Dr. Jeff Masters' WunderBlog

Gustav is intensifying again, and headed for Louisiana

By: JeffMasters, 8:20 PM GMT on August 31, 2008

Gustav is intensifying again, and threatens to bring a destructive storm surge that will offer a significant test to New Orleans' rebuilt levees when it strikes Louisiana on Monday. Gustav tore across Cuba with 150 mph winds last night, but Cuba exacted a heavy toll on the storm. Passage over Cuba allowed wind shear to penetrate into the core of the storm, disrupting the eyewall and cutting the storm's winds by 35-45 mph. However, Gustav's day over the warm Gulf of Mexico has begun to rejuvenate the storm, and the pressure is starting to fall again. At 3:17 pm EDT, the Hurricane Hunters reported a pressure of 957 mb, down 5 mb from the eye report at 9:48 am. Winds at the surface and at the flight altitude of 10,000 feet have not changed significantly since this morning, and still support classifying Gustav as a strong Category 2 hurricane with top winds of 105 mph. However, the winds should start to increase by late tonight in response to the falling pressure.

The satellite appearance of Gustav is steadily improving. Visible satellite loops of Gustav show that the storm has recently assumed a more symmetric appearance, though the heaviest thunderstorms are still just on the south side of the eye. Upper level winds from the south are creating about 15 knots of shear over Gustav, restricting the upper-level outflow on the south side. The 35-mile wide eye is not very distinct, and the Hurricane Hunters reported that the eyewall was missing a chunk on the southeast side. However, the eye was elliptical this morning, and has now assumed a more circular, well-formed appearance.

The outer spiral bands of Gustav are now visible on New Orleans radar, and some rain bands have already affected the Mississippi River Delta region.


Figure 1. Estimated storm surge from NHC's experimental storm surge model. Note that these values will often differ from the latest official NHC forecast, and one should consult the latest text advisory for the most current storm surge information.

The latest track forecast
The latest 12Z (8 am EDT) model runs have shifted slightly west, with a central Louisiana landfall still the most popular solution. The GFDL model, which has all along insisted that Gustav would arrive at the coast a day earlier than the other models, has proven to have the correct timing. We should not be surprised if the center comes ashore as far east as New Orleans, or as far west as western Louisiana, given the current spread in the model tracks. Once Gustav makes landfall, it will slow down, and pose a significant rainfall/flooding threat to Louisiana and Texas. Portions of this region are under moderate to severe drought, so the flooding could have been worse. Only the HWRF model is forecasting that Gustav will drift southwestward back over the Gulf of Mexico after landfall. I am not expecting Gustav to be reborn off the Texas coast late in the week.

The intensity forecast for Gustav
Wind shear has remained in the moderate range (about 15 knots) today, and is forecast by the SHIPS model to decrease to 10 knots tonight. This amount of shear will allow Gustav to intensify. Gustav is moving further away from the upper-level anticyclone that helped it intensify as it approached Cuba, however. Overall, the upper level wind environment is favorable for intensification, but not as favorable as during yesterday's rapid intensification. Gustav has now left the warm waters of the Loop Current, and is over waters of much lower heat content than it has had. The latest 12Z (8 am EDT) run of the GFDL model forecasts no strengthening, and brings Gustav to shore as a Category 3 hurricane with 115 mph winds. The latest SHIPS intensity model predicts the same thing. The 12Z (8 am EDT) run of the HWRF shows that Gustav will weaken to a Category 2 hurricane by landfall. Given the recent improvement in Gustav's organization, I believe that the storm has time to intensify into at least a Category 3 hurricane with 125-130 mph winds by landfall. Gustav should not intensify as rapidly as it did when approaching Cuba.

Gustav's storm surge is not likely to breach the New Orleans levees--if they perform as designed
Gustav is a very large storm. Like Katrina, Gustav may carry a larger storm surge to the coast than its wind speeds might suggest. Currently, Gustav's diameter of tropical storm force winds is 340 miles. By landfall, this number is forecast to increase to 360 miles, which would make Gustav 80% as large as Katrina was at landfall. NHC's current storm surge forecast calls for a storm surge of 10-14 feet to the right of where the center of Gustav comes ashore. The latest computer generated storm surge map shows that highest surge will be along the levee system along the east side of New Orleans. Storm surge levels of this magnitude are characteristic of a Category 3 hurricane. The levee system of New Orleans is designed to withstand a Category 3 storm surge. If Gustav intensifies more than the NHC forecast is calling for, there is a significant threat of multiple levee failures in the New Orleans levee system resulting in flooding of portions of the city. However, the latest 12Z (8 am EDT) model runs have shifted their landfall points a bit further west, reducing the odds of a Category 4 storm surge in New Orleans. My best guess is that New Orleans will suffer a Category 2 or 3-level storm surge. The levees will hold with that level of storm surge, if they perform as designed.

Comparing Gustav to "Billion-Dollar Betsy"
Gustav's track and expected intensity at landfall are similar to those of Hurricane Betsy of 1965. Betsy was a strong Category 4 hurricane as it crossed the Gulf of Mexico, which then weakened to a Category 3 at landfall, right where Gustav is predicted to make landfall (Figure 2). Betsy brought a storm surge of up to 15 feet to Louisiana (Figure 3). According to wikipedia, levees for the Mississippi River Gulf Outlet along Florida Avenue in the Lower Ninth Ward and on both sides of the Industrial Canal failed. The flood waters reached the eaves of houses in some places and over some one story roofs in the Lower Ninth Ward. These levee breaches flooded parts of Gentilly, the Upper Ninth Ward, and the Lower Ninth Ward of New Orleans as well as Arabi and Chalmette in neighboring St. Bernard Parish. Seventy-six people died in Louisiana, and "Billion-Dollar Betsy" became the first billion-dollar hurricane ($10 billion in 2008 dollars). As a result of the hurricane, the Army Corps of Engineers was tasked with the job of upgrading the New Orleans levee system to withstand "the most severe
combination of meteorological conditions reasonably expected". As of today, that means protection from a Category 3 hurricane, but no higher.


Figure 2. Track of Hurricane Betsy of 1965.


Figure 3. Simulated maximum storm surge from Hurricane Betsy of 1965, as modeled using the ADCIRC model. Image credit: ADCIRC Development Group.

Links to follow:
New Orleans radar
New Orleans weather

Tropical Storm Hanna
The Hurricane Hunters are flying their first mission into Tropical Storm Hanna, and have found a weak tropical storm with 50 mph winds and a central pressure of 997 mb, according the 1:36 pm EDT eye report. Watching satellite loops of Hanna is kind of like watching a stick caught in turbulent rapids--the atmosphere is extremely chaotic in Hanna's vicinity, and it's tough to follow what is going on. Hanna has moved underneath an upper-level low that is pumping cold, dry air in. Hanna is also experiencing wind shear from the upper-level outflow from Hurricane Gustav. These effects have combined to keep Hanna a weak tropical storm. While Hanna does have a large circulation and some respectable upper-level outflow to the north, heavy thunderstorm activity is limited and is removed from the center.

The track forecast for Hanna
The current steering flow driving Hanna to the west is expected to collapse soon, resulting in a slow, erratic motion or small loop for Hanna. By 4-5 days from now, a strong ridge of high pressure is expected to build in, forcing Hanna to the northwest. Most of the long-range models foresee a landfall along the U.S. East Coast late in the week. The most popular model solution (GFS, GFDL, and HWRF) is a landfall Friday in North Carolina, followed by a track up the East Coast. The UKMET model targets South Florida on Friday, and the NOGAPS and ECMWF models target Georgia/northern Florida on Thursday night.

The intensity forecast for Hanna
Hanna will not be able to intensify significantly over the next two days, due to upper low it is situated under, and the outflow from Hurricane Gustav. By Tuesday, both Gustav and the upper low should be weaker, potentially allowing Hanna to intensify. The GFDL, HWRF, and SHIPS intensity models all predict Hanna will be a Category 1 hurricane on Friday. However, due to the uncertain future evolution of Gustav, consider all intensity forecasts for Hanna beyond two days from now low confidence.

Middle Atlantic disturbance, 98L
An area of disturbed weather (98L) is located near 22N, 45W, in the mid-Atlantic Ocean. Visible satellite images show that this disturbance has a well-defined surface circulation. However, the disturbance has very little heavy thunderstorm activity, thanks to 20 knots of wind shear, and some dry air to the west. Wind shear should remain too high to permit development over the next two days, and is highly uncertain after that, due to the upper-atmosphere interactions occurring with Hanna and Gustav. NHC is giving this system a low (<20% chance) of developing into a tropical depression by Tuesday. None of the models develop 98L, but Bermuda should keep an eye it.

Cape Verdes Islands disturbance, 97L
A low pressure system near 16N 35W (97L), is a few hundred miles west-northwest of the Cape Verde Islands. Visible satellite loops show that 97L has a very large closed circulation, but little heavy thunderstorm activity. Dry air on the west side of the storm is getting wrapped into the circulation, interfering with development. Wind shear is a moderate 15 knots, and is expected to stay in the low to moderate range over the next four days. NHC has given this system a high (>50%) chance of developing into a tropical depression by Tuesday. Several of the models develop this system, and the storm is expected to pass close to the northern Lesser Antilles Islands on Friday.

There are two other impressive African tropical waves lined up behind 97L that are also likely to be a threat to develop once they move offshore Africa this week. The long-range GFS model develops both of these waves.

I'll have a short update tonight, if there's any major developments to report.

Jeff Masters

Updated: 1:27 AM GMT on September 01, 2008

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A weakened Gustav still presents a grave danger to New Orleans

By: JeffMasters, 12:18 PM GMT on August 31, 2008

Gustav roared over Cuba as a Category 4 hurricane with 150 mph winds last night, but Cuba exacted a heavy toll on the storm. Gustav weakened to a Category 3 storm after passage over the island, and is still weakening, according to the latest Hurricane Hunter reports. At 9:48 am EDT, the Hurricane Hunters reported a pressure of 962 mb, up 2 mb from their previous pass through the eye at 7 am. Surface and flight level winds measured by the Hurricane Hunters suggest that Gustav may only be at Category 2 strength. Top winds seen by the plane's SFMR instrument so far this morning have been about 105 mph. The top flight level winds of 103 knots at the flight altitude of 10,000 feet also support a surface wind of 105 mph. NHC has elected to keep Gustav at Category 3 120 mph strength with their 11 am advisory, probably because the satellite appearance still supports a Cat 3. The boundary between Category 2 and Category 3 is at 115 mph.

Visible satellite loops show a ragged and lopsided looking hurricane. Upper level winds from the south are creating 10-15 knots of shear over Gustav, and restricting the upper-level outflow on the south side. The 28-mile wide eye is not very distinct, and the Hurricane Hunters reported that the eyewall was missing a chunk on the south side. Long range radar from Key West also shows the missing southern portion of the eyewall.

The latest track forecast
The latest 00Z (8 pm EDT) model runs have united around a strike in central or southeast Louisiana late Monday morning or early afternoon. The GFDL model, which has all along insisted that Gustav would arrive at the coast a day earlier than the other models, has proven to be correct. We should not be surprised if the center comes ashore as far east as New Orleans, or as far west as western Louisiana, given the current spread in the model tracks. Once Gustav makes landfall, it will slow down, and pose a significant rainfall/flooding threat to Louisiana and Texas. Portions of this region are under moderate to severe drought, so the flooding could've been worse. None of the models are currently forecasting that Gustav will drift southwestward back over the Gulf of Mexico after landfall.

The intensity forecast for Gustav
Wind shear has remained in the moderate range (10-15 knots) the past day, and is forecast to increase to marginal late tonight (15-20 knots). Moderate to marginal shear will still allow Gustav to intensify. Gustav is moving further away from the upper-level anticyclone that helped it intensify as it approached Cuba. Overall, the upper level wind environment is favorable for intensification, but not as favorable as during yesterday's rapid intensification. Gustav is currently over the Loop Current, containing the highest heat content waters of the Atlantic (Figure 1). Both the GFDL and HWRF models forecast that these high heat content waters should result in a 15-20 mph increase in Gustav's winds today. By this evening, Gustav will be passing over a cold eddy. The heat content of the Gulf will decrease as Gustav approaches the coast. As seen in a simulation done yesterday using the GFDL model (Figure 2), the relatively shallow depth of warm water near the coast will allow Gustav to upwell large amounts of cold water from the depths. This will chill the surface waters down by up to 5°C (9°F), which should weaken Gustav's winds by about 15 mph. This cooling effect does not occur for Gustav's path over the southern Gulf of Mexico, due to the great depth of warm waters there. Both the GFDL and HWRF models respond to the lower heat content waters near Louisiana by weakening Gustav to a Category 3 hurricane with 115-120 mph winds at landfall. These models are the only ones that incorporate detailed depictions of the thermal structure of the Gulf of Mexico into their runs.


Figure 1. Tropical Cyclone Heat Potential (TCHP, in kJ/cm^2) for August 28, 2008. Values of TCHP greater than 80 are commonly associated with rapid intensification of hurricanes. The forecast points from the NHC 5 am Saturday forecast are overlaid. Gustav is currently crossing over a portion of the Loop Current with extremely high value of TCHP of 120. However, Gustav will then cross over a cold eddy, and will miss crossing the warm Loop Current eddy that broke off in July. Note that this forecast is old, and the newer forecasts bring Gustav much closer to New Orleans. Image credit: NOAA/AOML.


Figure 2. Forecast track and sea surface temperature response to the passage of Gustav, as simulated by the GFDL model at 8 am EDT Saturday 8/30/08. Passage of Gustav over the relatively shallow depth of warm water near the coast will allow Gustav to upwell large amounts of cold water from the depths. This will chill the surface waters down by up to 5°C (9°F). Note that this forecast is old, and the newer forecasts bring Gustav much closer to New Orleans. Image credit: Isaac Ginis, University of Rhode Island.

Gustav's storm surge may breach the New Orleans levees
NHC's current storm surge forecast calls for a storm surge of 12-16 feet to the right of where the center of Gustav comes ashore. The latest computer generated storm surge map shows that highest surge will be along the levee system along the east side of New Orleans. Storm surge levels of this magnitude are characteristic of a Category 3 to 4 hurricane. The levee system of New Orleans is designed to withstand a storm surge characteristic of a Category 3 storm. If the NHC storm surge forecast verifies, there is a significant threat of multiple levee failures in the New Orleans levee system resulting in flooding of portions of the city. The latest 06Z (2 am EDT) model runs have shifted their landfall points a bit further west, slightly reducing the odds of a Category 4 storm surge in New Orleans. My best guess is that New Orleans will suffer a Category 3-level storm surge. Let's hope that the Army Corps of Engineers' assertion that the levee system can withstand a Category 3-level storm surge is correct.

Comparing Gustav to "Billion-Dollar Betsy"
Gustav's track and expected intensity at landfall are similar to those of Hurricane Betsy of 1965. Betsy was a strong Category 4 hurricane as it crossed the Gulf of Mexico, which then weakened to a Category 3 at landfall, right where Gustav is predicted to make landfall (Figure 3). Betsy brought a storm surge of up to 15 feet to Louisiana (Figure 4). According to wikipedia, levees for the Mississippi River Gulf Outlet along Florida Avenue in the Lower Ninth Ward and on both sides of the Industrial Canal failed. The flood waters reached the eaves of houses in some places and over some one story roofs in the Lower Ninth Ward. These levee breaches flooded parts of Gentilly, the Upper Ninth Ward, and the Lower Ninth Ward of New Orleans as well as Arabi and Chalmette in neighboring St. Bernard Parish. Seventy-six people died in Louisiana, and "Billion-Dollar Betsy" became the first billion-dollar hurricane ($10 billion in 2008 dollars). As a result of the hurricane, the Army Corps of Engineers was tasked with the job of upgrading the New Orleans levee system to withstand a future hurricane of Besty's strength--but no stronger.


Figure 3. Track of Hurricane Betsy of 1965.


Figure 4. Simulated maximum storm surge from Hurricane Betsy of 1965, as modeled using the ADCIRC model. Image credit: ADCIRC Development Group.

Links to follow:
Key West radar
New Orleans weather

I'll have a new blog entry later today that will provide an update on Gustav. I'll also cover Hanna, which may hit the U.S. East Coast late this week.

Some prayers this morning for New Orleans would be in order!

Jeff Masters

Updated: 2:53 PM GMT on August 31, 2008

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Gustav rips through Cuba at Category 4 strength

By: JeffMasters, 8:28 PM GMT on August 30, 2008

Gustav roared over Cuba's Isle of Youth at 2 pm today, and is now clobbering the western tip of Cuba with 145 mph Cat 4 winds and a storm surge estimated at 18-23 feet. This will be a massive disaster for Cuba, as portions of the stretch of coast to the right of where Gustav will make landfall are heavily populated (Figure 2). Furthermore, Gustav's hurricane-force winds extend out 60 miles from the eye, meaning that the western reaches of the capital city of Havana will receive a formidable blow. Havana's construction is fairly dilapidated, and Category 1 hurricane winds will do heavy damage. Adding to the catastrophe will be rainfall amounts of up to 25 inches, causing dangerous fresh-water flooding. Gustav is likely to be one of the five most damaging hurricanes in Cuban history. Fortunately, Cuba has a top-notch hurricane civil defense operation, and I'm confident they have gotten all of the population at risk out of harm's way.


Figure 1. Visible satellite image of Hurricane Gustav at landfall in Cuba, 3:02 pm EDT 8/30/08. At the time, Gustav was a Category 4 hurricane with 145 mph winds. Tropical Storm Hanna is visible at the right. Image credit: NASA.

Visible satellite loops show a well-organized and intensifying major hurricane. Upper-level outflow is well-established. Gustav has a well-formed eye and Central Dense Overcast (CDO) of high cirrus clouds, characteristic of a major hurricane. Radar from Cuba's Isle of Youth shows impressive spiral banding and a solid 27-mile diameter eyewall. The latest Hurricane Hunter report at 4:09 pm EDT put Gustav's pressure at 942 mb, with 145 mph surface winds. Winds at flight level of 10,000 feet were a little higher than in the eye penetration completed at 2 pm, so Gustav is still intensifying, although at a slower rate than earlier today.


