Category 6™

El Niño chances rising for hurricane season

By: JeffMasters, 3:38 PM GMT on May 29, 2009

Sea surface temperatures in the Equatorial Eastern Pacific have been rising steadily for several months, and there is now a very real possibility that an El Niño event could occur during the peak of the Atlantic hurricane season, August - October. This is important, since the number and intensity of Atlantic tropical storms and hurricanes is usually reduced during an El Niño year, thanks to the increased wind shear such events bring to the tropical Atlantic. Last month, Columbia University's International Research Institute (IRI) was giving a 30% chance of an El Niño event for the coming hurricane season; this month, they have bumped their odds up to 45%. The Australian Bureau of Meteorology notes that "recent trends are consistent with the very early stages of a developing El Niño". NOAA's Climate Prediction Center forecasts the current neutral conditions in the Equatorial Eastern Pacific will continue into the summer, but shows that their CFS El Niño model is predicting a moderate El Niño event for the coming hurricane season.


Figure 1. Sea Surface Temperature (SST) departure from average for the equatorial Eastern Pacific (the area 5°N - 5°S, 120°W - 170°W, also called the "Niña 3.4 region"). The +0.5°C mark is the threshold for El Niño conditions, and we are very close to that mark now. Image credit: NOAA's Climate Prediction Center.

El Niño forecast models
The latest suite of runs by the various computer models used to forecast El Niño offer two main forecasts for the coming hurricane season: neutral conditions will persist, or an El Niño will develop (Figure 2). There are two types of models used to make these forecasts: statistical models and dynamical models. The statistical models have been around the longest, and they rely on statistics of how past El Niño episodes have developed in order to make a forecast. Dynamical models don't care about what has happened in the past. They make a forecast by taking the current state of the atmosphere, putting the data on a grid covering the entire globe, then solving the equations that govern the physics of the atmosphere and ocean on each point of this grid every few minutes, marching forward in time for many months. These dynamical models, in many cases, are simply modified versions of the same models we use to forecast the short-term weather. For example, the NOAA's Coupled Forecast System (CFS) model is based on the GFS model that we use to track hurricanes and make short-term weather forecasts. The main difference is that the CFS model runs for many months instead of just a few days.


Figure 2. El Niño model forecasts made in mid-May. Note that for the peak part of hurricane season, August-September-October (ASO), most of the dynamical models are forecasting an El Niño event (SST anomaly greater than 0.5°C in the Eastern Equatorial Pacific). Image credit: Columbia University's IRI.

Which model to believe?
As is the case with all seasonal forecasts, El Niño forecasts are not very good, and don't do much better than flipping a coin. However, thanks to intensive research efforts and the doubling of computer power that has been occurring every 1.5 years, the El Niño forecasts by the dynamical models have improved considerably over the past few years. These models now do about as well as the traditional statistical models, and should continue to improve as computer power continues to increase and our understanding of El Niño increases. Over the past two months, the dynamical models have increasingly been forecasting the development of an El Niño this Fall. To illustrate, in March only three of the thirteen dynamical models were predicting an El Niño event for hurricane season. By mid-May, this had increased to nine out of thirteen models. However, none of the eight statistical El Niño models are forecasting an El Niño event for the Fall, and their forecasts should be respected, as well. The IRI web site has a nice tool one can use to study the performance of the individual models. To my eye, the Japanese Meteorological Agency (JMA) dynamical model has made the best El Niño forecasts over the past two years (though I haven't done a rigorous error analysis to verify this). The JMA model is predicting a weak El Niño event for the coming hurricane season, and I am going to go along with that forecast.

What will an El Niño event do to hurricane numbers?
Since the active hurricane period we are in began in 1995, there have been four El Niño events (Figure 3). During these years, the number of named storms, hurricanes, and intense hurricanes 11 named storms, 5 hurricanes, and 3 intense hurricanes. This is close to the average levels we've seen over the past 60 years--10-11 named storms, 6 hurricanes, and 2 intense hurricanes. If, on the other hand, we look at the five years that had neutral conditions, the numbers are considerably higher--18 named storms, 10 hurricanes, and 5 intense hurricanes. So, let's hope for an El Niño this year. Note, though, that one of our worst hurricane years--2004, which featured hurricanes Ivan, Charlie, Frances and Jeanne, which all affected Florida with hurricane conditions--was an El Niño year. It seems that in years like 2004, there is a lag between the time a El Niño event develops and the response of the atmosphere over the Atlantic. There is no way of forecasting at this point whether this could be the case this year. One argument against a repeat of 2004 is the presence of much lower heat content and SSTs in the tropical Atlantic this year compared to 2004.


Figure 3. Looking at the numbers of Atlantic names storms, hurricanes, and intense hurricanes since 1995.

Tropical Depression One
The season's first tropical depression formed yesterday off the coast of North Carolina, but has missed its opportunity to become the Tropical Storm Ana. Tropical Depression One is headed east-northeastward out to sea, and is now entering a region with cooler water temperatures and increased wind shear of 15 - 20 knots. The heavy thunderstorm activity associated with TD One has shrunk this morning, and high wind shear has pushed these thunderstorms to the east side of the center, exposing the surface circulation to view. Tropical Depression One will not hold together much longer, and should be history by Saturday night.

Is the formation of TD One a harbinger of an active hurricane season?
Probably not. Early season storms occurring near the U.S. coast have not been shown to be correlated with an active main portion of hurricane season during August - October. However, the situation is different if we start getting June and July storms in the deep tropics between Africa and the Lesser Antilles Islands. This was the case last year, when the formation of Hurricane Bertha in the deep tropics in July presaged an active 2008 hurricane season. According to the Hurricane FAQ, "as shown in (Goldenberg 2000), if one looks only at the June-July Atlantic tropical storms and hurricanes occurring south of 22°N and east of 77°W (the eastern portion of the Main Development Region [MDR] for Atlantic hurricanes), there is a strong association with activity for the remainder of the year. According to the data from 1944-1999, total overall Atlantic activity for years that had a tropical storm or hurricane form in this region during June and July have been at least average and often times above average. So it could be said that a June/July storm in this region is pretty much a "sufficient" condition for a year to produce at least average activity."

I'll have a detailed outlook of the coming hurricane season on Monday, the first day of the Atlantic hurricane season.

Jeff Masters

Hurricane

Season's first tropical depression forms

By: JeffMasters, 3:43 PM GMT on May 28, 2009

Nature is jumping the gun a bit this year, with the season's first tropical depression forming four days before the official start to hurricane season. The area of disturbed weather (91L) that we've been watching, about 250 miles east-northeast of Cape Hatteras, North Carolina, has developed enough heavy thunderstorm activity and spin to be classified as a tropical depression. QuikSCAT imagery from last night revealed a closed surface circulation, but top winds of only 20 - 25 mph. Satellite estimates (using a cloud pattern recognition method called the "Dvorak" technique) were saying this was a tropical depression this morning, though, so the NHC elected to upgrade the system.

The disturbance is over the relatively warm waters of the Gulf Stream (25°C) and has wind shear of 5 - 10 knots over it, and these conditions are marginally favorable for some slow development to occur until Friday, when the system will likely move over waters too cold to support intensification. TD One is not a threat to any land areas. I give the storm a 60% chance of becoming Tropical Storm Ana.


Figure 1. Latest satellite image of TD One.

Is the formation of TD One a harbinger of an active hurricane season?
Probably not. Early season storms occurring near the U.S. coast have not been shown to be correlated with an active main portion of hurricane season during August - October. However, the situation is different if we start getting June and July storms in the deep tropics between Africa and the Lesser Antilles Islands. This was the case last year, when the formation of Hurricane Bertha in the deep tropics in July presaged an active 2008 hurricane season. According to the Hurricane FAQ, "as shown in (Goldenberg 2000), if one looks only at the June-July Atlantic tropical storms and hurricanes occurring south of 22°N and east of 77°W (the eastern portion of the Main Development Region [MDR] for Atlantic hurricanes), there is a strong association with activity for the remainder of the year. According to the data from 1944-1999, total overall Atlantic activity for years that had a tropical storm or hurricane form in this region during June and July have been at least average and often times above average. So it could be said that a June/July storm in this region is pretty much a "sufficient" condition for a year to produce at least average activity."

Portlight.org offering relief to Florida flood victims
Tropical disturbance 90L dropped as much as two feet of rain over Northeastern Florida last week, causing severe flooding. In Volusia County, at least 1500 homes were damaged by the flooding, and many of these were in low-income housing projects where the residents did not have flood insurance. Portlight Strategies, Inc., is now working to assist in this area by providing durable medical equipment to the disabled, elderly, or injured that have lost equipment due to the flooding. Equipment will also be provided to local shelters and other organizations working with flood victims. To help out, visit the Portlight disaster relief blog..


Figure 2. Rainfall amounts over Florida for the two weeks ending on May 27, 2009. Images credit: NOAA.

