and flooding July 20 - 24 that caused $1.2 billion in damage. With twelve billion-dollar weather disasters so far in 2015, Earth is on pace for its lowest number of such disasters since 2004, when sixteen occurred.


Disaster 1. Typhoon Chan-hom made landfall about 80 mi south-southeast of Shanghai, China on July 11, killing 16 people and doing at least $1.5 billion in damage. The typhoon did another $100 million in damage to Guam, Japan, Taiwan, and Korea. In this image, we see people watching huge waves from Chan-hom pounding Wenling, in east China's Zhejiang province, on July 10, 2015. Image credit: STR/AFP/Getty Images.


Disaster 2. Heavy rainfall in China from July 20 - 24 killed 28 people and did $1.2 billion in damage. More than 238,000 residents were evacuated as floods and landslides destroyed 7,770 homes and damaged 35,100. In this picture, we see vehicles stranded on a flooded road in Wuhan, Hubei Province of China, on July 23, 2015, when 160.2 millimeters (6.31") hit the city. This was their heaviest daily rainfall since 1998, according to Changjiang Times. Image credit: ChinaFotoPress/Getty Images.

Arctic sea ice falls to 8th lowest July extent on record
Arctic sea ice extent during July 2015 was the 8th lowest in the 36-year satellite record, according to the National Snow and Ice Data Center (NSIDC). A large area of high pressure set up shop north of Alaska, and a strong area of low pressure formed over Northeastern Eurasia. The circulation around these features brought sunny skies and a warm flow of air into the Arctic that led to rapid ice loss. This Arctic Dipole pattern also occurred in all the summer months of 2007, and helped support the record 2007 summer reduction in sea ice extent. (Note that the record was beaten in 2012, a year that did not feature an Arctic Dipole pattern.) The Arctic Dipole pattern diminished in early August 2015, but substantial melting has continued into the middle of the month.

Notable global heat and cold marks set for July 2015
Hottest temperature in the Northern Hemisphere: 52.8°C (127.0°F) at Mitribah, Kuwait, July 30
Coldest temperature in the Northern Hemisphere: -22.5°C (-8.5°F) at Summit, Greenland, July 30
Hottest temperature in the Southern Hemisphere: 37.6°C (99.7°F) at Floriano, Brazil, July 10
Coldest temperature in the Southern Hemisphere: -80.2°C (-112.4°F) at Dome A, Antarctica, July 2

Major stations that set (not tied) new all-time heat or cold records in July 2015
Akkuduk (Kazakhstan) max. 46.8°C July 1
Boulogne sur Mer (France) max. 35.4C° July 1
Melun (France) max. 39.4°C July 1
Dieppe (France) max. 38.3°C July 1
Urumita (Colombia) max. 42.2°C July 1
Dzhusaly (Kazakhstan) max. 46.2°C July 2
Volkel (Netherlands) max. 36.9°C July 2
Twenthe (Netherlands) max. 36.1°C July 2
Leeuwarden (Netherlands) max. 34.0°C July 2
Valledupar-Villa Rosa (Colombia) max. 42.4°C July 3
Bad Lippspringe (Germany) max. 37.9°C July 4
Giessen (Germany) max. 38.1°C July 4
Repelon (Colombia) max. 40.9°C July 4
Frankfurt (Germany) max. 39.0°C July 5
Ohringen (Germany) max. 38.5°C July 5
Wurzburg (Germany) max. 38.6°C July 5
Kiztingen (Germany) max. 40.3°C July 5, New national record high for Germany
Kahl (Germany) max. 39.8°C July 5
Bad Durkheim (Germany) max. 39.7°C July 5
Neunkirchen (Germany) max. 39.2°C July 5
Hannover City (Germany) max. 39.0°C July 5
Aigle (Switzerland) max. 36.1°C July 5
Gerona Airport (Spain) max. 41.3°C July 5
Gerona St Daniels (Spain) max. 42.2°C July 5
Cienfuegos (Cuba) max. 37.0°C July 6
Barcelonette (France) max. 34.3°C July 6
Mende (France) max. 36.1°C July 6
Gap (France) max. 36.9°C July 6
Saint-Martin-d’Heres (France) max. 40.7°C July 7
Lezigneux (France) max. 39.9°C July 7
Embrun (France) max. 36.7°C July 7
St Etienne (France) max. 41.1°C July 7
Sainte-Leocadie (France) max. 35.4°C July 7
Grenada Airport (Spain) max. 43.1°C July 7
Grenada City (Spain) max. 43.9°C July 7
Lerida (Spain) max. 43.1°C July 7
Zaragoza (Spain) max. 44.5°C July 7
Geneva (Switzerland) max. 39.7°C July 7
Nyon/Changins (Switzerland) max. 38.0°C July 7
Payerne (Switzerland) max. 37.9°C July 7
Neuchatel (Switzerland) max. 37.8°C July 7
Fribourg (Switzerland) max. 36.6°C July 7
Neuenburg (Switzerland) max. 37.8°C July 7
Wynau (Switzerland) max. 37.2°C July 7
Evolene (Switzerland) max. 28.4°C July 7
Plaffeien (Switzerland) max. 32.0°C July 7
La Fretaz (Switzerland) max. 29.9°C July 7
Oberstdorf (Germany) max. 35.6°C July 7
Innsbruck City (Austria) max. 38.2°C July 7
Qaanaaq (Greenland/Denmark) max. 20.4°C July 8
Ardebil (Iran) max. 40.2°C July 10
Jucaro (Cuba) max. 36.8°C July 10
Riohacha (Colombia) max. 40.6°C July 13
Yuzawa (Japan) max. 36.8°C July 13
Washikura (Japan) max. 29.0°C July 13
Tajima (Japan) max. 34.8°C July 13
Niitsu (Japan) max. 37.9°C July 13
Ogata (Japan) max. 38.3°C July 13
Uozu (Japan) max. 37.9°C July 13
Nanao (Japan) max. 37.4°C July 13
Yamada (Japan) max. 37.5°C July 14
Kasenuma (Japan) max. 36.7°C July 14
Marumori (Japan) max. 37.6°C July 14
Yanagawa (Japan) max. 39.1°C July 14
Kawauchi (Japan) max. 35.7°C July 14
Ononimachi (Japan) max. 35.8°C July 14
Buzaubaj (Uzbekistan) max. 48.2°C July 14
Limoges Airport (France) max. 37.3°C July 16
Grazzanise (Italy) max. 39.8°C July 17
Split Airport (Croatia) max. 39.4°C July 18
Krems (Austria) max. 38.3°C July 19
Senj (Croatia) max. 39.7°C July 22
Rab (Croatia) max. 39.3°C July 22
Zadar Airport (Croatia) max. 39.0°C July 22
Zavizan (Croatia) max. 28.3°C July 22
Ronchi dei Legionari (Italy) max. 39.2°C July 22
Aviano (Italy) max. 38.3°C July 22
Vsetin (Czech Republic) max. 36.8°C July 22
Osako (Japan) max. 36.4°C July 22
Esashi (Japan) max. 37.2°C July 22
Kanayama (Japan) max. 36.1°C July 22
Altai (China) max. 39.5°C July 22
Hoboksar (China) max. 37.7°C July 22
Kaba He (China) max. 41.0°C July 22
Korla (China) max. 40.5°C July 24
Jucaro (Cuba) max. 37.0°C July 28
Contramaestre (Cuba) max. 38.2°C July 29
Isabel Rubio Airport (Cuba) max. 36.3°C July 29
Indio Hatuey (Cuba) max. 38.1°C July 30
Kirkuk (Iraq) max. 50.0°C July 30
Najaf (Iraq) max. 51.5°C July 30
Kanaqin (Iraq) max. 52.0°C July 30
Salahaddin (Iraq) max. 41.1°C July 31
Meigetsu (Japan) max. 37.8°C July 31
Vize Island (Russia) max. 9.2°C July 31



New all-time national and territorial heat records set or tied in 2015
As of August 14, 2015, ten nations or territories tied or set all-time records for their hottest temperature in recorded history in 2015, and one (Israel) set an all-time cold temperature record. For comparison, only two nations or territories set all-time heat records in 2014, and nine did in 2013. The most all-time national heat records held by any year is nineteen in 2010. Most nations do not maintain official databases of extreme temperature records, so the national temperature records reported here are in many cases not official. I use as my source for 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. Wunderground's weather historian Christopher C. Burt maintains a database of these national heat and cold records for 235 nations and territories on wunderground.com's extremes page. Here are the all-time national or territorial heat and cold records set so far in 2015:

Hong Kong set its national heat record on August 9, when the mercury hit 37.9°C (100.2°F) at Happy Valley.
Germany set a new national heat record of 40.3°C (104.5°F) twice this year: on July 5 and on August 7, at the Kitzingen station in Bavaria.
Vietnam tied its national heat record of 42.7°C (108.9°F) at Con Cuong on May 30.
Palau tied its national heat record of 34.4°C (94.0°F) at Koror Airport on May 14.
Venezuela set a new national heat record of 43.6°C (110.5°F) at Coro on April 29.
Laos tied its national heat record of 42.0°C (107.6°F) at Thakhek on April 20.
Ghana set a new national heat record of 43.3°C (109.9°F) at Navrongo on April 10. This is the third time this year Ghana has tied or set a new all-time heat record.
Cocos Islands (Australian territory) tied their all-time heat record with 32.8°C (91.0°F) on April 8.
Equatorial Guinea set a new national heat record of 35.5°C (95.9°F) at Bata on March 17.
Wallis and Futuna Territory (France) set a new territorial heat record with 35.5°C (95.9°F) on January 19 at Futuna Airport.

Israel set a new national cold record of -14.2°C (6.4°F) at Merom Golan on January 10.

Special Mentions:
Antarctica set a new heat record for its mainland of 17.5°C (63.5°F) at Esperanza Base on March 24. Previous record: 17.4°C (63.3°F) at Marambio Base, set the previous day. The World Meteorological Organization (WMO) has appointed a committee to study this event and determine if this represents an official record for the continent. Note that this is a record for mainland Antarctica, not a territorial or continental record. The all-time maximum record for the continent and territory of Antarctica is 19.8°C (67.6°F) on January 30, 1982, in Signy Island, South Orkney, an island group located about 450 miles northeast of the tip of the Antarctic Peninsula, the northernmost portion of mainland Antarctica. Geologically, the South Orkney are on the Antarctic plate, and politically, they are part of Antarctica. This record was improperly listed as a territorial record for Antarctica in May's global summary.

Switzerland had its highest reliably measured temperature on record in Geneva on July 7, when the mercury hit 103.5°F (39.7°C). The only higher temperature ever measured in the country was a 106.7°F (41.5°C) reading on August 11, 2003 at Grono. As reported at the Swiss news site swissinfo.ch, this old record was achieved "using an old measurement technique of weather huts, which generally recorded temperatures a few degrees higher than modern instruments." Weather records researcher Maximiliano Herrera agrees that this year's 39.7°C reading in Geneva is the highest reliably measured temperature ever in Switzerland, though the August 11, 2003 temperature at Grono was probably warmer (near 40°C), after correcting for the known problems with the site.

Samoa was originally listed by Mr. Herrera as tying its national heat record with 36.5°C (97.7°F) on January 20 at Asau, but a subsequent review of the record revealed possible issues with the measurement equipment, so this record is dubious.

Kudos also to Mr. Herrera for supplying the data for the "Notable global heat and cold marks set for July 2015" and "Major stations that set (not tied) new all-time heat or cold records in July 2015" sections.

Danny strengthens slightly; 93C likely to become Hurricane Kilo and approach Hawaii
Tiny Hurricane Danny continues to gradually strengthen in the central Atlantic. At 5:00 p.m. EDT, Danny’s top sustained winds were up to 80 mph. Danny was still located far out to sea—more than 1000 miles east of the Windward Islands, moving west-northwest at just 10 mph—and there are no major changes to the outlook for Danny from our post this morning. Meanwhile, Invest 93C has been upgraded to Tropical Depression 3 in the central Pacific, and the Central Pacific Hurricane Center projects TD 3-C to become Hurricane Kilo by Saturday, perhaps curving toward the western Hawaiian islands as a Category 2 hurricane by Monday. We’ll have a full update on both systems by 1 PM ET Friday. See also Steve Gregory’s update from earlier this afternoon.

Jeff Masters and Bob Henson

Climate Summaries Climate Change

Danny Becomes First Atlantic Hurricane of 2015; Invest 93C Moves Toward Hawaii

By: Bob Henson , 4:42 PM GMT on August 20, 2015

The little storm that could, Danny, surged from weak tropical-storm status on Wednesday night to become the Atlantic’s first hurricane of the year on Thursday morning. At 11:00 am EDT Thursday, the top sustained winds in Hurricane Danny were estimated at 75 mph, or near minimal hurricane strength. Located in the remote central tropical Atlantic, near 12.5°N and 44.8°W, Danny remains far from land areas, roughly 1100 miles east of the Leeward Islands. Danny is moving west-northwest at about 12 mph, a fairly modest pace for tropical cyclones in this region.

Danny was largely stripped of its core convection (showers and thunderstorms) on Wednesday, as dry air filtered into its center, but the storm retained its overall structure and was able to rebuild central convection on Wednesday night, even managing to produce an eyewall formation and a visible eye over the last few hours. Danny’s quick intensification was facilitated by its small size (see Figure 3 below). Hurricane-force winds only spanned a region 20 miles in diameter on Thursday morning, with tropical-storm-force winds extending out up to 60 miles from the center. Smaller tropical cyclones are able to both intensify and weaken more rapidly, which makes intensity prediction especially challenging. That said, Danny’s rise to hurricane status was well forecast several days in advance by the recently upgraded HWRF model, and since Tuesday by the GFDL model. The leading statistics-based models, which are the most accurate guidance for intensity beyond about three days, also gave a solid heads-up that Danny could attain hurricane status by today.


Figure 1. A 1-km-resolution visible image of Tropical Storm Danny, collected by NOAA’s GOES-East floater satellite at 1315 GMT (11:15 am EDT) on Thursday, August 20. Image credit: University of Wisconsin/CIMSS.


Figure 2. Hurricane Danny as viewed from the International Space Station and tweeted on Thursday morning, August 20, by astronaut Scott Kelly. Image credit: NASA.


Figure 3. A side-by-side comparison of visible satellite imagery from Thursday, August 20, for Super Typhoon Atsani in the Pacific (left) and Hurricane Danny in the Atlantic (right), both at the same scale, shows how much tropical cyclones can vary in size. Even smaller systems like Danny can be devastating if they grow intense enough and strike a highly populated area. Composite image credit: Mark Lander, University of Guam.


The outlook for Danny
Despite the overnight growth spurt, Danny’s future as a hurricane remains highly uncertain. One of the two main obstacles it faces is a huge zone of relatively dry, dusty air that extends across most of the Atlantic around 20°N latitude (see Figure 4). Any large-scale intrusion of this air into Danny’s circulation would tamp down the instability that fuels convection. Thus far, Danny has been able to generate and consolidate enough shower and thunderstorm activity to fight off injections of dry air--another benefit of its small size. As it slowly gains latitude, Danny will be at increasing risk of falling victim to this zone of dry air, provided it does not shift to the northeast in tandem with Danny’s northwestward motion.


Figure 4. Hurricane Danny (small white area near the center of the Atlantic tropics) is tucked just south of a large belt of dry, dusty air, generated in part by the Saharan air layer (SAL) moving west from Africa. Danny’s projected west-northwest motion will bring it closer to this dry air mass. Image credit: University of Wisconsin/CIMSS and NOAA Hurricane Research Division.

Danny’s other nemesis is vertical wind shear, a perennial feature in the North Atlantic tropics during El Niño years. The widespread rising of warm air over the eastern tropical Pacific during El Niño helps foster unusually strong west winds at upper levels, pushing away from the Niño region into the Caribbean and western Atlantic. Such shear can easily disrupt the chimney-like vertical structure that helps to maintain a tropical cyclone’s strength. Currently, wind shear over the Caribbean is below the record values observed earlier this summer, but values of 30 - 60 mph remain widespread. (Anything above 20 knots, or about 23 mph, is problematic for tropical cyclone development.) For now, shear over Danny remains low, in part because its potent convection has produced a small zone of high pressure overhead. This weekend, Danny will be hard-pressed to avoid encountering a belt of higher shear, which would not only jeopardize its structure but also help push dry, dusty air into its circulation. If Danny survives that passage as a well-structured tropical cyclone, then it may encounter somewhat lower shear values ahead of its path early next week (see Figure 5), as a weak but persistent upper-level low is predicted to move northward from the Bahamas and a ridge builds in to replace it.



Figure 5. Predicted vertical wind shear between the 200 mb and 850 mb altitudes at at 0600 GMT on Tuesday, August 24, near the end of the five-day outlook shown below in Figure 6. Shear values above 20 knots (23 mph) are generally destructive to tropical cyclone circulation. Image credit: Levi Cowan, tropicaltidbits.com.


Figure 6. NHC’s outlook for Danny as of 11:00 am EDT Thursday.

Computer forecast models remain in general agreement on Danny’s path over the next five days, a west-northwest trek that would bring it close to the northernmost Leeward Islands, and perhaps eventually Puerto Rico, by early next week. NHC predicts that Danny will maintain Category 1 hurricane strength throughout the next four days, then weaken to strong tropical-storm levels by Day 5 (Tuesday, August 24). Assuming that Danny follows the projected track, its intensity would be modulated not only by the factors above but also by any interaction with islands. Danny should gradually grow in size, but if it remains a compact cyclone, any interaction with mountainous islands could have an unusually large detrimental effect on its intensity. Danny’s intensity forecast is more uncertain than usual because of its very small size: day-to-day conditions could produce dramatic spikes in strength, both upward and downward. Even so, the statistical and dynamical model guidance suggests that Danny is unlikely to strengthen beyond Category 1 levels over the next five days.


Figure 7. This infrared image from the GOES-East satellite, from 1445 GMT (10:45 am EDT) shows Invest 97L (top center) dwarfing Hurricane Danny (bottom center) in size, if not in organization. Image credit: NOAA/NHC.

Another disturbance in the North Atlantic
A disorganized cluster of convection--actually much larger than the entire circulation of Danny--has been designated Invest 97L. The storminess is associated with the upper-level low mentioned above, intersecting with a weak surface front. Sea-surface temperatures near 97L are 28-29°C (82-84°C), about 1-2°C above average, which is more than adequate for tropical development. Statistical guidance indicates that 97L could evolve into a subtropical or tropical storm as it drifts northward, but neither of the most reliable dynamical models for short-term intensity trends--the HWRF and GFDL--developed 97L in their 0000Z Thursday model runs. NHC gives 97L a 20% chance of development in the next two days and a 60% chance in the next five days.