Figure 2. Population density map of Cuba. The Isle of Youth, where the western eyewall of Gustav passed over, is lightly populated. However, the portion of the coast to the right of where Gustav is making its second landfall is highly populated. Image credit: University of Texas.

The latest computer models
Louisiana, here comes Gustav. The latest 12Z (8 am EDT) model runs have united around a strike in central Louisiana on Monday afternoon or evening. We should not be surprised if the center comes ashore as far east as Mississippi, or as far west as Galveston, though, given the current boundaries of the cone of uncertainty. A landfall on the western side of the cone of uncertainty, in Texas, is more likely than one on the eastern side, in Mississippi. Once Gustav makes landfall, it will slow down, and pose a significant rainfall/flooding threat to Louisiana and Texas. Portions of this region are under moderate to severe drought, so the flooding could've been worse. Several of the models are forecasting that Gustav will drift southwestward back over the Gulf of Mexico after landfall, and this is a distinct possibility. The motion of Gustav in the longer term may be influenced by Hanna, which I will talk about in the Hanna section.

The intensity forecast for Gustav
Wind shear has increased over Gustav, and is now moderate (10-15 knots). However, the storm is under an upper-level anticyclone that aids intensification, is over the highest heat content waters of the Atlantic, and has no dry air to interfere with it. Furthermore, the storm has armored itself with a formidable Cat 4 eyewall which will resist the effects of wind shear, thanks to its high level of angular momentum. Interaction with the flat land area of western Cuba will probably knock down Gustav's intensity by 10-20 mph. However, Gustav should regain its lost strength and more once it enters the Gulf of Mexico. A region of exceptionally high oceanic heat content, associated with the warm Loop Current, lies just north of Cuba along Gustav's track (Figure 3). Gustav will encounter a cool eddy in ocean after this, and will miss passing over the warm Loop Current Eddy that broke off in July. As Gustav approaches landfall, the total heat in the ocean will continue to decrease, and wind shear is expected to increase to 15-20 knots, as forecast by the SHIPS model. Thus, some modest weakening is to be expected as Gustav approaches Louisiana. Still, Gustav will likely be a Category 3 or 4 hurricane at landfall in Louisiana, as forecast by the HWRF and GFDL models.


Figure 3. Tropical Cyclone Heat Potential (TCHP, in kJ/cm^2) for August 28, 2008. Values of TCHP greater than 80 are commonly associated with rapid intensification of hurricanes. The forecast points from the NHC 5 am Saturday forecast are overlaid. Gustav is expected to cross over a portion of the Loop Current with extremely high value of TCHP of 120 after crossing Cuba. However, Gustav will then cross over a cold eddy, and will miss crossing the warm Loop Current eddy that broke off in July. Image credit: NOAA/AOML.

How big will Gustav get, and what will its storm surge be like?
Gustav has expanded significantly in size, and will be a large and dangerous hurricane the remainder of its life. But will it rival Katrina in size, bringing an enormous Katrina-like storm surge to the coast? Well, our ability to predict size changes in hurricanes is poor. We do know that as storms move further from the Equator, they grow in size. This is because the Coriolis force increases as you move away from the Equator. An increased Coriolis force provides more spin to the storm, and the hurricane responds by growing in size. Thus, expect Gustav to grow in size as it approaches landfall along the Gulf coast. Hurricanes also tend to grow in size as they intensify. These two factors are taken into account when NHC makes a wind radius forecast. NHC is forecasting that Gustav's current diameter of tropical storm force (about 320 miles across) will grow to about 350 miles across at landfall in Louisiana. This is about 80% as wide as the 440 mile-wide region of tropical storm force winds Hurricane Katrina had at landfall (5 am August 29, 2005).

A wind field of this size in a Category 4 hurricane traveling NW over the Gulf of Mexico is capable of carrying a 13-15 foot high storm tide to the coast in central Louisiana. NNW-moving storms bring the highest surge to this region of coast. Storm tide is the storm surge, adjusted upwards by 2 feet in case the storm hits at high tide.

Time to leave New Orleans
It should go without saying that it's time to leave New Orleans. But, I'll say it anyway: It's Time to Leave New Orleans! The risk of staying in New Orleans is unacceptable. This is a huge and dangerous storm that has already killed a lot of people. The projected track and strength of Gustav is very close to that of Hurricane Betsy of 1965, the Category 3 hurricane that overwhelmed New Orleans' levees, and killed 76 people. Get out now.

Links to follow:
Key West radar

Tropical Storm Hanna
Tropical Storm Hanna continues to struggle. The upper level low to Hanna's west has weakened some, but is still creating substantial wind shear over Hanna that is keeping the storm from putting its center of circulation underneath its heaviest thunderstorm activity. Visible satellite loops show the exposed surface circulation, with the heavy thunderstorm activity blown to the northeast side of the storm. The upper low is also pumping in plenty of dry air into Hanna's core, as seen on water vapor satellite loops. However, Hanna is a large storm, and appears to making headway against the shear in recent hours. Heavy thunderstorm have managed to wrap around the north side of the center. Hanna has two prominent upper-level outflow channels visible, on the north and east sides. This morning's QuikSCAT pass saw winds of 40-50 mph at the surface. The first Hurricane Hunter mission for Hanna is scheduled for Sunday afternoon.

The track forecast for Hanna
Steering currents imparted by the counter-clockwise flow around the upper-level low to its west will keep Hanna moving west to west-northwest, towards the Bahamas. As this upper low weakens and moves off, Hanna may begin a period of erratic motion three or so days from now. By 4-5 days from now, a strong ridge of high pressure is expected to build in to the north of Hanna, forcing it west or even southwest. The UKMET model pushes Hanna across Cuba into the western Caribbean by Friday, but the rest of the models foresee that Hanna will loop back to the northwest, eventually threatening Florida, Georgia, or South Carolina by Friday. With such a divergent set of models, it is impossible to predict where and when Hanna will eventually make landfall.

The tropical disturbance behind Hanna, near 22N 45W, is under too much wind shear to develop, and will not influence Hanna's track over the next 2-3 days. If Gustav stalls over the western Gulf of Mexico, as some models are predicting, Hanna's track may be influenced by the presence of another hurricane so close to it. The FujiWhara Effect occurs when two hurricanes approach within 900 miles of each other (roughly the diameter of the Gulf of Mexico). The two storms tend to rotate counter-clockwise around a common center. This effect would tend to send Gustav southwest, over Texas, and Hanna to the northwest. Since Hanna and Gustav are forecast to be roughly 1000 miles apart, it is unlikely that this effect will be a major player in the track.

The intensity forecast for Hanna
Hanna will not be able to intensify until the upper low to its west weakens and moves away. The computer models have, without much success, been predicting this will happen, and continue to do so. Assuming this does really happen, Hanna is large and well-organized enough that it could develop into a Category 1 hurricane fairly quickly once the upper low finally does go away. However, strong upper-level winds from an approaching trough of low pressure are expected to bring 20-30 knots of shear over Hanna beginning Tuesday, and this should substantially weaken the storm, perhaps even to a minimal tropical storm. Oddly, the GFDL model is forecasting Hanna to intensify to a Category 3 hurricane during this high wind shear, which is rather improbable.

Elsewhere in the tropics
An area of disturbed weather located near 22N, 45W, in the mid-Atlantic Ocean, is under 30-40 knots of wind shear. This is too high to allow development to occur, and the shear is expected to remain too high for development for at least 2-3 more days. NHC is giving this system a low (<20% chance) of developing into a tropical depression by Monday.

Cape Verdes Islands disturbance, 97L
A low pressure system (97L) with a large circulation and plenty of spin is located near the Cape Verde Islands off the coast of Africa. This morning's QuikSCAT pass missed 97L. This system has developed a significant amount of heavy thunderstorms, and appears poised to become a tropical depression in the next two days. NHC has given this system a high (>50%) chance of developing into a tropical depression by Monday. The models have been very aggressive developing this system, and both the GFDL and HWRF models predict 97L will be a hurricane five days from now. The storm is expected to pass to the north of the Lesser Antilles Islands, and it is too early to speculate whether this storm might end up recurving out to sea or not.

There are two other impressive African tropical waves lined up behind 97L that are also likely to be a threat to develop once they move offshore Africa next week. The long-range GFS model develops both of these waves.

My next blog will be Sunday morning, unless there's some significant development before then.

Jeff Masters

Updated: 8:48 PM GMT on August 30, 2008

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Gustav deepens explosively into a Category 3 hurricane

By: JeffMasters, 2:32 PM GMT on August 30, 2008

Hurricane Gustav is deepening explosively, and is now a major hurricane. At 8:20 am EDT, the Hurricane Hunters estimated surface winds of 120 mph, and a surface pressure of 954 mb. This is a 34 mb drop in the past 24 hours, since the storm left Jamaica. In Jamaica, Gustav may have killed 11 people, and left millions in damage. Earlier this week, Gustav killed 59 people on Haiti, and 8 in the Dominican Republic.


Figure 1. Current satellite image of Gustav.

Visible satellite loops show a well-organized and intensifying major hurricane. Upper-level outflow is well-established. Gustav has a well-formed eye and Central Dense Overcast (CDO) of high cirrus clouds, characteristic of a major hurricane. Radar from Cuba's Isle of Youth shows impressive spiral banding and a solid 35-mile diameter eyewall.

The latest computer models
The latest 00Z/06Z (8 pm/2 am EDT) model runs still offer two solutions. The main solution, offered by the GFDL, GFS, and NOGAPS models, and adopted by NHC, takes Gustav to the central Louisiana coast between Monday afternoon and early Tuesday morning. The GFDL forecasts a Category 4 storm at landfall, with Category 1 winds affecting New Orleans. It is the fastest model, bringing Gustav ashore Monday afternoon. The other solution, offered by the UKMET and ECMWF models, is to turn Gustav westwards towards Texas just before landfall in Central Louisiana Monday afternoon. The HWRF is in between, taking Gustav to the coast of western Louisiana as a Category 3 storm, then turning the storm southwestward along the Texas coast as a tropical storm. While the official NHC forecast taking Gustav ashore over central Louisiana is the most probable one, a significant chance exists of a landfall farther west, perhaps even as far west as Corpus Christi, Texas.


Figure 2. Tropical Cyclone Heat Potential (TCHP, in kJ/cm^2) for August 28, 2008. Values of TCHP greater than 80 are commonly associated with rapid intensification of hurricanes. The forecast points from the NHC 5 am Saturday forecast are overlaid. Gustav is expected to cross over a portion of the Loop Current with extremely high value of TCHP of 120 after crossing Cuba. However, Gustav will then cross over a cold eddy, and will miss crossing the warm Loop Current eddy that broke off in July. Image credit: NOAA/AOML.

The intensity forecast for Gustav
Wind shear has increased over Gustav, and is now moderate (10-15 knots). However, the storm is under an upper-level anticyclone that aids intensification, is over the highest heat content waters of the Atlantic, and has no dry air to interfere with it. I expect Gustav will continue to intensify until landfall in Cuba. Interaction with the flat land area of Cuba will probably knock down Gustav's intensity by 10-20 mph or so. However, Gustav should regain its lost strength and more once it enters the Gulf of Mexico. A region of exceptionally high oceanic heat content, associated with the warm Loop Current, lies just north of Cuba along Gustav's track (Figure 2). However, Gustav will encounter a cool eddy in ocean after this, and will miss passing over the warm Loop Current Eddy that broke off in July. As Gustav approaches landfall, the total heat in the ocean will continue to decrease, and wind shear is expected to increase to 15-20 knots, as forecast by the SHIPS model. Thus, some weakening is to be expected as Gustav approaches Louisiana.

How big will Gustav get, and what will its storm surge be like?
Gustav has expanded significantly in size, and will be a large and dangerous hurricane the remainder of its life. But will it rival Katrina in size, bringing an enormous Katrina-like storm surge to the coast? Well, our ability to predict size changes in hurricanes is poor. We do know that as storms move further from the Equator, they grow in size. This is because the Coriolis force increases as you move away from the Equator. An increased Coriolis force provides more spin to the storm, and the hurricane responds by growing in size. Thus, expect Gustav to grow in size as it approaches landfall along the Gulf coast. Hurricanes also tend to grow in size as they intensify. These two factors are taken into account when NHC makes a wind radius forecast. NHC is forecasting that Gustav's current diameter of tropical storm force (about 300 miles across) will remain about the same at landfall. This is about 70% as wide as the 440 mile-wide region of tropical storm force winds Hurricane Katrina had at landfall (5 am August 29, 2005).

A wind field of this size in a Category 4 hurricane traveling NW over the Gulf of Mexico is capable of carrying a 13-15 foot high storm tide to the coast in central Louisiana. NNW-moving storms bring the highest surge to this region of coast. Storm tide is the storm surge, adjusted upwards by 2 feet in case the storm hits at high tide.

Time to leave New Orleans
I've been criticized by some for recommending people evacuate New Orleans, since that's not my job, and for saying "it's not natural" to live in a city that lies partially below sea level. I apologize for my remarks, they could have been phrased better. We had to build New Orleans where it is, and it is a great city that needs to be protected. The fact remains that New Orleans is highly vulnerable to storms like Gustav. Gustav is capable of bringing a storm surge to the city that will overwhelm the levees. Pre-Katrina wisdom suggested that the city needed 72 hours to evacuate. With the population about half of the pre-Katrina population, that lead time is about 60 hours. With Gustav likely to bring tropical storm force winds to the city by Monday afternoon, that means it's time to leave. I'm not an emergency manager, but I am a hurricane scientist. I understand the danger this storm poses better than most. The risk of staying in New Orleans is unacceptable. This is a huge and dangerous storm that has already killed a lot of people. The projected track and strength of Gustav is very close to that of Hurricane Betsy of 1965, the Category 3 hurricane that overwhelmed New Orleans' levees, and killed 76 people. It's time to get out of New Orleans.

Why did Katrina get so huge?
We really don't understand why Katrina got so huge, though an interesting theory was provided by Pat Fitzpatrick of Mississippi State University at a recent hurricane conference. Here's the technical gist:

Katrina nearly doubled in size on 27 August, and by the end of that day tropical storm-force winds extended up to about 140 n mi from the center. A cursory examination of satellite imagery shows the possible influence of a trough or confluence zone to the north that may have contributed angular momentum to the intensifying cyclone.

Although the rapid intensification of Katrina was noteworthy, the expansion of the tropical storm-force winds is the key forecast issue. The devastation wrought by this storm upon landfall is attributable more to its size rather than its intensity, as it landed as a Category 3 hurricane. This large hurricane caused a record storm surge and exposed the coastal regions of Louisiana and Mississippi to hurricane-force winds for an extensive period of time.

Observations, as well as a Weather Research Forecast model simulation, suggest that an influx of vorticity associated with a remnant front near north Florida contributed to the wind field expansion.


Basically, Dr. Fitzpatrick is saying that satellite observations and computer modeling studies suggest that Katrina got extra spin that helped it grow in size by ingesting a portion of an old front that had stalled out over northern Florida. As Gustav approaches the U.S., we should be on the lookout for similar clumps of clouds with some extra spin that the hurricane could use to help grow in size.

Our interactive Wundermap is a good way to study the predicted size changes of a hurricane. Click on the "Hurricane" layer, then check the "Wind Radius" box to see the predicted extent of hurricane force (65 kt), storm force (50 kt) and tropical storm force (34 kt) winds, for the four quadrants of the storm.

Links to follow
Punta del Este, Cuba radar
Punta del Este, Cuba weather
Nueva Gerona, Cuba weather

Hanna and the rest of the tropics
I'll talk about Hanna and the rest of the tropics in my next blog, which will be this afternoon between 3-5 pm.

Jeff Masters

Updated: 5:14 PM GMT on August 30, 2008

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Time to leave New Orleans

By: JeffMasters, 8:24 PM GMT on August 29, 2008

Gustav is now a hurricane, according to wind data taken by the latest Hurricane Hunter flight. At 3:11 pm EDT, the Hurricane Hunters visually estimated surface winds of 90 mph, and the SFMR instrument also measured a patch of surface winds of hurricane force, 75-90 mph. Gustav's pressure has fallen 8 mb since it exited the western tip of Jamaica this morning, and now stands at 980 mb, its lowest pressure so far.


Figure 1. Current satellite image of Gustav.

Visible satellite loops continue to show a well-organized and intensifying storm that is growing larger in size. Upper-level outflow is established in all quadrants and is growing. Low-level spiral bands are multiplying and intensifying, and the amount and intensity of Gustav's heavy thunderstorms are steadily increasing. A well-defined eye has appeared, and Gustav appears poised to enter a phase of rapid intensification. Radar from Pilon, Cuba shows the developing spiral bands of Gustav quite well. Dry air is not evident anywhere close to Gustav.

It's time to leave New Orleans
Today is the 3rd anniversary of Hurricane Katrina's catastrophic hit on the Louisiana/Mississippi/Alabama coast. Unfortunately, I think that people living in New Orleans should mark the anniversary of Katrina by getting the heck out of the city. You live at the bottom of a bowl, much of it below sea level. While New Orleans must exist where it is, this is not natural. Nature wants to fill up this bowl with huge quantities of Gulf of Mexico sea water. There is a storm capable of doing that bearing down on you. If you live in New Orleans, I suggest you take a little Labor Day holiday--sooner, rather than later, to beat the rush--and get out of town. Gustav is going to come close to you, and there's no sense messing with a major hurricane capable of pushing a Category 3 storm surge to your doorstep. Don't test those Category 3 rated--but untested--levees. Conventional pre-Katrina wisdom suggested that the city needed 72 hours to evacuate. With the population about half of the pre-Katrina population, that lead time is about 60 hours. With Gustav likely to bring tropical storm force winds to the city by Monday afternoon, that means that tonight is a good time to start evacuating--Saturday morning at the latest. Voluntary evacuations have already begun, which is a good idea.

The latest computer models
After analyzing the latest 12Z (8 am EDT) model runs, I don't think Gustav is going to bring hurricane force winds to New Orleans, and I estimate there is 10% chance Gustav will bring a Category 3-level storm surge to the city. However, if I lived in the city, I'd get out tonight, because the storm is going to be plenty mean and is going to come uncomfortably close to you. A 10% chance is a big chance when it's your life.