Jeff Masters

Hurricane

Disturbance 91L more organized, but headed out to sea

By: JeffMasters, 1:18 PM GMT on May 28, 2009

An area of disturbed weather (91L), located about 250 miles east-northeast of Cape Hatteras North Carolina, has shown a modest increase in heavy thunderstorm activity this morning. QuikSCAT imagery from last night revealed a closed surface circulation, but top winds of only 20 - 25 mph.

The disturbance is over the relatively warm waters of the Gulf Stream (25°C) and has wind shear of 10 - 15 knots over it, and these conditions are marginally favorable for some slow development to occur until this evening, when the system will begin moving over waters too cold to support tropical cyclone development. The disturbance will track northeastward at 15 mph today, and and is not a threat to any land areas. In a Special Tropical Weather Outlook issued at 8am EDT this morning, NHC gave 91L a low (less than 30% chance) of developing into a tropical depression.


Figure 1. Latest satellite image of 91L.

Portlight.org offering relief to Florida flood victims
Tropical disturbance 90L dropped as much as two feet of rain over Northeastern Florida last week, causing severe flooding. In Volusia County, at least 1500 homes were damaged by the flooding, and many of these were in low-income housing projects where the residents did not have flood insurance. Portlight Strategies, Inc., is now working to assist in this area by providing durable medical equipment to the disabled, elderly, or injured that have lost equipment due to the flooding. Equipment will also be provided to local shelters and other organizations working with flood victims. To help out, visit the Portlight disaster relief blog..


Figure 2. Rainfall amounts over Florida for the two weeks ending on May 27, 2009. Images credit: NOAA.

Jeff Masters

Hurricane

Disturbance 91L spins towards Cape Hatteras; Cyclone Aila toll at 180

By: JeffMasters, 1:28 PM GMT on May 27, 2009

An area of disturbed weather, dubbed "91L" by the National Hurricane Center, is centered 120 miles south of Cape Hatteras, North Carolina. While the storm does not have much in the way of heavy thunderstorm activity, it does have a well-developed circulation, and the spin of the system is readily apparent on long range radar animations out of Morehead City, North Carolina. The disturbance is over waters of 25 - 26°C and has wind shear of 10 - 15 knots over it, and these conditions are marginally favorable for some slow development to occur until Thursday afternoon, when the system will begin moving over waters too cold to support tropical cyclone development. The disturbance will track north or north-northeastward at 10 - 15 mph towards North Carolina's Outer Banks today, then get swept northeastwards out to sea on Thursday. It is unlikely that the disturbance has enough time to develop into a tropical depression, but an Air Force hurricane hunter flight is on call to investigate the system this afternoon, if necessary. If the system does develop, the current location of the heaviest thunderstorm activity in a band well removed from the center suggests that 91L would be classified as a subtropical depression. The Outer Banks of North Carolina can expect 20 - 25 mph winds and heavy rain from this system tonight and Thursday morning. In a Special Tropical Weather Outlook issued at 8am EDT this morning, NHC gave 91L a low (less than 30% chance) of developing into a tropical depression.


Figure 1. Latest radar image from the Morehead City, NC radar.

Tropical Cyclone Aila death toll at 180
The year's deadliest tropical cyclone so far, Tropical Cyclone Aila, has killed at least 180 people in India and Bangladesh border region, according to the latest media reports. Aila hit the India/Bangladesh border region on May 25 as a borderline tropical storm/Category 1 hurricane, bringing sustained winds of 65 - 75 mph and a 3 - 4 meter (10 - 13 foot) storm surge to the coast. Aila has left over 150,000 homeless in India and 500,000 in Bangladesh. The cyclone destroyed over 180,000 homes in Bangladesh--a severe blow for a region still recovering from the devastation wrought by Category 4 Tropical Cyclone Sidr of November 2007, which killed 3,500 people. The death toll form Aila will likely go much higher, as over 500 people are still missing. The Bay of Bengal is no stranger to deadly cyclones--fifteen of the world's twenty deadliest tropical cyclones have been Bay of Bengal storms that have hit Bangladesh, India, or Myanmar. The most recent was last year's Cyclone Nargis, which killed 146,000 people in Myanmar.


Figure 2. Satellite image of Aila as it made landfall near the India/Bangladesh border. Image credit: NASA.

Interview tonight on hurricanetrack.com
I'll be doing one of my periodic spiels on Internet radio tonight at 9pm EDT. Tune your browsers to www.hurricanetrack.com and listen in to my interview with host Mark Sudduth. There is also a live chat to participate in. Hurricane season starts Monday!

Jeff Masters

Hurricane

Cyclone Aila kills at least 120 in India, Bangaldesh; 91L forms

By: JeffMasters, 1:57 PM GMT on May 26, 2009

The year's deadliest tropical cyclone so far, Tropical Cyclone Aila, hit the India/Bangladesh border region on May 25 as a borderline tropical storm/Category 1 hurricane. Aila brought sustained winds of 65 - 75 mph at landfall, and a 3 - 4 meter (10 - 13 foot) storm surge to coast of eastern India and western Bangladesh. Approximately 150,000 were left homeless In India, and at least 45 people were killed, many of them in the Kolkata (Calcutta) area. Damage was heavy in the city, which is India's second largest, with a population of 7.8 million. In Bangladesh, at least 89 are dead and ten of thousands homeless. The death toll will likely go higher, as over 100 people are missing in Bangladesh. The Bay of Bengal is no stranger to deadly cyclones--fifteen of the world's twenty deadliest tropical cyclones have been Bay of Bengal storms that have hit Bangladesh, India, or Myanmar. The most recent was last year's Cyclone Nargis, which killed 146,000 people in Myanmar.


Figure 1. Satellite image of Aila as it made landfall near the India/Bangladesh border. Image credit: Navy Research Lab, Monterey.

Aila began as a "monsoon depression"--a large cyclonic disturbance that formed within the advancing Southwest Monsoon. The monsoon depression filled the entire Bay of Bengal, then gradually intensified between May 23 and 25 to the threshold of Category 1 hurricane strength. Since Aila started as a monsoon depression, it was a huge storm, with an eye over 100 miles in diameter. The storm helped pull the welcome rains of the Southwest Monsoon into India and Bangladesh a week ahead of normal (Figure 2). For reasons we don't fully understand, tropical cyclones in the North Indian Ocean are very predictable using computer models. Dr. Peter Webster of Georgia Tech was predicting the formation of Aila ten days in advance.


Figure 2. Progress of the Southwest Monsoon over India this week was aided by Cyclone Aila. Image credit: India Meteorology Department.

Disturbance 91L forms near North Carolina
An area of disturbed weather, dubbed "91L" by the National Hurricane Center, has formed a few hundred miles southeast of North Carolina. The disturbance is over waters of 25 - 26°C and has wind shear of 10 - 15 knots over it, and these conditions are marginally favorable for some slow development to occur over the next 24 - 36 hours. The disturbance will track northwards towards North Carolina's Outer Banks over the next 24 - 36 hours, then get swept northeastwards out to sea. It is unlikely that the disturbance has enough time to develop into a tropical depression. However, the storm should bring winds of 20 - 25 mph and heavy rain to North Carolina's Cape Hatteras on Wednesday. An Air Force hurricane hunter flight is on call to investigate the system on Wednesday afternoon.

Jeff Masters

Hurricane

90L comes close to being the season's first named storm

By: JeffMasters, 5:30 PM GMT on May 23, 2009

The large extratropical storm (90L) that has been pounding Florida, the Bahamas, and Haiti this week with heavy rain and high surf has moved ashore over Mississippi. Last night and this morning, the storm put on an impressive burst of heavy thunderstorm activity and came close to becoming a subtropical or tropical depression. Winds at the Fort Morgan, AL station were 44 mph, gusting to 52 mph, and winds at Dauphin Island, Alabama coastal station were sustained at 39 mph, gusting to 48 mph at between 8am and 10am EDT this morning. While these winds were in excess of the 39 mph threshold of tropical storm force, both measurements were taken at elevations higher than the international wind measurement standard of ten meters, so the top measured winds of 90L were actually slightly less than tropical storm force. Had 90L spent another six hours over water, it very likely would have been declared a tropical/subtropical depression/storm.

The circulation around 90L continues to mean rain for Florida, but the rains will be lessening today. Rainfall amounts as high as 28 inches were measured this week in Northern Florida at Brunnell. Rainfall amounts of 2 - 3 inches have accumulated along the Alabama, Mississippi, and Florida Panhandle coasts from 90L so far (Figure 1).


Figure 1. Estimated rainfall from the Mobile, AL radar.

There are no other threat areas in the Atlantic to discuss today, so have a great weekend, and I'll have a new blog post by Tuesday.