Hawaiians need to keep a close watch on Invest 93C
Dynamical models are increasingly bullish on the odds that Invest 93C could become a historical rarity: a hurricane threatening the Hawaiian Islands. Now located about 900 miles south-southeast of Hilo, 93C is slowly organizing and has a large area of showers and thunderstorms associated with it. The Central Pacific Hurricane Center gives Invest 93C an 80% chance of development over the next 48 hours. The 0000Z and 0600Z Thursday runs of the GFDL model both strengthen 93C into a major hurricane by Tuesday, when the system could be within striking distance of the islands. While less extreme than the GFDL forecasts, the 0600Z Thursday run of the HWRF model also projected a solid Category 1 hurricane approaching the islands by Tuesday, and long-range statistical intensity models (the more reliable guidance beyond Day 3) agree that 93C has a good chance of reaching hurricane strength. Track models agree that 93C, now moving slowly west-northwest, will soon begin moving more briskly along a northwestward track that should continue through the weekend, putting it southwest of the islands by Sunday. The major uncertainty is how soon 93C will recurve toward the northeast, which in turn will determine whether 93C’s track intersects the northwest-southeast chain of Hawaii’s islands. As we noted yesterday, northward-moving hurricanes are very unusual near Hawaii, but the only two major hurricanes to have affected the islands since 1949, Hurricane Iniki of 1992 and Hurricane Dot of 1959, both came from the south.

Further west, the Central Pacific is also playing host to Invest 94C, far at sea, which shows no immediate signs of development.


Figure 8. Forecasts from various track models at 1200 GMT Thursday, August 20, bring Invest 93C northwestward over the next several days, putting it in position for a potential run at Hawaii early next week. These track forecasts are subject to large change over the next several days and should not be used as official guidance. A list of models and definitions can be found at the NHC website.


Figure 9. Typhoon Goni (left) and Super Typhoon Atsani (right) are unmissable in this enhanced infrared image from Japan’s Himiwari-8 satellite, collected at 1550Z (11:50 am EDT) on Thursday, August 20. Image credit: Japan Meteorological Agency and CIRA/RAMMB.

Atsani, Goni continue to rage across Northwest Pacific
Long-lived Super Typhoon Atsani and Typhoon Goni continue to make waves far out to sea in the Northwest Pacific. Atsani’s peak sustained 1-minute winds are 150 mph, down slightly from a peak yesterday of 160 mph, and a slow weakening trend should begin soon as Atsani starts to recurve and encounters cooler waters. Atsani could threaten southern Japan as a relatively weak typhoon early next week. Meanwhile, Goni is uncomfortably close to the northern Philippines and Taiwan, but models insist it will sharply recurve over the next several days, perhaps moving near Okinawa at Category 3 strength and approaching Kyushu, the southernmost of Japan’s large islands, early next week. Goni’s top winds are holding steady at 135 mph, making it a Category 4 storm, and it could briefly grow stronger over deep warm water before weakening as it approaches Japan.

Jeff Masters and I will have an update on the planet’s sizzling July later today. The next tropical update will be Friday morning at the latest.

Bob Henson




Hurricane

Danny’s Leg Up: A Convectively Coupled Kelvin Wave (CCKW)

By: Bob Henson , 11:38 PM GMT on August 19, 2015

Tropical Storm Danny might not be making the scene were it not for the help of a subtle but important atmospheric feature, called a convectively coupled Kelvin wave (CCKW), that’s contributed to a temporary break from hurricane-hostile El Niño conditions. Danny has changed little over the last few hours: as of 5:00 pm EDT Wednesday, peak sustained winds remained near 50 mph, and the latest National Hurricane Center outlook continues to bring Danny to hurricane status, though not until Friday. Tonight will be a good test of whether Danny can rebuild a solid convective core after its disruption from dry air and Saharan dust over the last 24 hours. For a more complete look at Danny and today’s other tropical activity, see our post from earlier this afternoon and the post from WU blogger Steve Gregory.

What’s a CCKW?
CCKWs are huge impulses, spanning thousands of miles, that move from west to east through the stratosphere, typically rolling along at about 30 to 40 mph. CCKWs are centered on the equator, with their effects progressively weaker as you move toward the subtropics. Like a giant chimney, each CCKW has a broad zone of rising air at its heart, tilted toward the west as you move up. The resulting circulation (see Figure 1) favors the development of showers and thunderstorms ahead of the CCKW, as low-level air converges. The resulting storms are then supported by upper-level divergence toward the center of the CCKW, plus low-level westerlies near the equator that can enhance cyclonic spin. An eastward-moving CCKW can intersect the train of westward-moving waves rolling through the Atlantic, giving one or more of them a boost that can help them consolidate into tropical cyclones.


Figure 1. Schematic cross section through a convectively coupled Kelvin wave (CCKW). Image credit: Michael Ventrice.


Global weather prediction models such as the GFS and ECMWF can now latch onto CCKWs and preserve them as they make their way around the globe. CCKWs are nondispersive waves, meaning they tend to maintain their structure and can survive long enough to make several trips around the world over the course of several weeks. This gives them a noteworthy predictive value, but it takes some time and training to analyze CCKWs, which are too subtle to be seen with the naked eye on satellite images. One of the most useful tools is a Hovmöller diagram (see Figures 2 and 4 below), a time-versus-longitude plot often used to analyze areas where outgoing infrared radiation is consistently strong or weak. A CCKW will often foster and suppress enough clouds along its path to appear in a Hovmöller diagram, where its signal can be separated from the noise of day-to-day storminess. Sometimes a CCKW will overtake an active phase of the slower-moving Madden-Julian Oscillation, providing a twofold boost to showers and thunderstorms at that location before the CCKW moves on.

“It can be hard to see CCKWs for sure in these [diagrams], as these waves often destructively interfere from time to time with other equatorial waves and/or standing oscillations,” Ventrice told me. “Thus, you'll see a clear eastward propagation over some location, a break, then continuation of the signal over another location to the east at a later time.”


Figure 2. A Hovmöller diagram showing CCKW activity in July-August 2006 as traced by reductions in outgoing infrared radiation (encircled bands), averaged by longitude (bottom axis) over the main development region of the Atlantic (7.5°N to 12.5°N). Negative values of longitude are °W; positive values are °E. The CCKWs occur in the context of other, stronger features, so it can take careful analysis to find them. The green “D” near the center of the image indicates where and when Tropical Storm Debby became a named storm, supported by a CCKW passing from upper left to lower right through the “D.” Image credit: Michael Ventrice.


WSI scientist Michael Ventrice unraveled some of the links between CCKWs and hurricane formation several years ago, as part of his doctoral work at the University at Albany, State University of New York. He and colleagues at WSI now consult the CCKW regularly, and Ventrice has helped NHC develop code that allows the center to analyze CCKWs on a daily basis. “It seems to have really caught on among forecasters in the last three years,” Ventrice told me. For example, a forthcoming paper in Monthly Weather Review by Carl Schreck (North Carolina State University) surveys CCKWs around the globe and their role in tropical cyclogenesis.

Ventrice’s own research includes a set of three CCKW-related papers published in Monthly Weather Review in April 2012, July 2012, and June 2013. Ventrice’s dissertation is online, including an analysis of how a CCKW assisted in the formation of Tropical Storm Debby in 2006. There’s much more about CCKWs, including daily analyses, on Ventrice’s personal website. He also discussed CCKWs as part of a guest post on this blog last year.

How could a CCKW help give Danny a kick-start?
I was intrigued on Tuesday when a tweet from @MJVentrice credited a weak CCKW with assisting in the formation of the tropical depression that became Danny. According to Ventrice, this CCKW formed near the International Date Line around August 8, then amplified as it moved into the Atlantic basin. It’s now centered over the heart of the main development region, the swath of deep tropics in the north Atlantic that plays host to many of each year’s tropical cyclones. Over the last several days, this CCKW has likely enhanced low-level moisture and convection on its forward flank, in the eastern Atlantic, which would have helped nourish the easterly wave that became Danny as it came off the African coast. Only a minority of such waves make it to tropical-storm status; Ventrice believes that CCKWs are one of the key factors that can make or break such a system.


Figure 3. Tropical Storm Danny is now on the western edge of an eastward-moving CCKW, as indicated by this composite of Atlantic rain rates (shaded), departures from average wind at the 200-millibar height (vectors), and velocity potential anomalies at the 200-millibar height (contours). The velocity potential anomalies are related to upper-level divergence (blue contours) and convergence (red contours), with divergence favoring upward motion. For the latest version of this image, see Michael Ventrice’s website. Image credit: Michael Ventrice.


As Danny and the CCKW move in opposite directions, Danny is finding itself left behind. “The environment behind the CCKW can stay favorable for a couple of days, where you have enhanced low-level spinning, reduced vertical wind shear, and enhanced outflow,” Ventrice told me. (Carl Schreck’s new paper also finds that tropical cyclogenesis is favored for several days behind the crest of a CCKW.) However, Danny appears to be already feeling a lack of support, with the peak of the CCKW having long passed it by. “This may be the reason why we are seeing convection decouple from Danny this afternoon [Wednesday], as the low-level circulation associated with Danny is becoming exposed,” he said.

Fairly soon, Danny will encounter the more suppressed environment well behind the CCKW, together with the likelihood of higher wind shear and drier air favored by the ongoing El Niño regime. “I’ve seen a lot of cases where a mature tropical cyclone gets run over by the suppression behind a CCKW,” said Ventrice. “Can the hurricane create its own environment to protect itself from this?” One thing is for sure: the more latitude Danny gains, the less it will be influenced--for better or worse--by this CCKW. Meanwhile, it’s possible that the CCKW that gave Danny a boost will go on during the next week to favor development of another one or two easterly waves now over Africa, as suggested by some long-range model runs. After that, said Ventrice, “it doesn’t look like there’ll be much in the way of CCKW activity over the next two weeks.”

On Thursday morning, we’ll have a complete update on Danny, as well as Invest 93C in the Central Pacific and Typhoon Goni and Super Typhoon Atsani in the Northwest Pacific. Later on Thursday, watch for our summary of July’s global climate. In the meantime, here's a fascinating infrared loop of Atsani's core, featuring 2-km-resolution imagery from the Himiwari-8 satellite, courtesy of the University of Wisconsin/CIMSS. Jeff Masters is offline most of this week but will be fully back on board next week.

Bob Henson



Figure 4. A Hovmöller plot for May-September 2015 from the Cooperative Institute of Climate and Satellites-North Carolina (CICS-NC) showing variations in 200-mb zonal (west-to-east) wind speed by longitude over time. After August 17, the plot draws on long-range predictions from the NOAA Climate Forecast System model. Tropical cyclones over the Atlantic and Pacific are shown as lettered hurricane symbols. CCKWs are encircled in blue. A weak CCKW at lower right may have played a role in the development of Tropical Depression 11 (“E”) in the Northeast Pacific as well as Danny (“F”) in the Atlantic. Regularly updated plots are available from the Cooperative Institute of Climate and Satellites–North Carolina. Image credit:

Hurricane

Danny Strengthens in Atlantic; Goni, Atsani Rev Up in Pacific

By: Bob Henson and Jeff Masters , 6:28 PM GMT on August 19, 2015

Tropical activity is on the upswing today, with two intensifying cyclones in the Northwest Pacific (one a super typhoon), a Central Pacific disturbance that could make a run at Hawaii, and a tropical storm in the Atlantic threatening to become the region’s first hurricane of the year. At 11:00 am EDT Wednesday, Tropical Storm Danny was located near 11.2°N, 41.1°W, or about 1400 miles east of the Lesser Antilles, moving just north of due west at about 12 mph. Danny was struggling this morning, in part due to having ingested dry air from its northwest. Showers and thunderstorms were more scattered than last night, and much of Danny’s central convective core had dissipated. However, in its 11:00 am EDT discussion, the National Hurricane Center notes that the structure of the inner core has improved somewhat since Tuesday, and core convection was beginning to rebuild on Wednesday afternoon. Apart from the vast swath of Saharan dust and dry air that lurks just to the north, conditions remain favorable for Danny to intensify, with sea-surface temperatures (SSTs) around 28°C (82°F) and vertical wind shear quite low (less than 10 mph) for at least the next couple of days.


Figure 1. Infrared satellite image of Tropical Storm Danny, collected by the GOES-East floater satellite at 1715 GMT (1:15 pm EDT) on Wednesday, August 19. Image credit: NOAA/NESDIS.


Figure 2. Dry air dominates the northeast Caribbean and north Atlantic around latitude 20°N, as Tropical Storm Danny (lower right of image) continues on a west-northwest track. Image credit: National Hurricane Center.


Figure 3. Various track models at 1200 GMT Wednesday largely agreed on a west-northwest track for Tropical Storm Danny over the next five days (120 hours). A list of models and definitions can be found at the NHC website. Image credit: NCAR/RAL Tropical Cyclone Guidance Project.


This morning’s models (1200 GMT Wednesday) have converged more strongly on a steady west-northwest bearing for Danny, predicted to continue throughout the next five days. Such a track would bring Danny close to the northernmost Leeward Islands by Sunday or Monday, as projected by the official NHC outlook (see Figure 4 below). There’s less model agreement on Danny’s future intensity, although it appears that Danny may peak in the next 2 or 3 days before entering a period of greater struggle. The 1200 GMT run of the recently upgraded HWRF model, which has been bullish on Danny from the start, make Danny a strong Category 1 hurricane by the end of this week, then weakens it back to tropical-storm strength. The GFDL model, the other of our two most reliable dynamical models for intensity, had retreated from its initial skepticism about Danny by Tuesday. The 1200 GMT GFDL run brings Danny to a similar peak intensity as the HWRF, with a minimal central pressure close to 970 millibars. Statistical intensity models, which are the most reliable guidance beyond Day 3, have begun to pull back from earlier forecasts that Danny would reach solid Category 2 strength by the weekend.


Figure 4. NHC’s outlook for Danny as of 11:00 am EDT Wednesday.

The official NHC outlook from 11:00 am EDT Wednesday projects Danny to reach Category 1 strength by Friday, with a slight decrease in strength over the weekend. Beyond the five-day outlook period, questions multiply as to how strong Danny will be (if it survives) and whether it might continue tracking toward the Bahamas or recurve out to sea. It remains far too soon to know how much of a threat Danny might pose to the United States next week, assuming it holds together. NOAA is scheduled to begin research flights around Danny on Friday, using its Gulfstream V and P-3 aircraft, while Air Force hurricane-hunter flights into the storm are currently slated to begin on Saturday afternoon.



Figure 5. Infrared satellite image of the central tropical Pacific, collected by the GOES-West satellite at 1630 GMT (12:30 pm EDT) Wednesday, August 19, showing Hawaii (center) along with Invest 93C, the large, poorly organized system to its southeast. Image credit: CIMMS Tropical Cyclones.

Invest 93C: a potential threat for Hawaii
Forecasters in Hawaii are already casting a wary eye toward Invest 93C, which was located about 915 miles south-southeast of Hilo, Hawaii, at 2:00 am HST (8:00 am EDT) Wednesday. Convection is disorganized but widespread in the vicinity of 93C. Now moving slowly toward the north, 93C is predicted to turn toward the northwest over the next several days before recurving northeastward. The particulars of that track—still to be determined--are crucial to whether 93C might impact Hawaii. The 0600 GMT Wednesday run of the GFDL model suggests the possibility that 93C could affect the Big Island as a strong tropical storm or minimal hurricane late in the weekend, while the 0600 GMT run of the HWRF model similarly develops 93C but recurves it well to the east of Hawaii. The 1200 GMT runs of both models paint a similar picture, and other models agree that 93C is likely to become Tropical Storm Kilo over the next several days. This year has been active in the Central Pacific, with an assist from unusually warm SSTs (currently running about 1-2°C above average south of Hawaii) partially associated with El Niño. SSTs south of Hawaii are well above the 26°C threshold for supporting tropical cyclones, and the high wind shear now over Hawaii (40 – 60 knots) is predicted to relax significantly over the next several days, as a weak upper trough is replaced by a building ridge. Tropical cyclones approaching Hawaii from the south can generally maintain their strength more easily than those approaching from the east, as they spend less time over marginally warm water and may experience less wind shear. The two most intense hurricanes to strike Hawaii in modern times—Iniki (1992) and Dot (1959)—both arrived at Kauai from the south. The Air Force is currently scheduled to begin reconnaissance flights into 93C starting on Friday afternoon.


Figure 6. Tracks of all tropical cyclones (tropical depressions, tropical storms, and hurricanes) to pass within 100 miles of the Hawaiian Islands, 1949 - 2014. Hurricanes approaching from the east typically fall apart before they reach Hawaii due to the cool waters and dry air that lie to the east of the islands. Only two named storms approaching from the east have hit the islands since 1949, an unnamed 1958 tropical storm and Tropical Storm Iselle of 2014, which hit the Big Island. Hurricanes approaching from the south represent the biggest danger to the islands, due to the warmer waters and more unstable air present to the south. The only two major hurricanes to have affected the islands since 1949, Hurricane Iniki of 1992 and Hurricane Dot of 1959, both came from the south. Image credit: NOAA/CSC.



Figure 7. Full-disk visible image from Japan’s Himiwari-8 satellite, collected at 1100 GMT on August 19, showing twin typhoons Goni (left) and Atsani (right). Image credit: Japan Meteorological Agency, courtesy Tom Niziol, The Weather Channel.

Goni and Atsani roil the Northwest Pacific
Like two ghostly eyes peering out from the sea, the centers of Typhoon Goni and Super Typhoon Atsani are gazing up at satellites monitoring their powerhouse development. Now a Category 3 storm, with peak sustained 1-minute winds of 125 mph as of 1200 GMT Wednesday, Goni is maintaining a steadily westerly course over very warm waters. Models now agree that Goni will sharply recurve this weekend, and the model trend has been for the recurvature to happen before reaching Taiwan—good news for that populated island, still picking up the pieces from Typhoon Soudelor a few days ago. However, the recurvature will put Okinawa and Japan’s southernmost islands more at risk from a weakening Goni. As it recurves, Goni is projected by the Joint Typhoon Warning Center (JTWC) to peak as Category 4 strength, with winds topping out around 145 mph.

Well to the east, Category 5 Atsani was packing sustained winds close to 160 mph at 1200 GMT Wednesday as it continued its northwesterly course. Favorable SSTs and light wind shear should allow Atsani to continue racking up accumulated cyclone energy (ACE). The JTWC keeps Atsani at super-typhoon strength (1-minute sustained winds of at least 150 mph) for at least another day before gradual weakening begins. Atsani is expected to recurve long before reaching Japan.

If Goni intensifies a bit more than predicted, it’s possible we will have two super typhoons at the same time. According to Phil Klotzbach (Colorado State University), this has happened only five times since 1950, most recently in 1997 (the last year we had a strong and intensifying El Niño):

Pamela and Nancy (9/11/1961)
Mary and Lucy (8/17-8/18/1965)
Alice and Cora (9-2-9/3/1966)
Owen and Page (11/27/1990)
Ivan and Joan (10/17-10/19/1997)

We’ll have a post later today on how a subtle but important atmospheric feature called a convectively coupled Kelvin wave (CCKW) may be influencing Tropical Storm Danny. For more on Danny, see today’s update from WU blogger Steve Gregory.

Bob Henson and Jeff Masters





Hurricane

Tropical Storm Danny Gathers Strength in Atlantic

By: Bob Henson , 11:33 PM GMT on August 18, 2015

After more than a month between named Atlantic storms--a somewhat unusual occurrence between late July and late August over the last few years--Tropical Storm Danny was christened at 5:00 pm EDT Tuesday. Located at 10.9°N, 37.5°W, or about 1600 miles east of the Windward Islands, Danny was moving west at 12 mph. Sustained winds were at minimal tropical storm strength: 40 mph. Danny has a small core of heavy showers and thunderstorms (convection), surrounded by a fairly large envelope of clouds and scattered storms. Visible and infrared satellite loops show convection rapidly strengthening over the last several hours near Danny’s center and in surrounding bands.