The models continue to fall into two camps: models that foresee a track over Louisiana, making landfall Monday night or Tuesday morning (the GFS and GFDL models), and the models that bring Gustav very close to the Louisiana coast on Monday, then turn the storm westward towards Texas. The HWRF model, which was in the former camp, has now gone over to the westward-turn-to-Texas camp. The general trend of most of the models has been to build a stronger ridge of high pressure over the southern U.S. on Monday and Tuesday, and I forecast that Gustav will come very close to the Louisiana coast on Monday as a Category 3 hurricane, stall, then move westwards towards Texas. This is the solution of the latest HWRF model.

Specifically, the GFDL model continues to be a day faster than the other models, and brings Gustav over New Orleans on Monday morning as a Category 3 hurricane. It is the furthest east model. The GFS predicts a western Louisiana landfall Tuesday morning, with Gustav stalling inland. The ECMWF predicts a landfall near the TX/LA border Monday night. The NOGAPS targets Galveston on Tuesday afternoon, and the UKMET targets Galveston on Wednesday. The HWRF does not bring Gustav to a landfall by the end of its forecast cycle (5.25 days). It predicts a Category 3 hurricane 100 miles south of the central Louisiana coast on Monday morning, that stalls and wanders west, weakening to a Category 1 hurricane.

The intensity forecast for Gustav
Gustav has low wind shear (less than 10 knots), is under an upper-level anticyclone that aids intensification, is over the highest heat content waters of the Atlantic, and has no land masses or dry air to interfere with it between now and landfall in Cuba. I expect Gustav will intensify into a Category 3 or 4 hurricane before hitting Cuba. These favorable conditions will continue once Gustav enters the Gulf of Mexico. However, on Monday afternoon, when Gustav will be nearing the Gulf Coast, the SHIPS intensity model is predicting wind shear will increase to 15-20 knots. This may act to weaken Gustav, as predicted by the HWRF model. If Gustav stalls near the coast, as I am expecting, this will churn up cold water from the depths that may also act to weaken the storm.

Yesterday, a NOAA Hurricane Hunter research aircraft dropped a network of 60 specialized buoys (Air eXpendable BathyThermographs, or AXBTs) in the Gulf of Mexico to provide precise measurements of ocean temperatures in order to aid intensification forecasts for Gustav. This data has not yet been assimilated into the GFDL and HWRF computer models, I am told, since the procedure has not yet been automated. The data must be entered manually by NHC staff into the models. Presumably, NHC has their hands full with Hanna and Gustav, so it is not certain this data will be used for real forecasts of Gustav. However, I'm told by Morris Bender of the GFDL lab that this year's intensity forecasts for hurricanes in the Gulf should be much better than in years past. To quote:

The detailed structure of the warm core ring and the Loop Current is getting assimilated into the initial ocean structure for BOTH GFDL and HWRF. There was a major advancement in this technique that became operational this season.


How big will Gustav get?
Gustav is currently expanding significantly in size, and will be a medium to large sized hurricane the remainder of its life. But will it rival Katrina in size, bringing an enormous Katrina-like storm surge to the coast? Well, our ability to predict size changes in hurricanes is poor. We do know that as storms move further from the Equator, they grow in size. This is because the Coriolis force increases as you move away from the Equator. An increased Coriolis force provides more spin to the storm, and the hurricane responds by growing in size. Thus, expect Gustav to grow in size as it approaches landfall along the Gulf coast. Hurricanes also tend to grow in size as they intensify. These two factors are taken into account when NHC makes a wind radius forecast (Figure 2). NHC is forecasting that Gustav's diameter of tropical storm force winds will increase from 210 miles (estimated at 11 am today) to 310 miles by 8 am EDT Monday, just before landfall. This is about 70% as wide as the 440 mile-wide region of tropical storm force winds Hurricane Katrina had at landfall (5 am August 29, 2005). We really don't understand why Katrina got so huge, though an interesting theory was provided by Pat Fitzpatrick of Mississippi State University at a recent hurricane conference. Here's the technical gist:

Katrina nearly doubled in size on 27 August, and by the end of that day tropical storm-force winds extended up to about 140 n mi from the center. A cursory examination of satellite imagery shows the possible influence of a trough or confluence zone to the north that may have contributed angular momentum to the intensifying cyclone.

Although the rapid intensification of Katrina was noteworthy, the expansion of the tropical storm-force winds is the key forecast issue. The devastation wrought by this storm upon landfall is attributable more to its size rather than its intensity, as it landed as a Category 3 hurricane. This large hurricane caused a record storm surge and exposed the coastal regions of Louisiana and Mississippi to hurricane-force winds for an extensive period of time.

Observations, as well as a Weather Research Forecast model simulation, suggest that an influx of vorticity associated with a remnant front near north Florida contributed to the wind field expansion.


Basically, Dr. Fitzpatrick is saying that satellite observations and computer modeling studies suggest that Katrina got extra spin that helped it grow in size by ingesting a portion of an old front that had stalled out over northern Florida. As Gustav approaches the U.S., we should be on the lookout for similar clumps of clouds with some extra spin that the hurricane could use to help grow in size.

Our interactive Wundermap is a good way to study the predicted size changes of a hurricane. Click on the "Hurricane" layer, then check the "Wind Radius" box to see the predicted extent of hurricane force (65 kt), storm force (50 kt) and tropical storm force (34 kt) winds, for the four quadrants of the storm.


Figure 2. Extent of hurricane force (65 kt, yellow), storm force (50 kt, yellow-green) and tropical storm force (34 kt, green) winds, for the four quadrants of Gustav, as predicted by the National Hurricane Center at 11 am EDT August 29, 2008. Gustav's tropical storm-force winds are predicted to have a diameter of 270 nm (310 miles) when it approaches the coast of Louisiana. For comparison, Hurricane Katrina had a 380 nm (440 mile) diameter region of tropical storm force winds at landfall. Thus, Gustav's wind field is predicted to be 70% as large as Katrina's. Image is from our interactive Wundermap. Hurricane force winds are not included in the final two forecast points, because NHC did not make a forecast of these winds for those times.

Links to follow
Wundermap for the Cayman Islands
Pilon, Cuba radar
Cayman Brac, Cayman Islands
Grand Cayman

Hanna and the rest of the tropics
Not much has changed in my thinking on Hanna and the rest of the tropics, which I presented in this morning's blog. The new set of model runs for Hanna continue to give a wide range of solutions for where the storm might go, with a track over Cuba into the western Caribbean, through the Keys into the Gulf of Mexico, and into the east coast of Florida all possibilities. I'll discuss Hanna more in my next blog, which will be Saturday morning.

In harm's way, on Cayman Brac Island
Wunderground member mangroveman is on Cayman Brac Island, directly in the projected path of Gustav. Here's his report from noon today:

Hi Dr. Jeff

Well, things are picking up here. Winds from the ENE have been steadily increasing over the past 3 or 4 hours. I'd estimate them at around 30 mph with 45 mph gusts. We live on the north side of the island, so have battened down E, N and S with shutters. W ready to set up so we're not in total darkness. Sea state is angry with white tops at about 6 feet. Rain bands giving short showers, none of them very heavy. No thunder and lightning as of yet, since last night.

Will give you more of an update in a couple of hours.

cheers
MM

And from 4pm:

OK. Wind speeds have increased, now from ENE. I'd say 40 mph and gusts now up over 50 mph. This started about an hour ago, and it's been raining solidly for the same period of time. Sea state (from what we can see) has gotten rougher, maybe 8 feet crests. We may be losing power. Just had a couple a brief power outages...probably tree branches on wires. We're watching the eye and movement on the satellite image and it looks like it's going to come pretty close. T& L just started a few minutes ago, probably have to unplug soon, anyway.

more later, I hope, Cayman Brac Power & Light willing.....

MM


Jeff Masters

Updated: 1:17 AM GMT on August 30, 2008

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Gustav clears Jamaica, grows significantly in size

By: JeffMasters, 3:26 PM GMT on August 29, 2008

Tropical Storm Gustav has completed its traverse of Jamaica and the center has now exited the western end of the island. At 7:06 am EDT, the Hurricane Hunters found the center of Gustav on the western tip of Jamaica. The storm weathered the passage over the island with relative ease, with the pressure only rising 5 mb, from 983 to 988 mb. Gustav's top winds remain below hurricane force, near 65 mph. Top winds measured in Jamaica during Gustav's passage were in the capital of Kingston, where sustained winds of 50 mph were measured last night. Preliminary news reports indicate that Jamaica suffered no major damage or injuries. However, the death toll has risen to 59 on Hispaniola, where flooding from Gustav killed 51 people in Haiti and 8 in the Dominican Republic.


Figure 1. Current satellite image of Gustav.

Visible satellite loops show that Gustav has significantly expanded in size in the past few hours, and now presents a rather intimidating appearance. Gustav is going to be a large and dangerous hurricane by Saturday. Upper-level outflow is established in all quadrants, low-level spiral bands are multiplying and intensifying, and the amount and intensity of Gustav's heavy thunderstorms are steadily increasing. There is no eye yet, but that should appear by this evening. Radar from Pilon, Cuba shows the developing spiral bands of Gustav quite well. Dry air is not evident anywhere close to Gustav.

The track forecast for Gustav
The latest 0Z/6Z (8pm/2 am EDT) model runs are more divergent than yesterday's runs, and the uncertainty of the long-term landfall location on the U.S. Gulf Coast remains high. In the short term, we're confident that Gustav will track over the Cayman Islands today, then cross the western tip of Cuba Saturday. Passage over the relatively flat terrain of western Cuba will probably decrease Gustav's winds by 20 mph, which it will regain within 12 or so hours.

The trough of low pressure moving across the Midwest U.S. expected to pull Gustav northwest towards the Gulf Coast is expected to weaken and be replaced by a ridge of high pressire on Monday. This means that Gustav may stall just before or just after landfall near Louisiana, and be forced westwards. The models are split: the GFS, HWRF, and GFDL models all carry Gustav into Louisaina or Mississippi on Monday, with a possible turn to the west after landfall. The ECMWF, NOGAPS, and UKMET all foresee a turn to the west before Gustav reaches the coast. The ECMWF and NOGAPS models predict an eventual landfall near Corpus Christi, Texas, while the UKMET bends Gustav southwestwards towards Mexico. NHC usually makes their forecast a blend of the GFS, GFDL, NOGAPS, and UKMET model forecasts. A forecast of that blend would show a track to the Texas/Louisiana border region. However, the current (11 am EDT) forecast cycle throws this cautious approach out the window, and gambles that the GFS/HWRF/GFDL model solution is correct. These models are currently forecasting a stronger hurricane, which would be less likely to be forced westwards. Residents of Texas should be prepared in case the alternative solution materializes, and Gustav takes a sharp turn to the west on Monday.

The intensity forecast for Gustav
Gustav has low wind shear (<10 knots), the highest heat content waters of the Atlantic, and no land masses or dry air to interfere with it. I expect it will intensify into a Category 3 or 4 hurricane before hitting Cuba.

Yesterday, a NOAA Hurricane Hunter research aircraft dropped a network of 60 specialized buoys (Air eXpendable BathyThermographs, or AXBTs) in the Gulf of Mexico to provide precise measurements of ocean temperatures in order to aid intensification forecasts for Gustav. This data are now feeding directly into the GFDL and HWRF computer models, but not into the other global models (GFS, NOGAPS, UKMET, and ECMWF). In theory, the intensity forecasts from the GFDL and HWRF models should be better, for the portion of Gustav's track over the Gulf. Both of these models are currently calling for Gustav to be a Category 3 hurricane at landfall. The HWRF is saying this landfall will be near New Orleans Monday night, and the GFDL predicts a Mississippi landfall Monday morning.

Gustav's impact on Cuba and Mexico's Cancun/Cozumel region
Cancun and Cozumel are no longer in the cone of uncertainty, so a direct hit from Gustav is unlikely. Remember, though, that the cone is only right about 2/3 of the time--historically, over the past five years, about 1/3 of storm positions have fallen outside the cone. For example, the NHC forecasts issued at 5 pm, 8pm and 11pm Wednesday all put the cone of uncertainty along the northern portion of Jamaica, and the eventual track of Gustav directly over the island fell outside the cone of uncertainty. With that caveat in mind, those of you planning to visit Cancun/Cozumel will probably only have one day of heavy rain (Saturday), with some wind gusts of 40-50 mph. The Yucatan will be on the weak (left) side of Gustav, where tropical storm force winds do not extend out as far. The odds of Cozumel getting sustained winds of tropical storm force (39 mph or greater) have decreased to 23%, as indicated in NHC's wind probability product. The odds of hurricane force winds are 3%.

The portion of the world most likely to suffer a major hurricane strike from Gustav will be western Cuba. If Gustav makes landfall as a major hurricane somewhere along this stretch of coast, it may bring a storm surge of 10-15 feet to the right of where the eye come ashore.

I'll have much more to say about Gustav this afternoon, including an analysis of its likely size at landfall, and discussion about when it would be good to evacuate New Orleans.

Links to follow
Wundermap for the Cayman Islands
Pilon, Cuba radar
Cayman Brac, Cayman Islands
Grand Cayman

Tropical Storm Hanna
Tropical Storm Hanna is beginning to get its act together. The upper level low to Hanna's west has weakened some and moved away, allowing Hanna to position its center of circulation underneath its heaviest thunderstorm activity. Visible satellite loops show that the amount and intensity of heavy thunderstorm activity has increased considerably, and there are now two prominent upper-level outflow channels visible, on the north and east sides. Strong upper-level winds from the upper-level low to Hanna's west are still interfering with the thunderstorm development and upper-level outflow on Hanna's west side. Radar out of Martinique shows a large area of heavy thunderstorms that are poorly organized (no spiral bands). This morning's QuikSCAT pass saw winds of 40-55 mph at the surface. The first Hurricane Hunter mission for Hanna is scheduled for Sunday.

The forecast for Hanna
Steering currents imparted by the counterclockwise flow around the upper-level low to its west will keep Hanna moving northwest, to a point midway between Bermuda and the Bahama Islands. About 3-4 days from now, a strong blocking ridge of high pressure is forecast by most of the models to build over Hanna, forcing it to the southwest towards the Bahamas. This is an unusual motion for a hurricane, and it would be surprise to see Hanna move as far south as some of the models are predicting--all the way into Cuba. However, the Bahamas are at high risk from this storm 4-5 days from now. Hanna may be weakening at that time, as wind shear from an upper-level trough to the north of the storm is expected to bring 15-25 knots of shear to the storm. In the longer term, both the ECMWF and GFS models are predicting Hanna will pass through South Florida or the Florida Keys into the Gulf of Mexico by Thursday or Friday next week.

A major complicating factor in forecasting both Hanna's track and intensity may be the possible development of a tropical disturbance behind it, near 18N 41W (see discussion below, under "Elsewhere in the tropics". This disturbance is forecast to develop into a tropical storm 3-5 days from now by some of the models. If so, the new storm could substantially alter the path and strength of Hanna. Don't believe that Hanna will be going through South Florida quite yet; the models do very poorly with hurricane-hurricane interactions, and the long term fate of Hanna is still highly uncertain.

Elsewhere in the tropics
A large circulation is located near 18N, 41W, in the mid-Atlantic Ocean. This morning's QuikSCAT pass show an elongated, poorly formed circulation, with top winds of 25 mph. Heavy thunderstorm activity has increased since yesterday, but the disturbance is battling marginal wind shear of 15-20 knots and dry air on its south side. Wind shear is expected to stay marginal for development over the next few days, limiting this system to only slow development. NHC is giving this system a low (<20% chance) of developing into a tropical depression by Sunday. Some slow development is possible over the next few days, but the system has a better chance 3-5 days from now, when its environment will be moister. Several of the models develop it, and predict the system will be 500 miles or so north of the northern Lesser Antilles Islands by Monday or Tuesday.

Cape Verdes Islands disturbance, 97L
A low pressure system (97L) with a large circulation and plenty of spin emerged from the coast of Africa last night, and is now located near 13N 19W, a few hundred miles southeast of the Cape Verde Islands. This morning's QuikSCAT pass revealed a closed surface circulation and top winds of 25 mph. This system is already developing some concentrated heavy thunderstorms over the waters south of the Cape Verde Islands. Wind shear is a moderate 10-15 knots over the storm, and is expected to remain in the low to moderate range the next few days. NHC has given this system a medium (20%-50%) chance of developing into a tropical depression by Sunday. The models have been very aggressive developing this system over the past few days. It is too early to speculate whether this storm might end up recurving out to sea or not.

There are two other impressive African tropical waves lined up behind 97L that are also likely to be a threat to develop once they move offshore Africa next week. The long-range GFS model develops all three of these waves.

Disturbance 96L in the Bay of Campeche
Some good news: the blob in the southern Gulf of Mexico, in Mexico's Bay of Campeche, dissipated last night.

My next blog will be by 5pm this afternoon.

Jeff Masters

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Gustav pounds Jamaica

By: JeffMasters, 8:31 PM GMT on August 28, 2008

Jamaica is feeling the full force of Tropical Storm Gustav, as torrential rains and near-hurricane force winds pound the island. At 1:55 pm EDT, the Hurricane Hunters found the eye of Gustav on the eastern tip of Jamaica. Gustav's central pressure had dropped another 2 mb, to 983 mb, and the surface winds were at 70 mph, just below hurricane strength. Kingston, Jamaica recently reported sustained winds of 35 mph, and the pressure falling, at 995 mb.

It's a surprise the Gustav is here at all, because the atmosphere pulled a major trick out of The Joker's book last night. The ridge of high pressure that was forcing Gustav to the west shifted positions to be oriented southwest-to-northeast. This pushed Gustav to the southwest, and pumped in some dry air into the northwest side of Gustav. As a result of this dry air, and the weakening of the circulation due to interaction with Haiti's mountains, Gustav was forced to form a new center under heavy thunderstorms on its south side, away from Haiti and the dry air. As a result, Gustav is now pounding Jamaica. The ridge that forced Gustav southwest has now re-oriented itself, and is pushing Gustav due west again. Rainfall will be the main threat from Gustav in Jamaica, as it was when the storm hit Hispaniola. Gustav's torrential rains accumulated to over 12 inches in southern Hispaniola, triggering floods and landslides that killed 22 people. It appears now that eastern Cuba will not get much rain from Gustav.