Jeff Masters

Hurricane

Gulf of Mexico storm not likely to become a depression; storm kills 11 in Haiti

By: JeffMasters, 1:38 PM GMT on May 22, 2009

The large extratropical storm that has been pounding Florida and the Bahamas this week with heavy rain and high surf extended its reach yesterday, killing at least 11 people in Haiti. According to Reuters, most of the victims were killed while crossing rivers or when their flimsy homes collapsed. Approximately six inches of rain fell on Haiti's southwest peninsula in the 24 hours ending at 8pm EDT. Hundreds of homes were flooded and dozens destroyed in the flooding, which left 40% of the southern city of Cayes underwater. The impoverished Caribbean country is still struggling to recover from the massive flooding that killed over 800 people during the hurricane season of 2008. In March, UN chief Ban Ki-moon and Bill Clinton--who was this week appointed UN special envoy to the island--toured Haiti together and urged the international community to continue to aid the country. Participants at a Washington international donors conference in April agreed to donate $324 million to help Haiti rebuild.

The worst of the rain and flooding is over for Florida, which has seen rainfall amounts this week as high as 23.75 inches at the Flagler County Fairgrounds. Another 1 - 2 inches are expected in Northeast Florida today from the storm, along with beach erosion, 6 - 8 foot waves, and tides 1 - 2 feet above normal. The bad weather in Northeast Florida should subside substantially on Saturday.


Figure 1. Long range radar out of New Orleans.

The storm responsible for the heavy rains is now headed north-northwest, and should make landfall Saturday near the Louisiana/Mississippi border. The storm has developed a warm core at low levels, and NHC designated it Invest 90L late this morning. Latest visible satellite loops show a modest amount of heavy thunderstorm activity on the east side of the storm's circulation, but development is being hindered by dry air, and wind shear of 20 knots. Long range radar out of New Orleans (Figure 1) shows little organization or banding of the radar echoes. With only 24 hours to go until the system moves inland, it does not have enough time to develop into a depression. Since there will be strong winds from the west creating wind shear over the storm, plus dry air to the storm's west, most of the heavy rain should be confined to the east of the storm's center, along a stretch of coast from New Orleans to Pensacola. Rainfall totals of 2 - 4 inches can be expected.


Figure 2. Latest satellite image of the Gulf of Mexico storm.

I'll have an update Saturday.

Portlight.org/wunderground shirts are now available on Ebay.

Jeff Masters

Hurricane

Gulf of Mexico low growing more organized

By: JeffMasters, 1:51 PM GMT on May 21, 2009

The large extratropical storm that has been drenching Florida over the past few days continues to pour rain on the state as it treks slowly west-northwestward over the Gulf of Mexico. Rainfall amounts as high as 23.75 inches have been measured in Northern Florida at the Flagler County Fairgrounds over the past four days, and Daytona Beach has seen over 14 inches, forcing closure of numerous roads. Another 1 - 3 inches are expected in Northeast Florida today from the storm, along with beach erosion, 7 - 10 foot waves, and tides 2 - 3 feet above normal. The bad weather in Northeast Florida should gradually subside Friday and the rains should diminish substantially on Saturday, when the center of the storm is expected to move ashore over Louisiana.


Figure 1. Storm total rainfall estimated from the Melbourne radar.

Latest visible satellite loops show that the circulation of the low has grown more circular and better defined, and heavy thunderstorm activity is starting to build off the Gulf Coast of Florida on the low's east side. These thunderstorms are beginning to form spiral bands characteristic of a developing tropical depression. Water vapor satellite imagery shows that the low is moving into an area of dry continental air, though, and this dry air will slow development. Wind shear is about 20 knots, which is low enough to allow development. While none of the computer models develop the storm, they generally predict the storm should move northwestwards towards Louisiana over the next two days, and the shear is lower to the north. The low is currently over Sea Surface Temperatures (SSTs) of about 27°C in the Loop Current, which is about 1°C (1.8°F) warmer than average. SSTs of at least 26°C are typically needed to support a tropical storm, so I expect the low will continue to develop through Friday. By Friday night, the low will be approaching the Louisiana coast, and SSTs will cool to 26°C. Still, with wind shear expected to drop, this low has a medium (30 - 50%) chance of developing into a subtropical or tropical depression before its expected landfall Saturday over Louisiana. Since there will be strong winds from the west creating wind shear over the storm, plus dry air to the storm's west, most of the heavy rain should be confined to the east of the storm's center, along a stretch of coast from New Orleans to Pensacola. Rainfall totals of 3 - 6 inches can be expected.


Figure 2. Latest satellite image of the Gulf of Mexico storm.

I'll have an update Friday morning, or sooner if NHC declares this storm an area of interest (an "Invest").

Jeff Masters

Hurricane

Florida's soaking continues; 5th warmest April for the globe

By: JeffMasters, 12:51 PM GMT on May 20, 2009

The large extratropical storm that has been drenching Florida over the past few days continues to pour rain on the state as it treks slowly westward over the Gulf of Mexico. Rainfall amounts as high as 13.3 inches have been measured in Northern Florida in Flagler County over the past three days, with another 2 - 4 inches expected from the storm. The storm is bringing beach erosion, 7 - 10 foot waves, and tides 2 - 3 feet above normal to the Northeast Florida coast, along with damaging thunderstorm winds. While the storm's center is located just offshore Southwest Florida, a large band of precipitation arcs to the north and east, extending over northern Florida. This type of structure is typical of subtropical storms, though the storm does not have enough heavy thunderstorm activity or warm core air to qualify as a subtropical storm. The storm is under about 30 knots of wind shear, and shows no signs of developing more tropical characteristics. The computer models generally predict the storm should weaken this week as it moves towards Louisiana or Texas. As the center moves farther north over the next few days, it will be moving into a region of lower wind shear, and I still give the storm a 20% chance of becoming a subtropical depression 2 - 7 days from now.


Figure 1. Latest satellite image of the Gulf of Mexico storm.

Fifth warmest April on record
The globe recorded its 5th warmest April on record, according to the National Climatic Data Center. The period January - April was tied for the sixth warmest such period on record. April marked the first time since October 2008 that the planet has recorded a monthly temperature anomaly in the top five warmest months. The warming may be due to the fact that a La Niña event ended in the Eastern Pacific in April. Global temperature records go back to 1880.

A cool, wet April for the U.S.
For the contiguous U.S., April temperatures were the 36th coolest in the 115-year record, according to the National Climatic Data Center. The month was also quite wet, ranking as the 35th wettest April. The warmest state was New Hampshire, which recorded its 8th warmest April. The coldest state was South Dakota, which wad its 25th coldest April. Through April, U.S. tornado activity was very close to the mean observed during the past five years, according to NOAA's Storm Prediction Center. However, there were just 15 tornado deaths through April, compared to 70 deaths through April of 2008, and the 3-year average of 60 deaths.

On May 12, 2009, 17% of the contiguous United States was in moderate-to-exceptional drought. This is a drop from the 19% figure observed at the beginning of the year. However, The amount of the U.S. in the highest levels of drought, extreme to exceptional, had increased from 1.2% at the beginning of the year to 2.6% by May 12. These extreme drought regions were in South Texas and South Florida.

La Niña officially over
The La Niña event of September 2008 - March 2009 is officially over, according to NOAA's Climate Prediction Center. They define La Niña conditions as occurring when the 1-month mean temperature anomaly in the equatorial Eastern Pacific (the area 5°N - 5°S, 120°W - 170°W, also called the "Niña 3.4 region") cools below -0.5°C and is expected to persist for three consecutive months. Temperatures warmed significantly in the Equatorial Eastern Pacific during April, and are now near average in the Niña 3.4 region. Most of the model forecasts for the Niño 3.4 region predict neutral conditions for the August - October peak of hurricane season. Three out of 16 El Niño models are predicting an El Niño event for hurricane season. Columbia University's International Research Institute for Climate and Society (IRI) is predicting a 30% chance of an El Niño event during the coming hurricane season. The number of Atlantic hurricanes is typically reduced in an El Niño year, due to increased wind shear from strong high-level winds.

Sea ice in the Arctic below average, but not greatly so
April 2009 Northern Hemisphere sea ice extent was the 10th lowest since 1979, according to the National Snow and Ice Data Center. The record April low was set in 2007. The rate of ice decline in April was the third slowest on record, thanks to cooler than usual temperatures over the Arctic. Nevertheless, the Arctic remains vulnerable to near-record melting this summer if much warmer than average temperatures return to the region. Strong winter winds pushed a considerable amount of multi-year-old ice out of the Arctic this year, leaving the Arctic with the lowest amount of old sea ice on record in March. The amount of ice more than two years old fell below 10% for the first time since satellites began observing the ice in 1979.

Jeff Masters

Climate Summaries

First Invest of the year peters out; Florida gets soaked

By: JeffMasters, 1:00 PM GMT on May 19, 2009

A complex weather system is bringing showers and thunderstorms over Florida and the Bahamas, in association with a trough of low pressure. Two 1010 mb surface lows have developed--one over the Florida Keys, near 24N 81W, and the other over the south-central Bahamas, near 23N 77W. This second low was designated Invest 90L by the National Hurricane Center yesterday afternoon, and was the first area of interest so designated this year. At that time, they gave it a 30% chance of developing into a tropical cyclone by Wednesday. However, current satellite imagery shows little organization of the cloud pattern and no signs of a surface circulation, and the region is under high shear of 30 knots. 90L is being absorbed into the larger low over Florida, and is no longer a threat to develop, according to a Special Tropical Weather Outlook issued by NHC.