Figure 1. Visible image of Danny collected by the GOES-East floater satellite at 1945 GMT Tuesday (3:45 pm EDT), just before it was upgraded to tropical-storm status. Image credit: NOAA/NESDIS.


Figure 2. NHC’s outlook for Tropical Storm Danny as of 5:00 pm EDT Tuesday.

There was little change in the prognosis for Danny from the NHC’s 11:00 am EDT advisory to the 5:00 pm edition (see Figure 2 above). Wind shear around Danny should remain below 10 - 15 mph for most of the next five days, and sea-surface temperatures along Danny’s path will average near 28°C (82°F) for that entire period. Both of these factors point toward a good chance that Danny will strengthen over the next several days. The NHC projects that Danny will become the year’s first Atlantic hurricane by Thursday morning and grow to Category 2 strength (sustained winds of 100 mph) by Saturday. This long-range forecast is consistent with the statistical models that show more skill than dynamical models at intensity prediction beyond 3 days. Of the two dynamical models most trusted for intensity forecasting, the recently upgraded HWRF has consistently called for Danny to develop into at least a strong Category 1 hurricane. The GFDL failed to develop Danny until today’s runs, but its 1200 GMT Tuesday run brings Danny well above the hurricane-strength threshold, much more in line with the HWRF. If Danny continues to develop at a healthy clip, a period of more rapid intensification later this week cannot be ruled out. Such phases remain very difficult to predict.

Most of the dynamical track models now move Danny toward the west-northwest at a fairly modest pace until this weekend, when a building ridge to the north should help push it at a faster rate. By that point, Danny would draw on oceanic heat content that gradually increases along its path. However, a large area of dry air and Saharan dust north of Danny may inhibit its development at times. With a solid convective core, Danny might be able to fend off interference from this dry, dusty air until it encounters pockets of stronger wind shear, a possibility that long-range models are suggesting for this weekend into early next week. Thus, there is no guarantee that Danny would maintain whatever strength it attains in the deep tropics, and it is still far too early to predict with any confidence how much of a threat Danny might pose to the United States if it survives the long trek. A small change in trajectory now would have big implications for the track many days from now.


Figure 3. Intensity forecasts for Tropical Storm Danny as of 1800 GMT Tuesday, August 18. Models shown are GFDL=Geophysical Fluid Dynamic Laboratory model; HWRF=Hurricane Weather Research Forecasting model; ICON and IVCN = blends of statistical and dynamical model guidance used at the National Hurricane Center.


Figure 4. Infrared satellite image of Typhoon Atsani from NASA’s Visible Infrared Imaging Radiometer Suite (VIIRS) at 1509 GMT Tuesday (9:09 am EDT). Image credit: NOAA/NASA and RAMMB/CIRA, courtesy Dan Lindsey.

In the Pacific: Goni, Atsani continue to rage
Two massive tropical cyclones in the Northwest Pacific--Typhoon Goni and Typhoon Atsani--continue to churn over open water, with no significant changes in strength over the last few hours. See this morning’s post for more on Goni and Atsani.

We’ll have a full update on Danny, Goni, and Atsani by midday Wednesday.

Bob Henson

Hurricane

Tropical Depression Forms in the North Atlantic

By: Bob Henson , 4:46 PM GMT on August 18, 2015

The fourth tropical depression of 2015 has developed in the central tropical Atlantic, and it could become the year’s first Atlantic hurricane by later this week. Advisories on TD 4 were initiated by the National Hurricane Center (NHC) at 11:00 am EDT Tuesday. The depression was located near 10.6°N, 36.5°W, moving west at about 13 mph. Top sustained winds were estimated at 35 mph, just below tropical-storm strength (40 mph). TD 4 is expected to become Tropical Storm Danny, the season’s fourth named system, by later tonight, according to the NHC. Interestingly, this is the first named system of the 2015 Atlantic season to begin its official life as a tropical depression. Ana began as a subtropical storm, while Bill and Claudette were tracked as tropical storms by NHC from the outset.


Figure 1. Infrared satellite image of a gradually organizing TD 4, collected by the GOES-East floater satellite at 1545 GMT (11:45 am EDT) Tuesday. Image credit: NOAA/NESDIS.


Figure 2. GOES-East floater visible image of TD 4 at 1545 GMT (11:45 am EDT) Tuesday. Image credit: RAMMB/CIRA.


The outlook for TD 4
As the first Cape Verde depression of the year, TD 4 formed well east of the Caribbean, where El Niño has been producing record amounts of vertical wind shear this summer. Even though the Atlantic tropics have been largely suppressed, as expected with this year’s strong El Niño event, TD 4 appears to be finding a window in time and space where some development is possible. Wind shear near the system is quite low, only around 5 knots (see Figure 3), and the region of stronger upper-level westerlies to the north of TD 4 could end up helping to support development by serving as an outflow channel. A massive area of Saharan dust and dry air lies just north of TD 4, but as mentioned in Tuesday’s morning’s NHC discussion, it appears that the depression is surrounded by enough moisture that it may be able to intensify even if it ingests some of this dry, dusty air.


Figure 3. Wind shear between upper and lower layers of the atmosphere across the North Atlantic. Lower values of shear, as shown above TD 4 (far right), support tropical development. Image credit: University of Wisconsin-CIMMS/NESDIS.


Figure 4. NHC’s outlook for TD 4 as of 11:00 am EDT Tuesday.

The 11 am EDT outlook from NHC brings TD 4 to hurricane strength by Friday morning and to Category 2 strength (sustained winds of 100 mph) by Saturday. This long-range forecast is consistent with the statistical models that show more skill than dynamical models at intensity prediction beyond 3 days. Of the two dynamical models most trusted for intensity forecasting, the HWRF has consistently called for TD 4 to develop into at least a strong Category 1 hurricane, while the GFDL has failed to develop TD 4, so the recently upgraded HWRF may end up closer to the mark in this case. Intensity prediction is still very challenging, so it is quite possible that TD 4 could be substantially weaker or stronger by this weekend than the current NHC forecast indicates. Over the last two years, the average 5-day error in NHC intensity projections was around 15 mph, or a bit more than half a category on the Saffir-Simpson Hurricane Wind Scale. This is substantially better than prior years: between 2000 and 2010, intensity errors at 5 days averaged more than 25 mph.

Looking ahead
TD 4 is many days away from any threat to the Leeward Islands. Most of the dynamical track models keep TD 4 moving west to west-northwest at a modest pace. The model consensus keeps TD 4 east of 50°W longitude until this weekend, when a building ridge to the north of TD 4 should help push it at a faster rate toward the islands. By that point, the system would draw on oceanic heat content that gradually increases along its path. It is far too early to predict with any confidence how much of a threat TD 4 might pose to the United States next week. Only a small change in trajectory this far out can have big implications for the track many days from now, and it remains to be seen whether dry air and dust will keep TD 4 from maximizing its potential for development.


Figure 5. Enhanced image from the MTSAT satellite, collected from the Northwest Pacific at 1532 GMT Tuesday, showing twin typhoons Goni (left) and Atsani (right). Image credit: NOAA/NESDIS.

Twin typhoons raking the open waters of the Northwest Pacific
Two intense, well-structured typhoons are churning their way toward Asia, posing no immediate threat to large land areas. Typhoon Goni, which surged to Category 4 strength on Sunday, has weakened somewhat as a result of an eyewall replacement cycle, although Goni remains a powerful, well-structured system. Now packing top 1-minute sustained winds of 115 mph, Category 3 Goni was located near 18.7°N, 132.9°E at 1200 GMT Tuesday, moving due west at about 18 mph. Typhoon Atsani is the tortoise to Goni’s hare: though it developed more slowly than Goni, it is now more powerful, with top 1-minute sustained winds of 140 mph. At 1200 GMT Tuesday, Category 4 Atsani was located near 17.0°N, 154.8°E, moving northwest at about 9 mph.

Goni is currently traveling near waters that were left slightly cooler in the wake of Typhoon Soudelor, which peaked at Category 5 strength in this region less than two weeks ago. Over the next couple of days, Goni will move into an area of progressively richer oceanic heat content (see Figure 6 below), which should support reintensification. Strong ridges over China and over the Northwest Pacific east of Japan will help keep Goni traveling on a due-west track until late this week, when the typhoon may attempt to recurve in between ridges. The latest JTWC outlook has recurvature occuring on a northward track just east of Taiwan, which would put the island on Goni’s weaker left-hand side; that would be good news for a population still reeling from Typhoon Soudelor. Meanwhile, Atsani is already traveling over rich oceanic heat content, and with upper-level shear relatively weak, Atsani is predicted in the latest JTWC outlook to reach Category 5 strength (around 160 mph) before recurvature and weakening begin over the weekend. It appears likely that Atsani will recurve before reaching Japan, but residents should not let their guard down just yet.

Bob Henson



Figure 6. Oceanic heat content over the western Pacific as of August 17. Image credit: NOAA/AOML.

Hurricane

96L Slowly Organizing; Two Powerhouse Typhoons in NW Pacific

By: Bob Henson , 8:02 PM GMT on August 17, 2015

While Invest 96L takes its time developing in the tropical Atlantic, typhoons Goni and Atsani haven’t been wasting much time in the Northwest Pacific, both becoming major typhoons in the last 24 hours. Neither typhoon poses an immediate threat to land, although Goni could spell big trouble for Taiwan or neighboring areas by week’s end (see below).


Figure 1. Visible image of Invest 96L, collected by the GOES-Floater satellite at 1745 GMT (2:45 pm EDT) on Monday, August 17. Image credit: NOAA/NESDIS.

David vs. Goliath? 96L fights the El Niño factor
As of 2:00 pm EDT Monday, Invest 96L was located near 10°N and 31°W, moving west at about 10 mph. Located along a broad monsoon trough that coincides with the Intertropical Convergence Zone, 96L remains only loosely organized, with a large but unconsolidated area of showers and thunderstorms. Vertical wind shear is light (less than 10 knots), and 96L will encounter warmer sea-surface temperatures as it moves west-northwest (up to 28°C, or 82°F, by later this week), so the large-scale conditions favor gradual strengthening. The National Hurricane Center has been increasing the odds that 96L will develop: in its 8:00 am and 2:00 pm EDT updates, NHC gave the system a 50% chance of becoming a tropical depression in the next 48 hours and a 70% chance over the next 5 days. The RAMMB/CIRA Tropical Genesis Index is also maintaining high odds for development. Among the favored models for intensity, the 1200 GMT Monday runs of the statistics-based LGEM and SHIPS models, which rely heavily on climatology, bring 96L to Category 2 strength by Thursday. The dynamics-based HWRF and GFDL models, which simulate tropical systems within nested high-resolution grids, diverge on the future of 96L. The HWRF develops 96L into a Category 1 hurricane by Thursday, while the GFDL fails to develop 96L significantly. As we discussed in last week’s post on tropical cyclone modeling, HWRF features a dramatic increase in resolution this year, so it will be interesting to see if it correctly pegs the fate of 96L.


Figure 2. A large swath of dry air and dust from the Saharan Desert dust is sweeping across the Atlantic just to the north of Invest 96L. Image credit: University of Wisconsin-CIMMS and NOAA Huricane Research Division.


While it seems that 96L has a reasonable shot at becoming a tropical storm (which would be named Danny), it also faces some obstacles. Foremost is a huge area of dry air and Saharan dust that extends across the tropical Atlantic just north of 96L’s path. As the system grows in size and strength, it would become more likely to ingest some of the dry, dusty air, which would hinder shower and thunderstorm activity. 96L may also encounter an increasing amount of vertical wind shear as it approaches the longitude of the Leeward and Windward Islands this weekend, assuming it survives up to that point. Over the northern Caribbean, shear has actually lessened from the near-record values observed earlier this summer, although shear values of 20 to 40 knots continue to prevail across the southern Caribbean. The ever-strengthening El Niño favors westerly wind at upper levels across this region, though it’s possible that the relative lull in shear over the northern Caribbean will continue as 96L approaches. A weak upper-level low is forecast to become pinched off near the Bahamas, south of a building ridge over the northwest Atlantic; this low could become a growing influence on 96L’s track and intensity as it moves west of longitude 60°W.

So far, a rare storm-free August
It’s been a very quiet August so far in the tropical Atlantic. Chris Dolce at weather.com points out that if we go another two weeks without a named storm, this will be the first such August since 1997—which happens to be the last year that featured an El Niño ramping up as strongly as the current one. Prior to 1997, the last August without any named storms was 1961 (which wasn’t an El Niño year).



Figure 3. Enhanced infrared image of Typhoon Goni from the MTSAT satellite at 1901 GMT (3:01 pm EDT) Monday, August 17. Image credit: NOAA/NESDIS.

Goni goes gonzo
The rapid strengthening of Typhoon Goni was a sight to behold on Sunday. As with Typhoon Soudelor, Goni wrapped an intense core of convection (showers and thunderstorms) around a tiny eye, allowing it to intensify at a phenomenal rate. Goni zoomed from tropical-storm strength (1-minute sustained winds of 65 mph) to Category 4 status (135 mph) from 0000 GMT Sunday to 0000 GMT Monday, according to the Joint Typhoon Warning Center. Most of that strengthening occurred in just 12 hours (1200 GMT Sunday to 0000 GMT Monday), when JTWC bumped up Goni’s estimated sustained winds from 75 mph to 135 mph. (There have been even more rapid intensifications: in 1983, Typhoon Forrest went from 75- to 173-mph sustained winds in 24 hours.)

Over the last 12 - 18 hours, an eyewall replacement cycle has put at least a temporary brake on Goni’s headlong strengthening. The original tiny eye has dissolved within a larger, somewhat elongated feature, and JTWC has held the estimated peak winds at 135 mph. Goni’s overall structure remains potent, with ample strong convection at its core and extensive outflow aloft. Goni will be traveling in a low-shear environment over the next several days across very warm waters with relatively high heat content, so there is every reason to expect further intensification after the eyewall replacement cycle is complete. JTWC brings Goni’s sustained winds up to the super-typhoon threshold (130 knots, or about 150 mph) by Thursday, and I would not be surprised at all to see Goni reach Category 5 status, perhaps sooner than Thursday. Although Goni is projected to weaken somewhat after that point, it would still remain a formidable typhoon. Steering currents are also expected to weaken, which leaves a great deal of uncertainty over where and how Goni might affect land. The most recent JTWC outlook has Goni moving slowly northward just east of Taiwan as a strong typhoon toward the end of this week, a track roughly consistent with the 1200 GMT Monday runs of the operational GFS and HWRF models. Any slowdown in Goni’s progress near the mountainous terrain of Taiwan or nearby islands could lead to torrential, destructive rainfall. Taiwan’s worst natural disaster was in 2009, when Typhoon Morakot (which peaked at Category 2 strength) slowed just as it approached and moved over the island. Morakot brought Taiwan its all-time rainfall records for a 24-hour period (1403 mm or 55.2” at Weiliaoshan) and for a 48-hour period (2327 mm or 91.6” at Alishan).


Figure 4. Projected track for Typhoon Goni from 1200 GMT Monday, August 17, to 1200 GMT Friday, August 22. Image credit: Joint Typhoon Warning Center.


Figure 5. Infrared image of Typhoons Goni (left) and Atsani (right), collected by Japan’s Himiwari-8 satellite at 1920 GMT on Monday, August 17. Image credit: JMA/MSC.

While nearby Typhoon Goni dazzled typhoon watchers with its rapid strengthening on Sunday, the larger, slower-growing Typhoon Atsani more than held its own, transitioning from a minimal hurricane (75 mph) to high-end Category 2 status (110 mph) from 1200 GMT Sunday to 1200 GMT Monday. At 1200 GMT Monday, Atsani was located near 15.0°N and 158.0°E, moving to the northwest at about 7 mph. Having completed a well-predicted turn toward the northwest, Atsani should continue heading in that direction over the next several days, with a gradual increase in forward speed and at least gradual strengthening until at least Thursday. The latest JTWC outlook has Atsani peaking in tandem with Goni at 1200 GMT Thursday, both with sustained winds of around 150 mph. Japan will need to watch Atsani closely, as any delay in recurvature beyond the 5-day period would increase the potential risk.

Will the Fujiwhara effect influence the twin typhoons?
Given how close they appear on satellite, a sharp-eyed observer might wonder if Goni and Atsani will be influenced by the Fujiwhara effect, where two closely spaced hurricanes or typhoons begin rotating around a point midway between the two. In a website on the Fujiwhara effect, the Hong Kong Observatory explains that two tropical cyclones need to be located within at least 1350 km in order for the effect to be a major factor. Goni and Atsani are currently about 2000 km (1250 miles) apart, and their projected tracks only increase that distance, so the two typhoons will most likely remain free agents.

Bob Henson


Hurricane

First Cape Verde Storm of 2015 Possible This Week

By: Bob Henson , 11:56 PM GMT on August 16, 2015

A tropical wave in the central Atlantic has the potential to develop into a tropical storm over the next several days. Invest 96L was gradually organizing near 10.0°N and 28.3°W at 1800 GMT Sunday (2:00 pm EDT), moving west at about 15 mph. Showers and thunderstorms (convection) blossomed around the wave on Saturday night before weakening on Sunday. Convection often subsides during the daytime and redevelops at night over incipient tropical cyclones. Invest 96L has a fairly large shield of moist air around it, separating it from a large area of dust and dry area further north and west (see Figure 2 below).

Figure 1. Enhanced infrared imagery (4 km resolution) from the Meteosat satellite, showing Invest 96L at 2145 GMT (5:45 pm EDT) on Sunday, August 16. Image credit: RAMMB/CIRA.

The outlook for 96L
The National Hurricane Center boosted the odds of development for 96L on Sunday afternoon, with a 50% chance of a tropical depression within 48 hours and a 60% chance in 120 hours. The 0000 GMT Monday update of the Atlantic Genesis Index from RAMMB/CIRA (Colorado State University) shows a 96% chance that 96L will develop into a depression over the next 48 hours and a 99% chance by 120 hours. The main factors behind these high probabilities are converging air at low levels, climatology, and relatively low vertical wind shear. Shear affecting 96L is projected to average only about 9 mph over the next five days, compared to an seasonal average of 22 mph, although shear will be gradually increasing through that five-day period. Sea-surface temperatures along 96L’s path will increase from about 26°C to 28°C, close to the climatological norm for the region and more than adequate for development.


Figure 2. Water-vapor imagery from the GOES-East satellite, collected at 2215 GMT (6:15 pm EDT) on Sunday, August 16. Invest 96L is located at the far lower right of the image. Dry air covering much the central Atlantic (light patch near center of image) could eventually interfere with the evolution of 96L. Image credit: NOAA/NHC.

Both the GFS and ECMWF operational models, our most reliable models for tropical cyclone track, bring 96L along a nearly due-west track over the next five days, with a gentle west-northwest turn by late in the week. The latest GFS ensemble includes some runs that move the system more rapidly toward the west-northwest. If the operational models are correct, 96L will remain south of 15°N though the upcoming week, keeping it over warm waters and well away from midlatitude systems that might interfere with its development. An upper-level trough will be sagging from the western Atlantic toward the Bahamas, potentially affecting 96L toward the weekend, but no large, deep troughs will be in place to steer the system dramatically northward for at least the next 5 or 6 days.