Figure 1. Tropical Storm Gustav at landfall in Jamaica.

Visible satellite loops show that Gustav has already suffered from its impact on Jamaica. A large Central Dense Overcast (CDO) of high cirrus clouds has become asymmetric, and the eye that appeared briefly before landfall has disappeared. Radar from Gran Piedra, Cuba still shows several well-formed spiral bands on the east and south sides of the storm. However, the northwest side of the storm shows evidence that dry air is interfering with Gustav's organization.

The track forecast for Gustav
The latest 12Z (8 am EDT) model runs continue to show a significant shift westward in Gustav's track, thanks to the southwestward motion and center re-formation of the storm this morning. Gustav may now pass through the narrow Yucatan Channel, and not be significantly weakened by Cuba.

By Saturday, a trough of low pressure moving across the Midwest U.S. should weaken the ridge, and allow Gustav to turn north near the tip of western Cuba into the Gulf of Mexico. The final landfall location of Gustav depends on the strength and speed of the trough. This trough may not be strong enough to pull Gustav to a landfall in the central Gulf Coast. Instead, a blocking ridge of high pressure may build in over the southern U.S. by Monday, forcing Gustav to move slowly westward towards Texas. This continues to be the solution of the 12Z run of the GFS model, but it is now the only model predicting this. The best guess now is that the ridge will slow Gustav down, but allow it to make landfall Tuesday. The GFS is the only model calling for a Texas landfall; the other main models (UKMET, NOGAPS, ECMWF, GFDL, and HWRF) all foresee a landfall between Alabama and western Louisiana.

The intensity forecast for Gustav
Gustav has two factors inhibiting its development in the short term: land interaction with Jamaica, and dry air. Jamaica has some 7,000 foot high mountains on it, and may be able to keep Gustav from strengthening today, as the center passes near or over Jamaica. As we saw yesterday, Gustav is a very small storm, and is prone to disruption by mountains. A large region of dry air to the north is also hurting Gustav, and this dry air may continue to be a problem for the storm over the next 2-3 days. Wind shear is not a problem for Gustav--it is currently under low wind shear (5-10 knots). This shear is expected to remain in the low to moderate range (0-15 knots) for the remainder of the week. By Friday, as Gustav approaches the Cayman Islands, the storm will be underneath an upper level anticyclone, and over the highest heat content waters of the Atlantic. The intensity of Gustav as it passes through the Caymans depend critically on how much of a favor Jamaica does today by keeping Gustav from intensifying. My best guess continues to be that the Caymans will get hit by an intensifying Category 1 hurricane on Friday.

If Gustav crosses the western tip of Cuba, it will lose intensity-- by perhaps 25 mph or so--but should easily regain that lost strength within 12-24 hours. Gustav will likely be a major Category 3 or 4 hurricane in the Gulf of Mexico. However, Gustav may not be able to maintain that strength all the way to landfall, if it slows down close to landfall, as expected. The GFDL is currently predicting a landfall Monday morning near the Alabama/Mississippi border as a Category 2 hurricane; the HWRF is predicting a Category 3 storm making landfall in central Louisiana, on Tuesday morning. The GFDL continues to be a day or so faster than the other models in bringing Gustav to landfall, resulting in less time for intensification. Since it is the outlier, a landfall Tuesday seems more likely.

A NOAA Hurricane Hunter research aircraft is currently dropping a network of 60 specialized buoys (Air eXpendable BathyThermographs, or AXBTs) in the Gulf of Mexico to provide precise measurements of ocean temperatures in order to aid intensification forecasts for Gustav. This data will feed directly into the GFDL and HWRF computer models, but not into the other global models (GFS, NOGAPS, UKMET, and ECMWF).

I just got an email from one of my old Hurricane Hunter colleagues, Terry Lynch, who is currently on the flight deploying the AXBTs. He says:

We're currently just a tad over 1/2 way through the pattern...AXBT #33 just went out. About 1/2 an hour into the flight the chief scientist started second guessing and wished that he had us load up 80 BT's. And I thought 60 was a lot!

How big will Gustav get?
Gustav is currently a very small storm, similar in size to Hurricane Andrew of 1992. Tropical storm force winds extend out only 50 miles from the center. NHC is forecasting that Gustav will remain small over the next three days, with tropical storm-force winds expanding out only to a 75 miles radius. For comparison, Hurricane Katrina's tropical storm-force winds extended out about 200 miles from the center. Can Gustav become as large as Katrina? Well, we don't understand the processes that govern hurricane size very well. Katrina started out much larger than Gustav, so I doubt Gustav will grow that large. It is a fact that as storms move further from the Equator, they grow in size. This is because the Coriolis force increases as you move away from the Equator. An increased Coriolis force provides more spin to the storm, and the hurricane responds by growing in size. Thus, expect Gustav to grow in size as it approaches landfall along the Gulf coast. However, I'm guessing that the wind field will be half the size of Katrina's.


Gustav's impact on Cuba and Mexico's Cancun/Cozumel region
If you're in the cone of uncertainty, you're at risk. Hurricane forecasts are uncertain, and this uncertainty is graphically represented by the cone of uncertainty around the central "best guess" forecast positions. Cancun and Cozumel are both in the cone of uncertainty, so could get a direct hit from Gustav. At present, though, it appears that Mexico's Yucatan will only have one day of heavy rain (Saturday) with some winds gusts of 40-50 mph. The odds of Cozumel getting sustained winds of tropical storm force (39 mph or greater) are not much higher, though, and have increased to 29%, as indicated in NHC's wind probability product. The odds of hurricane force winds are 6%.

The portion of the world most likely to suffer a major hurricane strike from Gustav will be western Cuba. If Gustav makes landfall as a major hurricane somewhere along this stretch of coast, it may bring a storm surge of 10-15 feet to the right of where the eye come ashore.

Gustav's potential impact on the oil and gas industry
The price of U.S. crude oil has jumped about 2%, and the price of U.S. natural gas has increased 11% in the past two days, in anticipation that Gustav might rip through the oil and gas production areas of the Gulf of Mexico. About 25% of U.S. crude oil and 15% of its natural gas are produced in the Gulf of Mexico. As seen in Figure 2, the oil production areas are concentrated along the Louisiana and Texas coast. If Gustav makes a landfall on the right side of its cone of uncertainty, in Alabama or the Florida Panhandle, the oil and gas infrastructure might not be significantly affected. However, most of the cone of uncertainty lies in the major oil and gas producing areas, and I give an increased 70% chance that Gustav will significantly hurt oil and gas production in the Gulf.


Figure 2. Location of major oil platforms in the Gulf of Mexico. The data was taken from the Department of Interior Mineral Management Services

Links to follow
Wundermap for Eastern Cuba/Jamaica
Gran Piedra, Cuba radar

Tropical Storm Hanna
Tropical Storm Hanna continues to suffer from wind shear, and remains a minimal tropical storm. Visible satellite loops show a well-defined surface circulation that is exposed to view, thank to strong upper-level winds from the west that are blowing all of Hanna's heavy thunderstorm activity to the east side of the storm. These upper-level winds are creating a moderate 10-20 knots of wind shear over Hanna. This morning's QuikSCAT pass found top winds of 45 mph.

The forecast for Hanna
Since the center of circulation is fairly far removed from Hanna's heaviest thunderstorms, Hanna's center could reform to the southeast later today. This re-formation would alter the storm's eventual track, and make it less likely to recurve out to sea. Steering currents imparted by the counterclockwise flow around the upper-level low to its west will keep Hanna moving northwest, to a point midway between Bermuda and the Bahama Islands. About the four days from now, a strong blocking ridge of high pressure is forecast by most of the models to build over Hanna, forcing it to the southwest towards the Bahamas. The exception is the GFS model, which takes Hanna northeast out to sea. I'm discounting this solution at present, since it is the outlier. About five day from now, wind shear is expected to increase due to an upper level trough to the north. Upper level outflow from Gustav may also create some shear. The models respond by weakening Hanna to a tropical storm. In the very long range, the ECMWF model predicts Hanna will hit Cuba on Tuesday, then pass through the Florida Keys next Thursday. The NOGAPS model brings Hanna just off the South Carolina coast, and the other models stall it out northeast of the Bahamas. At this time, Hanna's intentions are a mystery. No Hurricane Hunter missions are scheduled into Hanna yet.


Figure 3. Visible satellite image from 7:30 am EDT Thursday August 28, 2008. A long line of impressive tropical waves is lined up over the Atlantic and Africa. Image credit: U.S. Navy.

Elsewhere in the tropics
Most of the computer models forecast the development of at least one additional tropical wave between Africa and the Lesser Antilles Islands during the coming week. The first candidate is a large circulation located near 18N, 39W, 800 hundred miles west of the Cape Verde Islands. While the circulation of this system is impressive, the system is devoid of any heavy thunderstorm activity, and is surrounded by a large are of dry air to the west and north. Wind shear is a moderate to marginal 10-20 knots in the region, and forecast to remain in the moderate to marginal range for the next few days. NHC is giving this system a low (<20% chance) of developing into a tropical depression by Saturday. Some slow development is possible over the next few days, but the system has a better chance 3-5 days from now, when its environment will be moister. Several of the models develop it, and predict the system will be 500 miles or so north of the northern Lesser Antilles Islands by Monday Tuesday. This system would probably only be a threat to Bermuda if it develops.

Like the ocean, the atmosphere also has waves. Large-scale atmospheric waves form over Africa during the African summer monsoon season, track east to west, and emerge over the Atlantic Ocean near the Cape Verde Islands, where they often serve as the nucleus for a powerful Cape Verdes-type hurricane. Well, the African Easterly Wave Factory is exceptionally busy right now, and there are three very impressive looking waves with plenty of spin lined up across the continent (Figure 3).
The western-most wave, just coming off the coast of Africa today, is particularly impressive. This system has a very large circulation with plenty of spin, and is already developing some concentrated heavy thunderstorms over the waters south of the Cape Verde Islands. This morning's QuikSCAT pass saw winds of 50 mph near the heaviest thunderstorms. Wind shear is a moderate 10-20 knots over the storm, and is expected to remain in the low to moderate range the next few days. NHC has given this system a medium (20%-50%) chance of developing into a tropical depression by Saturday. The models have been very aggressive developing this system over the past few days, and chances are good that this system will become a large and powerful Cape Verdes-type hurricane next week. It is too early to speculate whether this storm might end up recurving out to sea or not.

The other two waves lined up behind the wave moving off the coast are also likely to be a threat to develop once they move offshore Africa next week. The long-range GFS model develops all three of these waves.

Disturbance 96L in the Bay of Campeche
Finally, there's a new blob (96L) in the southern Gulf of Mexico, in Mexico's Bay of Campeche. An ASCAT pass at 12:15 pm EDT captured the right side of 96L, and revealed that the storm probably has a surface circulation. A small area of heavy thunderstorm activity has developed on the north side of 96L, as seen on visible satellite loops. NHC has given 96L a medium (20%-50% chance) of developing into a tropical depression by Saturday. By Saturday, 96L will be very close to the coast of Mexico, several hundred miles south of the Texas border. There is a 20% chance that this storm will grow large enough to influence the steering currents and wind shear that might impact Gustav when it enters the Gulf of Mexico.

On Cayman Brac, in Gustav's path:
I got this report last night from wunderground user mangroveman, who lives on Cayman Brac: The general feeling here is to be prepared and wait and see. Pu blic Works has gotten most of the shutters in place on schools and other public buildings. They will finish up in the morning. Hurricane shelters are ready incl uding the new Humane Society shelter for pets. The shelters are almost all on th e Bluff, the limestone spine of the Brac which rises to 140 feet on the east end of the island and gives the Brac its name, and so are well out of the way of an y storm surge.

Since Hurricane Ivan virtually destroyed Grand Cayman in 2004, many folks from t here (as well as Brackers) have built themselves hurricane cottages on the Bluff , also well out of the way of any storm surge. I would say that about half the h omes here have their shutters up, and the rest should all be up -- or ready to g o depending on the storm's strength -- by noon tomorrow.

Cayman Airways has added several jet flights between the islands--we are usual ly served by Twin Otters from Grand Cayman--for people who wish to get out of here--or there!

Business at the grocery and hardware stores has been steady, with the usual stocking up of supples. A lot of people went through this exercise not so long ago with Fay, and so most folks already have plenty of supplies.

The biggest concern here about the nature of the storm is that it has slowed down to a crawl, and, experience tells us that the slower it moves, the stronger it can become. However, general feeling is that Gustav is disorganized and will ne ed to really pick up strength quickly if it is going to be a major hurricane that would threaten the Brac. One of the historic memory points on the island is 1932 when the Brac was whacked by a mega-storm which not only destroyed the island and killed 110 people, but also caused many families to leave. The '32 Storm has become a living legend, and most any hurricane that hits the Brac is compared to that storm. Most people feel Gustav will not measure up against the '32 monster. Which is a good thing, of course!


Next blog
OK, now I feel like the guy who paints the bridge and gets done, only to realize that coat of paint he put on at the beginning already needs to be re-done! Anyway, it's time to post this, and start painting the bridge again Friday morning--

Jeff Masters

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Gustav and Hanna--and many more to come

By: JeffMasters, 5:21 PM GMT on August 28, 2008

It's time to get familiar with the names Hanna, Josephine, Ike, and Kyle, because the tropical Atlantic is about to put on a rare burst of very high activity in the coming weeks.

The atmosphere pulled a major surprise last night and this morning, substantially altering the short and long-term fate of Tropical Storm Gustav. The ridge of high pressure that was forcing Gustav to the west shifted positions, and is now oriented southwest-to-northeast. This has pushed Gustav to the southwest, and pumped in some dry air into the northwest side of Gustav. As a result of this dry air, and the weakening of the circulation due to interaction with Haiti's mountains, Gustav was forced to form a new center under heavy thunderstorms on the its south side, away from the dry air and Haiti. As a result, the center of Gustav is now passing very close to Jamaica, and Gustav will pound that island today with winds near hurricane force. Rainfall will continue to be the main threat from Gustav today, as it was when it hit Hispaniola. Gustav's torrential rains accumulated to over 12 inches in southern Hispaniola, triggering floods and landslides that killed 22 people. The rain are mostly over in Hispaniola, but are just getting cranked up over Jamaica. It appears now that eastern Cuba will not get much rain from Gustav.


Figure 1. Current satellite image of Gustav.

Visible satellite loops and radar from Gran Piedra, Cuba show that Gustav has become better organized. A large Central Dense Overcast (CDO) of high cirrus clouds has covered the storm, and an eye has formed, right on the east coast of Jamaica. Several well-formed spiral bands have developed on the east and south sides of the storm, but the northwest side of the storm shows evidence that dry air is interfering with Gustav's organization. The latest 7:33 am EDT center report from the Hurricane Hunters found a significant drop in pressure, to 985 mb. This will likely mean that Gustav's winds will increase to hurricane force later this afternoon. The highest surface winds measured by this morning's mission were 70 knots (80 mph), with one spot of higher winds. However, winds at the aircraft's flight level of 10,000 feet were quite a bit lower.

The track forecast for Gustav
The latest 6Z (2 am EDT) model runs show a significant shift westward in Gustav's track, thanks to the southwestward motion and center re-formation of the storm. Gustav may now pass through the narrow Yucatan Channel, and not be significantly weakened by Cuba.

By Saturday, a trough of low pressure moving across the Midwest U.S. should weaken the ridge, and allow Gustav to turn north across western Cuba into the Gulf of Mexico. The final landfall location of Gustav depends on the strength and speed of the trough. This trough may not be strong enough to pull Gustav to a landfall in the central Gulf Coast. Instead, a blocking ridge of high pressure may build in over the southern U.S. by Monday, forcing Gustav to move slowly westward towards Texas. The ridge will at least slow Gustav down, and the long term fate of Gustav remains highly uncertain.

The intensity forecast for Gustav
Gustav has two factors inhibiting its development in the short term: land interaction with Jamaica, and dry air. Jamaica has some 7,000 foot high mountains on it, and may be able to keep Gustav from strengthening today, as the center passes near or over Jamaica. As we saw yesterday, Gustav is a very small storm, and is prone to disruption by mountains. A large region of dry air to the north is also hurting Gustav, and this dry air may continue to be a problem for the storm over the next 2-3 days. Wind shear is not a problem for Gustav--it is currently under low wind shear (5-10 knots). This shear is expected to remain in the low to moderate range (0-15 knots) for the remainder of the week. By Friday, as Gustav approaches the Cayman Islands, the storm will be underneath an upper level anticyclone, and over the highest heat content waters of the Atlantic. The intensity of Gustav as it passes through the Caymans depend critically on how much of a favor Jamaica does today by keeping Gustav from intensifying. My best guess is that the Caymans will get hit by an intensifying Category 1 hurricane on Friday.

If Gustav crosses the western tip of Cuba, it will lose intensity-- by perhaps 25 mph or so--but should easily regain that lost strength within 12-24 hours. Gustav will likely be a major Category 3 or 4 hurricane in the Gulf of Mexico. However, Gustav not be able to maintain that strength all the way to landfall, if it slows down close to landfall, as some of the models are predicting. The GFDL is currently predicting a landfall in Alabama as a Category 2 hurricane; the HWRF is predicting a Category 3 storm making landfall in central Louisiana.

A NOAA Hurricane Hunter research aircraft is scheduled to drop a network of 60 specialized buoys (Air eXpendable BathyThermographs, or AXBTs) in the Gulf of Mexico today to provide precise measurements of ocean temperatures in order to aid intensification forecasts for Gustav. This data will feed directly into the GFDL and HWRF computer models, but not into the other global models (GFS, NOGAPS, UKMET, and ECMWF).

Gustav's impact on Cuba and Mexico's Cancun/Cozumel region
If you're in the cone of uncertainty, you're at risk. Hurricane forecasts are uncertain, and this uncertainty is graphically represented by the cone of uncertainty around the central "best guess" forecast positions. Cancun and Cozumel are both in the cone of uncertainty, so could get a direct hit from Gustav. At present, though, it appears that Mexico's Yucatan will only have one day of heavy rain (Saturday) with some winds gusts of 40-50 mph. The odds of Cozumel getting sustained winds of tropical storm force (39 mph or greater) are not much higher, though, and have increased to 29%, as indicated in NHC's wind probability product. The odds of hurricane force winds are 6%.