Figure 1. Current radar-estimated precipitation from the Melbourne radar.

The latest 00Z and 06Z runs of the computer models continue to forecast the intensification of an extratropical low near Florida over the next day. The low should bring heavy rain and possible flooding problems to Florida and the Bahamas this week as it moves west or west-northwest into the Gulf of Mexico. Up to eight inches of rain have already fallen over Florida so far (Figure 1), thanks in most part to a cold front that moved over the state during the past two days. Florida could use the rain--most of South Florida is under extreme drought, and Central Florida is under severe drought. The Lake Okeechobee water level is at 10.58 feet, which is about 3 feet below average. During the past week, the lake fell below the level that triggers water conservation measures for the first time since Tropical Storm Fay filled up the lake in August.

I don't expect development of a tropical or subtropical storm over the next two days, due to high wind shear. However, once the system moves into the Gulf of Mexico later this week, the ECMWF and UKMET models are predicting wind shear will drop enough over the northern Gulf of Mexico to allow some development. The GFS and NOGAPS models portray an unfavorable environment with higher shear. I'll give a 20% chance of this system eventually developing into a tropical or subtropical depression in the next seven days. The storm is expected to come ashore over Louisiana on Saturday (NOGAPS and UKMET models), or Sunday over Texas (ECMWF model). The GFS model dissipates the storm over the southern Gulf.

Jeff Masters

Hurricane

Florida poised for a substantial soaking

By: JeffMasters, 1:03 PM GMT on May 18, 2009

An area of showers and thunderstorms is over the southeastern Bahamas and eastern Cuba, in association with a trough of low pressure. The latest 00Z and 06Z runs of the GFS, UKMET, NOGAPS, and ECMWF global dynamical computer models continue to forecast that this disturbance will develop into an extratropical low by Tuesday. The low should bring heavy rain and possible flooding problems to Florida and the Bahamas this week as it lifts northwestward over Florida. Up to nine inches of rain may fall over Florida by Friday (Figure 1), thanks also to a cold front expected to move over the state over the next two days. The exact timing and location of the rains over Florida are still uncertain, as the GFS model predicts development of the low over the Bahamas, while the ECMWF and UKMET models predict development over South Florida. At present, it appears wind shear will be too high to allow the extratropical storm to transition into a subtropical or tropical storm. However, if the storm's center emerges into the northern Gulf of Mexico late this week, wind shear may be low enough to allow a transition to a subtropical storm (10% chance).

Florida could use the rain--most of South Florida is under extreme drought, and Central Florida is under severe drought. The Lake Okeechobee water level is at 10.58 feet (Figure 2), which is about 3 feet below average. During the past week, the lake fell below the level that triggers water conservation measures for the first time since Tropical Storm Fay filled up the lake in August.


Figure 1. Forecast precipitation amount between 8am EDT Monday 5/18/09 and 8am EDT Saturday 5/23/09. Image credit: NOAA/HPC.


Figure 2. Water levels in Florida's Lake Okeechobee were under the level that triggers conservation measures between January 2007 - August 2008. Tropical Storm Fay then filled up the lake, which has gradually declined in level since, reaching water shortage management levels again in May 2009. Image credit: U.S. Army Corps of Engineers.

There has been little change to the large upper-level cold low spinning in the mid-Atlantic a few hundred miles east of Bermuda. The low may spin in place long enough over the next week to develop a warm core and be classified as a subtropical storm. It is unlikely that a subtropical storm forming that far out in the Atlantic this early in the year would pose a threat to any land areas, with the possible exception of Bermuda.

Climatology of early-season Atlantic tropical cyclones
Tropical storms are uncommon in the Atlantic before June 1, with only 26 named storms on record between 1851 - 2008. Five of these have made it to hurricane strength, and only one--Hurricane Able of 1951--made it to major hurricane status. Last year's Tropical Storm Arthur may be the deadliest May tropical cyclone on record. Though only a 40 mph tropical storm at landfall, Arthur killed five people in Belize and caused $78 million in damage. Three early-season storms have brought hurricane-force winds to land. The March 1908 hurricane swept through the northern Lesser Antilles Islands as a Category 1 or 2 hurricane, destroying at least 24 boats and causing damage to buildings on St. Bartholomew. Hurricane Able of 1951 brought sustained winds of 90 - 95 mph to the northern Bahama Islands, but caused little damage. Hurricane 2 of May 1908 hit North Carolina's Outer Banks as a Category 1 hurricane, but also caused little damage.

List of all early season (formed in January - May) Atlantic named storms
May 31, 2008: Tropical Storm Arthur
May 6, 2007: Subtropical Storm Andrea
April 18, 2003: Tropical Storm Ana
April 21, 1992: Subtropical Storm 1
May 6, 1981: Tropical Storm Arlene
January 18, 1978: Subtropical Storm 1
May 21, 1976: Subtropical Storm 1
May 23, 1972: Subtropical Storm Alpha
May 17, 1970: Hurricane Alma (Category 1)
May 28, 1959: Tropical Storm Arlene
February 2, 1953: Tropical Storm Alice
May 25, 1952: Tropical Storm 1
May 15, 1951: Hurricane Able (Category 3)
May 22, 1948: Tropical Storm 1
May 19, 1940: Tropical Storm 1
May 27, 1934: Tropical Storm 1
May 14, 1933: Tropical Storm 1
May 5, 1932: Tropical Storm 1
May 13, 1916: Tropical Storm 1
May 24, 1908: Hurricane 2 (Category 1)
March 6, 1908: Hurricane 1 (Category 2)
May 27, 1890: Tropical Storm 1
May 16, 1889: Hurricane 1 (Category 1)
May 17, 1887: Tropical Storm 2
May 15, 1887: Tropical Storm 1
May 30, 1865: Tropical Storm 1

I'll have an update Tuesday.

Jeff Masters

Hurricane

Gathering extratropical storm set to drench Florida, Cuba, and the Bahamas

By: JeffMasters, 3:11 PM GMT on May 17, 2009

A concentrated area of heavy thunderstorms has developed near Jamaica and eastern Cuba, in association with a mid- to upper-level trough of low pressure. The latest 00Z and 06Z runs of the GFS, UKMET, NOGAPS, and ECMWF global dynamical computer models continue to forecast that this disturbance will develop into an extratropical low by Tuesday. The low should bring heavy rain and possible flooding problems to Cuba, Florida, and the Bahamas this week as it lifts northwestward over Florida. Up to eight inches of rain may fall over Florida by Friday (Figure 2), thanks also to a cold front expected to move over the state over the next two days. At present, it appears wind shear will be too high to allow the extratropical storm developing over Cuba to transition into a subtropical or tropical storm. However, if the center emerges into the northern Gulf of Mexico late this week, wind shear may be low enough to allow a transition to a subtropical storm (10% chance).


Figure 1. Latest IR satellite image of the Atlantic, showing the gathering extratropical storm near the eastern tip of Cuba, and the cut-off low spinning in the mid-Atlantic.


Figure 2. Forecast precipitation amount between 8am EDT Sunday 5/17/09 and 8am EDT Friday 5/22/09. Image credit: NOAA/HPC.

There has been little change to the large upper-level cold low spinning in the mid-Atlantic a few hundred miles east of Bermuda. The low may spin in place long enough over the next week to develop a warm core and be classified as a subtropical storm. It is unlikely that a subtropical storm forming that far out in the Atlantic this early in the year would pose a threat to any land areas, with the possible exception of Bermuda.

Climatology of early-season Atlantic tropical cyclones
Tropical storms are uncommon in the Atlantic before June 1, with only 26 named storms on record between 1851 - 2008. Five of these have made it to hurricane strength, and only one--Hurricane Able of 1951--made it to major hurricane status. Last year's Tropical Storm Arthur may be the deadliest May tropical cyclone on record. Though only a 40 mph tropical storm at landfall, Arthur killed five people in Belize and caused $78 million in damage. Three early-season storms have brought hurricane-force winds to land. The March 1908 hurricane swept through the northern Lesser Antilles Islands as a Category 1 or 2 hurricane, destroying at least 24 boats and causing damage to buildings on St. Bartholomew. Hurricane Able of 1951 brought sustained winds of 90 - 95 mph to the northern Bahama Islands, but caused little damage. Hurricane 2 of May 1908 hit North Carolina's Outer Banks as a Category 1 hurricane, but also caused little damage.