The intensity forecast for 96L is a bit more uncertain. Of the two most trusted models for intensity, the 1200 GMT Sunday run of the GFDL model brings 96L up to weak tropical-storm strength by midweek before a subsequent weakening. The 1200 GMT Sunday run of the HWRF model failed to develop 96L, but the 1800 GMT run brings 96L to midrange tropical storm strength by late Wednesday. Given the overall favorable conditions and the apparent healthiness of 96L’s structure, I’d rate the odds as being at least 50-50 that we will have a tropical storm in the central Atlantic before Friday.

Cape Verde season gears up
If 96L becomes a tropical storm, it will be Danny, the fourth named storm of this year’s Atlantic season. Based on data from 1966 through 2009, the fourth named storm typically occurs around August 23, so a tropical storm this week would be more or less on schedule. It would also be the first named storm to form in the Cape Verde region, where some of the most powerful Atlantic hurricanes originate. An informal definition of the peak Cape Verde season is from August 20 to September 20, although tropical storm formation becomes increasingly common throughout August across the deep tropics of the Atlantic. Already, a string of increasingly potent waves has been shuttling from Africa into the eastern Atlantic. However, all of these waves have collapsed as they approach the western Atlantic and the hostile conditions fostered by El Niño, including very high wind shear over the Caribbean and relatively stable air over much of the North Atlantic. Tropical cyclone activity often tapers off prematurely in the Atlantic during El Niño years, so it may become even more difficult to get Cape Verde storms toward September and October. We’ll see if this week’s wave happens to encounter El Niño at a weak point.

Figure 3. An infrared image from the MTSAT satellite, collected at 2132 GMT (5:32 pm EDT) on Sunday, August 16, showing typhoons Goni (left) and Atsani (right). Image credit: CIMMS Tropical Cyclone Group.

Twin typhoons in the Pacific
As expected, a pair of typhoons has developed in the Northwest Pacific over the weekend, and at least one appear destined to make a run at super typhoon status. Typhoon Atsani, packing minimal-typhoon winds of around 75 mph, was located near 14.4°N and 159.4°E at 2100 GMT Sunday. Over the weekend, Atsani has been a lackadaisical mover, drifting west-southwest at 7 mph, but the Joint Typhoon Warning Center (JTWC) projects that Atsani will take a somewhat speedier course this week toward the northwest. The JTWC outlook brings Atsani to Category 4 strength in the next four days, but there is a good chance that the typhoon will recurve before threatening any major land areas. Further west, Typhoon Goni is set to pose a more serious threat to East Asia. With winds already above 90 mph at 1800 GMT Sunday, Goni was located near 16.0°N and 142.7°E, moving west-northwest at about 10 mph. While Atsani has a sprawling, less focused circulation, Goni already has a large, symmetric shield of strong convection and appears to be intensifying rapidly. The JWTC projects Goni to reach the threshold of super typhoon strength (130 knots or about 150 mph) in about three days, as it moves in the general direction of Taiwan and the northern Philippines. One very worrisome factor is the projected weakening of the upper-level steering flow later this week. As noted by the JWTC, this could leave Goni moving very slowly in the vicinity of Taiwan, a scenario that could lead to potentially devastating rainfall.

I’ll have a full update on the Atlantic and Pacific by midday Monday. For more on Invest 96L, including a wealth of imagery, see the update posted on Sunday afternoon by Steve Gregory. Steve has also posted an special in-depth look at the evolution of this year’s El Niño event, including comparisons with prior events and some of the mechanisms that brought it about. He’ll follow up on Monday with a Part II post on some of the forecasts for this El Niño event.

Bob Henson

Hurricane

What's New in Tropical Cyclone Modeling? An Update from the Trenches

By: Bob Henson , 11:35 AM GMT on August 14, 2015

With each passing year, forecasters have ever-more-accurate numerical guidance on where tropical storms and hurricanes are most likely to track and how strong they’ll get.  Several of the leading models have undergone noteworthy improvements over the past year. Track models have gotten steadily better over the last couple of decades, whereas improvements in forecasting intensity have been much more difficult to come by (see Figures 1 and 2 below), so a great deal of energy has been focused on the latter. Below is a summary of what’s new and cool, based on interviews and email exchanges with the following experts:



--Richard Pasch, Senior Hurricane Specialist, NOAA National Hurricane Center (NHC)

--David Richardson, Head of Evaluation, Forecast Department, European Center for Medium-Range Weather Forecasts (ECMWF)

--Julian Heming, Tropical Cyclone Specialist, UK Met Office (UKMET)
 
If you’re unfamiliar with the major models discussed below, see this overview by Jeff Masters and a somewhat more technical summary from NHC. Jeff’s post “Which Model Should You Trust?”, from August 2014, offers another excellent set of guideposts.
 
How are NHC’s hurricane forecasts doing?
Before diving into model improvements, let’s take a look at how NHC has fared over recent years in its own predictions, which rely heavily on the models below. Here are two graphics from NHC’s 2014 Forecast Verification Report, released in March 2015.
 

Figure 1. Verification of official NHC hurricane track forecasts for the Atlantic, 1990 - 2014. Over the past 25 years, 1 - 3 day track forecast errors have been reduced by 60 - 75%. Track forecast error reductions of 30 - 40% have occurred over the past ten years for 4- and 5-day forecasts. Image credit: 2014 National Hurricane Center Forecast Verification Report.




Figure 2. Verification of official NHC hurricane intensity forecasts for the Atlantic, 1990 - 2015. After more than two decades with little improvement, intensity forecasts have become notably more accurate in the 2010s, due to model improvements as well as a relative lack of strong and/or rapidly developing hurricanes.  Image credit: 2014 National Hurricane Center Forecast Verification Report. 



ECMWF

The ECMWF’s Integrated Forecast System is widely considered to be the world's best at tropical cyclone (TC) track forecasting, although it is sometimes beaten out by the GFS model (below). The ECMWF got a big boost in attention after it performed admirably during 2012’s Hurricane/Superstorm Sandy. The ECMWF was the first major model to call for Sandy to arc westward into New Jersey, a full week before the storm made landfall and several days ahead of the GFS. The high-resolution version of the ECMWF (16 km between grid points) is run every 12 hours; it is accompanied by a 51-member ensemble that’s run at lower resolution (32 km between grid points). Each ensemble run starts out with slightly different initial conditions, generated randomly to simulate the actual uncertainty in starting-point observations of the atmosphere. Ensembles help flesh out when the future of a tropical cyclone is fairly straightforward to predict, or when it's highly uncertain. Another strength of the ECMWF ensemble runs is that they include interaction between the atmosphere and ocean, which can help improve intensity prediction. “Recent work shows the importance of this coupling, at least in some situations,” said ECMWF's David Richardson.



The latest version of the ECMWF model, introduced in May, has significant changes to model physics and the ways in which observations are brought into and used within the model. The overall improvements include better portrayal of clouds and precipitation, including a more accurate depiction of intense rainfall. The main effect of the model upgrade for tropical cyclones is slightly lower central pressure. During the first 3 days of a forecast, the ECMWF has tended to have a slight weak bias on tropical cyclones; the new version is closer to the mark. From Day 5 onward, however, the new version adds to the preexisting tendency in those time frames to make hurricanes and typhoons too deep.

UKMET

Forecasters at the UK Met Office are already seeing benefits from two major upgrades to its Global Model. One is the adoption of a new dynamic core, improvements in model physics, and an increase in horizontal resolution (now 17 km between grid points) in July 2014, which affected many characteristics of the model. This provided a big improvement in TC track forecasts: errors were reduced by about 8% when the new version was tested offline in 2012, and by about 18% when it was run alongside the old one during actual events from April to July 2014.

“The Met Office Global Model has historically been far too weak for most TCs,” Julian Heming told me. “However, the new model configuration is far more energetic, and TC intensity errors are significantly reduced, particularly at longer lead times.” To produce even more improvement in TC intensity forecasting, a new assimilation scheme was introduced in February. Each model run now incorporates the observed locations and central pressures of tropical cyclones at the model initialization time, as provided by the various warning centers around the world. These data are also interpolated for the six hours before the model run and extrapolated for the two subsequent hours, again based on warning-center advisories. “This is a totally new method of initializing TCs for us,” Heming said. The change not only further reduced the UKMET’s weak bias on TC strength, but it had a somewhat unexpected benefit: track errors in testing went down by 6%. Together, the updates of July 2014 and February 2015 reduced track error by as much as 30% in one test period, according to Heming.
 
UKMET also runs a 24-member ensemble system, dubbed MOGREPS-G, that includes the upgrades above. The ensemble is run every 6 hours, out to 7 days ahead, with a grid spacing of around 33 km.





Figure 3. The last two years have seen marked improvement in the UKMET Global Model predictions of tropical cyclone intensity, especially at longer time frames. Shown here are Northern Hemisphere results through July 20, 2015, with the average error for each year (left axis) shown for various lead times (bottom axis). Image credit: UK Met Office, courtesy Julian Heming.

GFS
A well-publicized upgrade to the GFS model at the start of 2015 was made possible by a large increase in available computing power. The upgrade significantly boosted the model’s horizontal resolution, which increased from 26 km to 13 km (a fourfold jump, since it includes both east-west and north-south directions). However, there may not be major improvements evident right away in TC track or intensity, because the representation of atmospheric physics in the new GFS has not yet been tweaked to maximize the value of the higher resolution. “The new GFS has been doing fairly well this year, although it lags other global models in predicting east Pacific tropical cyclone formation,” said NHC’s Richard Pasch. The ensemble version of the GFS (GEFS), which includes 20 members, is run at the coarser resolution of 55 km.



GFDL
The highly regarded GFDL model has gotten a few tweaks this year, but changes are relatively minor compared to the other models above. According to Pasch, "we're going to see some improvement, but nothing earth-shattering." GFDL and HWRF (below) are the two leading models used by NHC in recent years for intensity prediction, along with statistics-based models.
 
HWRF

NOAA’s version of the Weather Research and Forecasting model specifically tailored for hurricanes (the Hurricane WRF, or HWRF) has undergone a major improvement in resolution, implemented in June. HWRF features a triple nest of concentric model domains that narrow in resolution as they zero in on hurricanes. The previous resolutions of 27, 9, and 3 km are now 18, 6, and 2 km. "There’s no other regional operational model in the world at that resolution that I know of,” Pasch told me. The new version also features a number of physics upgrades, including an advanced land surface model and a new radiation scheme that allows for better depiction of nocturnal peaks in shower and thunderstorm activity within TCs. According to Pasch, “the new HWRF has been a good performer thus far for track and intensity.”

Blending the models

The most powerful approach to hurricane prediction is model consensus: averaging the results from a large number of model runs, so that the most consistent signals come to the forefront and the outliers fade into the background. One type of consensus is the average of all the lower-resolution ensemble runs from a single model (such as the GFS or ECMWF). While this has some value, its usefulness is limited by the coarse resolution of ensemble runs, and by the particular strengths and weakness of any particular model. Forecasters usually favor multi-model ensembles, where the higher-resolution runs from several different models are averaged. This approach reduces the negative impact of any one model's idiosyncrasies, and random errors are more likely to cancel each other out. In making its forecasts, NHC calls on a variety of model blends, which usually outperform any individual model. Official NHC track predictions are often very close to the output from a model blend called TVCA, which employs the five models above (ECMWF, UKMET, GFS, GFDL, and HWRF). Going even further in this direction are "superensembles," such as the one developed at Florida State University. A superensemble not only blends multiple models, but it also weighs each model based on its past performance and includes bias corrections for each. As NHC puts it, "The [Florida State superensemble] is constantly learning from the past performance of the models that it comprises."




Figure 4. Successive forecasts of wind speed (in knots) from the GFS operational model for Cyclone Pam in the Southwest Pacific, beginning at 0000 GMT on March 10, 2015. Each red line represents a single GFS model run. The blue line indicates the best estimate of Pam's maximum wind speed based on satellite imagery. Image credit: Kerry Emanuel, Massachusetts Institute of Technology.


Impressive results from Cyclone Pam

As it tore through the Southwest Pacific in March with sustained winds that topped out at 165 mph, fearsome Tropical Cyclone Pam lived up to the ominous projections from long-range models. Several days ahead of time, operational runs of both the GFS and ECMWF models indicated that Pam had the potential to become a severe cyclone. Figure 4 shows the intensity forecasts (in maximum wind speed) from a number of successive GFS runs starting on March 10, by which point the model was already consistently and correctly predicting that Pam would become a Category 5 cyclone (if anything, the GFS was overdoing Pam's strength).
 
At the ECMWF, Pam offered forecasters an encouraging preview of the next version of their model. This upgrade, scheduled for early 2016, will bring the top resolution of the ECMWF ensemble members to 17 km, which is comparable to the current resolution of the ECMWF operational runs. Figure 5 shows that the average from the present-day ensemble (part a) brought Pam down to a minimal central pressure of 950 mb, whereas the average from the higher-resolution version to be implemented in 2016 (part b) produced a central pressure of 915 mb. The present-day operational model (gray line in part a) did even better, giving several days’ notice that Pam’s central pressure could dip below 900 mb. It bottomed out on March 14 at 896 mb.
 
The UKMET operational model predicted a minimum central pressure for Pam of 916 mb, the lowest in that model's history. Even lower values have shown up in UKMET runs for other recent tropical cyclones. “Prior to the two model changes made in the last year, it would not have gotten anywhere near these central pressure values,” said Julian Heming.
 
This year’s tepid Atlantic hurricane season won’t provide many case studies for the model improvements discussed above, but the active Pacific is giving the new incarnations a solid workout. Should the Atlantic perk up in 2016, even more upgrades will be in place by that point, giving forecasters and the public an even better sense of what to expect.
 
Bob Henson
 

Figure 5. Forecasts of Pam’s central pressure at mean sea level from 10 March 1200 UTC, showing (a) the operational high-resolution forecast (HRES) and the operational ensemble mean forecast (ENS mean) with vertical lines indicating the extreme members and blue bars representing the 25th to 75th percentile of the ensemble distribution, and (b) a higher-resolution (17 km) ensemble mean forecast. Image credit: ECMWF, reproduced with permission from the ECMWF Newsletter, Summer 2015, courtesy David Richardson.


Hurricane Computer Modeling

NOAA: Major El Niño Still on Track

By: Bob Henson , 5:12 PM GMT on August 13, 2015

In its latest monthly outlook, issued on Thursday (see PDF), NOAA continues to project that the ongoing El Niño event, already close to record strength for August, will at least approach the highest overall strength observed at any time of year since 1950. As of last week (see PDF), sea-surface temperatures across a key part of the eastern tropical Pacific called Niño3.4 were running 1.9°C above the long-term average for this time of year. This month’s Niño3.4 values could end up warmer than those for any other August in the official NOAA database, which goes back to 1950. The most recent value of NOAA’s closely watched Oceanic Niño Index, which is based on three-month averages for Niño3.4, was +1.0°C for May-July 2015, which ranks behind only 1987 (+1.1°C) for May-July readings. The NOAA outlook released on Thursday notes that the atmosphere-ocean coupling remains strong across the tropical Pacific, with weaker-than-average trade winds. Also, showers and thunderstorms have shifted toward the central and eastern equatorial Pacific from the west. “Collectively, these atmospheric and oceanic features reflect a significant and strengthening El Niño,” noted the outlook.


Figure 1. Early-August status of the 1997 and 2015 El NIño events in terms of satellite-derived data showing departure from average sea-surface height for a given time of year, which is correlated with warmth in the upper ocean. This animation shows the side-by-side evolution of both events. Image credit: NASA/JPL.


More signs of a barn-burner El Niño can be gleaned from the international array of computer models scrutinized by the Australian Bureau of Meteorology. Each month BOM calculates a multimodel average of Niño3.4 values going out for several months. In July, BOM’s multimodel average indicated that the Niño3.4 anomaly (departure from the seasonal norm) will rise to around +2.6°C by October and +2.7°C by December. This would imply a three-month average (Oct-Dec) of between +2.6° and +2.7°C. Were this to materialize, it would be well above the previous record three-month average in the NOAA database of +2.3°C, observed in Sep-Nov and Oct-Dec 1997. Update (15 August]: The outlook from NOAA's Climate Forecast System, version 2 (CFSv2], which is regularly cited in the BOM outlook, has hovered close to a maximum Niño3.4 anomaly of +3.0°C in its forecasts over the last few weeks. Shown below is the "PDF-corrected" version of NOAA's CFSv2 outlook, which has been consistently in the neighborhood of a +2.0°C maximum. Studies indicate that the PDF correction can improve the anomaly predictions for the Niño3.4 region, although in this case the PDF-corrected outlook lies substantially below the multimodel average cited above. Thanks to WU reader Cheyne Mosher for calling attention to this.


Figure 2. NOAA’s Climate Forecast System model (CFSv2) continues to show El Niño intensifying into this autumn, then decreasing in early 2016, in a fairly typical pattern for a strong El Niño event. Niño3.4 sea-surface temperatures in this PDF-corrected version are projected by CFSv2 to rise to about 2.0°C above the seasonal norm. Without the PDF correction, CFSv2 predicts a maximum closer to 3.0°C. Both versions can be tracked here. The panels at right show SSTs for three-month windows from Aug-Oct 2015 (top left) to Feb-Apr 2016 (lower right). Image credit: NOAA.


There are well-known limits to how well models can simulate El Niño, and even a solid model can be temporarily “fooled” by short-term changes in the tropical Pacific. Nevertheless, the general consistency in El Niño outlooks across models and across time--and the steadily building warmth across the eastern tropical Pacific, both at and below the surface--suggests that an event as strong or stronger than any observed in modern times is still a real possibility. That said, NOAA forecasters stressed in a Thursday morning news conference that El Niño is not guaranteed to bring drought relief to California. In the crucial water-storage region of the central Sierra, for examples, the last four years brought only 56% of the cumulative average precipitation from October 2011 through July 2015, leaving a 71-inch deficit. To make this up, the region would need roughly 2.5 to 3 times its annual average precipitation over the coming year, said Kevin Werner, director of climate services for the National Weather Service’s Western Region. “We’d need something in excess of the wettest year on record to balance the four-year deficit,” Werner said. This message was reinforced by Mike Halpert, deputy director of NOAA’s Climate Prediction Center. “Just because something is favored, it doesn’t guarantee it will happen. One season of above-normal rain and snow is very unlikely to erase four years of drought,” said Halpert.

For more background on what impacts we might expect from El Niño over the next few months, see our North American roundup post from July 28 and our special post on potential Northeast U.S. impacts. Jeff Masters will take a look at global impacts of El Niño in a forthcoming post.

What happened to the El Niño of 2014-15?
If you thought we were already in an El Niño episode a few months ago, you might be puzzled to see that the official NOAA database no longer shows it. This change is due to a fairly minor update in the ocean temperature record that pulled one key period just below the required threshold.


Figure 3. Departures from seasonal average for sea-surface temperatures (SSTs) across the Niño3.4 region over three-month intervals from 1997 to 2015. The shaded box (Jan-Mar 2015), originally 0.5°C, was “demoted” to 0.4°C with a July upgrade to SST analysis techniques. Image credit: NOAA Climate Prediction Center.