The portion of the world most likely to suffer a major hurricane strike from Gustav will be western Cuba. If Gustav makes landfall as a major hurricane somewhere along this stretch of coast, it may bring a storm surge of 10-15 feet to the right of where the eye come ashore.

Gustav's potential impact on the oil and gas industry
The price of U.S. crude oil has jumped about 2%, and the price of U.S. natural gas has increased 11% in the past two days, in anticipation that Gustav might rip through the oil and gas production areas of the Gulf of Mexico. About 25% of U.S. crude oil and 15% of its natural gas are produced in the Gulf of Mexico. As seen in Figure 2, the oil production areas are concentrated along the Louisiana and Texas coast. If Gustav makes a landfall on the right side of its cone of uncertainty, in Alabama or the Florida Panhandle, the oil and gas infrastructure might not be significantly affected. However, most of the cone of uncertainty lies in the major oil and gas producing areas, and I give an increased 70% chance that Gustav will significantly hurt oil and gas production in the Gulf.


Figure 2. Location of major oil platforms in the Gulf of Mexico. The data was taken from the Department of Interior Mineral Management Services

Links to follow
Wundermap for Eastern Cuba/Jamaica
Gradn Piedra, Cuba radar

Tropical Storm Hanna
Tropical Storm Hanna formed this morning near 19N 57W, a few hundred miles east-northeast of Puerto Rico. This storm may intensify into a hurricane and become a threat next week to the Bahamas, Cuba, and Florida. This morning's QuikSCAT pass found top winds of 45 mph. Visible satellite loops show a well-defined surface circulation that is exposed to view, thank to strong upper-level winds from the west that are blowing all of Hanna's heavy thunderstorm activity to the east side of the storm. These upper-level winds are creating a moderate 10-20 knots of wind shear over Hanna.

The forecast for Hanna
The upper-level low to the west of Hanna that is creating the shear is expected to most westward over the next few days, relaxing the shear over Hanna. Wind shear is expected to fall below 10 knots later today and remain below 10 knots for the next four days. This should allow Hanna to strengthen into a hurricane three to four days from now, as predicted by the HWRF, GFDL, and SHIPS intensity models. Steering currents imparted by the counterclockwise flow around the upper-level low to its west will keep Hanna moving northwest, to a point midway between Bermuda and the Bahama Islands. About the four days from now, a strong blocking ridge of high pressure is forecast by most of the models to build over Hanna, forcing it to the southwest towards the Bahamas. The exception is the GFS model, which takes Hanna northeast out to sea. I'm discounting this solution at present, since it is the outlier. About five day from now, wind shear is expected to increase, and the models respond by weakening the system to a tropical storm. In the very long range, the ECMWF model predicts Hanna will hit Cuba, then move northwest again over Florida by September 6. We don't have any other models that extend out long enough in time to show what the eventual long-term fate of this storm. At this time, it appears that the Bahamas are the main region at risk from Hanna. No Hurricane Hunter missions are scheduled into Hanna yet.


Figure 3. Visible satellite image from 7:30 am EDT Thursday August 28, 2008. A long line of impressive tropical waves is lined up over the Atlantic and Africa. Image credit: U.S. Navy.

Elsewhere in the tropics
Most of the computer models forecast the development of at least one additional tropical wave between Africa and the Lesser Antilles Islands during the coming week. The first candidate is a large circulation located near 18N, 39W, 800 hundred miles west of the Cape Verde Islands. While the circulation of this system is impressive, the system is devoid of any heavy thunderstorm activity, and is surrounded by a large are of dry air to the west and north. Wind shear is a moderate to marginal 10-20 knots in the region, and forecast to remain in the moderate to marginal range for the next few days. NHC is giving this system a low (<20% chance) of developing into a tropical depression by Saturday. Some slow development is possible over the next few days, but the system has a better chance 3-5 days from now, when its environment will be moister. Several of the models develop it, and predict the system will be a few hundred miles north of the northern Lesser Antilles Islands by Monday Tuesday.

Like the ocean, the atmosphere also has waves. Large-scale atmospheric waves form over Africa during the African summer monsoon season, track east to west, and emerge over the Atlantic Ocean near the Cape Verde Islands, where they often serve as the nucleus for a powerful Cape Verdes-type hurricane. Well, the African Easterly Wave Factory is exceptionally busy right now, and there are three very impressive looking waves with plenty of spin lined up across the continent (Figure 3).
The western-most wave, just coming off the coast of Africa today, is particularly impressive. This system has a very large circulation with plenty of spin, and is already developing some concentrated heavy thunderstorms over the waters south of the Cape Verde Islands. This morning's QuikSCAT pass saw winds of 50 mph near the heaviest thunderstorms. Wind shear is a moderate 10-20 knots over the storm, and is expected to remain in the low to moderate range the next few days. NHC has given this system a medium (20%-50%) chance of developing into a tropical depression by Saturday. The models have been very aggressive developing this system over the past few days, and chances are good that this system will become a large and powerful Cape Verdes-type hurricane next week. It is too early to speculate whether this storm might end up recurving out to sea or not.

The other two waves lined up behind the wave moving off the coast are also likely to be a threat to develop once they move offshore Africa next week. The long-range GFS model develops all three of these waves.

Finally, there's a new blob (96L) in the southern Gulf of Mexico, in Mexico's Bay of Campeche. I haven't had time to look at this one yet, and this blog is way overdue, so I'll report on 96L in my 4 pm blog.

Jeff Masters

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Gustav pulls away from Hispaniola after killing at least 22

By: JeffMasters, 8:46 PM GMT on August 27, 2008

Tropical Storm Gustav continues to pound Haiti with torrential rains and tropical storm-force winds, as the storm slowly moves westward towards Jamaica and Cuba. Flooding form Gustav has killed at least 22 people--14 on Haiti, and 8 in the Dominican Republic. Satellite rainfall estimates show that many portions of the southern peninsula of Haiti have received 8 or more inches of rain, and an additional 2-4 inches are likely over the worst-hit regions before Gustav finally moves away. The encounter with Haiti's high mountains has weakened Gustav. The latest center fix from the Hurricane Hunters at 3:34 pm EDT put Gustav's central pressure at 999 mb, and the aircraft was not able to find surface winds above 55 mph.


Figure 1. Current satellite image of Gustav.

Visible satellite loops and radar from Gran Piedra, Cuba show that Gustav starting to become better organized, with an increase in heavy thunderstorm activity and better spiral banding. The pressure should start to fall tonight, and the winds should increase to hurricane strength by Thursday night.

The track forecast for Gustav
The latest 12Z (8 am EDT) model runs show not much change to the short term track of Gustav, but there is a major long-term forecast change to report. In the short term, the models agree on a track through the narrow channel between Cuba and Jamaica. However, the HWRF model takes Gustav over Jamaica, and residents there should anticipate the possibility of a Category 1 hurricane strike on Thursday. However, heavy rain is probably the primary threat to Jamaica and eastern Cuba. The storm is small enough that high winds will be confined to a small region around the center, and a near miss will not cause much wind damage.

A ridge of high pressure is expected to force Gustav west through Friday. By Saturday, a trough of low pressure moving across the Midwest U.S. should weaken the ridge, and allow Gustav to turn north across western Cuba into the Gulf of Mexico. The final landfall location of Gustav depends on the strength and speed of the trough. In this morning's runs, we now have some indication that this trough of low pressure may not be strong enough to pull Gustav to the coast by Monday. In fact, the NOGAPS model predicts Gustav will stall offshore the Alabama coast on Monday, before finally edging ashore three days later. The UKMET model is also much slower with its latest run, and slows Gustav down as it approaches the Louisiana/Texas coast on Monday. The latest HWRF model is also slower than the last run, and doesn't bring Gustav to the coast by the end of its forecast period (Monday). The HWRF foresees a Category 3 or 4 hurricane a few hundred miles south of the Louisiana coast on Monday. The latest ECMWF run is not much slower than the previous run, but does stall Gustav out over central Louisiana once it makes landfall near New Orleans Monday night. The latest GFDL model, though, is not much slower, and predicts landfall in Mississippi early Monday morning as a Category 3 hurricane. In summary, Gustav may slow down considerably just before landfall in the U.S., making its long-term track and landfall location very uncertain at this time.

The intensity forecast for Gustav
Now that Gustav is clearing the southwest Peninsula of Haiti, it should begin to strengthen. Gustav is currently under low wind shear (5-10 knots). This shear is expected to remain in the low to moderate range (0-15 knots) for the remainder of the week. By Friday, as Gustav approaches the Cayman Islands, the storm will be underneath an upper level anticyclone, and over the highest heat content waters of the Atlantic. The GFDL model responds by intensifying Gustav to a Category 3 hurricane as it passes through the Caymans. I think it is plausible that Gustav could intensify further, to Category 4 strength, before hitting the Caymans, though the most likely intensity is Category 2. Keep in mind that our skill forecasting intensity changes is poor. If you have plans to be on the northern Cayman Islands--Cayman Brac or Little Cayman Island--on Friday, be prepared to be stuck there for several days, as Gustav may heavily damage these islands. Grand Cayman Island is also at risk--the HWRF model predicts Gustav will pass over Grand Cayman on Friday afternoon.

Gustav will lose intensity when it crosses the western tip of Cuba--perhaps by 25 mph or so--but should easily regain that lost strength within 12-24 hours. Gustav will likely be a major Category 3 or 4 hurricane in the Gulf of Mexico. However, Gustav not be able to maintain that strength all the way to landfall, if it slows down close to landfall, as some of the models are predicting.

Gustav's impact on Cuba and Mexico's Cancun/Cozumel region
If you're in the cone of uncertainty, you're at risk. Hurricane forecasts are uncertain, and this uncertainty is graphically represented by the cone of uncertainty around the central "best guess" forecast positions. Cancun and Cozumel are both at the edge of the cone of uncertainty, so could get a direct hit from Gustav. At present, though, it appears that Mexico's Yucatan will only have one day of heavy rain (Saturday) with some winds gusts of 40 mph. The odds of Cozumel getting sustained winds of tropical storm force (39 mph or greater) are 20%, as indicated in NHC's wind probability product. The odds of hurricane force winds are 9%.

The portion of the world most likely to suffer a major hurricane strike from Gustav will be western Cuba. Gustav is likely to make landfall as a major hurricane somewhere along this stretch of coast, bringing a storm surge of 10-15 feet to the right of where the eye come ashore. Gustav may bring hurricane-force winds to the capital, Havana (on the north shore), which would cause heavy damage to the many poorly-built structures in the city.

Gustav's potential impact on the oil and gas industry
The price of U.S. crude oil has jumped about 2%, and the price of U.S. natural gas has increased 11% in the past two days, in anticipation that Gustav might rip through the oil and gas production areas of the Gulf of Mexico. About 25% of U.S. crude oil and 15% of its natural gas are produced in the Gulf of Mexico. As seen in Figure 3, the oil production areas are concentrated along the Louisiana and Texas coast. If Gustav makes a landfall on the right side of its cone of uncertainty, in Alabama or the Florida Panhandle, the oil and gas infrastructure might not be significantly affected. However, most of the cone of uncertainty lies in the major oil and gas producing areas, and I give a 60% chance that Gustav will significantly hurt oil and gas production in the Gulf.


Figure 2. Location of major oil platforms in the Gulf of Mexico. The data was taken from the Department of Interior Mineral Management Services

Gustav's intensification potential in the Gulf of Mexico
As we saw in 2005 with Katrina and Rita, the large amounts of deep, warm water brought into the Gulf of Mexico by the Loop Current can help intensify hurricanes to Category 5 intensity. As explained in my Loop Current tutorial, the Loop Current is an ocean current that transports warm Caribbean water through the Yucatan Channel between Cuba and Mexico. The current flows northward into the Gulf of Mexico, then loops southeastward through the Florida Keys. The Loop Current commonly bulges out in the northern Gulf of Mexico and sometimes will shed a clockwise rotating ring of warm water that separates from the main current. This ring of warm water slowly drifts west-southwestward towards Texas or Mexico at about 3-5 km per day. This feature is called a "Loop Current Ring", "Loop Current Eddy", or "Warm Core Ring", and can provide a key source of energy to fuel rapid intensification of hurricanes that cross the Gulf. The Loop Current itself can also fuel rapid intensification, such as happened with Hurricane Charley in 2004. When a Loop Current Eddy breaks off in the Gulf of Mexico at the height of hurricane season, it can lead to a dangerous situation where a vast reservoir of energy is available to any hurricane that might cross over. This occurred in 2005, when a Loop Current Eddy separated in July, just before Hurricane Katrina passed over and "bombed" into a Category 5 hurricane. The eddy remained in the Gulf and slowly drifted westward during September. Hurricane Rita passed over the same Loop Current Eddy three weeks after Katrina, and also explosively deepened to a Category 5 storm.

This year, we had another Loop Current Eddy break off in July. This eddy is now positioned due south of New Orleans (Figure 2), and this eddy has similar levels of heat energy to the 2005 eddy that powered Katrina and Rita. Should Gustav pass over or just to the left of this eddy, we can expect the storm to significantly intensify. There is also a weaker eddy present in the western Gulf; this eddy broke off from the Loop Current in April, and is much cooler then the eddy that broke off in July. Should Gustav pass over the April eddy, it shouldn't make much difference.

A NOAA Hurricane Hunter research aircraft is scheduled to drop a network of 20 specialized buoys (Air eXpendable BathyThermographs, or AXBTs) in the Gulf of Mexico Thursday to provide precise measurements of ocean temperatures in order to aid intensification forecasts for Gustav. This data will feed directly into the GFDL and HWRF computer models, but not into the other global models (GFS, NOGAPS, UKMET, and ECMWF).


Figure 2.Tropical Cyclone Heat Potential (TCHP) for the Gulf of Mexico on August 24, 2008. TCHP values in excess of 80 kJ/cm^2 (yellow colors) have been found to promote rapid intensification of hurricanes. The Loop Current is shown by the arrows at lower right, and begins in the Caribbean, flows north through the Yucatan Channel into the Gulf of Mexico, then loops back to the south and turns eastward over the Florida Keys. Two Loop Current Eddies have broken off from the Loop Current this year--one in April, and one in July. These eddies have drifted slowly westward, and still maintain heat from the Loop Current. Image credit: NOAA/AOML.

Links to follow
Wundermap for Cuba/Haiti/Jamaica
Gran Piedra, Cuba radar

Disturbance 95L east-northeast of Puerto Rico
A tropical disturbance (95L) near 19N 57W, a few hundred miles east-northeast of Puerto Rico, has become better organized today. Visible satellite loops show an increase in heavy thunderstorm activity, and the center of circulation has jumped about 100 miles to the southeast to be underneath the heaviest thunderstorms. This new location is also in a region of lower wind shear, and also means the latest 12Z (8 am EDT) computer model runs are likely to not be so good. Wind shear is a moderate 10-20 knots over 95L today, but is forecast to decrease below 5 knots on Thursday and remain below 15 knots for most of the remainder of the week. Dry air should not be a problem for 95L. NHC is giving 95L a medium (20%-50% chance) of developing into a tropical depression by Friday. I believe it is very likely (>70% chance) that this will be a tropical storm sometime in the next 2-5 days. Most of the models develop 95L, foreseeing a northwest track and a threat to Bermuda 4-6 days from now. The latest (8 am EDT) GFDL model run develops 95L into a strong Category 1 hurricane that passes very close to Bermuda on Sunday. The HWRF model predicts a Category 1 hurricane near Bermuda on Monday. In the longer range, it is possible that 95L will stall in the Bermuda area and move slowly, as steering currents collapse early next week. The storm may then get forced westward over Florida late next week, as forecast by the ECMWF model. However, the GFS and NOGAPS models are taking 95L northwards towards Canada late next week. It is too early to guess which of these solutions is more likely. There are currently no Hurricane Hunter missions scheduled into 95L.

Elsewhere in the tropics
Most of the computer models forecast the development of at least one additional tropical wave between Africa and the Lesser Antilles Islands during the coming week. The first candidate is a large circulation located near 17N, 34W, a few hundred miles west of the Cape Verde Islands. Wind shear is a moderate 10-15 knots in the region, and forecast to remain in the moderate to marginal range (10-20 knots) for the next few days. NHC is giving this system a low (<20% chance) of developing into a tropical depression by Friday, and some slow development is possible over the next few days. Only the GFS model develops it, and predicts the system will be near the northern Lesser Antilles Islands Monday or Tuesday.

My next blog will be Thursday morning.

P.S., a little humor from The Onion might be what is needed for some. Check out this morning's video, Hurricane Bound For Texas Slowed By Large Land Mass To The South.

Jeff Masters

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Gustav kills 5 in Haiti, continues to dump heavy rains

By: JeffMasters, 2:44 PM GMT on August 27, 2008

Tropical Storm Gustav continues to pound Haiti with torrential rains and tropical storm-force winds, as the storm slowly lumbers westward towards Cuba. The storm has killed at least five people on Haiti, according to early news reports. Satellite rainfall estimates show that the southern peninsula of Haiti has probably received 4-8 inches of rain, and an additional 2-4 inches are likely over the worst-hit regions before Gustav finally moves away. Yesterday, Gustav plowed into Haiti as a Category 1 hurricane with 90 mph winds. The encounter with Haiti's high mountains has weakened Gustav to a tropical storm. The latest center fix from the Hurricane Hunters at 7:03 am EDT put Gustav's central pressure at 999 mb, and the aircraft was not able to find surface winds above 50 mph. The Hurricane Hunters have left Gustav, and a new airplane is due in the storm this afternoon around 2 pm EDT.


Figure 1. Current satellite image of Gustav.

Visible satellite loops and radar from Gran Piedra, Cuba show that Gustav is not well-organized, and the eyewall has mostly collapsed. However, the appearance of the storm on both radar and satellite is slowly improving.

The track forecast for Gustav
The latest 6Z (2 am EDT) model runs are in good agreement that Gustav will turn west and pass through the narrow channel between Cuba and Jamaica after leaving Haiti. Gustav is small enough that the Kingston, Jamaica airport on the south side of the island may be able to stay open Thursday as the storm passes to the north. However, the Montego Bay airport on the north side of the island is likely to close. Heavy rain will be the primary threat to Jamaica and eastern Cuba.