List of all early season (formed in January - May) Atlantic named storms
May 31, 2008: Tropical Storm Arthur
May 6, 2007: Subtropical Storm Andrea
April 18, 2003: Tropical Storm Ana
April 21, 1992: Subtropical Storm 1
May 6, 1981: Tropical Storm Arlene
January 18, 1978: Subtropical Storm 1
May 21, 1976: Subtropical Storm 1
May 23, 1972: Subtropical Storm Alpha
May 17, 1970: Hurricane Alma (Category 1)
May 28, 1959: Tropical Storm Arlene
February 2, 1953: Tropical Storm Alice
May 25, 1952: Tropical Storm 1
May 15, 1951: Hurricane Able (Category 3)
May 22, 1948: Tropical Storm 1
May 19, 1940: Tropical Storm 1
May 27, 1934: Tropical Storm 1
May 14, 1933: Tropical Storm 1
May 5, 1932: Tropical Storm 1
May 13, 1916: Tropical Storm 1
May 24, 1908: Hurricane 2 (Category 1)
March 6, 1908: Hurricane 1 (Category 2)
May 27, 1890: Tropical Storm 1
May 16, 1889: Hurricane 1 (Category 1)
May 17, 1887: Tropical Storm 2
May 15, 1887: Tropical Storm 1
May 30, 1865: Tropical Storm 1

I'll have an update Monday.

Jeff Masters

Hurricane

Still watching for a potential early-season Atlantic subtropical storm

By: JeffMasters, 7:10 PM GMT on May 16, 2009

The latest 00Z and 12Z runs of the GFS, UKMET, NOGAPS, and ECMWF global dynamical computer models continue to indicate the possibility of an early season subtropical storm forming in the Gulf of Mexico 3 - 7 days from now. A modest area of low shear air is expected to open up between the polar jet stream to the north and the subtropical jet stream to the south, between Cuba and the northern Gulf of Mexico. The models have not been consistent with the timing or the size of this low shear area, and I wouldn't be surprised to see them back off again from this forecast with Sunday's set of runs. Nevertheless, I'll call once again for a 10% chance of development of a subtropical depression in the eastern Gulf of Mexico, 4 - 7 days from now. There isn't much of a disturbance at present to look at--there are some scattered showers between Cuba and Jamaica, but they are under a hefty 30 knots of wind shear. These showers should gradually increase in intensity and areal coverage over the next two days, and phase space diagrams from Florida State University indicate that an extratropical low may form near the western tip of Cuba 2 - 3 days from now. The storm may then gradually transition to a subtropical or tropical system later in the week, if the shear is low enough. Even if the shear is high and the storm remains extratropical, it could be a substantial rain-maker where it comes ashore. The models target the northern Gulf Coast between Louisiana and the Florida Panhandle as the most likely landfall location, but it is too early to place any confidence in this track.

There is also about a 10% chance a subtropical storm may form in the mid-Atlantic a few hundred miles east of Bermuda. There, a large upper-level cold low has cut off from the jet stream, and it may spin in place long enough over the next week to develop a warm core and be classified as a subtropical storm. It is unlikely that a subtropical storm forming that far out in the Atlantic this early in the year would pose a threat to any land areas, with the possible exception of Bermuda.

Climatology of early-season Atlantic tropical cyclones
Tropical storms are uncommon in the Atlantic before June 1, with only 26 named storms on record between 1851 - 2008. Five of these have made it to hurricane strength, and only one--Hurricane Able of 1951--made it to major hurricane status. Last year's Tropical Storm Arthur may be the deadliest May tropical cyclone on record. Though only a 40 mph tropical storm at landfall, Arthur killed five people in Belize and caused $78 million in damage. Three early-season storms have brought hurricane-force winds to land. The March 1908 hurricane swept through the northern Lesser Antilles Islands as a Category 1 or 2 hurricane, destroying at least 24 boats and causing damage to buildings on St. Bartholomew. Hurricane Able of 1951 brought sustained winds of 90 - 95 mph to the northern Bahama Islands, but caused little damage. Hurricane 2 of May 1908 hit North Carolina's Outer Banks as a Category 1 hurricane, but also caused little damage.


Figure 1. Tracking map for the earliest hurricane to make landfall, the March 1908 hurricane in the northern Lesser Antilles Islands.

List of all early season (formed in January - May) Atlantic named storms
May 31, 2008: Tropical Storm Arthur
May 6, 2007: Subtropical Storm Andrea
April 18, 2003: Tropical Storm Ana
April 21, 1992: Subtropical Storm 1
May 6, 1981: Tropical Storm Arlene
January 18, 1978: Subtropical Storm 1
May 21, 1976: Subtropical Storm 1
May 23, 1972: Subtropical Storm Alpha
May 17, 1970: Hurricane Alma (Category 1)
May 28, 1959: Tropical Storm Arlene
February 2, 1953: Tropical Storm Alice
May 25, 1952: Tropical Storm 1
May 15, 1951: Hurricane Able (Category 3)
May 22, 1948: Tropical Storm 1
May 19, 1940: Tropical Storm 1
May 27, 1934: Tropical Storm 1
May 14, 1933: Tropical Storm 1
May 5, 1932: Tropical Storm 1
May 13, 1916: Tropical Storm 1
May 24, 1908: Hurricane 2 (Category 1)
March 6, 1908: Hurricane 1 (Category 2)
May 27, 1890: Tropical Storm 1
May 16, 1889: Hurricane 1 (Category 1)
May 17, 1887: Tropical Storm 2
May 15, 1887: Tropical Storm 1
May 30, 1865: Tropical Storm 1

I'll have an update Sunday.

Jeff Masters

Hurricane

An early season Atlantic named storm looking less likely

By: JeffMasters, 2:46 PM GMT on May 15, 2009

The possibility of a May subtropical storm forming near Florida next week now appear dim, according to the latest suite of runs by the GFS, UKMET, NOGAPS, and ECMWF global dynamical computer models. Yesterday's runs indicated that a small area of low shear air might develop over Florida next week, between the polar jet stream to the north and the subtropical jet stream to the south. However, the latest set of runs are showing a much reduced area of low shear, too small for a subtropical storm to form. Perhaps a better chance for a subtropical storm to form is in the mid-Atlantic a few hundred miles east of Bermuda. There, a large upper-level cold low has cut off from the jet stream, and it may spin in place long enough over the next week to develop a warm core and be classified as Subtropical Storm Ana. It is unlikely that a subtropical storm forming that far out in the Atlantic this early in the year would pose a threat to any land areas, with the possible exception of Bermuda.

Climatology of early-season Atlantic tropical cyclones
Tropical storms are uncommon in the Atlantic before June 1, with only 26 named storms on record between 1851 - 2008. Five of these have made it to hurricane strength, and only one--Hurricane Able of 1951--made it to major hurricane status. Last year's Tropical Storm Arthur may be the deadliest May tropical cyclone on record. Though only a 40 mph tropical storm at landfall, Arthur killed five people in Belize and caused $78 million in damage. Three early-season storms have brought hurricane-force winds to land. The March 1908 hurricane swept through the northern Lesser Antilles Islands as a Category 1 or 2 hurricane, destroying at least 24 boats and causing damage to buildings on St. Bartholomew. Hurricane Able of 1951 brought sustained winds of 90 - 95 mph to the northern Bahama Islands, but caused little damage. Hurricane 2 of May 1908 hit North Carolina's Outer Banks as a Category 1 hurricane, but also caused little damage.


Figure 1. Tracking map for the earliest hurricane to make landfall, the March 1908 hurricane in the northern Lesser Antilles Islands.

List of all early season (formed in January - May) Atlantic named storms
May 31, 2008: Tropical Storm Arthur
May 6, 2007: Subtropical Storm Andrea
April 18, 2003: Tropical Storm Ana
April 21, 1992: Subtropical Storm 1
May 6, 1981: Tropical Storm Arlene
January 18, 1978: Subtropical Storm 1
May 21, 1976: Subtropical Storm 1
May 23, 1972: Subtropical Storm Alpha
May 17, 1970: Hurricane Alma (Category 1)
May 28, 1959: Tropical Storm Arlene
February 2, 1953: Tropical Storm Alice
May 25, 1952: Tropical Storm 1
May 15, 1951: Hurricane Able (Category 3)
May 22, 1948: Tropical Storm 1
May 19, 1940: Tropical Storm 1
May 27, 1934: Tropical Storm 1
May 14, 1933: Tropical Storm 1
May 5, 1932: Tropical Storm 1
May 13, 1916: Tropical Storm 1
May 24, 1908: Hurricane 2 (Category 1)
March 6, 1908: Hurricane 1 (Category 2)
May 27, 1890: Tropical Storm 1
May 16, 1889: Hurricane 1 (Category 1)
May 17, 1887: Tropical Storm 2
May 15, 1887: Tropical Storm 1
May 30, 1865: Tropical Storm 1

I'll have an update over the weekend if the models indicate a renewed tropical threat; otherwise, have a great weekend, and I'll be back on Monday.