To qualify as a full-fledged El Niño episode, the Niño3.4 departure must be sustained at +0.5°C or greater for at least five overlapping three-month-long periods. When NOAA analysts are tracking El Niño in real time, they rely on a series of daily and weekly analyses of sea-surface temperature called OISST (optimum interpolation SST). OISST incorporates data from a variety of sources, including satellite-based measurements that are useful for short-term needs but which can introduce biases if they’re folded into a longer-term dataset that predates a particular satellite. For those longer-term purposes, the NOAA National Centers for Environmental Information (NCEI, formerly NCDC) produces the Extended Reconstructed Sea Surface Temperature (ERSST), a monthly dataset that goes back to 1854 and that uses statistical techniques to fill in data gaps.

Every few years, the techniques used in ERSST are updated and a new version of the full dataset is released. This occurred with the advent of ERSSTv4 this past July. As it happens, ERSSTv4 brought down the Niño3.4 anomaly for January through March 2015 from +0.5°C to +0.4°C (see Figure 3). Even though every other three-month window since Oct-Dec 2014 is still at +0.5°C or greater, this “demotion” of Jan-Mar 2015 means that we have yet to see five consecutive three-month periods of El Niño. Without those five periods, we haven’t yet met the formal definition for an El Niño episode (shown as red intervals in the NOAA historical database and in Figure 3 above). By next month, though, we’ll have five consecutive periods, and the El Niño episode will again become official, extending back to Feb-Apr 2015. The ERSSTv4 introduction also downgraded a few other El Niño and La Niña events from the last 65 years. These are identified by Jan Null (Golden Gate Weather Services) on a website that classifies El Niño and La Niña events by strength. One more wrinkle: it’s possible that Jan-Mar 2015 could be “undemoted” next year, when the climatology that underlies the above- and below-normal categorizations goes through a scheduled five-year update that takes long-term warming into account. In any case, the atmospheric response to the warm Niño3.4 readings in the winter of 2014-15 fell short of the usual El Niño standards, according to Michelle L’Heureux (NOAA Climate Prediction Center), so it’s best not to place too much emphasis on this borderline event.

The Case of the Not-Quite El Niño Episode reminds us of an important point stressed by L’Heureux: “Our SST observations are estimates. This is why we always encourage looking at multiple indices and datasets when trying to assess the state of ENSO. No one dataset or index will ever be perfect.” The further back in time we go, the more piecemeal is our knowledge of the SSTs that prevailed at the time. This is why NOAA’s most commonly used database of ENSO episodes only extends back to 1950. It’s quite possible to use ERSSTv4 or analyses from other research centers to calculate Niño3.4 values prior to 1950. However, these must be used with caution, as the data become increasingly scant going back in time (see Figure 4 below). To help flesh out the picture, scientists look to independent measurements related to El Niño, such as the Southern Oscillation Index, based on the observed difference in barometeric pressure between Darwin, Australia, and Tahiti. The greatest confidence in pre-1950 ENSO history is for the very strongest events, which are typically reflected in a wide range of land-based repercussions consistent with El Niño and La Niña behavior. For example, the El Niño of 1877-78 appears to have been at least as strong as the “super” 1982-83 and 1997-98 events. Calculations based on ERSSTv4 by wunderground member Eric Webb (@webberweather) suggest that the Niño3.4 value topped 2.5°C for several months. Drought associated with the 1877-78 El Niño may have contributed to horrific multiyear famines that took an estimated 5 million lives in India and 9-13 million lives in China.

For an update on the latest tropical cyclone action, including a fizzling Tropical Storm Hilda and twin typhoons predicted to develop next week, see this morning's post by Jeff Masters.

Bob Henson


Figure 4. Distribution of sea surface temperature observations from the International Comprehensive Ocean Atmosphere Data Set for each 20-year period from 1860 to 1979. This dataset underpins the NOAA ERRST long-term reanalysis discussed above. Color shading indicates the percentage of months that have at least one measurement within a 2°-latitude by 2°-longitude grid box (roughly 140 by 140 miles near the equator). Image credit: Used with permission from “Sea Surface Temperature Variability: Patterns and Mechanisms,” Annual Review of Marine Science 2009, doi: 10.1146/annurev-marine-120408-151453, courtesy Clara Deser, National Center for Atmospheric Research.



El Niño

Hilda Fizzling as it Approaches Hawaii; Twin Typhoons Coming to the Western Pacific

By: Jeff Masters , 3:29 PM GMT on August 13, 2015

The Tropical Storm Watch for the Big Island of Hawaii has been dropped as Tropical Storm Hilda heads westwards at 9 mph on a course that will take it no closer than 150 miles south of the Big Island. High wind shear of 30 - 40 knots continues to disrupt Hilda, and satellite loops on Thursday morning showed the storm struggling to keep a respectable amount of heavy thunderstorm activity near its core. Shear will remain a high 25 - 40 knots through Friday, the surrounding atmosphere will grow increasingly dry, and sea surface temperatures will cool slightly, which should cause Hilda to weaken to a tropical depression on Friday, if not sooner. However, even if Hilda dissipates before reaching Hawaii, it will still be capable of bringing heavy rains to the islands, particularly to the Big Island. The 2 am EDT (06Z) Thursday run of the HWRF model predicted that the eastern portion of the Big Island would see 4 - 8" of rain over the next few days from Hilda, with the southern portion of Maui receiving 2 - 4". Rains of this magnitude will be capable of causing dangerous floods and mudslides. Hawaii could use some rain, though--today's Drought Monitor classified 26% of the state as being in moderate or greater drought.


Figure 1. When a hurricane unravels: high wind shear due to strong upper-level westerly winds on Wedneday, August 12, 2015, exposed the low level center of Hurricane Hilda to view. Hilda's heavy thunderstorms were all on the east side of the center of circulation in this MODIS satellite image from approximately 9 pm EDT, when Hilda had top winds of 45 mph. Image credit: NASA.

Twin typhoons likely next week in the Western Pacific
In the Western Pacific, a pair of tropical disturbances, 97W and 98W, appear destined to become twin typhoons early next week, according to the latest runs of the European and GFS models. Both of these storms will have the potential to cause trouble for Asia late next week. The twin storms will be close enough together that they could influence each other, making prediction of their track and intensity more difficult than usual.


Figure 2. Surface winds over the Pacific Ocean on Thursday morning, August 13, 2015. Image credit: http://earth.nullschool.net/.

Ten years ago today
Tropical Depression Ten formed on August 13, 2015, from a tropical wave that emerged from the west coast of Africa on August 8. As a result of strong wind shear, the depression remained weak and did not strengthen beyond tropical depression status, and degenerated on August 14. Its remnants partially contributed to the formation of Tropical Depression Twelve, which eventually intensified into Hurricane Katrina. Thanks go to wunderground members MonsterTrough and BiloxiIsle for posting this in my blog comments.

The Atlantic remains quiet today, with no tropical cyclone development likely over the next five days.

Bob Henson will have a new post by 2 pm EDT this afternoon on the latest monthly El Niño update issued by NOAA.

Jeff Masters

Hurricane

Tropical Storm Hilda Degenerating as it Approaches Hawaii

By: Jeff Masters , 3:26 PM GMT on August 12, 2015

A Tropical Storm Watch continues for the Big Island of Hawaii as Tropical Storm Hilda heads northwest at 5 mph towards the islands. Hilda degraded significantly over the past day due to high wind shear of 30 - 40 knots, but satellite loops on Wednesday morning showed that the storm continued to generate a respectable amount of heavy thunderstorm activity near its core. Shear will remain a high 25 - 40 knots through Friday, the surrounding atmosphere will grow increasingly dry, and sea surface temperatures will cool slightly, which should cause Hilda to weaken to a tropical depression by the time of its closet approach to Hawaii. The Wednesday morning runs of our two most reliable models for predicting hurricane tracks, the European and GFS models, both showed Hilda dissipating before making its closest approach to Hawaii on Thursday evening. However, even if Hilda dissipates before reaching Hawaii, it will still be capable of bringing heavy rains to the islands, particularly to the Big Island. In their 11 am EDT Wednesday Public Advisory, the Central Pacific Hurricane Center cautioned that Hilda could bring 6 - 12" of rain, with isolated amounts up to 18". These amounts will likely only fall over ocean areas, since the center of Hilda is expected to pass about 150 miles south of the Big Island. The 2 am EDT (06Z) Wednesday run of the HWRF model predicted that the Big Island would see 2 - 4" of rain from Hilda, with the extreme southeast corner getting 4 - 8". Maui would get less than 1". I expect that the Big Island will receive maximum rainfall amounts of 3 - 6" from Hilda, with Maui getting 1 - 2".


Figure 1. When a hurricane unravels: high wind shear due to strong upper-level westerly winds on Tuesday, August 11, 2015, exposed the low level center of Hurricane Hilda to view. Hilda's heavy thunderstorms were all on the east side of the center of circulation in this MODIS satellite image from 5 pm EDT, when Hilda had top winds of 75 mph. Image credit: NASA.

In the Western Pacific, the European and GFS models predict that twin tropical storms will form in the waters midway between Hawaii and the Philippines' Luzon Island this weekend. Both of these storms will have the potential to cause trouble for Asia late next week. The twin storms will be close enough together that they could influence each other, making prediction of their track and intensity more difficult than usual.


Figure 2. Tracks of the 22 tropical cyclones (tropical depressions, tropical storms, and hurricanes) to pass within 150 miles of the Hawaiian Islands, 1949 - 2015. Hawaii has seen an unusual number of these storms (three) affect them in the past three years: Flossie (2013), Iselle (2014), and Guillermo (2015). Hilda (2015) may become the fourth such storm if it survives into Thursday night. Hurricanes approaching from the east typically fall apart before they reach Hawaii due to the cool waters and dry air that lie to the east of the islands. Only two named storms approaching from the east has hit the islands since 1949, an unnamed 1958 tropical storm that hit the Big Island, and Tropical Storm Iselle of 2014. Hurricanes approaching from the south represent the biggest danger to the islands, due to the warmer waters and more unstable air present to the south. The only two major hurricanes to have affected the islands since 1949, Hurricane Iniki of 1992 and Hurricane Dot of 1959, both came from the south. Image credit: NOAA/CSC.

An unusual amount of hurricane activity for Hawaii in recent years
An unusually high number of tropical storms and hurricane have roamed the waters near Hawaii over the past three years. This year, there has been Guillermo and Hilda, last year had Iselle and Julio, and 2013 had Flossie. Since 1949, 22 tropical cyclones (tropical depressions, tropical storms, and hurricanes) have passed within 150 miles of the Hawaiian Islands, an average of one tropical cyclone every three years. So, to have three tropical cyclones (four if we count Hilda) pass so close in three years (see Figure 2) is an unusual amount of activity. Part of the blame for the activity can be placed on unusually warm sea surface temperatures: these temperatures were warmest on record for the waters south and east of Hawaii this summer, and were also well above average in 2014. It is also possible that we are seeing the beginning of a shift in the tracks of the Eastern Pacific hurricanes due to climate change, though it is too early to say. In my August 2014 blog post, Climate Change May Increase the Number of Hawaiian Hurricanes, I reported on a study done last year which projected a doubling or tripling of the number of hurricanes affecting Hawaii by the end of the century, due to climate change.


Figure 3. Double trouble for Hawaii: True-color VIIRS image of Hurricane Iselle (left) and Tropical Storm Julio (right) approaching Hawaii, taken between 3 - 6 pm EDT August 5, 2014. At the time, Iselle was a Category 2 hurricane with 110 mph winds, and Julio had 65 mph winds. Image credit: NOAA Visualization Lab.

Hawaii's hurricane history
On average, between four and five tropical cyclones are observed in the Central Pacific every year. This number has ranged from zero, most recently as 1979, to as many as eleven in 1992 and 1994. August is the peak month, followed by July, then September. Tropical storms and hurricanes are rare in the Hawaiian Islands. Since 1949, the Hawaiian Islands have received a direct hit from just two hurricanes--Dot in 1959, and Iniki in 1992. Both hit the island of Kauai. Only two tropical storms have hit the islands since 1949--an unnamed 1958 storm that hit the Big Island, and Tropical Storm Iselle of 2014, which hit the Big Island with sustained winds of 60 mph, causing $79 million in damage. A brief summary of the three most significant hurricanes to affect Hawaii in modern times:

September 1992: Hurricane Iniki was the strongest, deadliest, and most damaging hurricane to affect Hawaii since records began. It hit the island of Kauai as a Category 4 on September 11, killing six and causing $2 billion in damage. The filming of the original "Jurassic Park" was interrupted by Iniki.

November 1982: Hurricane Iwa was one of Hawaii's most damaging hurricanes. Although it was only a Category 1 storm, it passed just miles west of Kauai, moving at a speed of nearly 50 miles per hour (80 km/h). Iwa killed one person and did $250 million in damage, making it the second most damaging hurricane to ever hit Hawaii. All the islands reported some surf damage along their southwest facing shores, and wind damage was widespread on Kauai.

August 1959: Hurricane Dot entered the Central Pacific as a Category 4 hurricane just south of Hawaii, but weakened to a Category 1 storm before making landfall on Kauai. Dot brought sustained winds of 81 mph with gusts to 103 mph to Kilauea Light. Damage was in excess of $6 million. No Dot-related deaths were recorded.

Jeff Masters

Hurricane

South Asia's Deadly Rains of 2015; Tropical Storm Watch in Hawaii

By: Bob Henson and Jeff Masters , 3:55 PM GMT on August 11, 2015

A multi-week series of disastrous rains, floods, and mudslides has taken more than 400 lives and affected millions in a broad swath from Pakistan to Vietnam over the last several weeks. This is the height of the South Asia and Southeast Asia monsoon season, when life-giving rains sweep from the Indian Ocean across India and neighboring countries after the parched, scorching conditions that typify spring. Monsoon-related floods often produce hundreds of deaths across India each year. This year’s Indian monsoon has brought an distinct patchwork of impacts. Although several regions have been affected by torrential downpours, others are wrestling with unusually dry conditions, in a monsoon that’s actually been skimpy for the nation as a whole.


Figure 1. An aerial view shows floodwaters inundating houses and vegetation in Kalay, upper Myanmar's Sagaing region, on August 3, 2015. Image credit: Ye Aung Thu/AFP/Getty Images.

The most widespread flood impacts have been in the Bay of Bengal region, from far eastern India across parts of Pakistan and Myanmar. Three weeks of flooding across five provinces of Pakistan have killed at least 151 people, and the government of Myanmar is seeking international assistance, as more than a million acres of farmland have been inundated and at least 46 people have died. In the South Bengal state of eastern India, floods have taken at least 83 lives, and more than 300,000 homes have been destroyed or seriously damaged. Further west, in the central state of Madhya Pradesh, flash flooding was the apparent cause of two train derailments that led to at least 29 deaths.


Figure 2. Two Indian passenger trains lay next to each other following a derailment after flash floods struck a bridge outside the town of Harda in Madhya Pradesh state on August 5, 2015. At least 27 people died as a result of the derailment. Image credit: STR/AFP/Getty Images.

The worst of the rains were associated with Tropical Cyclone Komen. A weak but wet system, with top 3-minute sustained winds of only 45 mph, Komen moved inland over Bangladesh on July 29 but left behind a remnant low that drifted west near the India-Bangladesh border last weekend, according to weather.com. Moist southwesterly flow on Komen’s east flank led to huge rainfall totals, especially along the hilly southeast coast of Bangladesh, where several locations reported between 40” and 50” for the 10-day period from July 24 to August 2.

A separate area of monsoon-related flooding has affected more than a million people in Pakistan and far western India. At least 166 deaths have been reported in Pakistan, where catastrophic flooding in 2010--the nation’s worst natural disaster on record--caused more than 1,700 deaths and left some 11 million people homeless.


Figure 3. A Pakistani resident drinks water from a hand-pump at a flooded area in Nowshera district, in Khyber Pakhtunkhwa province, on August 3, 2015. Image credit: A. Majeed/AFP/Getty Images.

India’s monsoon falling short of average
Despite the flooding disasters scattered across parts of India, the nation is actually seeing a mix of drought and deluge this summer, leaning toward the dry side. The total amount of rainfall in the 2015 monsoon season to date across India (June 1 - August 11) has been only 91% of the long-term average. The nationally weighted rainfall total for the Jun 1 - Aug 11 period was 501.8 mm (19.76”), compared to a typical value to date of 553.1 mm (21.78”). The 91% value is only a slight improvement over the start-of-monsoon outlook from the India Meteorological Department (IMD), which called for 88% of average over the entire monsoon season (June-September). The driest areas have been concentrated along the nation’s northern tier, from Punjab to Assam states (20% to 40% below average) and in the southwest peninsula (20% to more than 50% below average). The central state of Telangana is making contingency plans for drought response should the second half of monsoon season turn out as disappointing as the first half did. In a mid-monsoon update on August 3, the IMD held to its 88% outlook. Such a deficit can cause severe stress on agriculture and the power grid, which relies heavily on hydroelectric power. However, a 12% reduction in rains would not rank in the top five for worst monsoons on record (see below).




Figure 4. Cumulative rainfall from June 1 to August 11 across India has produced a patchwork of above- and below-average accumulations compared to a typical year. Within each state, the average rainfall from June 1 to August 11 is shown at bottom, in millimeters, with the actual 2015 total at top and the percentage anomaly following in parentheses. A total of 16 states have deficient rainfall, while 5 have an excess and 15 are near normal. The nationwide total rainfall for the period was 91% of average. Image credit: India Meteorological Department.

El Niño and the Indian monsoon
In recent decades, El Niño has been closely associated with deficient monsoon rainfall over India, so this year’s underwhelming rainfall is not a total surprise. But the relationship isn’t iron-clad. Monsoon rainfall was 2% above average in 1997 even as a strong El Niño very similar to the current one was building, and rainfall deficits are possible even without El Niño. El Niño Modoki, the type where warming is focused in the central tropical Pacific rather than toward the east, tends to be more effective at suppressing the monsoon than a classic east-Pacific El Niño. The monsoon is also influenced by the Indian Ocean Dipole, measured by the east-west difference in sea-surface temperatures across the Indian Ocean. A positive IOD event tends to enhance moisture in the southwesterly flow over India that brings monsoon rain.

If the 2015 monsoon does end up falling short of the norm, it will be for the second consecutive year, as the monsoon of 2014 produced only 88% of average rainfall. Since most significant El Niño events last just a year, it’s unusual to have two low-rainfall years in a row. The last time two consecutive Indian monsoons saw below-average rain was during the two-year El Niño event of 1986 and 1987. There’s also marked multi-decadal variability in monsoon rains, according to the Indian Institute of Tropical Meteorology. From 1921 to 1964, only three monsoon seasons produced less than 90% of average rainfall. Over the following period, 1965 - 1989, 10 of 24 years fell short of the 90% mark. The most devastating monsoon rainfall deficits since modern records began were as follows:

1) 1877, -33%
2) 1899, -29%
3) 1918, -25%
4) 1972, -24%
5) 2009, -22%

Averaged across the globe, the planet’s major monsoons appear to be collectively producing more rain in recent decades. A 2014 study in the Bulletin of the American Meteorological Society found evidence for increased global monsoon precipitation between 1979 and 2011, but with substantial year-to-year variability in the mix. Theory, modeling, and observations all support the general trend toward intensified precipitation events in many areas, together with exacerbated impacts when drought does strike (the “wet get wetter, dry get drier” concept). For more on the dynamics that drive the Indian monsoon, and its relationship to climate change in India, see this 2013 post from Jeff Masters.