The trough of low pressure that was steering Gustav northwest has moved off to the east, allowing a ridge of high pressure to build in. This ridge of high pressure will force Gustav west. There is now better agreement in the longer term track of Gustav. All of the models foresee a turn to the north over western Cuba, followed by a landfall on the central U.S. Gulf Coast Sunday or Monday. The final landfall location of Gustav depends on the strength and speed of a trough of low pressure forecast to move across the Midwest U.S. late this week. While it currently appears that Louisiana is the most likely target, keep in mind that if you're in the cone of uncertainty, you're not safe. Final landfall of Gustav could occur anywhere from Texas to the Florida Panhandle. The latest GFDL model predicts landfall near New Orleans on Sunday night as a Category 3 hurricane. The HWRF model picks Mississippi on Monday morning, as a Category 3 hurricane. The NOGAPS prefers the Florida Panhandle on Sunday night, and the ECMWF targets central Louisiana on Monday morning.

The intensity forecast for Gustav
One key question controlling Gustav's short-term intensity is how close it will stay to land. Gustav will not be able to intensify much until it clears the southwest Peninsula of Haiti, which should occur late this afternoon. If Gustav passes close to the rugged southeastern tip of Cuba, this will also impede its intensification. However, all the models are forecasting that Gustav will be able to thread the narrow gap between Cuba and Jamaica, allowing Gustav to intensify back to hurricane status on Thursday. Gustav is currently under moderate wind shear (10-15 knots). This shear is expected to remain in the low to moderate range (0-15 knots) for the remainder of the week. By Friday, as Gustav approaches the Cayman Islands, the storm will be underneath an upper level anticyclone, and over the highest heat content waters of the Atlantic. The HWRF and GFDL models respond by intensifying Gustav to a Category 3 hurricane as it passes through the Caymans. I think it is plausible that Gustav could intensify further, to Category 4 strength, before hitting the Caymans. If you have plans to be on the northern Cayman Islands--Cayman Brac or Little Cayman Island--on Friday, be prepared to be stuck there for several days, as Gustav may heavily damage these islands. Grand Cayman Island is also at risk--the GFDL model predicts Gustav will pass very close to Grand Cayman on Friday afternoon.

Gustav will lose intensity when it crosses the western tip of Cuba--perhaps by 25 mph or so--but should easily regain that lost strength within 12-24 hours. Gustav will likely be a major Category 3 or 4 hurricane in the Gulf of Mexico, and make landfall in the U.S. as a major hurricane.

Gustav's intensification potential in the Gulf of Mexico
As we saw in 2005 with Katrina and Rita, the large amounts of deep, warm water brought into the Gulf of Mexico by the Loop Current can help intensify hurricanes to Category 5 intensity. As explained in my Loop Current tutorial, the Loop Current is an ocean current that transports warm Caribbean water through the Yucatan Channel between Cuba and Mexico. The current flows northward into the Gulf of Mexico, then loops southeastward through the Florida Keys. The Loop Current commonly bulges out in the northern Gulf of Mexico and sometimes will shed a clockwise rotating ring of warm water that separates from the main current. This ring of warm water slowly drifts west-southwestward towards Texas or Mexico at about 3-5 km per day. This feature is called a "Loop Current Ring", "Loop Current Eddy", or "Warm Core Ring", and can provide a key source of energy to fuel rapid intensification of hurricanes that cross the Gulf. The Loop Current itself can also fuel rapid intensification, such as happened with Hurricane Charley in 2004. When a Loop Current Eddy breaks off in the Gulf of Mexico at the height of hurricane season, it can lead to a dangerous situation where a vast reservoir of energy is available to any hurricane that might cross over. This occurred in 2005, when a Loop Current Eddy separated in July, just before Hurricane Katrina passed over and "bombed" into a Category 5 hurricane. The eddy remained in the Gulf and slowly drifted westward during September. Hurricane Rita passed over the same Loop Current Eddy three weeks after Katrina, and also explosively deepened to a Category 5 storm.

This year, we had another Loop Current Eddy break off in July. This eddy is now positioned due south of New Orleans (Figure 2), and this eddy has similar levels of heat energy to the 2005 eddy that powered Katrina and Rita. Should Gustav pass over or just to the left of this eddy, we can expect the storm to significantly intensify. There is also a weaker eddy present in the western Gulf; this eddy broke off from the Loop Current in April, and is much cooler then the eddy that broke off in July. Should Gustav pass over the April eddy, it shouldn't make much difference.

A NOAA Hurricane Hunter research aircraft is scheduled to drop a network of 20-30 specialized buoys (AXBTs) in the Gulf of Mexico Thursday to provide precise measurements of ocean temperatures in order to aid intensification forecasts for Gustav.


Figure 2.Tropical Cyclone Heat Potential (TCHP) for the Gulf of Mexico on August 24, 2008. TCHP values in excess of 80 kJ/cm^2 (yellow colors) have been found to promote rapid intensification of hurricanes. The Loop Current is shown by the arrows at lower right, and begins in the Caribbean, flows north through the Yucatan Channel into the Gulf of Mexico, then loops back to the south and turns eastward over the Florida Keys. Two Loop Current Eddies have broken off from the Loop Current this year--one in April, and one in July. These eddies have drifted slowly westward, and still maintain heat from the Loop Current. Image credit: NOAA/AOML.

Links to follow
Wundermap for Cuba/Haiti/Jamaica
Gran Piedra, Cuba radar


Figure 3. QuikSCAT image from 5:43 am EDT Wednesday August 27, 2008 clearly shows the surface circulation associated with disturbance 95L (marked with an "L"). The black wind barbs indicated "flagged" regions where rain is falling and the wind estimates cannot be trusted. Highest unflagged winds were 20-25 knots (24-28 mph). Image credit: NOAA.

Disturbance 95L east-northeast of Puerto Rico
A tropical disturbance (95L) near 20N 58W, a few hundred miles east-northeast of Puerto Rico, has a closed surface circulation, as seen on this morning's QuikSCAT pass (Figure 3). This system could develop and be a problem for Bermuda by Monday, and the U.S. East Coast late next week. Visible satellite loops show that wind shear continues to play havoc with this system--strong upper-level winds from an upper level low pressure system to the west are pushing 95L's heavy thunderstorms to the east side of the center of circulation. Wind shear is a high 20 knots over 95L today, but is forecast to decrease below 5 knots on Thursday and remain below 15 knots for most of the remainder of the week. Dry air should not be a problem for 95L. NHC is giving 95L a medium (20%-50% chance) of developing into a tropical depression by Friday. Most of the models develop 95L, foreseeing a northwest track and a threat to Bermuda 4-7 days from now. The latest (2 am EDT) GFDL model run develops 95L into a strong Category 1 hurricane that passes very close to Bermuda on Monday. The HWRF model is more aggressive, predicting a Category 3 hurricane by Saturday, that then weakens to a Category 1 hurricane by the time it nears Bermuda on Monday. In the longer range, it appears that 95L will stall in the Bermuda area and move slowly, as steering currents collapse early next week. By the end of next week, the storm may scoot northward towards Canada (as predicted by the GFS model), or head west-southwestward into the Bahamas and Florida (as predicted by the ECMWF model). It is too early to guess which of these solutions is more likely.

Elsewhere in the tropics
Most of the computer models forecast the development of one additional tropical wave between Africa and the Lesser Antilles Islands during the coming week. The first candidate is a system a few hundred miles west-southwest of the Cape Verde Islands. Wind shear is a low 5-10 knots in the region, and forecast to remain in the low to moderate range for the next few days. None of the models are developing this region, and NHC is giving this system a low (<20% chance) of developing into a tropical depression by Friday. The models predict this system will be near the northern Lesser Antilles Islands Monday or Tuesday.

My next blog will be this afternoon.

P.S., a little humor from The Onion might be what is needed for some. Check out this morning's video, Hurricane Bound For Texas Slowed By Large Land Mass To The South.

Jeff Masters

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Gustav hits Haiti and weakens

By: JeffMasters, 8:25 PM GMT on August 26, 2008

Hurricane Gustav plowed into Haiti as a Category 1 hurricane with 90 mph winds around 1 pm AST today. The encounter with Haiti's high mountains has weakened Gustav to a tropical storm, according to the latest data from the Hurricane Hunters. At 1:56 pm EDT, the Hurricane Hunters measured a central pressure of 992 mb in Gustav's eye--an increase of 11 mb from the previous reading. Gustav's top winds have decreased to tropical storm strength, according to the SFMR instrument, which measured winds of 65-70 mph at the surface, just south of Haiti's southwest peninsula. The eye of Gustav is still over Haiti, moving west-northwest along this peninsula. This longer than expected track of the eye over Haiti is bad news for that country, but good news for Jamaica and eastern Cuba.


Figure 1. Hurricane Gustav just after landfall in Haiti. A faint eye was visible in this visible light photo.

Gustav's impact on Haiti and the Dominican Republic
Visible satellite loops show that Gustav's eye has disappeared. Gustav is a small storm, and wind damage from Gustav will be confined to a 50-mile diameter area along Haiti's southwest peninsula. Gustav's hurricane-force winds will miss Haiti's capital, Port-au-Prince. As always, heavy rain will be Haiti's main concern, due to the heavy deforestation on the steep mountainsides that allow deadly flash floods to pour unchecked into Haiti's populated areas. Flooding from Tropical Storm Fay killed 20 Haitians last week, and we can expect serious flooding along Haiti's southern reaches from Gustav. Gustav is moving a modest 9 mph, but is expected to slow down later today and Wednesday, extending the period of time Haiti is exposed to the storm's flooding rains. Heavy rainfall will also cause flooding problems in the Dominican Republic, but these will not be as severe as in Haiti, and will mostly be confined to the southwestern portion of the country. The tourist areas of Santo Domingo, Punta Cana, and Puerto Plata will escape the worst of Gustav's rains.

The track forecast for Gustav
The latest 12Z (8 am EDT) model runs continue to be in good agreement on the 1-3 day track of Gustav, and we can be confident that Gustav will turn west and pass south of Cuba after leaving Haiti. The trough of low pressure currently exiting the U.S. East Coast and pulling Gustav northwest is expected to move off to the east, allowing a ridge of high pressure to build in and force Gustav due west or slightly south of due west. After three days, there is more divergence in the models. The NOGAPS model no longer foresees landfall on Mexico's Yucatan, and now takes Gustav to a final landfall in the Florida Panhandle on Monday. The ECMWF is now the only model predicting a landfall in the Yucatan. This model predicts a second landfall in Texas. The GFDL is a little slower than its previous run, but still forecasts a Category 3/4 hurricane hitting Louisiana on Sunday evening. The UKMET prefers a Texas landfall. The GFS is not much help--it dissipates Gustav.

The final landfall location of Gustav depends on the strength and speed of a trough of low pressure forecast to move across the Midwest U.S. late this week. At present, there is no way to guess which location in the Gulf of Mexico is the most likely. Keep in mind that the cone of uncertainty is correct only about 2/3 of the time--1/3 of the time, we can expect the storm's position to be in error by more than what the cone of uncertainty suggests.

The intensity forecast for Gustav
As long as Gustav is over water, it will intensify. One key question is how close Gustav will pass to the rugged southeastern tip of Cuba. The HWRF model forecasts that Gustav will move along a significant stretch of the southeastern coast of Cuba, weakening the storm to a minimal tropical storm. This is a plausible scenario, should Gustav indeed follow this path. Gustav is currently under moderate wind shear (10-15 knots). This shear is expected to remain in the low to moderate range (0-15 knots) for the remainder of the week. Gustav is over the highest heat content waters in the Atlantic. Given these two factors, intensification is likely whenever the storm is over water, at least 50 miles from land. It should take 24-36 hours for Gustav to recover from its encounter with the high mountains of Hispaniola and become a hurricane again--or longer, if the storm passes close to or over Cuba. I give Gustav a 70% chance of being a tropical storm when it makes its closest approach to Jamaica. By Wednesday night, Gustav should be past the mountains of southeast Cuba, and will be underneath an upper-level anticyclone. These upper atmosphere high pressure systems can greatly intensify a tropical storm, since the clockwise flow of air at the top of the storm acts to efficiently vent away air pulled aloft by the storm's heavy thunderstorms. With high oceanic heat content also present in the waters off western Cuba, the potential for rapid intensification exists should the center stay more than 50 miles from the Cuban coast.

Gustav's intensification potential in the Gulf of Mexico
As we saw in 2005 with Katrina and Rita, the large amounts of deep, warm water brought into the Gulf of Mexico by the Loop Current can help intensify hurricanes to Category 5 intensity. As explained in my Loop Current tutorial, the Loop Current is an ocean current that transports warm Caribbean water through the Yucatan Channel between Cuba and Mexico. The current flows northward into the Gulf of Mexico, then loops southeastward through the Florida Keys. The Loop Current commonly bulges out in the northern Gulf of Mexico and sometimes will shed a clockwise rotating ring of warm water that separates from the main current. This ring of warm water slowly drifts west-southwestward towards Texas or Mexico at about 3-5 km per day. This feature is called a "Loop Current Ring", "Loop Current Eddy", or "Warm Core Ring", and can provide a key source of energy to fuel rapid intensification of hurricanes that cross the Gulf. The Loop Current itself can also fuel rapid intensification, such as happened with Hurricane Charley in 2004. When a Loop Current Eddy breaks off in the Gulf of Mexico at the height of hurricane season, it can lead to a dangerous situation where a vast reservoir of energy is available to any hurricane that might cross over. This occurred in 2005, when a Loop Current Eddy separated in July, just before Hurricane Katrina passed over and "bombed" into a Category 5 hurricane. The eddy remained in the Gulf and slowly drifted westward during September. Hurricane Rita passed over the same Loop Current Eddy three weeks after Katrina, and also explosively deepened to a Category 5 storm.

This year, we had another Loop Current Eddy break off in July. This eddy is now positioned due south of New Orleans (Figure 2), and this eddy has similar levels of heat energy to the 2005 eddy that powered Katrina and Rita. Should Gustav pass over or just to the left of this eddy, we can expect the storm to significantly intensify. There is also a weaker eddy present in the western Gulf; this eddy broke off from the Loop Current in April, and is much cooler then the eddy that broke off in July. Should Gustav pass over the April eddy, it shouldn't make much difference.


Figure 2.Tropical Cyclone Heat Potential (TCHP) for the Gulf of Mexico on August 24, 2008. TCHP values in excess of 80 kJ/cm^2 (yellow colors) have been found to promote rapid intensification of hurricanes. The Loop Current is shown by the arrows at lower right, and begins in the Caribbean, flows north through the Yucatan Channel into the Gulf of Mexico, then loops back to the south and turns eastward over the Florida Keys. Two Loop Current Eddies have broken off from the Loop Current this year--one in April, and one in July. These eddies have drifted slowly westward, and still maintain heat from the Loop Current. Image credit: NOAA/AOML.

Links to follow
Wundermap for Cuba/Haiti
Gran Piedra, Cuba radar

Disturbance 95L east-northeast of Puerto Rico
A tropical disturbance (95L) near 21N 57W, a few hundred miles east-northeast of Puerto Rico, remains weak and disorganized. However, if this disturbance survives the next 24 hours, it could develop and be a problem for Bermuda this weekend and the U.S. East Coast next week. Visible satellite loops show that wind shear is playing havoc with this system--strong upper-level winds from the west are allowing only a small amount of heavy thunderstorms to cling to the southeast side of the center of circulation. The surface circulation appears to be weakening, and the wind shear may be able to destroy 95L today. Wind shear is a high 20 knots over 95L today, but is forecast to decrease to zero by Thursday and remain below 15 knots for most of the remainder of the week. NHC is giving 95L a medium (20%-50% chance) of developing into a tropical depression by Thursday. Most of the models develop 95L, foreseeing a northwest track and a threat to Bermuda 5-7 days from now. The latest (8 am EDT) GFDL model run develops 95L into a strong Category 2 hurricane that passes very close to Bermuda on Saturday. The HWRF model is less aggressive, predicting a 55 mph tropical storm passing near Bermuda on Sunday. The NOGAPS and the GFS are similar, and predict 95L will stall near Bermuda. The GFS foresees that 95L will wander westward for a few days after encountering Bermuda, then scoot northwards along the U.S. East Coast, passing very close to North Carolina and New England. The ECMWF model takes 95L near Bermuda, then northwards out to sea.

Elsewhere in the tropics
Most of the computer models forecast the development of two more tropical waves between Africa and the Lesser Antilles Islands in the coming week, and it is possible we will have three or four simultaneous named storms in the Atlantic a week from now (Figure 3).


Figure 3. The ECMWF 8-day forecast valid Tuesday, September 2 at 8 pm EDT. The ECMWF model was initialized at 00 GMT Tuesday, August 26, 2008. The model is predicting a parade of four tropical storms or hurricanes stretched out across the Atlantic: Gustav, 95L, and the as yet hypothetical 96L and 97L. Image credit: ECMWF.

My next blog will be Wednesday morning.

Jeff Masters

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Intensifying Gustav bears down on Haiti

By: JeffMasters, 2:48 PM GMT on August 26, 2008

Hurricane Gustav intensified remarkably overnight, and is poised to deliver a heavy blow to Haiti early this afternoon as a Category 1 or 2 hurricane. Gustav intensified from a tropical depression at 11 am yesterday to a Category 1 hurricane last night in just 16 hours, tying Hurricane Humberto's record--set just last year--for the fastest intensification from first advisory to a Category 1 hurricane. (Actually, Humberto did the feat in 14 1/4 hours, but both Humberto's intensification feat and Gustav's will get rounded off to 18 hours in the final data base, which stores points only every six hours).


Figure 1. Latest satellite image of Tropical Storm Gustav.

Since reliable record keeping of intensification rates of Atlantic hurricanes began in 1970, only Humberto and Gustav have managed to intensify to a hurricane in less than 24 hours after the first advisory was issued. There have been six storms that accomplished the feat in 24 hours--Hurricane Florence of 2000, Hurricane Erin of 1995, Hurricane Bonnie of 1992, Hurricane Earl of 1986, Hurricane Kate of 1985, and Hurricane Kendra of 1978.