Jeff Masters

Hurricane

An early start to hurricane season? And, tornado kills 3 in Missouri

By: JeffMasters, 2:38 PM GMT on May 14, 2009

The 2009 Atlantic hurricane season may get off to an early start. For the past two days, most of our reliable hurricane forecast models have been predicting the possibility of a subtropical depression forming near Florida or Western Cuba Monday - Thursday next week. An area of disturbed weather associated with a mid- to upper-level trough of low pressure near eastern Cuba and the southeastern Bahama Islands is predicted to slide west-northwest over the next few days. Wind shear over this low, current a prohibitively high 40 knots, is expected to relax to just 10 knots by Monday in the region surrounding Florida. This may allow a disturbance with a surface warm core to develop, according to phase space analyses from Florida State. However, since the upper atmosphere will still be cold, any development of this system will likely be subtropical in nature. If a subtropical storm does form, it may be fairly dry, like Subtropical Storm Andrea of May 2007. This storm ended up fanning fires in Florida, instead of putting them out. Water vapor loops show plenty of dry, continental air in the region, and it will take many days for the atmosphere to moisten enough to support formation of a subtropical depression.


Figure 1. Water vapor image showing moisture from a weak mid- to upper-level low of low pressure over the southeastern Bahama Islands, surrounded by a large area of dry air, and sandwiched between the polar and subtropical jet stream. Image credit: NOAA/SSD.

The GFS and NOGAPS models predict a subtropical depression could form by Tuesday, while the ECMWF shows development later in the week. The latest UKMET model forecast puts development Wednesday or Thursday near Haiti. The area of predicted development is sandwiched in a relatively narrow band of low wind shear between two branches of the jet stream. This is not a typical set-up for formation of a May tropical cyclone. With so much shear and dry air around, I put the probability of a subtropical depression forming next week at about 10%. Any developing system will also have to contend with the arrival Tuesday of a strong upper-level low pressure system that is expected to drop down over the northern Gulf of Mexico. The eventual track of any depression that forms is highly uncertain, and the models support tracks up the U.S. East Coast towards South Carolina, or up the west coast of Florida in the Gulf of Mexico.

Season's first tropical wave arrives
The season's first tropical wave rolled off the coast of Africa yesterday, and is now located near 10N 20W, a few hundred miles off the coast of Africa, according to total precipitable water loops. Mid-May is a fairly typical time for these waves to begin moving across the Atlantic, though they usually don't start developing into tropical depressions until August. Last year was an exception, when Hurricane Bertha formed from a July tropical wave. Wunderground blogger Weather456 has been tracking the date of the first African wave each year since 2004, and these dates have ranged from May 2 to May 21. With Sea Surface Temperatures near average this year in the tropical Atlantic, an early season major hurricane in the eastern Atlantic like Bertha is unlikely.


Figure 2. The line of severe thunderstorms that spawned the Kirksville, MO tornado.

A wild night in Tornado Alley
Northern Missouri took a hard hit by a powerful tornado last night, when a twister passed through Kirksville, killing three people. The tornado flipped cars and damaged 30 - 40 buildings as it tore through the north end of town. The Storm Prediction Center recorded 23 tornado reports across Missouri, Illinois, Oklahoma, and Kansas yesterday.

The Vortex2 field study, the world's largest-ever tornado research project, caught last night's tornado outbreak with its armada of 40 research vehicles and radars. You can read about yesterday's chase on our new featured Vortex2 blog. Our team of University of Michigan students caught some severe thunderstorms, but were forced by darkness to quit before the storms spawned tornadoes.

Portlight/wunderground shirts for sale
The portlight.org disaster relief charity is selling spiffy new shirts sporting the portlight and wunderground logos, to help raise funds for relief operations during the coming hurricane season. I expect that their services will be needed this year, and I encourage you to shell out the $20 to get this fine piece or wunderwear.

I'll have an update on the tropics Friday.
Jeff Masters

Tornado Hurricane

Vortex2: world's largest tornado research project ever, is underway

By: JeffMasters, 1:49 PM GMT on May 12, 2009

Tornado season is in full swing, and researchers are now poised in America's Great Plains with the largest armada of storm chasing vehicles and equipment ever assembled, in order to learn more about these enigmatic and violent storms. The massive Vortex2 field study began Sunday, and for the next seven weeks over 100 scientists in up to 40 science and support vehicles will be roaming through Tornado Alley, seeking to catch tornadoes on the rampage. The three basic questions the $10 million study will attempt to answer are:

- How, when, and why do tornadoes form? Why some are violent and long lasting while others are weak and short lived?

- What is the structure of tornadoes? How strong are the winds near the ground? How exactly do they do damage?

- How can we learn to forecast tornadoes better? Current warnings have an only 13 minute average lead time and a 70% false alarm rate. Can we make warnings more accurate? Can we warn 30, 45, 60 minutes ahead?'


Figure 1. Tornado over Matador, Texas on April 29, 2009. Photo taken by Texas Tech meteorology graduate student Danielle Turner.

Major tornado outbreak possible Wednesday
The Vortex2 project will have its first good chance to help answer these questions on Wednesday, when a strong cold front is expected to pass through an unstable air mass over Missouri and Illinois, triggering severe thunderstorms with tornadoes. The Storm Prediction Center has given these states a "Moderate" chance of severe weather, the second highest alert level. Today, the Vortex2 armada is stationed in western Oklahoma. The cold front that is expected to trigger Wednesday's severe weather outbreak will be moving through Oklahoma today, bringing a slight chance of severe weather to that state. You can follow the progress of the Vortex2 field project this Spring through our new featured Vortex2 blog. This blog is being written by a team of six University of Michigan students that will help deploy the Texas Tech "Sticknet" sensors during a tornado.


Figure 2. Severe weather outlook from NOAA's Storm Prediction Center for Wednesday, May 13.

An average tornado season so far over the U.S.
Through April, U.S. tornado activity was very close to the mean observed during the past five years, according to NOAA's Storm Prediction Center. However, there were just 15 tornado deaths through April, compared to 70 deaths through April of 2008, and the 3-year average of 60 deaths. According to the unofficial seasonal stats at Wikipedia, we've had 57 strong EF2 and EF3 tornadoes so far this year, and two violent EF4 tornadoes. These are fairly typical numbers of strong and violent tornadoes for this point in the season. The season's first EF4 hit Lone Grove, Oklahoma on February 10, killing eight, injuring 46, and destroying 114 homes, and was the strongest February tornado to hit Oklahoma since 1950. The season's second EF4 hit Murfreesboro, Tennessee on April 10, killing two.

Wunderground launches high-definition radar product
In case you missed my post on this in December, wunderground is now providing imagery from a network of 45 Terminal Doppler Weather Radar (TDWR) units located at airports across the U.S. The radars were developed and deployed by the Federal Aviation Administration (FAA) beginning in 1994, as a response to several disastrous jetliner crashes in the 1970s and 1980s caused by strong thunderstorm winds. The crashes occurred because of wind shear--a sudden change in wind speed and direction. Wind shear is common in thunderstorms, due to a downward rush of air called a microburst or downburst. The TDWRs can detect such dangerous wind shear conditions, and have been instrumental in enhancing aviation safety in the U.S. over the past 15 years. The TDWRs also measure the same quantities as our familiar network of 148 NEXRAD WSR-88D Doppler radars--precipitation intensity, winds, rainfall rate, echo tops, etc. However, the newer Terminal Doppler Weather Radars are higher resolution, and can "see" details in much finer detail close to the radar. This high-resolution data has generally not been available to the public until now. Thanks to a collaboration between the National Weather Service (NWS) and the FAA, the data for all 44 of 45 TDWRs is now available in real time. We're calling them "High-Def" stations on our NEXRAD radar page, and they are denoted by a yellow "+" symbol. Only one TDWR radar (Las Vegas) remains to be added; this will happen in June. For more info on how to interpret the new TDWR images, see our radar FAQ page.

Jeff Masters

Tornado

Storm surge reduction by wetlands

By: JeffMasters, 10:31 AM GMT on May 08, 2009

It has long been argued that coastal wetlands provide critical protection against incoming hurricane storm surges, and that restoration of lost wetlands should be a key component of any strategy to protect vulnerable regions such as New Orleans. But exactly how much protection do wetlands afford? This has been a contentious issue, and there is no one number that works to define the value of wetlands in protecting against storm surge. In fact, in some cases, wetlands do not reduce the storm surge at all.

The traditional rule of thumb: each 2.7 miles of marsh knocks down the storm surge by 1 foot
Historically, many people have used the rule of thumb that each 2.7 miles of marsh knocks down the storm surge by 1 foot (1 meter reduction per 14.5 km of marsh). This estimate is based on a US Army Corps of Engineers report from 1963 (USACE, 1963), which examined the inland penetration of the storm surge from seven storms in southern Louisiana between 1909 and 1957. However, the data from this study varied by about a factor of three--attenuation rates as high as 1 foot per 1.3 miles of marsh were seen in one storm, and as low as 1 foot per 3.8 miles of marsh in another.