Bob Henson

Tropical Storm Watch posted for Hawaii's Big Island for Hurricane Hilda
In the Eastern Pacific, a Tropical Storm Watch has been issued for the Big Island of Hawaii as Hurricane Hilda heads northwest at 6 mph towards Hawaii. Hilda is under high wind shear of 25 - 35 knots, and the shear will increase to 30 - 40 knots by Wednesday. Although Hilda has remained remarkably intact in the face of this high shear, the storm's increasingly degraded appearance on Tuesday morning gives me confidence that the increasing shear will cause the storm to unravel rather quickly. I expect that Hilda will weaken to a tropical depression by Wednesday evening; the Tuesday morning runs of our two most reliable models for predicting hurricane tracks, the European and GFS models, both showed Hilda weakening to a tropical depression before reaching Hawaii on Thursday. However, even if Hilda dissipates before reaching Hawaii, it will still be capable of bringing dangerous flooding rains to the islands, particularly to the Big Island. Wunderblogger Steve Gregory has a more detailed look at Hilda in his Tuesday afternoon post.


Figure 5. Latest satellite image of Hilda.

In the Western Pacific, the European and GFS models predict that twin tropical storms will form in the waters midway between Hawaii and the Philippines' Luzon Island this weekend. Both of the these storms will have the potential to cause trouble for Asia late next week.

Jeff Masters


Hurricane Flood Extreme Weather

Quiet in the Atlantic; Hilda Poised to Bother Hawaii

By: Jeff Masters , 3:16 PM GMT on August 10, 2015

With mid-August at hand, the stress level of residents along the Atlantic's Hurricane Alley rises as African tropical wave season enters its climatological peak period. A steady supply of spinning disturbances emerge from the coast of Africa from mid-August through early October, providing weeks of suspense as we watch them develop and decay as they march across the hurricane Main Development Region (MDR) from the coast of Africa and into the Caribbean. However, this year is atypical--there has only been decay. This, despite the fact that wind shear has been a moderate 10 - 20 knots over the eastern portion of the MDR, and sea surface temperatures have been near average. The credit for the quiet start to hurricane season goes to an atmospheric circulation that has brought high pressure and dry, sinking air to the tropical Atlantic--due in part to one of the strongest El Niño events in recorded history that is underway in the Eastern Pacific. In addition, frequent outbreaks of dry, dusty air from the Saharan Air Layer (SAL) have brought even more dry air, making the atmosphere so stable that tropical waves coming off the coast of Africa have quickly decayed. El Niño is creating strong upper-level winds over the Caribbean that were generating a very high 30 - 50 knots of wind shear over the Caribbean on Monday, making tropical storm formation virtually impossible there. The high wind shear and low instability is forecast to persist in the Caribbean and tropical Atlantic for at least the next week. Wind shear will be lower, at times, in the waters of the Gulf of Mexico and off the U.S. East Coast, so if we get any tropical storms forming in mid-August, those would be the most likely locations. Keep in mind, though, that the last time we had an El Niño event this strong--back in 1997--no named storms formed in the Atlantic during August, and only one named storm (Hurricane Erika) formed in September. It would not be a surprise to see similar behavior in 2015. In their 8 am EDT Monday Tropical Weather Outlook, NHC continued to predict quiet conditions in the Atlantic for at least five days.


Figure 1. Vertical instability as of August 9, 2015 over the tropical Atlantic, from the coast of Africa to the Lesser Antilles Islands. The instability is plotted in °C, as a difference in temperature from near the surface to the upper atmosphere. Thunderstorms grow much more readily when vertical instability is high. Normal instability is the black line, and this year's instability levels are in blue. The atmosphere has been dominated by high pressure and dry, sinking air all summer, which has made it difficult for thunderstorms to develop. Instability has also been unusually low in the Caribbean, but has been near average over the Gulf of Mexico and waters off the U.S. East Coast. Image credit: NOAA/CIRA.


Figure 2. Saharan Air Layer (SAL) analysis for 8 am EDT Monday, August 10, 2015, from the University of Wisconsin CIMSS, shows plenty of dry air dominating the tropical Atlantic.

Hilda poised to bother Hawaii
In the Eastern Pacific, Hurricane Hilda continues to weaken as it heads northwest at 9 mph towards Hawaii. Hilda is under high wind shear of 15 - 25 knots, and the shear will increase to 30 - 40 knots by Tuesday. This increasing shear should cause the hurricane to weaken to a tropical depression by Wednesday. The speed with which Hilda weakens will be crucial for determining whether or not the storm will track over the Hawaiian Islands late this week; a weaker Hilda will tend to track more due west, caught in the low-level trade wind flow near the surface, while a stronger Hilda will tend to track more to the northwest, potentially leading to a landfall on Thursday. The Monday morning run of the European model favored this latter scenario, while the GFS model run showed Hilda turning due west and missing the Hawaiian Islands to the south. Either scenario is possible, and we will have to wait and see how the situation plays out.


Figure 3. Latest satellite image of Hilda.

In the Western Pacific, all looks to be quiet until late this week, when the European and GFS models predicts a new tropical depression will form in the waters midway between Hawaii and the Philippines' Luzon Island.

Jeff Masters

Hurricane

Soudelor Winds Down; Hilda Hits Category 4; Hong Kong Sets All-Time Heat Record

By: Bob Henson , 12:07 AM GMT on August 09, 2015

Former Typhoon Soudelor, now a 50-mph tropical storm, is inland over eastern China after giving storm-savvy Taiwan one of its most powerful typhoon strikes on record. The damage in Taiwan appears to be widespread but not catastrophic, although at least 10 people are dead or missing, according to a Washington Post report. Power was knocked out to a reported 3.22 million residents, the largest storm-related outage in Taiwan history, and high winds toppled more than 2,000 trees in Taipei. Soudelor made landfall around 5:00 am Saturday local time as a Category 3 typhoon, with its strong right-hand flank passing over the island’s northern third, including the city of Taipei. As classified using the extended Beaufort scale, winds speeds at the Taipei airport topped out in the Level 13 range of 83-93 mph. These are the second-highest speeds on record for Taipei, behind only the Level 14 winds (93-103 mph) observed during 1996’s Typhoon Herb. While rainfall during Soudelor appears to have fallen short of at least two other typhoons (Herb and 2009’s Typhoon Morakot), the amounts were still impressive over wide areas, with a total of 52.52” reported at Datong Township in far northeast Taiwan.

Soudelor made a second landfall as a Category 1 typhoon just after 10:00 pm local time Saturday in China’s Fujian province near Putian City. See this weather.com report for more details on Soudelor.




Figure 1. Trees torn down by strong wind along a street in Jinjiang, on the east coast of China in the Fujian province, on August 8, 2015, as Typhoon Soudelor drew near. Image credit: STR/AFP/Getty Images.



Figure 2. Hurricane Hilda as seen at approximately 19 UTC (3 pm EDT) Saturday, August 8, 2015, from the MODIS instrument on NASA's Aqua satellite. At the time, Hilda was a Category 4 storm with 140 mph winds. The Big Island of Hawaii is visible in the upper left of the image. Image credit: NASA.


Hilda vaults to Category 4 status
A burst of rapid intensification brought Hurricane Hilda from Category 1 to Category 4 status in just 24 hours, with sustained winds estimated at 140 mph by the Central Pacific Hurricane Center at 11:00 am Hawaii time on Saturday (5:00 pm EDT). Hilda showed little in the way of spiral banding late Saturday, taking on more of an annular configuration. Already one of the strongest hurricanes in recent years over the Central Pacific, Hilda could intensify further over the next 24 hours. If Hilda manages to reach Category 5 strength, it will join the elite group of hurricanes of that intensity in the Central Pacific. The year to beat is 1994, when three Category 5 systems plowed across the basin in just five weeks: Emilia, Gilma, and John. El Niño tends to enhance hurricane activity in the Central Pacific; interestingly, an El Niño was not underway during July-August 1994, though one did develop later in the year.

As Hilda gradually gains latitude on its west-northwest course, wind shear will take an increasing toll. Models continue to diverge on Hilda’s strength and track toward the end of the five-day forecast period, though most models recurve the hurricane well east of Hawaii. A weaker Hilda would be steered more by low-level easterly flow, perhaps nearing the islands as a tropical storm, while a stronger system would be influenced more by the upper-level westerlies and would be more likely to angle north.

Elsewhere in the Pacific, Tropical Storm Molave continues to spin harmlessly well southeast of Japan, and Invest 93 should remain weak as it traverses the open waters of the Northeast Pacific.



Figure 3. Hilda will be traveling over warm seas for the next several days, but increasing wind shear will keep the hurricane from taking full advantage of the oceanic heat. Image credit: NOAA Central Pacific Hurricane Center.


Broken records piling up in Europe, Asia
As 2015 continues marching toward a new global record high temperature, the heat is making itself felt this weekend at a variety of locations across the globe. Excessive heat warnings are out for several south-central U.S. states, but records aren’t tumbling at the pace that they are in several other countries.

On Saturday, the venerable Hong Kong Observatory reached 36.3°C (97.3°F), the hottest temperature in its 132-year history. The heat in Hong Kong was likely enhanced by sinking air around the southern periphery of Typhoon Soudelor. Likewise, subsidence on the north side of Soudelor helped keep Tokyo toasty, as the city notched its eight consecutive day of at least 95°F temperatures on Friday. The streak was twice the previous record length of four days, recorded on five different occasions between 1978 and 2013, as noted in a full report by The Weather Channel’s Nick Wiltgen. Records in Tokyo began in 1875. The streak was broken on Saturday, as temperatures topped out at 91°F, and the heat should stay just short of the 95°F threshold over the coming week.

Meanwhile, central and eastern Europe continues to broil in a sustained heat wave. In Poland, the Wroclaw Observatory hit an all-time record high on Saturday of 38.9°C (102.0°F), and highs soared above 95°F over a broad swath from Lithuania to the Mediterranean. Some areas will see relief over the next several days, but others are facing at least a solid week of torrid readings (see Figure 4, below).

Bob Henson


Figure 4. On Saturday night, WU was predicting temperatures in Vienna, Austria, to hit the upper 90s Fahrenheit each day during the coming workweek. Vienna’s all-time high is 39.5°C (103.1°F), according to weather records researcher Maximiliano Herrera.

Hurricane Heat

Soudelor Approaches Taiwan; All-Time Record Heat Returns to Germany

By: Bob Henson , 6:56 PM GMT on August 07, 2015

It's been a nail-biting Friday night for residents of Taiwan as Category 3 Typhoon Soudelor approaches the island. At 1745 GMT Friday (1:45 pm EDT), the Japan Meteorological Agency placed the center of Soudelor at 23.2°N, 122.5°E, or about 60 miles east-southeast of the east-central coast of Taiwan. Soudelor’s peak 10-minute sustained winds were 105 mph, according to JMA, whereas the 1500 GMT (11:00 am EDT) update from the Joint Typhoon Warning Center kept Soudelor as a Category 3 cyclone with 120-mph sustained winds, using the 1-minute definition that is commonly associated with the Saffir-Simpson scale.


Figure 1. Streamline imagery from the earth.nullschool.net visualization site shows the circulation around Typhoon Soudelor in stunning detail. Thanks to wunderground member PlazaRed for creating and posting this visualization. Image credit: earth.nullschool.net.

Soudelor was moving west-northwest at about 11 mph on a track that would take it directly into the east-central coast of Taiwan around 8:00 am local time on Saturday morning (about 6:00 pm EDT Friday). Soudelor is a powerful, well-structured cyclone with an expanding shield of heavy rain. Hurricane-force winds extended up to 45 miles from the center, and gale-force winds covered an area some 450 miles in diameter. A peak gust of 123 mph was clocked on the Japanese island of Ishigakijima at 11:51 p.m. local time Friday (10:51 a.m. EDT), according to weather.com. Between 1700 and 1800 GMT, winds gusted to about 80 mph at Su-ao, a fishing port on Taiwan’s northeast coast, as reported by Taiwan’s Central Weather Bureau (thanks to wunderground member bwi for this tip).

Reintensification over the past day has been partially thwarted by intrusions of dry air at times. The typhoon also embarked on a second eyewall replacement cycle (ERC), following one earlier in the week. This time, a once-50-mile-wide eye contracted to about 20 miles in width and has been fragmenting, while a larger ring of convection morphed into an partial outer secondary eyewall. It can take 24-48 hours for an ERC to be completed, after which the newly restructured tropical cyclone has another chance to restrengthen. Soudelor does not have enough time for that process to conclude before landfall. However, as it approaches the coast, Soudelor’s interaction with land will help increase low-level convergence into the storm’s center, and a slight bit of additional intensification could occur before Soudelor strikes the central Taiwan coast on Saturday morning local time. The strongest winds and heaviest rains will be on the north (right-hand) side of the eye, toward the northern third of Taiwan (including the city of Taipei).



Figure 2. Radar imagery shows a somewhat elongated central core of Soudelor, with an open eyewall to the northwest. Extremely heavy rain is funneling into far northern Taiwan, including the Taipei area. Image credit: Central Weather Bureau, Taiwan.


Figure 3. A composite (RGB) satellite image of Typhoon Soudelor, collected by MTSAT at 1732 GMT Friday (11:32 am EDT). Image credit: NOAA/NESDIS.

Soudelor may be the strongest typhoon to make landfall in Taiwan in three years, as noted by weather.com. The last Category 4 equivalent typhoon to landfall in Taiwan was Tembin in August 2012, according to hurricane specialist Michael Lowry. In all, Taiwan has seen 18 Category 4 or stronger equivalent typhoon landfalls since 1958, says Lowry.

Torrential rain, already widespread across Taiwan, will continue through Saturday local time. As of 1730 GMT Friday, Yilan County in the far northeast had reported 522 mm (20.55”), with several reports above 8” in the Taipei area. Especially massive amounts of rain will fall where Soudelor slams into the north-south mountain range that spans most of Taiwan, and substantial local flooding and mudslides can be expected. Soudelor’s steady movement will help at least to some extent in keeping rainfall totals below the even more prodigious amounts that slower-moving systems such as 2009’s Typhoon Morakot can produce. Morakot was only a Category 1 storm, but it moved in a leisurely cyclonic loop across northern Taiwan, prolonging the widespread intense rainfall. An almost unbelievable total of 2777 mm (109.33”) was reported at the mountainside resort of Alishan, far outstripping the previous record of 1736 mm (68.35”) set during Typhoon Herb in 1996. Morakot caused more than 450 deaths and some $3.3 billion US in damage.

On its relatively steady west-northwest track, Soudelor will strike the coast of China on Saturday night local time. The China Meteorological Administration (CMA) is calling for a landfall in Fujian Province, near the cities of Lianjiang and Longhai, which together have about 1.4 million residents. The CMA has launched a level-three emergency response, the second highest category in China’s four-tier system, to address the arrival of Soudelor. The passage over Taiwan’s mountains will markedly disrupt Soudelor so that its winds may be at or just below hurricane strength by the time it reaches China. However, the typhoon’s large envelope of rich moisture will produce heavy rains near the coast and for some distance inland, as the center recurves toward the Yangtze Valley.

Elsewhere in the tropics
Although there are no immediate threats on the scale of Soudelor, the Pacific remains active. Tropical Depression Molave, well east of Soudelor, should recurve before threatening Japan and is unlikely to reach hurricane strength. In the Northeast Pacific, Tropical Storm Hilda could reach hurricane strength over the weekend as it move west-northwest over open water. Hilda could approach Hawaii by later next week, although track models continue to diverge on Hilda’s ultimate trajectory, so it is far too soon to know if any real threat will emerge. On the heels of Hilda is Invest 93E, which appears to have little chance of major development on its westward track.


Figure 4. An electronic display at a pharmacy in Lyon, France, shows a temperature of 42°C (107.6°F) on Friday afternoon, August 7. While many such outdoor displays are compromised by poor placement of thermometers, official temperatures did reach 102°F at Lyon’s airport, compared to an average high for the date of 77°F. Image credit: Philippe Desmazes/AFP/Getty Images.

Europe again slathered with all-time record heat; Berlin has hottest day on record
Just one month after setting its all-time national heat record, Germany tied that mark on Friday at the same location, as yet another multiway heat wave swept across much of Europe. The German meteorological agency (Deutscher Wetterdienst) confirms that the town of Kitzingen reached 40.3°C (104.5°F) on Friday, the same national record it reached on July 5. According to Michael Theusner (Klimahaus), more than 100 towns and cities in Germany either tied or broke their all-time record highs on Friday. Berlin's Kaniswall station hit 38.9°C (102.0°F)--the hottest temperature ever observed in the Berlin area, beating the old record of 38.6°C (101.5°F).

Record heat extended far across other parts of Europe on Friday. According to international weather records expert Maximiliano Herrera, who maintains a comprehensive list of extreme temperature records for every nation in the world on his website, Friday’s high of 38.3C (100.9°F] at Genoa, Italy, topped the all-time airport record by a full 4°F. Records at the airport extend back to 1962; the previous reporting site for Genoa was located further inland, with a warmer microclimate. Even at that location, the previous Genoa record was 37.8°C (100.0°F) in July 1952. We’ll continue to keep an eye on Europe this weekend, as several nationwide all-time records could be approached or toppled.

Jeff Masters will be back on deck next week. In the meantime, have a great weekend, everyone!

Bob Henson

Hurricane Heat

Soudelor Makes Beeline for Taiwan; CSU, NOAA Update Atlantic Hurricane Outlooks

By: Bob Henson , 5:48 PM GMT on August 06, 2015

Although its peak sustained winds have dropped from 180 mph to 105 mph over the last several days, Typhoon Soudelor may embark on a final burst of intensification before striking Taiwan on Saturday. At 1500 GMT (11:00 am EDT) Thursday, the Joint Typhoon Warning Center (JTWC) positioned Soudelor at 21.3°N, 127.5°E, which is about 550 miles east-southeast of Taiwan’s largest city, Taipei, on the north end of the island. Soudelor is moving just north of west at about 11 mph. The JTWC’s 1500 GMT Thursday forecast track brings Soudelor onshore across central Taiwan just before 0000 GMT Saturday, or about 8:00 am Saturday local time. The projected track, arcing slightly toward the northwest over time, is a classic trajectory for this latitude, and is consistent with the GFS and ECMWF operational model runs and ensemble averages. The GFDL model bumps the track a bit more to the northwest, which would take Soudelor into the northern part of Taiwan, closer to Taipei.


Figure 1. An enhanced infrared image of Typhoon Soudelor from 1550 GMT Thursday (11:50 am EDT). Image credit: RAMMB/CIRA.