Gustav's impact on Haiti and the Dominican Republic
The last Hurricane Hunter data we have is from 8 am EDT this morning, when an Air Force plane measured a 981 mb pressure and top surface winds of 90 mph. A new aircraft is due in the storm about the time of landfall in Haiti, at 2 pm EDT. Visible satellite loops show that Gustav continues to intensify, but is a relatively small storm, and wind damage from Gustav will be confined to a 50-mile diameter area on Haiti's southwest peninsula. Gustav's hurricane-force winds will pass just south of Haiti's capital, Port-au-Prince. As always, heavy rain will be Haiti's main concern, due to the heavy deforestation on the steep mountainsides that allow deadly flash floods to pour unchecked into Haiti's populated areas. Flooding from Tropical Storm Fay killed 20 Haitians last week, and we can expect serious flooding along Haiti's southern reaches from Gustav. Gustav is moving a modest 9 mph, but is expected to slow down later today and Wednesday, extending the period of time Haiti is exposed to the storm's flooding rains. Heavy rainfall will also cause flooding problems in the Dominican Republic, but these will not be as severe as in Haiti, and will mostly be confined to the southwestern portion of the country. The tourist areas of Santo Domingo, Punta Cana, and Puerto Plata will escape the worst of Gustav's rains.

Gustav's impact on Cuba, Jamaica, and the Cayman Islands
Cuba, Jamaica, and the Cayman Islands will suffer moderate to severe flooding from Gustav's rains over the next few days. In particular, the northern side of Jamaica may see heavy weather that will close the Montego Bay airport Wednesday afternoon through Friday morning. However, I'm not confident of this forecast, since Gustav is such a small storm. The Kingston airport is likely to remain open except for a few hours when Gustav's spiral bands pass through. Airport closures in the Cayman Islands are likely to begin on Thursday afternoon, and extend into Saturday. Those of you traveling to Cancun/Cozumel can expect airport closures beginning Saturday afternoon.

The track forecast for Gustav
The models are in good agreement on the 1-3 day track of Gustav, and we can be confident that Gustav will turn west and pass south of Cuba after a close encounter with the southwest peninsula of Haiti. The trough of low pressure currently exiting the U.S. East Coast and pulling Gustav northwest is expected to move off to the east, allowing a ridge of high pressure to build in and force Gustav due west or slightly south of due west. After three days, there is more divergence in the models. The ECMWF and NOGAPS models foresee a landfall in the Cancun/Cozumel region on Mexico's Yucatan Peninsula, followed by a second Mexican landfall south of Brownsville, Texas, early next week. This solution assumes the trough of low pressure moving across the Midwest U.S. late this week will not be strong enough to turn Gustav to the north. The other models predict that this trough will be strong enough to turn Gustav northward, and foresee a landfall on the Gulf Coast between the Florida Panhandle and Texas border 6-8 days from now. The GFDL is the fastest, bringing Gustav to New Orleans on Sunday afternoon. This is a plausible forecast, but at this point, virtually any point along the Gulf Coast has a roughly equal chance of a direct hit by Gustav.

Which set of model should we trust? I plotted up the errors for some of the computer model forecasts made during Fay. While Fay was over Hispaniola and Cuba, the GFDL model made the best track forecasts, among the four main models used by NHC: GFS, GFDL, NOGAPS, and UKMET. This makes me more inclined to trust the GFDL model's forecasts for Gustav, since Fay and Gustav are similar storms.

The intensity forecast for Gustav
As long as Gustav is over water, it will intensify. Gustav is currently under moderate wind shear (15 knots) . This shear is expected to remain in the low to moderate range (0-15 knots) for the remainder of the week. Gustav is over the highest heat content waters in the Atlantic. Given these two factors, intensification is likely whenever the storm is over water, at least 50 miles from land. Expect the high mountains of Hispaniola to take a toll on Gustav. Recall in 2006 that Hurricane Ernesto hit the southwest tip of Haiti as a Category 1 hurricane with 75 mph winds. Haiti's mountains knocked Ernesto down to a tropical storm with 50 mph winds, which decreased further to 40 mph when the storm crossed over into Cuba. Expect at least a 25 mph decrease in Gustav's winds by Wednesday, after it encounters Haiti. Further weakening is likely if the storm passes close to or over Cuba. By Wednesday, Gustav will be underneath an upper-level anticyclone. These upper atmosphere high pressure systems can greatly intensify a tropical storm, since the clockwise flow of air at the top of the storm acts to efficiently vent away air pulled aloft by the storm's heavy thunderstorms. With high oceanic heat content also present in the waters off western Cuba, the potential for rapid intensification exists should the center stay more than 50 miles from the Cuban coast. Once in the Gulf of Mexico, Gustav is likely to intensify into a major Category 3 or higher storm. I give a 60% chance that Gustav will cause significant disruption to the oil and gas industry in the Gulf.

Links to follow
Wundermap for Haiti
Gran Piedra, Cuba radar

Disturbance 95L east-northeast of Puerto Rico
Another tropical wave (95L) near 19N 55W, a few hundred miles east-northeast of Puerto Rico, remains disorganized. However, this disturbance has the potential to be trouble for Bermuda or the U.S. East Coast, and needs to be monitored. Visible satellite loops show only a small amount of heavy thunderstorm activity, but there is some evidence of rotation in the clouds. This morning's QuickSCAT pass missed 95L, but last night's pass showed a wind shift (no surface circulation). Wind shear is a marginal 15-20 knots over 95L, but is forecast to decrease to zero by Thursday and remain below 15 knots for most of the remainder of the week. NHC is giving 95L a medium (20%-50% chance) of developing into a tropical depression by Thursday. All of the models develop 95L, with most of them foreseeing northwest track and a threat to Bermuda 5-7 days from now. However, the ECMWF model takes 95L westward into Florida 7-8 days from now.

Elsewhere in the tropics
Most of the computer models forecast the development of two more tropical waves between Africa and the Lesser Antilles Islands in the coming week, and it is possible we will have three or four simultaneous named storms in the Atlantic a week from now (Figure 2).


Figure 2. The ECMWF 8-day forecast valid Tuesday, September 2 at 8 pm EDT. The ECMWF model was initialized at 00 GMT Tuesday, August 26, 2008. The model is predicting a parade of four tropical storms or hurricanes stretched out across the Atlantic: Gustav, 95L, and the as yet hypothetical 96L and 97L. Image credit: ECMWF.

My next blog will be this afternoon.

Jeff Masters

Updated: 3:25 PM GMT on August 26, 2008

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A strengthening Gustav takes aim at Haiti

By: JeffMasters, 8:14 PM GMT on August 25, 2008

Tropical Storm Gustav intensified rapidly from a mere disturbance to a strong tropical storm in just a few short hours. At 1:33 pm EDT, an Air Force Hurricane Hunter aircraft measured a surface pressure of 996 mb at 15.8N, 70.5W, at the center of a closed circulation. Top winds at the aircraft's flight level of 2200 feet were hurricane force, 74 mph. Top winds measured at the surface by the SFMR instrument were 60 mph, on the southeast side of the storm. Large regions of winds in excess of tropical storm force (39 mph) were measured on both the northwest and southeast sides of the storm. Visible satellite loops show a steadily increase in the intensity and areal coverage of Gustav's heavy thunderstorm activity. A cloud-free center (not a true eye) formed late this morning, and Gustav is now starting to build an eyewall of heavy thunderstorms around the cloud-free center. Gustav has an impressive spiral band to its north, and this band has now moved ashore over the southern Dominican Republic, as seen on Punta Cana radar. These rains have also spread to Puerto Rico, as seen on Puerto Rico radar. The Hurricane Hunters have left Gustav, and a new aircraft is scheduled to be in the storm by 2 am Tuesday.


Figure 1. Latest satellite image of Tropical Storm Gustav.

The track forecast for Gustav
The models have come into better agreement on the future track of Gustav. Gustav is likely to continue northwest across the southwestern peninsula of Haiti, and is being drawn this direction by a trough of low pressure currently exiting the U.S. East Coast. By Wednesday, the trough is expected to move eastward, leaving Gustav in a region of weak steering currents. The storm will slow down, and is then expected to turn westward, or even slightly south of west, as a ridge of high pressure builds in, forcing Gustav to move parallel to Cuba. The outlier model is the NOGAPS, which takes Gustav northward through the Bahamas and parallel to the U.S. East Coast. For now, I am discounting this solution. The NOGAPS was also the outlier model during Fay, and consistently made the worst track forecasts among the major models. I expect Gustav to track over or just south of Cuba. As Gustav nears western Cuba on Friday, another trough of low pressure moving across the central U.S. may be strong enough to turn Gustav northward, into the Gulf of Mexico. This is the solution given by the GFDL, HWRF, and ECMWF models, which foresee a Category 1 or 2 hurricane just west of Key West, Florida, on Saturday. The GFS, UKMET, and Canadian models disagree, and forecast that the new trough of low pressure will not be strong enough to turn Gustav to the north. Instead, these models predict that Gustav will continue west into the Yucatan Peninsula.

Which set of models do you trust? Both solutions are plausible. I plotted up the errors for some of the computer model forecasts made during Fay. While Fay was over Hispaniola and Cuba, the GFDL model made the best track forecasts, among the four main models used by NHC: GFS, GFDL, NOGAPS, and UKMET. This makes me more inclined to trust the GFDL model's forecasts for Gustav, since Fay and Gustav are similar storms.

The intensity forecast for Gustav
As long as Gustav is over water, it will intensify. Gustav is currently under low wind shear (5 knots) . This shear is expected to remain in the low range (0-10 knots) through Tuesday morning, then increase to a moderate 15 knots by Tuesday afternoon, as Gustav makes its closest approach to the trough of low pressure to its north. Gustav is over the highest heat content waters in the Atlantic. Given these two factors, intensification to a strong Category 1 hurricane with 90 mph winds is possible before the storm makes landfall in Haiti. Expect the high mountains of Hispaniola to take a significant toll on Gustav. Recall in 2006 that Hurricane Ernesto hit the southwest tip of Haiti as a Category 1 hurricane with 75 mph winds. Haiti's mountains knocked Ernesto down to a tropical storm with 50 mph winds, which decreased further to 40 mph when the storm crossed over into Cuba. Expect at least a 25 mph decrease in Gustav's winds by Wednesday, after it encounters Haiti. Further weakening is likely if the storm passes close to or over Cuba. By Wednesday, wind shear is expected to fall to the low range (0-10 knots) again, as the trough of low pressure to its north moves off to the east. Once Gustav reaches central or western Cuba, it will be underneath an upper-level anticyclone. These upper atmosphere high pressure systems can greatly intensify a tropical storm, since the clockwise flow of air at the top of the storm acts to efficiently vent away air pulled aloft by the storm's heavy thunderstorms. With high oceanic heat content also present in the waters off western Cuba, the potential for rapid intensification exists should the center stay more than 50 miles from the Cuban coast.

Links to follow
Wundermap for Hispaniola
Dominican Republic radar (Punta Cana)

My next blog will be Tuesday morning.

Jeff Masters

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Tropical Storm Gustav forms

By: JeffMasters, 6:07 PM GMT on August 25, 2008

The Hurricane Hunters have found data supporting the upgrade of Tropical Depression Seven to Tropical Storm Gustav. At 1:33 pm EDT, an Air Force Hurricane Hunter aircraft measured a surface pressure of 996 mb at 15.8N, 70.5W, at the center of a closed circulation. Top winds at the aircraft's flight level of 2200 feet were hurricane force, 74 mph. Top winds measured at the surface by the SFMR instrument were 60 mph, on the southeast side of the storm. Large regions of winds in excess of tropical storm force (39 mph) were measured on both the northwest and southeast sides of the storm.


Figure 1. Latest satellite image of what will soon be called Tropical Storm Gustav.

Links to follow
Wundermap for Hispaniola
Dominican Republic radar (Punta Cana)

I'll have a full blog by 4pm EDT.

Jeff Masters

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Dangerous Caribbean disturbance 94L: deja vu?

By: JeffMasters, 1:47 PM GMT on August 25, 2008

Heavy thunderstorms associated with a tropical wave (94L) near 15N, 70W, in the central Caribbean south of the Dominican Republic, have grown more concentrated over the past day. This morning's QuikSCAT pass (Figure 1) did now show a closed surface circulation, but did show sustained winds of 52 mph north of the center. Visible satellite loops show evidence of rotation in the clouds at middle levels of the atmosphere, but no surface circulation as yet. The area covered by the heaviest thunderstorms is relatively modest. The storm has moistened its environment considerably, and dry air should no longer be a problem for 94L. Wind shear has fallen to a very low 2 knots over 94L. NHC is giving this system a high (>50% chance) of developing into a tropical depression by Wednesday morning, and has scheduled a Hurricane Hunter flight to investigate the storm this afternoon.


Figure 1. QuikSCAT image from 6:33 am EDT Monday August 25, 2008. The red "L" denotes the center of disturbance 94L, which has no closed circulation. The wind barbs show there are south winds to the east of the center and winds from the east to the north of the center, but there are no winds from the west to the south of the center, which one would have to have in order for there to be a closed surface circulation. Note the purple wind barb just north of the center, with four long tines and one short tine, indicating that we had winds of 45 knots (52 mph) there. Image credit: Paul Chang, NOAA/NESDIS/ORA.

The deja vu forecast for 94L
The models have had a tough time properly handling 94L, due to the fact they have been getting this system entangled with the other disturbance (95L) a few hundred miles to the northeast of Puerto Rico. The latest (2 am EDT Monday) GFDL forecast appears believable--and presents a strong case of deja vu. It's an almost exact repeat of Fay's track. The GFDL predicts 94L will continue to move northwest and hit the Haiti/Dominican Republic region on Tuesday, then get turned to the west by a strengthening ridge of high pressure. The storm will cross over to eastern Cuba on Wednesday, then travel along the length of Cuba through Friday night. On Saturday, the GFDL has 94L popping off the coast of Cuba at the same spot Fay did, then intensifying into a Category 1 hurricane that moves over Key West towards a landfall in Southwest Florida. While it is unlikely that the exact details of this deja vu forecast will come true, it does give one a general idea of the land areas 94L is likely to affect. The Dominican Republic and Haiti can expect 4-8 inches of rain on Tuesday and Wednesday from 94L, with isolated amounts up to 12 inches. These rains will cause flash flooding and dangerous mudslides. Wednesday through Friday, 94L is likely to bring heavy rains to Cuba, Jamaica, the northern Cayman Islands, and the southeast Bahamas. There are some models calling for 94L to track through the Bahamas and then northeast out to sea, so it may end up that the Bahamas will end up taking the brunt of this storm. However, I don't think this is likely, and a more westerly track into the Gulf of Mexico will occur.

Sea surface temperatures and total oceanic heat content in the Central Caribbean are very high, and 94L is in a environment very favorable for intensification. Wind shear is predicted to remain very low to moderate, 0-15 knots, for the next five days. An upper level high pressure system is currently sitting over central Cuba, and if 94L can position itself under this high, it will provide very favorable upper-level outflow conditions for the storm later this week. The main restriction on 94L's development will, like for Fay, be interaction with land. The islands of Hispaniola and Cuba will provide formidable obstacles to intensification.

Links to follow
Buoy 42059, north of 94L, has measured sustained winds of 30 mph this morning.

Fay
Tropical Depression Fay continues to spin away over the Southeast U.S., and is now centered over southern Mississippi. With its center over land, Fay is cut off from its oceanic source of sustenance, and now has no hope of reaching tropical storm strength again. Satellite loops show that Fay still has a large circulation with plenty of rain bands soaking the Southeast, and the storm is now headed east-northeast towards Alabama and Georgia. It will take 2-3 days for the huge amount of angular momentum energy stored in such a large vortex to dissipate as the storm slowly spins down. No models are calling for Fay's center to emerge over water again, and the "The Joker" is finally finished. Some maximum rain amounts from Fay as of 1 am CDT Monday (by state): Florida: 25.28" (Melbourne Beach); Georgia: 17.43" (Thomasville); Alabama: 6.55" (Camden); South Carolina: 5.84" (Beaufort); Mississippi: 2.88" (Starkville); Louisiana: 2.80" (Baton Rouge).

On the plus side, Fay's rains are now moving into some of the most drought-stricken regions of the country--northern Alabama and northern Georgia. In fact, there is now concern about flooding in these regions later this week, if the remains of Fay continue to linger and bring eight or more inches of rain (Figure 2). Fay's rains have now increased the level of Lake Okeechobee by 2.2 feet. The lake level stands at 13.41 feet, which is still about 2 feet below normal.


Figure 2. Total forecast rainfall for the five days beginning at 8 am EDT Monday August 25. Image credit: National Weather Service.

Disturbance 95L northeast of Puerto Rico
Another tropical wave (95L) a few hundred miles northeast of Puerto Rico, remains disorganized. Wind shear has increased to a marginal 15 knots over 95L, and is forecast to increase to a high 15-25 knots in coming days. NHC is giving 95L a low (<20% chance) of developing into a tropical depression by Wednesday.

Elsewhere in the tropics
Several of the computer models forecast that a tropical wave that moved off the coast of Africa yesterday will develop late this week.

My next blog will be this afternoon, once the Hurricane Hunters find out more on 94L.

Jeff Masters

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Fay slowly winding down; Caribbean disturbance 94L slowly winding up

By: JeffMasters, 6:07 PM GMT on August 24, 2008

Tropical Depression Fay has made the fatal mistake of heading inland, away from its oceanic source of sustenance, and now has little hope of reaching tropical storm strength again. Satellite loops show that Fay still has a large circulation covering most of the Southeast U.S., and it will take 2-3 days for the huge amount of angular momentum energy stored in such a large vortex to dissipate as the storm slowly spins down. No models are calling for Fay's center to emerge over water again, and it appears that "The Joker" is finally finished.

Fay's rains
Fay's rains continue to pile up over the Southeast (Figure 1). In Florida, Fay has dumped up to 25 inches of rain over the Melbourne area, 19 inches in Tallahassee, and 14 inches in Jacksonville. In Alabama, rainfall amounts up to seven inches have fallen, with up to eight inches in Georgia (Valdosta) and four inches in Mississippi (Jackson). Additional rains of 4-8 inches are likely over Louisiana, Mississippi, Alabama, Georgia, and the Florida Panhandle over the next 2-3 days.