Thus, the simple rule of thumb of a 1 foot storm surge reduction per 2.7 miles of marsh is not a very good one to use in most situations. The inland penetration of the storm surge is an extremely complicated function of storm track, speed, duration, size, and associated waves; the regional topography, geometry of the shore, presence of barrier islands, and slope of the ocean bottom; plus the type and thickness of vegetation, and presence or absence of levees. Wetlands will always act to slow down the inland penetration of a storm surge, so the surge will not be able to advance very far inland before the winds die down if a region is exposed to strong winds for a short period of time. One example of this was in western Louisiana during Hurricane Rita of 2005. As the hurricane approached western Louisiana at 11 - 14 mph, the coast was initially subjected to offshore winds that blew water away from land. In the final few hours before landfall, the counter-clockwise circulation of air around the hurricane brought on-shore winds and a storm surge of up to 15 feet to the western Louisiana coast. However, this portion of the coast was only subject to on-shore winds for a few hours, and the surge was reduced by the wetlands by 1 foot per 2.1 - 3.6 miles of inland penetration, according to an ADCIRC storm surge model simulation by Resio and Westerink (2008).

Wetlands don't always decrease the storm surge
However, the situation is very different for slow moving storms, or for portions of the coast subjected to strong winds for many hours. If a marshland is subject to strong winds for long enough, the wetlands will completely flood, and there will be no reduction of storm surge at all--and an increase in storm surge is even possible, according to the mathematical equations governing the surge (Resio and Westerink, 2008). This has occurred in Louisiana during a number of storms--Hurricanes Rita, Katrina, Gustav, Ike, and Hurricane Betsy of 1965, along the eastern side of the protruding delta of the Mississippi River (Figure 1). Resio and Westerlink (2008) found that during Hurricane Rita of 2005, strong winds blew along the east side of the Mississippi for almost a full day, completely flooding the 25 miles of wetlands fronting the Mississippi River levee at English Turn. In fact, the model results show that the surge probably increased in height, by 1 foot per 8.7 miles of inland penetration in the Hurricane Rita simulation, since the day-long period of strong winds allowed the surge to pile up against the levee. Thus, while the wetlands were able to slow down the speed with which the surge reached the levee, the wetlands had no impact on the surge height in that location. A similar effect was seen during Hurricane Carla in 1961, a ferocious Category 4 hurricane that brought the highest storm surge ever observed to the Texas coast--a massive 22.7 feet at Port Lavaca. Carla moved so slowly--just 8 mph--that the surge had plenty of time to inundate marshes, and along one inland bluff fronted by wetlands, the surge was higher than at the coast.


Figure 1. For portions of the coast subjected to strong winds for a long period of time, wetlands do little to prevent high storm surges. This NOAA SLOSH model simulation of Category 3 Hurricane Betsy as it tracked west of New Orleans shows the highest storm surge occurred (pink colors) in a region where the surge had passed over 25 miles of wetlands. The Mississippi River levee at English Turn dammed up the storm surge.

In a 2008 conference presentation, Dr. Pat Fitzpatick used a SLOSH storm surge model to show that wetlands reduced the inland penetration of Katrina's storm surge near the hurricane's inital landfall over the Mississippi River "bird's foot" by 1 foot per 1.5 miles of wetlands traversed. The effect varied with the depth of the surge--an eight foot high surge was knocked down about 13% by wetlands, while a one foot high surge was reduced 59%. However, he found that where people lived along the river's levees (Venice), the surge piled up, and the marshes did not decrease the surge at all.

How much will future wetland loss increase storm surge?
Over the next century along the Gulf Coast, sea levels will continue to rise, coastal land will continue to subside, and human impacts due to shipping and the oil and gas industry will continue to cause erosion that will reduce wetland acreage. According to Harold Wanless of the University of Miami, global sea level is presently rising at 3 mm per year, and the land along the Louisiana Mississippi River delta is subsiding at 4-11 mm per year. Relative sea level is, thus, presently rising at 70-140 cm (2.3 - 4.6 feet) per century. Global warming may increase this sea relative sea level rise even further. Wetlands are being lost at an average rate of more than 23 square miles (60 square km) per year, with higher pulses during hurricanes. Water is about one meter deep across recently lost marshes and up to two meters deep in earlier lost marshes. This is of concern not only for the potential loss of hurricane storm surge protection, but because wetlands serve to increase fisheries production, filter pollutants out of water, and provide wave protection.

Wamsley et al. (2007) performed a surge surge simulation using the ADCIRC model of what would happen if the wetlands were allowed to continue to deteriorate with no restorative efforts over the next 50 years. Their results suggested that 50 years from now, storm surge heights would increase by 10-15% along Louisiana coastal areas to the east of New Orleans. These results held for both a severe Katrina-like hurricane, and a more modest hurricane (both making landfall at about 12 mph). However, the authors cautioned that "the impact of landscape features on surge propagation is a relatively new application for surge models and research is required". To underscore this lack of understanding, a White Paper put together by 25 coastal scientists and engineers held in July, 2007 found that adequate storm surge data do not exist for calibrating and verifying the models used to predict the impact of wetlands (or other features) on storm surge. Wamsley et al. are working on a field program in southern Louisiana to reduce these uncertainties. They intend to measure water level and wave attenuation across a wetland between Lake Borgne and the Mississippi River Gulf Outlet channel, using four non-directional water level/ wave gauges, an anemometer, and a periodic characterization of the wetland, including elevation, plant type, plant density, and plant height. The data collected will be analyzed to determine the surge and wave attenuation based on the vegetation type, density, and height.

Some expert opinions: Do wetlands reduce storm surges?
At an October 2008 meeting of the Geological Society of America, Dr. Robert Young of Western Carolina University stated that while he agreed with proposed ambitious efforts to restore wetlands in Louisiana, the potential storm protection benefits were "unknowable, but are most likely to be minimal".

Dr. Stephen Baig, who retired in 2008 as the head of the National Hurricane Center's storm surge unit, commented to me: Marshes are functionally useless as storm surge dissipators--disregarding for the moment their acknowledged utility for ocean breeding stock and other necessary and/or desirable functions. Once a marsh has more than a few feet of water overlying it the frictional effect of the grass is erased. The mythical "2.7 feet of surge reduction per inland mile of marsh" is just that, a myth. Also, it's unfortunate that the sand islands that front the shoreline are called "barrier" islands. They are certainly not barriers to storm surge. They get over-topped or breached with regularity. They are functionally useless as surge protection.

Dr. Joannes Westerink, who was the originator of the ADCIRC storm surge model, had this to say: "I think it depends on geography, storm direction, speed and size. For storms that track to the west of the Mississippi River with their sustained easterly winds impacting the eastern side of the Louisiana Mississippi River region, the marshes are essentially irrelevant. For the east-west coastline in western LA, there does appear to be some attenuation of surge elevations for many storms.

Outspoken hurricane scientist Dr. Ivor van Heerden, who as I reported, will be removed from his position at the Louisiana State Hurricane Center in 2010, had this to say in his 2006 book, The Storm: Wetlands can protect us from storm surge. Along with barrier islands, they are the best, most natural, least expensive buffer available...Joe Suhayda has already published computer studies showing that the 9.3-foot surge at Cocodrie, LA during Hurricane Andrew in 1992 would have been a foot higher without the barrier islands. Joe has calculated that a completely healthy marsh system could cut the storm surge in New Orleans by half.


Figure 2. Hurricane Ike of 2008 pushed a massive storm surge far inland over Texas and Louisiana, as the brown areas along the coast in this NASA Terra satellite image show.

Summary
The take home message from all this is that the degree of protection wetlands provide from storm surges is extremely complicated and is largely unknown. A simple rule of thumb that "X" miles of marshland will knock down the surge by "Y" number of feet is not going to be valid for most situations. Storm surge models do have equations to estimate the attenuation of the storm surge by wetlands, but these equations have not been validated using real world data. Thus, any model estimates of storm surge reduction by wetlands must be considered suspect. Wetlands will slow down the progress of a storm surge, and so will be most effective for for weaker and faster-moving storms. But if the wind blows strongly enough for long enough, it doesn't matter how many miles of wetlands you have, the storm surge will come. Thus, efforts to restore wetlands for the primary purpose of reducing hurricane surge will be ineffective in many cases, particularly for large, slow-moving hurricanes.

References
Resio, D.T., and J.J. Westerink, 2008, "Modeling the physics of storm surges", Physics Today, September 2008, pp. 33-38.

Corps of Engineers, US Army Engineer District, New Orleans, Interim Survey Report, Morgan City, Louisiana and Vicinity, serial no. 63, US Army Engineer District, New Orleans, LA (November 1963).

Fitzpatrick, P., 2008, "The impact of Louisiana's levees and wetlands on Katrina's storm surge", 28th Conference on Hurricanes and Tropical Meteorology, American Meteorological Society, May 2008.

Wamsley et al., 2007, "Influence of Wetland Degradation on Surge", Proc. 10th International Workshop on Wave Hindcasting and Forecasting and Coastal Hazard Symposium.