A bigger question mark is how strong Soudelor will be when it reaches Taiwan. Soudelor currently has a rather large eye (roughly 55 miles across, close to half a degree of longitude), with an intensifying solid ring of convection surrounded by a large field of spiral banding. Mesovortices (small-scale circulations) could be seen spinning within the large eye in visible satellite imagery. Soudelor’s brief stint as a super typhoon earlier this week was interrupted by an eyewall replacement cycle, with the much larger eye succeeding the typhoon’s originally tiny eye (about 5 miles in diameter, one of the smallest on record for a landfalling system, when Soudelor struck Saipan). Since that point, Soudelor has had some trouble rebuilding a large, solid core of convection, despite its excellent overall structure and the gradually increasing size of its envelope of moisture and banding. Dvorak imagery from the MTSAT satellite over the last few hours shows that very cold cloud tops (a sign of intense convection) are beginning to wrap around Soudelor’s eye once again, and temperatures within the eye are warming, another sign of intensification. Soudelor is now moving over slightly warmer SSTs (29-30°C, or 84-86°F, about 1°C above average) as it approaches Taiwan, and wind shear will be low (only around 5 - 10 mph).

Dynamical models generally agree in bringing Soudelor back to at least Category 3 strength before landfall in Taiwan, and I would not be at all surprised to see Soudelor pass the Category 4 threshold. The nation’s urbanized areas are well prepared for high typhoon winds, though widespread power outages and transportation disruptions are likely. Massive amounts of rain will fall where Soudelor slams into the north-south mountain range that spans most of Taiwan, and substantial local flooding and mudslides can be expected. Fortunately, Soudelor’s steady movement will help keep its rainfall totals below the truly prodigious amounts that slower-moving systems such as 2009’s Typhoon Morakot can produce. Morakot was only a Category 1 storm, but it moved in a leisurely cyclonic loop across northern Taiwan, prolonging the widespread intense rainfall. Morakot caused more than 450 deaths and some $3.3 billion US in damage.


Figure 2. This 3-D view of Soudelor’s eyewall and convective banding was produced using infrared and visible imagery from MTSAT combined with data from the Dual-Frequency Precipitation Radar aboard NASA’s Global Precipitation Satellite at 0006Z GMT on Thursday, August 6. The highest storm tops with Soudelor--extending up to more than 14.7 km (48,000 feet)--were located southwest of the typhoon's eye. Image credit: NASA.


Figure 3. Soudelor is likely to bring at least 8” of rain to most of Taiwan and parts of east-central China, according to these projections of precipitation from the 0600 GMT Thursday ensemble of the GFDL hurricane model. Much higher amounts can be expected in and near mountainous areas of Taiwan. Image credit: NOAA/GFDL.

Guillermo peters out north of Hawaii
Barely a tropical storm, Guillermo continues its slow decline as it tracks north of the Hawaiian islands. At 6:00 am Thursday HST (noon EDT), Guillermo was located about 225 miles east of Honolulu, heading west at 12 mph, with sustained winds of just 40 mph. Tropical storm warnings were in effect offshore, with high surf warning across the east- and north-facing shores of the Hawaiian islands. Guillermo is expected to weaken to tropical-depression status before sweeping just north of Honolulu; it could make landfall on Lihu’e, the northernmost major Hawaiian island, but little impact is expected other than localized heavy rain, gusty winds, and high seas.

Well to the southeast, newly christened Tropical Storm Hilda was located more than 1500 miles southwest of Cabo San Lucas with sustained winds of 40 mph. Hilda is wrapped in a moist environment and already showing a healthy amount of convection. Low wind shear (5-10 mph) and warm water (28-29°C) favor strengthening, and the National Hurricane Center projects Hilda to become a hurricane by Saturday. Hilda will move in the general direction of Hawaii, although it is far too soon to know whether it might pose a threat to the islands late next week.

Meanwhile, the Atlantic remains quiet, with any waves coming off Africa struggling against dry air, Saharan dust, marginal sea-surface temperatures, persistently high wind shear, or some combination of these.

CSU and NOAA: Atlantic to be even quieter than predicted
Unsurprisingly, both Colorado State University and NOAA have reduced their projected seasonal totals for 2015 tropical cyclones in the Atlantic below the predictions released in May/June, which were already among the lowest in years. The elephant in the room (and in the Atlantic) is the ever-strengthening El Niño, whose hurricane-suppressing effects on the deep atmosphere over the Atlantic are in some cases unprecedented for this time of year. For a full rundown on why El Niño is so hostile to tropical cyclones in the Atlantic, see last week’s post by new WU contributor Dr. Phil Klotzbach, lead author for the CSU hurricane seasonal outlooks.


Figure 4. Rising motion associated with El Niño across the eastern tropical Pacific has led to strong westerly winds at high levels across the Caribbean and deep tropical Atlantic, helping to suppress hurricane formation. This image shows vertical wind shear as measured by the difference between winds at 200 mb (about 40,000 feet) and 850 mb (about 5000 feet). Values of greater than 16 m/s (about 35 mph) dominate the main development region (MDR) of the tropical Atlantic, shown in the green box. Within the red box, shear values for June-July 2015 were well beyond record values for those months in a database extending back to 1970. Image credit: NOAA.


The CSU forecast (see PDF), issued on Tuesday, calls for the following end-of-season totals, including this year’s activity to date (Tropical Storms Ana, Bill, and Claudette), as compared to its start-of-season outlook issued on 1 June:

Named storms: 8 (no change)
Named storm days: 25 (down from 30)
Hurricanes: 2 (down from 3)
Hurricane days: 8 (down from 10)
Major hurricanes: 1 (unchanged)
Major hurricane days: 0.5 (unchanged)
Accumulated cyclone energy (ACE): 35 (down from 40)
Net tropical cyclone activity: 40 (down from 45)

Landfall probabilities from CSU for various sections of the Gulf and Atlantic coast from August onward are generally only about one-third of their full-season averages for the past century. For the entire U.S. Gulf and Atlantic coastline, CSU pegs the odds of a major hurricane landfall at 23%, compared to a full-season, century-long average of 52%. (Even that 23% may be on the high side, considering that no hurricanes have made a U.S. landfall at Category 3 or stronger since Wilma in 2005.)

NOAA’s outlook, issued on Thursday, also brings down all of the forecast indices previously released in their pre-season May outlook. Below are the new NOAA numbers, again incorporating this year’s activity to date. For each range, the likelihood assigned by NOAA that the numbers will fall within the range is 70%, based on years with conditions similar to those now present.

Named storms: 6-10 (lowered from 6-11)
Hurricanes: 1-4 (lowered from 3-6)
Major hurricanes: 1 (lowered from 2)
Accumulated cyclone energy (ACE): 25-70% of average (lowered from 40-80%)

NOAA Is calling for a 90% chance of a below-average hurricane season, the highest odds for a relatively quiet year assigned at any point since the outlooks began in 1998 (right after 1997, the last season that saw an El Niño comparable in strength to the current one.) NOAA reminds us: “For the U.S. and the region around the Caribbean Sea, tropical storms and hurricanes can and do strike even during seasons with El Niño.” For evidence, one need look no further than Hurricane Andrew, a catastrophic landfall in South Florida that occurred during El Niño. As with Andrew, any substantial hurricane this year would be most likely to intensify at or near subtropical latitudes, rather than in the deep tropical Atlantic, where the sinking air and strong shear produced by El Niño will be at their strongest.

For a recap of seasonal hurricane outlooks issued by other entities earlier this year, see the Jeff Masters post from May 27.

Bob Henson



Hurricane

Major Typhoon SOUDELOR Heading for Taiwan

By: Steve Gregory , 6:43 PM GMT on August 05, 2015





WEDNESDAY: 05-AUG-15
(By Steve Gregory - Substituting for Dr. Masters and Bob Henson.)

MAJOR TYPHOON TO STRIKE TAIWAN FRIDAY

What was the strongest Super Typhoon of the year just 36 hours ago with top winds estimated near 200mph, has weakened to a Major, CAT 3 Storm some 460NM SSE of Okinawa as outflow, especially to the north, has weakened considerably and the upper level Outflow weakened proportionately. Even in the WPAC, Super Typhoons like SOUDELOR with extremely small ‘pinhole’ eyes (at one point, less than 4NM across) rarely are able to maintain their MAX intensity following a ERC (Eyewall Replacement Cycle) – and in this case, the significant reduction in the outflow Jet speeds has only accelerated the current weakening phase. In addition, the symmetric and well developed outflow pattern of SOUDELOR is likely being negatively impacted by developing INVEST 96W well to the east of SOUDELOR.

That said – as the storm continues on a generally westward heading (280˚-290˚), the global models show a significant increase in the poleward outflow speeds in 36-48 hrs which should lead to re-intensification – possibly to a CAT 4 intensity by the time it reaches Taiwan late Friday/early Saturday. With virtually every global and specialized Hurricane Forecast model in excellent agreement on the track, there is a very high probability of a direct, full-force strike to the Island nation. After crossing the Island, the cyclone should weaken to a CAT 2 before striking the mainland China coast.

Next UP: INVEST 96W

Another large, developing TC (INVEST 96W) is estimated near 18˚N/147˚E, still well east of the Mariana Islands, is likely to intensify into a Typhoon during the next 48 hrs as it’s already developing an outflow pattern aloft and is in an area of low shears and very warm SST’s.

TROPICAL STORM GUILLERMO CONTINUES TO WEAKEN

Former Hurricane GUILLERMO has continued to weaken over the past several days to Tropical Storm strength with sustained winds now estimated near 50Kts and is located about 280NM east of the Big Island. The storm is expected to pass parallel to, but north of, the Hawaiian Island chain on Thursday and Friday, and will likely have minimal impact on any of the Islands as all significant convection (and max winds) is now only found to the north of the cyclone center.

Numerous RECON flights - including G-IV missions – confirm the steady increase in westerly wind shear observed in Satellite (SAT) imagery over GUILLERMO. This has greatly disrupted the inner circulation field as the vortex now has a strong ‘tilt’ in the mid and upper levels, and a major degradation in high level outflow.

Considering the increasing shear environment and current trends, it would not be surprising to see the storm weaken to depression intensity by the time it passing north of Hilo, and could easily become a remnant Low well north of Honolulu by Friday.

THERE’S MORE COMING WHERE GUILLERMO CAME FROM…

The next system to potentially pose a threat to Hawaii is INVEST 92E, now near 12˚N/127W, or about 1,400NM SW of the southern tip of Baja. The system is still quite close to the ITCZ but should extradite itself from its influence within the next 24-36 hours, allowing the system to begin developing. The latest model runs call for the system to slowly intensify to Tropical Storm intensity in 36-48 hours, as it heads WNW. This system is at least 6-8 days away from it becoming a possible threat to Hawaii.

Several global models are calling for a series of cyclone formations in the EPAC over the next 10-14 days, all of which have the potential to threaten Hawaii. It certainly appears that this year’s strong El Niño environment will be producing numerous EPAC storms with a significant potential to reach the Islands.

TROPICAL ATLANTIC – BELIEVE IT OR NOT - IT REALLY IS HURRICANE SEASON

The Tropical Atlantic is ‘stirring’ – but don’t expect a cyclone formation anytime soon.

Even though former INVEST 94L can still be identified in the central Atlantic, and ‘silly’ INVEST 95L exits from any real concern – we do see a more active environment continuing to develop over the far eastern Atlantic, with a significant Tropical Wave (TW) now just off the African coast, with distinct mid-level rotation observed in the TPW imagery loops. However, this system is solidly embedded within the ITCZ, and considering its current track that will send it towards a more stable environment, it’s unlikely this system will be able to spin-up.

On the ‘plus’ side (for those anxious to have a bona-fide cyclone to track), conditions have become more conducive for cyclone formation over the past week or so. SST’s in the EATL have finally risen to a level that can support ongoing convection, and wind shear is generally low enough south of 20˚N to not be a major hindrance. Probably more importantly at this point is the very large drop-off in the SAL (Saharan Air Layer) flow that has been a major hostile force all summer as the warm, dry air layer has produced a very stable environment across much of the tropical Atlantic.

This seasonal drop-off in the SAL is likely to continue, though occasional surges are likely at times over the next few weeks, especially if/when especially strong TW’s come off the African coast at ‘higher’ latitudes – which typically happens by the end of AUG thru mid-September – coinciding with ‘Cape Verde Season’.

As I’ve pointed out in earlier posts, wind shear in the CARIB has been extremely high this season – primarily induced by the strong El Niño event in progress over the EPAC. This will likely to continue for the rest of Hurricane season, though no doubt, there will be some periods of reduced shear especially in SEP and early OCT.

For additional coverage on national weather, El Nino and other related topics - I will be posting a new Blog update late this afternoon on my own WU Blog.

**

Fig 1: 48 Hour Color Enhanced IR (infrared) image loop of Typhoon SOUDELOR


Fig 2: Water Vapor Imagery of SOUDELOR from earlier today highlighting the banding structure of the storm, along with cloud top Temps that are as low as -70˚C. The CDO signature has weakened compared to yesterday, and while not cloud filled, there are some low level clouds within the eye – characteristic of a weakening system.


Fig 3: WPAC SST’S (left) and total OHC (Ocean Heat Content - right) SST’s are well above levels needed to support a CAT 5 intensity, and remain above 30˚C until reaching the Taiwanese coast. OHC along the projected track for SOUDELOR does drop off some, but remains high enough to support a Major storm as long as the storm’s forward motion does not slow appreciably. A major slowdown (not expected) would lead to widespread, strong upwelling which could weaken the storm – or at least prevent any strengthening.


Fig 4: Micro-wave Image highlighting possible ERC. This early AM image shows the most intense convective feeder band and eye wall of SOUDELOR. On this image, you can see what appears to be major weakening of the inner eyewall as an outer wall develops.


Fig 5: Upper level Wind analysis from CIMSSshows a much reduced outflow jet towards the north, and a bit weaker equatorward jet which has led to the weakening observed over the past 36 hours. In addition, a developing outflow over INVEST 96W to the east of SOUDELOR is also interfering with the Typhoon’s outflow.


Fig 6: Navy Track and Intensity Forecast from this AM is in excellent agreement with all global and specialized hurricane model forecasts – especially on the track – with a direct hit to Taiwan expected by early Saturday. Note the forecast for an increase in intensity prior to landfall as upper level outflow – especially poleward – should increase in 36-48 hrs, allowing an increase in low level surface winds.


Fig 7: Last VIS image showing SOUDELOR on the far left, with developing INVEST 96W about 950NM east of SOUDELOR. It’s can be tough to determine which system’s circulation field will be most impacted by the other’s!


Fig 8: Color Enhanced IR image and Shear analysis of T.S. GUILLERMO shows most of the significant convection is now only found to the north of the storm, with wind shear values approaching 40Kts. With shears increasing to over 60Kts near Honolulu, GUILLERMO’s days are numbered.


Fig 9: Navy Track and Intensity Forecast for GUILLERMO calls for the tropical Storm to weaken to depression intensity in within 48-72 hrs. This is likely too generous – as the system is likely to weaken to Depression intensity within 36-48 hrs.

CLICK IMAGE to open full size image in a new window
Fig 10: Color Enhanced IR (infrared) image of the Tropical Atlantic The remnant disturbance that was INVEST 94L is still discernable as a very weak Low near the base of the TW along 48W which is westbound at about 15Kts. Aside from several other very weak TW’s, a stronger wave with mid-level rotation is located just off the African coast, but is solidly embedded within the ITCZ which is inhibiting development. Most notable in today’s overview of the tropical ATL is the major reduction in the SAL flow – with little dust filled air found anywhere over the ATL ocean. However, the forecast for the SAL (below) does show a strong, but limited SAL flow spreading over the EATL in the next day or two. While none of the global models call for this TW to develop into a cyclone, an upstream wave over central Africa and still a couple days away from reaching the coast, is forecast by the ECMWF to spin-up into a cyclone by early next week – though none of the other global models concurs with this outlook.



Fig 11: The TPW (Total Precipitable Water) Loop (Top) and latest image frame (bottom) highlights areas with deep layered moisture. ‘Bluish’ colors represent dryer air while the darker, ‘orange’ tones highlight areas of deep moisture in the column of air above the surface. The 72 hour looped imagery shows a clear rotation in vicinity of the African coast – embedded within the ITCZ.


Fig 12: Specialized ‘Dust’ focused imagery from the EUMETSAT product suite. The magenta/pinkish colors correspond to specific light wavelengths that have been shown to correlate with atmospheric dust.If you've been following my blogs this summer, you know by now there are quite a few different products that aim to highlight Dust and other aerosols associated with the SAL (Saharan Air Layer). Many of these products tend to ‘overdo’ dust concentration - for example, the Univ. Of Wisconsin CIMMSS product which tends to rely on moisture content analysis in the lower portion of the atmosphere. The above 24 hour loop tends to underplay actual dust content; but, the 3 different products I've posted this season, when used together, usually results in a fairly accurate analysis. While I haven’t posted any this season, even ‘normal’ Visible imagery taken early in the day, along with MODIS True Color imagery can do an excellent job of highlighting aerosol content. What makes the ABOVE imagery so useful is the simultaneous depiction of deep convection over Africa at hourly intervals, enabling us to get a good handle on Tropical Waves moving across the African continent. The strongly highlighted area of low-level dust north of the ITCZ is forecast to come off the African coast during the next day or two (See Below), while we can also see the strong area of convection over central Africa that the ECMWF is forecasting to develop into a cyclone early next week.


Fig 13: Specialized ‘Dust’ analysis / forecast from NASA calls for a strong burst of dust filled air over the next few days – but is significantly less ‘dense’ and ultimately less expansive than we’ve seen over the past couple months.

For additional coverage on national weather, El Nino and other related topics - I will be posting a new Blog update late this afternoon on my own WU Blog.

Steve

TYPHOON TROPICS

Tropical Storm Watches for Guillermo; 95L Clips Coast; Soudelor Still a Super Typhoon

By: Bob Henson , 11:06 PM GMT on August 04, 2015

Tropical storm watches were hoisted Tuesday morning for the islands of Hawaii, Maui, Molokai, Lanai and Kahoolawe as Tropical Storm Guillermo headed toward the island chain. At 5:00 pm EDT Tuesday, Guillermo was located about 400 miles east of Hilo, packing sustained winds of 70 mph. Wind shear continues taking a toll on Guillermo, with most of the storm’s convection shunted to its northeast side. Drier air and increasing wind shear along Guillermo’s path should prompt a weakening trend later tonight or tomorrow, despite sea-surface temperatures that remain above 27°C (1°C to 2°C above average) along Guillermo’s path. Even the southernmost dynamical model (ECMWF) keeps Guillermo’s west-northwest track far enough north of the islands to avoid tropical storm impacts. NOAA’s probabilistic wind guidance gives less than a 10% chance of tropical storm force winds over Hawaii in the next 120 hours.


Figure 1. A composite (RGB) satellite image of the increasingly sheared Tropical Storm Guillermo, collected by the GOES West satellite at 2200 GMT Tuesday (6:00 pm EDT). Image credit: NOAA/NESDIS.


Figure 2. GOES infrared satellite image of Invest 95L, from 2145 GMT Tuesday (5:45 pm EDT), shows convection mainly offshore of North Carolina. Image credit: NOAA/NESDIS.