Jeff Masters

Sundowner winds fan California fires

By: JeffMasters, 1:16 PM GMT on May 07, 2009

Wildfires fanned by winds gusting over 60 mph roared through the hills of Santa Barbarba, California yesterday, and continue to burn out of control today. A strong high pressure system to the north of Santa Barbara is driving a variant of the famed Santa Ana winds called the "Sundowner" winds. These winds blow from north to south from the mountains to the sea. Since the winds blow downhill, the air compresses and heats as it descends, resulting in very hot, dry conditions perfect for fires. The winds tend to peak in strength at sundown, thus the name "Sundowner" winds. A number of record high temperatures were set in the region yesterday, with 100°F recorded at the Santa Barbara airport, smashing the old record of 86° set in 1991. Record heat was also observed in Los Angeles, where the UCLA weather station reported a record high of 93° (old record: 89° in 1990). Humidities as low as 14% were observed yesterday in Santa Barbara.

Winds this morning were sustained at 44 mph gusting to 62 mph at Montecito, CA at 1500 feet in the mountains near Santa Barbara, and these strong winds are expected to continue today before dying down on Friday. Precipitation since the rainy season began October 1, 2008, in the Santa Barbara region, has been only about half or normal (Figure 1). With the winter/spring rainy season pretty much over, California can expect another severe fire year in 2009. The snowpack in the Sierras was mostly below average this winter, so water shortages will also be a problem this summer in California.


Figure 1. Percent of normal precipitation for the Southwest U.S. for October 1, 2008 - May 6, 2009. Image credit: NOAA AHPS.

For more information
Take a look at the wundermap for Santa Barbara, and turn on the "fire" layer.

Jeff Masters

Fire

Influenza and the weather

By: JeffMasters, 1:14 PM GMT on May 04, 2009

It is well known that influenza hits hardest in winter--November to March in the Northern Hemisphere, and May to September in the Southern Hemisphere. In fact, the name influenza comes from the Italian word influenza, meaning "influence"--referring to the "influence of the season" (winter) in causing the illness. In the tropics, where there is little change in seasons, influenza occurs year-round (though increased incidence has been noted in rainy seasons--Viboud et al., 2006). Do the cold temperatures and lower humidities of winter cause increased transmission of the flu virus? If so, why is the current H1N1 swine flu outbreak doing so well, now that it is May, traditionally the end of flu season in the Northern Hemisphere? Or could it be that indoor crowding, lack of sunlight lowering vitamin D levels, and a more depressed immune system in winter are largely responsible, as some researchers have suggested?

Flu infections increase under cold or dry conditions
To test these hypotheses, researchers at the Mount Sinai School of Medicine in New York did a study in 2007 that looked at flu transmission among guinea pigs, which are highly susceptible to human influenza and easily transmit the virus to other guinea pigs (Lowen et al., 2007). The animals were placed in adjacent cages, so that infections could occur by airborne transmission, but not by direct contact (guinea pig lovers will be happy to know that the influenza virus-infected guinea pigs did not display detectable symptoms of disease--weight loss, fever, sneezing, and coughing--during the experiments). By carefully controlling temperature and humidity, the scientists were able to study the effects of each. They found that the animals shed much more of the virus--and over a longer period of time--at cold temperatures, which led to increased infection rates. The animals' immune system showed no signs of stress from the cold weather, arguing against the idea that cold conditions lead to increased infections by lowering the immune system. Lower humidities were also found to increase flu transmission rate, though the variation of infection rate with humidity was more complicated. The scientists built a model (Figure 1) to fit the data, and proposed that lower humidity increased infection rates through two mechanisms:

1) The stability of influenza virons in the suspended aerosol particles infected creatures cough out is dependent upon the humidity. Viruses are most stable at low RH (20%-40%), least stable at intermediate RH (50%), and stable again at high RH (60%-80%) (Schaffer et al., 1976). Thus, the virus has better staying power at the low moisture levels typical of winter.

2) At high RH (80%), exhaled respiratory droplets grow quite large as water vapor condenses around them, and these drops quickly settle to the ground under the force of gravity. Thus, even though the virus is stable at high humidities, it settles out of the atmosphere quickly, and cannot contribute to influenza virus spread.


Figure 1. A model of influenza transmission rates at 68°F (20°C) (dashed line) and 41°F (5°C) (solid line), as a function of relative humidity. Transmission efficiency is highest at low relative humidity, when influenza virions in an aerosol are relatively stable, and exhaled respiratory droplets stay small and don't settle out under the force of gravity. Transmission is diminished at intermediate humidity when virus particles are relatively unstable, but improves in parallel with influenza virus stability at higher humidities. At high humidity, evaporation from exhaled particles is limited, respiratory droplets settle out of the air, and transmission is blocked. At cold temperatures (solid line), transmission is more efficient than at warm temperatures (dashed line), but is reduced to a rate of 50% at higher humidities. Image credit: Lowen, A.C., S. Mubareka, J. Steel, and P. Palese, 2007, "Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature", PLos Pathogons, October 2007.

The researchers found no guinea pig infections at 86°F (30°C), which implies that in tropical climates, people may transmit the virus by direct contact rather than by coughing and sneezing. A second study Lowen et al., 2009) confirmed this idea--at least among guinea pigs. The authors concluded, "To our knowledge, we demonstrate for the first time that cold temperatures and low relative humidity are favorable to the spread of influenza virus. Although other factors likely contribute to the periodicity of influenza epidemics, it is clear that air temperature and RH could play an important role. Influenza virus transmission indoors could potentially be curtailed by simply maintaining room air at warm temperatures (>20 °C) and either intermediate (50%) or high (80%) RHs".

Climate change and influenza
The results of this study imply that global warming may significantly reduce influenza world-wide, since a warmer climate will also be more humid. Typically, there are between three and five million cases of severe flu and up to 500,000 deaths worldwide each year. In the United States alone, an average of 41,400 deaths and 1.68 million hospitalizations are attributed to influenza each year. A warmer world should reduce these numbers, if the current research is correct. However, these gains must be balanced against the possibility that malaria will become more widespread in a warmer world, since malaria kills about one million people per year.


Figure 2. Combined flu and pneumonia deaths in the United Kingdom during the great 1918 flu pandemic showed that the flu had three distinct peaks: one in June - July (a relatively mild form of the disease), followed by an extremely deadly outbreak in October, then another during the winter of 1918 - 1919. The 1918 flu pandemic infected 1/3 of the world's population, killing 50 - 100 million people. Strangely, the October peak occurred almost world-wide, with Bombay, India and Boston, Massachusetts peaking the same week. Image credit: Jordan, E., "Epidemic influenza: a survey", Chicago: American Medical Association, 1927.

Flu pandemics show little seasonality
The current Mexican H1N1 swine flu outbreak is seemingly unusual, since it is hitting at the end of the traditional flu season, in April - May. However, when a new flu strain develops that humans have no immunity to, the new strain is less constrained by seasonality. According to Dr. Jeffery Taubenberger, the virologist who helped isolate the genetic code of the virus responsible for the great 1918 flu pandemic, "Historical records since the 16th century suggest that new influenza pandemics may appear at any time of year, not necessarily in the familiar annual winter patterns of inter-pandemic years, presumably because newly shifted influenza viruses behave differently when they find a universal or highly susceptible human population." Indeed, the 1918 flu pandemic reached its peak in September - October (Figure 2), and the most recent flu pandemic, the 1968 Hong Kong flu, began in July. It wouldn't surprise me if the current flu outbreak dies down in the Northern Hemisphere over the summer months, as the combined effects of high temperatures, higher humidities, less indoor crowding, and increased sunlight interfere with its spread. However, we need to be ready for the virus to reappear in the Fall--potentially in a mutated, more virulent form--such as occurred during the 1918 flu pandemic.

References
Lowen, A.C., S. Mubareka, J. Steel, and P. Palese, 2007, "Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature", PLos Pathogons, October 2007.

Lowen, A.C., S. Mubareka, J. Steel, and P. Palese, 2009, "High Temperature (30°C) Blocks Aerosol but Not Contact Transmission of Influenza Virus", Journal of Virology, June 2008, p. 5650-5652, Vol. 82, No. 11 0022-538X/08/$08.00+0 doi:10.1128/JVI.00325-08

Schaffer, F.L., M.E. Soergel, and D.C. Straube, 1976, "Survival of airborne influenza virus: effects of propagating host, relative humidity, and composition of spray fluids", Arch Virol 51: 263-273.

Viboud, C, W.J. Alonso, and L. Simonsen, 2006, "Influenza in tropical regions", PLoS Med 3: e89 doi:10.1371/journal.pmed.0030089.

Vitamin D and influenza links:
http://www.medicalnewstoday.com/articles/51913.ph p
http://www.sciencedaily.com/releases/2008/08/0808 11195629.htm
http://www.vitamindcouncil.org/science/research/v itamin-d-and-influenza.shtml

Jeff Masters

Climate Change


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

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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