Invest 95L clips the Carolina coast
After it triggered widespread flash flooding and river flooding over central Florida, especially in the Tampa Bay area, Invest 95L has been kinder to the southeastern states. At 4:35 pm EDT Tuesday, the National Hurricane Center placed the center of 95L near Wilmington, NC, moving northeast very near the coastline. A large but disorganized shield of showers and thunderstorms has remained mainly southeast of 95L’s center, sparing the Carolinas from much impact side from pockets of heavy rain near the Outer Banks. Reconnaissance flights found offshore flight-level winds of 60 mph at 4:35 pm EDT Tuesday in gales southeast of the center, but winds have been quite light along the coast near 95L, generally less than 20 mph both inland and at near-shore buoys. Phase-space diagrams from Florida State University show that 95L is already exhibiting characteristics of a cold-core system, and the asymmetry of 95L should grow as it shoots northeast and becomes entangled in a midlatitude frontal system. Still, NHC gives 95L a 30% chance of organization over the next several days, and several dynamical models suggest the system could intensify just enough to potentially earn it a name over the next 24-36 hours.


Figure 3. Visible image of Super Typhoon Soudelor, collected by the MTSAT satellite at 2201 GMT Tuesday (6:01 pm EDT). Image credit: NOAA/NESDIS.

A slightly weaker Soudelor threatens Taiwan
Having completed an eyewall regeneration cycle, Super Typhoon Soudelor was sporting a less impressive convective shield and slightly weaker sustained winds on Tuesday, but it remained a formidable storm. Peak sustained winds were estimated by the Joint Typhoon Warning Center (JTWC) at 130 knots (around 150 mph) at 1800 GMT (2:00 pm EDT) Tuesday. Soudelor remains on a classic west-northwest path around the south side of an upper-level ridge, and models are still in close agreement on a landfall in Taiwan on Friday night or Saturday morning local time. The JTWC is calling for gradual weakening over the next couple of days, mainly due to subsidence related to a nearby tropospheric upper-level trough (TUTT). Apart from the TUTT, conditions are highly supportive of some eventual restrengthening. Category 4 strength is possible at landfall, according to the JTWC forecast from 2100 GMT Tuesday (5:00 pm EDT). Soudelor could end up even stronger than predicted if another rapid intensification cycle were to take place (such cycles are difficult to predict in advance). Soudelor will likely go on to strike mainland China as a strong tropical storm or Category 1 typhoon.


Figure 4. Flooding caused by heavy rain brought by Typhoon Morakot across Pingtung county, southern Taiwan, on August 8, 2009. Morakot, which means 'emerald' in Thai, dumped more than 1,255 millimetres (49 inches) of rain on southern Pingtung county. Image credit: AFP/AFP/Getty Images.

Taiwan is familiar with the strong winds and torrential rain brought by major typhoons. From 1897 to 2003, the island recorded 383 typhoon landfalls, an average of about 3.5 per year. Despite this experience, the nation remains vulnerable to flooding and landslides/mudslides, given its dense infrastructure and population and the unavoidable nature of its highly mountainous terrain. The most disastrous storm in recent years to strike Taiwan was 2009’s Typhoon Morakot, which caused more than 450 deaths and some $3.3 billion US in damage. Morakot was only a Category 1 storm, with peak 1-minute sustained winds of 90 mph, but it moved in a leisurely cyclonic loop across northern Taiwan, prolonging the widespread intense rainfall. A 2011study in the Journal of Geophysical Research found that Morakot’s slow motion and interaction with a monsoon-related belt of southwesterly winds played major roles in the disaster. Morakot brought Taiwan its all-time rainfall records for a 24-hour period (1403 mm or 55.2” at Weiliaoshan) and for a 48-hour period (2327 mm or 91.6” at Alishan). Soudelor is a powerful typhoon, but its expected motion at a steady clip across Taiwan should help reduce the risk of such exorbitant rainfall totals.

Our next blog update will be courtesy of wunderblogger Steve Gregory, who is planning a guest post early Wednesday afternoon (around 2PM CDT). You can follow Steve’s regular posts at his WU site.

Bob Henson

Hurricane

Another Category 5 Cyclone: Super Typhoon Soudelor

By: Bob Henson , 1:36 AM GMT on August 04, 2015

Super Typhoon Soudelor vaulted to Category 5 status on Monday, making it the planet's sixth (at least--see below) Category 5 storm of the year. At 2:00 pm EDT Monday, Soudelor’s sustained winds were estimated at 180 mph, with the strength unchanged in the 8:00 pm EDT Monday update from the Joint Typhoon Warning Center (JTWC). For the year thus far, Soudelor is Earth’s strongest tropical cyclone in terms of estimated wind speed. The Weather Channel’s Nick Wiltgen notes that Soudelor’s estimated central pressure of 900 mb is the lowest in a typhoon since last year’s Super Typhoon Vongfong, also 900 mb. Five prior Category 5 storms this year were described and illustrated in a May 19 post. They include Tropical Cyclone Eunice, Cyclone Pam, Super Typhoon Maysak, and Super Typhoon Noul. Update: Our initial survey of JTWC products showed that Cyclone Bansi fell short of Category 5 status. However, JTWC data for Cyclone Bansi archived by RAMMB-CIRA indicate that Bansi's estimated winds peaked at 140 knots (about 160 mph) at 0000 GMT on January 13. If we include Bansi, then we're now up to a startling seven Category 5 storms so far in 2015. This compares to a yearly average of 4.6 Category 5 storms for the period 1990-2014. It's not out of the question we could break the record total of 12 Category 5 storms notched in 1997, when--much like this year--a strong El Niño was ramping up. Thanks to wunderground member 1900hurricane for bringing Bansi data to our attention.

Models are in quite close agreement on keeping Soudelor rolling along a steady west-northwest track around the southwest side of a strong upper-level high. In its 0300 GMT Tuesday forecast update, the JTWC projected that Soudelor would strike the northern end of Taiwan on Friday or Saturday local time, potentially as a Category 3 or 4 storm. On such a track, Soudelor would move very close to Taiwan’s largest city, Taipei. The island’s rugged topography can lead to massive rainfall and hugely destructive flooding when strong typhoons make landfall, so Soudelor will have to be watched very closely.


Figures 1 and 2. These images of Super Typhoon Soudelor (Figure 1= infrared, Figure 2 = visible) show incredible detail collected at 1633 GMT Monday (12:33 pm EDT) by the Visible Infrared Imaging Radiometer Suite (VIIRS) sensor aboard NASA’s Suomi NPP satellite. For an animated-GIF version that switches between these two images, see the embedded tweet from Ari Salsalari (The Weather Channel) at bottom of this post. Image credit: NOAA/NASA, courtesy Dan Lindsey, RAMMB/CIRA, and Stu Ostro, Weather Channel.


Along with the strikingly crisp eyewall, the images of Soudelor in Figures 1 and 2 reveal a “cloud cliff”--a sharp, distinctly linear feature that abuts the northeast side of the eyewall. The cause of such cloud cliffs is unknown, but it is not uncommon to see them in super typhoons over the western Pacific. Similar features were observed in 2012 in Super Typhoon Bopha (visible on the south side of the eye in this blow-up image from the University of Wisconsin CIMSS Satellite Blog), and in Super Typhoon Jelawat (visible on the north side of the eye.) For several images showing a cloud cliff in last year’s Typhoon Neoguri, see this July 2014 post by Jeff Masters.



Figure 3. An infrared image of the increasingly sheared Tropical Storm Guillermo, collected by the GOES West satellite at 0000 GMT Tuesday (8:00 pm EDT Monday). Image credit: NOAA/NESDIS.

Elsewhere in the tropics
Moving into increasingly hostile conditions, Tropical Storm Guillermo continued its slow weakening trend on Monday. Guillermo’s sustained winds were reduced to 65 mph in the 3:00 pm EDT Monday advisory from the Central Pacific Hurricane Center. As expected, westerly wind shear is ramping up along Guillermo’s track as the storm gains latitude, with vertical shear values likely to exceed 30 mph by Wednesday. With regular input from hurricane-hunter reconnaissance flights, computer models have nudged Guillermo’s west-northwest track a bit further away from the Hawaiian islands. Tropical storm watches may be issued for parts of the state, although it appears that large swells and localized heavy rains will be the main threat from Guillermo. The scenario is very reminiscent of Tropical Storm Flossie, which moved along a similar track paralleling and just north of the Hawaiian Islands while weakening from a tropical storm to a tropical depression. Meanwhile, a new system in the Northeast Pacific, Invest 92E, shows potential for developing into a tropical storm later this week, but it is unlikely to affect any major land areas.

In the Atlantic, tenacious Invest 95L is now hugging the coast of Georgia as it parallels the southeast U.S. coast on its slow northeastward path. Though still poorly organized, 95L brought heavy rains to the central Florida peninsula, especially across the Tampa Bay area, with flooding a concern throughout the day on Monday. Tampa notched 4.39” of rain on Monday, eclipsing the daily record of 2.57” from 1913. These rains came on top of 3.89” observed in Tampa on Saturday and 11.84” through July (most of it during the last two weeks of the month). Chances of 95L developing into a tropical cyclone while moving so close to the southeast U.S. coast are minimal. By Wednesday, 95L should be accelerating out to sea off the North Carolina coast.

A large and healthy tropical wave was just coming off the African coast on Monday night, with some hints that it could develop into an invest-worthy system over the next several days. For more on what’s brewing there and elsewhere, check out this afternoon’s post from WU blogger Steve Gregory. I’ll have a new post by Tuesday afternoon.

Bob Henson



Hurricane

Invest 95L Drenches Florida’s West Coast; Super Typhoon Soudelor Threatens Taiwan

By: Bob Henson , 4:17 PM GMT on August 03, 2015

Already reeling from days of heavy rain, the Florida coastline from Tampa to the Big Bend area got an unwelcome visitor on Sunday night in the form of Invest 95L. A well-defined swirl was evident Sunday on satellite imagery just west of Florida’s Big Bend, near the weak south end of a large upper-level low covering much of eastern North America. The National Hurricane Center began tracking the system as Invest 95L at 0000 GMT Monday, with an initial surface wind speed of 30 mph. At 8:00 am EDT Monday, NHC placed the weak center of low pressure over north central Florida. Winds were strongest offshore, with sustained westerly winds of 20 – 30 mph reported on Sunday night at several buoys in the far northeast Gulf. A gust to 42.5 mph occurred at Buoy 42036, according to The Weather Channel’s Stu Ostro. Most of the shower and thunderstorm activity associated with 95L has been on its south side, extending into parts of the Florida west coast just north of Tampa Bay.


Figure 1. Infrared image shows a disorganized 95L producing extensive rainfall across central Florida. Image credit:. NASA MSFC Earth Science Office.


Figure 2. Parts of the northwest Florida coast picked up widespread rainfall totals of 10” to 20" in the two weeks ending at 8:00 am EDT Monday, August 3, with additional rain falling later on Monday. Image credit: NOAA Advanced Hydrological Prediction Service.

As the upper low shifts east, it will gradually nudge the system northeastward across the Florida peninsula over the next day or so. Some acceleration is expected by midweek as the center finally moves away from Florida. There will be little chance for 95L to intensify much while it remains so close to land, although there is a slender possibility of some strengthening (most likely in a subtropical or hybrid mode) after 95L clears Florida, assuming it travels along the Gulf Stream. NHC gives the system only 10% odds of developing into a tropical cyclone over the next 5 days. The extremely rich moisture associated with the circulation will continue to sweep onshore, contributing to more episodes of heavy rain. A lingering frontal boundary has kept the northwest Florida coast in deep moisture with frequent rounds of showers and thunderstorms, now leading to rainfall totals that are posing serious problems. Tampa International Airport received 4.08” of rain between 6:00 and 11:00 am EDT Monday, on the heels of 3.89” observed on Saturday and 11.84” through July (most of it during the last two weeks of the month). Water rescues took place on Monday morning in Pinellas County, with evacuations ordered of at least one neighborhood near Elfers, Florida, northwest of St. Petersburg. Flash flood watches extended along the coastal counties from Tampa to the Big Bend on Monday morning, with the heaviest rains expected to shift inland later today.



Figure 3. Typhoon Soudelor inflicted mostly minor but widespread damage across Saipan. Image credit: American Red Cross, via Pacific Daily News.

Soudelor brings extensive damage to Saipan
Trees were snapped and power lines brought down across much of Saipan after a miniscule but mighty Super Typhoon Soudelor (not yet a super typhoon at that point) barrelled into the southern half of the island. At 10:54 pm local time on Sunday (8:54 am EDT Sunday), sustained winds of 54 mph and gusts to 91 were reported at Saipan International Airport, located on the south end of the island and close to the tiny south eyewall of Soudelor. Stronger winds most likely occurred in the north eyewall, where the westward motion of the storm would have added to wind speeds. Soudelor was officially classified as a Category 1 storm at this point, but its small-scale structure was virtually impossible to resolve in standard satellite imagery and the typhoon was already rapidly intensifying, so peak winds on Saipan might have been considerably stronger. The Pacific Daily News reported that about 350 people were in shelters at midday Monday, and a state of disaster has been declared. Fortunately, initial reports listed only a few minor injuries.

Part of the U.S. Commonwealth of the Northern Marianas, Saipan extends only about 12 miles from north to south, but Soudelor was even more compact than that. Just before landfall, a National Weather Service NEXRAD radar image showed the eye of Soudelor to be only about 4 nautical miles in diameter, surrounded by an miniature ring of showers and thunderstorms (see Figure 4). This is among the smallest eyes and eyewalls observed anywhere as a tropical cyclone was making landfall. According to the final hurricane-hunter reconnaisance report, 2004’s destructive Hurricane Charley had an eye diameter of 5 nm, within a larger zone of convection than Soudelor, just before making landfall in southwest Florida on August 13, 2004, at the end of a period of sudden strengthening to Category 4 strength. In the open Atlantic, the record-holder for smallest observed eye is another compact tropical cyclone, Hurricane Wilma, whose eye shrank to an amazing 2 nm in diameter while hurricane hunters were in the vicinity. Wilma was another rapidly intensifying cyclone, with peak winds jumping from 150 to 184 mph in less than six hours. (A reconniasance report from 1993’s Hurricane Emily also showed a 2-nm eye, but Emily was only at tropical storm strength and its eyewall was partially open at the time, so it does not reflect the same processes discussed above, and the report could be an error. Thanks to The Weather Channel’s Michael Lowry and Stu Ostro for these examples.)


Figure 4. The pinhole eye of Typhoon Soudelor was evident in NEXRAD radar imagery from 1229 GMT (8:29 pm local time) on August 2, as the typhoon bore down on Saipan from the east-southeast. The island extends roughly 12 miles from its southern to northern tip. Image credit: NOAA, courtesy Mike Middlebrooke, Senior Forecaster, NWS/Guam.


Figure 5. A colorized infrared image of Hurricane Wilma near its peak intensity, collected at 1245 GMT on October 19, 2005. Unlike Soudelor, Wilma had a large shield of intense convection surrounding its tiny eye, which was only 2 nm wide at its smallest. Image credit: NOAA, courtesy Stu Ostro, The Weather Channel.

Now a super typhoon, Soudelor bears close watching
Soudelor carried out a spectacular burst of intensification on Sunday, no doubt aided by its ultra-compact size, which allows for more rapid strengthening. Sustained winds rocketed from 70 mph at 0000 GMT Sunday to 135 mph at 0000 GMT Monday. During the day on Monday, Soudelor continued to impress by carrying out an eyewall replacement cycle clearly evident in 1-km-resolution imagery collected by Japan’s Himiwari-8 satellite, with the pinhole eye decaying as a much larger eye developed around it. (Here’s a pair of not-to-be-missed Himiwari-8 satellite loops from Sunday and Monday, courtesy Dan Lindsey/CIRA and Japan Meteorological Agency). The Joint Typhoon Warning Center reported peak winds of 155 mph in its 1200 GMT Monday advisory, making Soudelor a super typhoon.


Figure 6. An infrared MTSAT image of Super Typhoon Soudelor, collected at 1401 GMT on August 3. Image credit: NOAA/NESDIS.


Figure 7. Japan’s Himiwari-8 satellite captured this large-perspective view of Soudelor at 1240 GMT on August 3. The island of Taiwan is shown at top left. Image credit: CIRA and Japan Meteorological Agency

With very warm sea-surface temperatures (31°C, or 88°F) along its path and upper conditions remaining favorable, Soudelor could intensify even more by Tuesday. Models agree on a straightforward west-northwest path over the next several days, which puts the typhoon uncomfortably close to northern Taiwan by the end of the week. The latest JTWC outlook (1200 GMT Monday) has Soudelor clipping the island at Category 3 strength. Soudelor could pose a serious threat to Taiwan, where typhoons crashing against high terrain can produce some of the heaviest rainfall rates measured on Earth. Track uncertainty in the 4-5 day time range means that the entire island needs to be on alert. Soudelor will likely move on to strike the east coast of China, although its intensity at this point will be strongly influenced by its eventual trajectory over or near Taiwan.

Guillermo weakens to a tropical storm
At 5:00 am local time (11:00 am EDT) Monday, the Central Pacific Hurricane Center downgraded former Hurricane Guillermo to tropical-storm status, with peak winds of 70 mph. Located near 16.6°N and 146.1°W, Guillermo continued to move west-northwest at about 10 mph. Upper-level wind shear of about 15 knots has contributed to Guillermo’s gradual decline, and further slow weakening is expected, so Guillermo will probably be a fairly weak tropical storm by the time it nears Hawaii late Wednesday. Data from the NOAA Gulfstream IV large-scale reconnaissance mission entered last night’s 0000 GMT Monday model runs, which has resulted in an eastward nudge to Guillermo’s expected track. This further reduces the threat to Hawaii; although the track will take Guillermo on an unusual trajectory paralleling the entire island chain, most models now place Hawaii on the weaker left-hand side of Guillermo. However, only a slight westward shift would put the islands on the stronger right-hand side. In any event, dangerously large swells will affect Hawaii’s east-facing coastlines over the next several days.

Rare August cyclone in the South Pacific
One of the first storm-strength tropical cyclones, if not the first, known to occur in the South Pacific during August is making its way southward with minimal impact. At 0900 GMT, Tropical Cyclone One (not yet assigned a name) was located near 10.9°S, 172.6°E, with sustained winds of 40 mph, just above minimal tropical storm strength. Unusually warm water temperatures associated with El Niño have made the South Pacific unseasonably active. Last month produced Tropical Cyclone Raquel, which struck the Solomon Islands at hurricane strength as the only tropical cyclone on record to occur in the basin during July. Moderate wind shear and cooler sea-surface temperatures downstream are expected to keep TC1 at minimal intensity for the next couple of days. Currently drifting eastward, TC1 is expected to gradually head southward before eventually dissipating, far from any populated areas.

Pacific Northwest continues to sizzle
Seattle, Washington, and Salem, Oregon, endured their hottest month in more than 120 years of recordkeeping during July, as high pressure kept rains at bay and hot weather in control of the Pacific Northwest. WU weather historian Christopher Burt has a full report on the many records smashed in July across Oregon and Washington, which are on their way to the warmest summer on record. If there’s any upside, it’s that wildfire activity (as measured by acreage burned) is actually below average this summer over the two states, despite the tinder-dry conditions. Cliff Mass (University of Washington) explains the paradox in a recent blog post. For starters, the atmosphere is so suppressed that little of the normal “dry lightning” from low-precipitation thunderstorms is occurring, and winds are too light to kick up any fires that do happen to start. Mass also credits the rapid response of state and federal firefighters this year in snuffing out potential fire disasters.

Bob Henson

Hurricane


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™

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