Figure 1. Total rainfall for the seven days ending Sunday, August 24, at 8 am EDT. Image credit: National Weather Service.

On the plus side, Fay's rains are now moving into some of the most drought-stricken regions of the country--northern Alabama and northern Georgia. In fact, there is now concern about flooding in these regions later this week, if the remains of Fay continue to linger and bring eight or more inches of rain (Figure 2). Fay's rains have now increased the level of Lake Okeechobee by two feet. The lake level stands at 13.11 feet, which is still about 2 feet below normal.


Figure 1. Total forecast rainfall for the five days beginning at 8 am EDT Sunday August 24. Image credit: National Weather Service.

Disturbance 94L in the eastern Caribbean
Heavy thunderstorms associated with a tropical wave (94L) near 12N, 65W, in the eastern Caribbean, have grown more widespread over the past day. This morning's QuikSCAT pass missed 94L, but did show sustained winds of 40 mph north of the center. Visible satellite loops show evidence of rotation in the clouds at middle levels of the atmosphere, but no surface circulation as yet. The area covered by the heaviest thunderstorms is relatively modest. The storm has moistened its environment considerably, and dry air should be less and less of a problem for the storm over the next few days. Wind shear has fallen to a modest 10 knots over 94L and is decreasing. NHC is giving this system a medium (20%-50% chance) of developing into a tropical depression by Tuesday afternoon, and has scheduled a Hurricane Hunter flight to investigate on Monday afternoon.

The forecast for 94L
The models have had a tough time properly handling 94L, and have not been making believable forecasts of the storm's track and speed. The latest (8 am EDT Sunday) GFDL forecast appears to have the most believable forecast I've seen, developing 94L into a tropical storm that tracks over extreme Southwest Haiti, then into the region between Jamaica and Cuba. The NOGAPS and ECMWF models have also been developing 94L with each run the past few days. Heavy rain and high winds from 94L's circulation should affect Haiti's southern Peninsula on Tuesday morning, and spread to Jamaica and eastern Cuba by Tuesday night. The wind shear forecast for the Caribbean calls for very low values of wind shear around 5 knots for most of the coming week. Residents of Haiti, Jamaica, and Cuba should keep a careful eye on this potentially dangerous disturbance. I expect 94L will be a tropical depression by Tuesday, and will eventually grow into a tropical storm or hurricane that will threaten Cuba, the U.S., and/or the Bahama Islands late this week.

Links to follow
Aruba radar

Disturbance 95L in the middle Atlantic
Another tropical wave (95L), near 22N 55W, in the middle Atlantic Ocean, remains disorganized. Wind shear has increased to a marginal 15 knots over 95L, and is forecast to increase further in coming days, ranging between 15-25 knots over the next five days. Visible satellite loops show only a small amount of heavy thunderstorm activity, and 95L is battling a large amount of dry air that surrounds the storm and is interfering with development. NHC is giving 95L a low (<20% chance) of developing into a tropical depression by Tuesday. Several of the models do develop 95L late this week, and Bermuda should keep an eye on 95L. However, I doubt 95L will ever develop into a tropical depression.

Elsewhere in the tropics
Several of the computer models forecast that a tropical wave that is moving off the coast of Africa today will develop late this week.

My next blog will be Monday morning.

Jeff Masters

Updated: 1:48 PM GMT on August 25, 2008

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Fay's rains and tornadoes continue; disturbance 94L entering Lesser Antilles today

By: JeffMasters, 2:18 PM GMT on August 23, 2008

Tropical Storm Fay continues its trek over the Florida Panhandle, and it now centered over land just north of Apalachicola, after making its record 4th landfall on the state this morning. Radar and satellite loops show no major changes to Fay. The storm still has a large circulation with plenty of rain bands sprawling over much of northern Florida and southern Georgia. The center of Fay should emerge over the Gulf of Mexico just west of Panama City this afternoon, and we can expect a modest increase in strength as it feeds off the warm Gulf waters. Yesterday, Fay intensified from 45 mph to 50 mph when its center emerged over the water for 12 or so hours, and I expect a 5-15 mph increase in the winds by Sunday evening, before the center moves back over land again.

Fay's rains
Fay's rains continue to pile up over northern Florida, but have mostly ended over the Melbourne region, which endured it's greatest single-storm rainfall on record. Fay dumped an official 22.83" of rain on Cape Canaveral. According to the latest public information statement from the NWS office in Melbourne, we have an unofficial public observation at Melbourne/Windover Farms (through 6 am 8/22/08) of 26.65". The previous rainfall record for a tropical cyclone in the region was set in 1950, when Hurricane King dumped 15.44" of rain on Patrick Air Force Base near Cape Canaveral. Hurricane Wilma of 2005 holds third place--it dumped 13.26" on Kennedy Space Center.

Several locations around Cape Canaveral reported rainfall amount in excess of 20 inches, including four personal weather stations that send data to the Weather Underground:

Satellite Beach: 23.02"
@The-Beach, Cocoa Beach: 22.10"
LongWood, Melbourne: 21.44"
Jericho Backyard, Palm Bay:20.93"

Rainfall amounts in the Jacksonville region have alse exceeded 20 inches in some spots, according to radar estimates. On the plus side, Fay's rains have now increased the level of Lake Okeechobee by 1.6 feet. The lake level now stands at 12.83 feet, which is still about 2 feet below normal.


Figure 1. One of Fay's tornadoes touched down on Friday, August 22, at 6:25 pm EDT, at 3000 Camp Posalie Road near Kissimmee, in Polk County. The tornado damaged four mobile homes and a bridge. Image credit: Polk County Sheriff's office.

Fay's tornadoes
Fay has produced a total of 15 tornadoes so far: 4 on Monday, 5 on Tuesday, and 6 on Friday. More tornadoes are possible over the next three days. Figure 1 shows a tornado photo kindly forwarded to me by Bryan Farrow, a freelance news photographer in Florida. The tornado touched down yesterday in Polk County, damaging four homes and a bridge. NOAA's Storm Prediction Center has tornado reports, and we've also added tornado reports to our interactive Wundermap. It's essentially a clone of our Tornado page, showing current tornado warnings, storm reports, and past tornadoes back to the 1950s. It also shows radar loops (when available) and photos from WunderPhotos of the storm, damage, etc.

Links to follow
Wundermap for Central Florida
Tallahassee, FL radar

Disturbance 94L approaching the Lesser Antilles
Heavy thunderstorms associated with a tropical wave (94L) near 11N, 57W, about 200 miles east of the Lesser Antilles Islands, have steadily grown more concentrated over the past day. This morning's QuikSCAT pass showed no closed circulation, but did show a pronounced wind shift associated with the disturbance. Top sustained winds were about 30 mph. Visible satellite loops show no evidence of rotation, and the area covered by heavy thunderstorms is relatively modest. The storm has moistened its environment considerably, and dry air should be less and less of a problem for the storm over the next few days. Wind shear has fallen to a modest 10 knots over 94L and is expected to remain in the low range, 5-10 knots, for the next four days. NHC is giving this system a medium (20%-50% chance) of developing into a tropical depression by Monday morning.

The forecast for 94L
The GFDL model, which with yesterday's run was developing 94L into a powerful hurricane that threatens Jamaica, is no longer developing the storm. The ECMWF, NOGAPS, and Canadian model all develop 94L. They foresee the system will enter the eastern Caribbean Sunday, jog northwest and affect the Dominican Republic on Tuesday, then develop into a tropical depression by Thursday near the Dominican Republic or eastern Bahama Islands. The wind shear forecast for the Caribbean calls for very low values of wind shear below 5 knots for most of the coming week (Figure 2). Residents of the Lesser Antilles can expect heavy rain and 40 mph wind gusts from 94L when it blows through Saturday night through Sunday. Residents of the Dominican Republic, Haiti, and Jamaica should keep a careful eye on this potentially dangerous disturbance.


Figure 2. Wind shear forecast for the Atlantic for 2 am EDT Monday August 25 2008, as predicted by this morning's 2 am EDT run of the GFS model. Very low values of winds shear (red colors) are predicted for the entire Caribbean and Gulf of Mexico. Winds shear values less than 8 m/s (approximately 16 knots, the three red colors) are conducive for troical storm formation.

Disturbance 95L in the middle Atlantic
Another tropical wave (95L), near 19N 41W, halfway between Africa and the Lesser Antilles Islands, remains disorganized. Last night's QuikSCAT pass showed only a slight wind shift associated with 95L, and no closed circulation. Top winds were about 30 mph. Wind shear is a modest 10 knots over 95L, and is forecast to remain near 10 knots for the next five days. Visible satellite loops show only a small amount of heavy thunderstorm activity, and 95L is battling a large amount of dry air that surrounds the storm and is interfering with development. It will be several days before the storm can moisten it environment enough to potentially permit development, and NHC has downgraded 95L's chance of developing into a tropical depression in the next two days to low (<20% chance).

My next blog will be Sunday, or later today, if there's some significant development to report.

Jeff Masters

Updated: 6:09 PM GMT on August 24, 2008

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Fay chugs on; potentially dangerous disturbance 94L approaches Lesser Antilles

By: JeffMasters, 9:23 PM GMT on August 22, 2008

Tropical Storm Fay continues its leisurely tour of Florida, chugging slowly westward along the northern Gulf Coast. Radar and satellite loops show no major changes to Fay. The storm still has a large circulation with plenty of rain bands sprawling over much of northern Florida and southern Georgia. Fay is headed towards the waters of the extreme northern Gulf or Mexico near the Big Bend region of Florida, and should be able to pull in enough moisture from the Gulf to at least maintain its current strength through Sunday. Fay may be able to intensify to a 50-55 mph tropical storm, but I don't see it reaching hurricane strength, due to the close proximity of the storm to land.

Fay's rains
Fay has brought the Melbourne, Florida region its greatest single-storm rainfall on record. By 1 am EDT today, Fay had dumped 22.83" of rain on Cape Canaveral. The previous rainfall record for a tropical cyclone in the region was set in 1950, when Hurricane King dumped 15.44" of rain on Patrick Air Force Base near Cape Canaveral. Hurricane Wilma of 2005 holds third place--it dumped 13.26" on Kennedy Space Center.

Fay is also one of Florida's rainiest storms on record. According to Wikipedia and NOAA, the eleven rainiest Florida tropical cyclones of all time were:

Easy (1950) 38.70" Yankeetown
Georges (1998) 38.46" Munson
Unnamed (1941) 35.00" Trenton
Dennis (1981) 25.56" Homestead
TD 1A (1992) 25.00" Arcadia Tower
Jeanne (1980) 24.98" Key West
Dora (1964) 23.73" Mayo
TD (1969) 23.40" Havana
Unnamed (1924) 23.22" Marco Island
Bob (1985) 21.50" Everglades City
Alberto (1994) 21.38 Niceville

According to the latest public information statement from the NWS office in Melbourne, we have an unofficial public observation at Melbourne/Windover Farms (through 6 am 8/22/08) of 26.65". If verified, that would make Fay the 4th rainiest Florida tropical cyclone on record.

Rainfall over the Melbourne area will continue Saturday, but should gradually diminish as Fay moves away from the area.


Figure 1. Satellite image of Fay stalled out over the Melbourne, Florida region at 2:35 pm EDT Wednesday, August 20, 2008. Image credit: NASA Earth Observatory.

Links to follow
Wundermap for Central Florida
Tallahassee, FL radar

Disturbance 94L approaching the Lesser Antilles
Heavy thunderstorms associated with a tropical wave (94L) near 11N, 54W, about 500 miles east of the Lesser Antilles Islands, have grown more concentrated in recent hours. This disturbance could be trouble, and bears careful scrutiny. This morning's QuikSCAT pass mostly missed 94L, but did show a pronounced wind shift at the western edge of the disturbance. Visible satellite loops show some evidence of rotation and increase in heavy thunderstorms, but I'm also seeing surface outflow boundaries to the southeast, indicating that dry is getting sucked into the thunderstorms and creating downdrafts that rob the disturbance of energy. Wind shear has fallen to a modest 10 knots over 94L, and the dry air surrounding it has been steadily moistening. Wind shear is expected to drop to a low 5 knots and remain low for the next four days, and NHC is giving this system a medium (20%-50% chance) of developing into a tropical depression by Sunday afternoon. I would put the odds at 40%-70%. Three of our reliable computer models develop the storm. It's easy to see why--the wind shear forecast for the Caribbean calls for very low values of wind shear below 5 knots for most of the coming week (Figure 2). The GFDL calls for 94L to develop into a Category 2 hurricane that threatens Jamaica on Tuesday. The HWRF doesn't develop 94L at all, and the ECMWF and NOGAPS models call for 94L to eventually develop late next week after it moves north of Hispaniola, into the Bahama Islands. All of these models used the wrong starting position of 94L, since the storm re-formed 150 miles to the east of where the models were expecting it to be. Thus, we can put little faith in the details of forecast track of 94L predicted by the models. A track through the Caribbean towards Jamaica currently appears to be the highest probability track to me. Residents of the Lesser Antilles can expect heavy rain and 40 mph wind gusts from 94L when it blows through Saturday and Sunday. Residents of the Dominican Republic, Haiti, and Jamaica should keep a careful eye on this potentially dangerous disturbance.


Figure 2. Wind shear forecast for the Atlantic for 2 am EDT Monday August 25 2008, as predicted by this morning's 2 am EDT run of the GFS model. Very low values of winds shear (red colors) are predicted for the entire Caribbean and Gulf of Mexico. Winds shear values less than 8 m/s (approximately 16 knots, the three red colors) are conducive for tropical storm formation.

Disturbance 95L in the middle Atlantic
Another tropical wave (95L), near 19N 41W, halfway between Africa and the Lesser Antilles Islands, remains disorganized. This morning's QuikSCAT pass showed only a slight wind shift associated with 95L , and no closed circulation. Top winds were about 30 mph. Wind shear is a modest 10 knots over 95L, and is forecast to remain near 10 knots for the next five days. Visible satellite loops show only a small clump of heavy thunderstorm activity, and NHC has downgraded 95L's chance of developing into a tropical depression in the next two days to low (<20% chance).

My next blog will be Saturday.

Jeff Masters

Updated: 9:36 PM GMT on August 22, 2008

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Fay continues its tour of Florida, dropping prodigious rains along the way

By: JeffMasters, 2:39 PM GMT on August 21, 2008

The rains are beginning to pick along Florida's northern Gulf Coast, as Tropical Storm Fay continues its extensive tour of the state. Radar and satellite loops show that Fay has changed little during its trek across northern Florida, and still has a large circulation with plenty of rainbands sprawling over much of northern Florida and southern Georgia. Fay is headed towards the waters of the extreme northern Gulf or Mexico near the Big Bend region of Florida, and should be able to pull in enough moisture from the Gulf to at least maintain its current strength through Sunday. Fay may be able to intensify to a 60-65 mph tropical storm, but I don't see it reaching hurricane strength, due to the close proximity of the storm to land. Fay is the first storm since Donna of 1960 to make three landfalls in Florida. If Fay crosses the Big Bend region of the Gulf of Mexico and then makes landfall again around Apalachicola, FL, it will be the first tropical cyclone on record to hit the same state four times. "The Joker" has it in for the state of Florida!

Fay's rains
Fay has brought the Melbourne, Florida region its greatest single-storm rainfall on record. By 1 am EDT today, Fay had dumped 22.83" of rain on Cape Canaveral. The previous rainfall record for a tropical cyclone in the region was set in 1950, when Hurricane King dumped 15.44" of rain on Patrick Air Force Base near Cape Canaveral. Hurricane Wilma of 2005 holds third place--it dumped 13.26" on Kennedy Space Center.

Fay is also one of Florida's rainiest storms on record. According to Wikipedia and NOAA, the eleven rainiest Florida tropical cyclones of all time were:

Easy (1950) 38.70" Yankeetown
Georges (1998) 38.46" Munson
Unnamed (1941) 35.00" Trenton
Dennis (1985) 25.56" Homestead
TD 1A (1992) 25.00" Arcadia Tower
Jeanne (1980) 24.98" Key West
Dora (1964) 23.73" Mayo
TD (1969) 23.40" Havana
Unnamed (1924) 23.22" Marco Island
Bob (1985) 21.50" Everglades City
Alberto (1994) 21.38 Niceville

According to the latest public information statement from the NWS office in Melbourne, we have an unofficial public observation at Melbourne/Windover Farms (through 5 am 8/21/08) of 26.20". If verified, that would make Fay the 4th rainiest Florida tropical cyclone on record.

Rainfall over the Melbourne area will continue today, but should gradually diminish as Fay moves away from the area.


Figure 1. Current radar-estimated precipitation from Fay. Fifteen to twenty inches of rain has fallen just northwest of Melbourne, Florida.

Links to follow
Wundermap for Central Florida
Tallahassee, FL radar
Melbourne, FL weather

Disturbance 94L approaching the Lesser Antilles
A tropical wave (94L), 400 miles east of the Lesser Antilles Islands, consists of a disorganized region with three clumps of heavy thunderstorms. This one could be trouble, and we'll have to watch it carefully. This morning's QuikSCAT pass mostly missed 94L, but did show a pronounced wind shift at the western edge of the disturbance. Visible satellite imagery shows little chance to this disturbance so far today. Wind shear has fallen to a modest 10 knots over 94L, and the dry air surrounding it has been steadily moistening. Wind shear is expected to drop to a low 5 knots and remain low for the next four days, and NHC is now giving this system a medium (20%-50% chance) of developing into a tropical depression by Sunday morning. Once 94L moves into the eastern Caribbean on Sunday, three of our reliable computer models are developing the storm. Puerto Rico could see rains from 94L by Monday.

Disturbance 95L in the middle Atlantic
Another tropical wave (95L), near 18N 39W, halfway between Africa and the Lesser Antilles Islands, remains disorganized. This morning's QuikSCAT pass showed only a slight wind shift associated with 95L , and no closed circulation. Top winds were about 30 mph. Wind shear is a modest 10 knots over 95L, and is forecast to remain near 10 knots for the next five days. Visible satellite images show only a small clump of heavy thunderstorm activity. NHC has given 95L a moderate (20%-50% chance) of developing into a tropical depression by Sunday morning. None of the computer models develop 95L.


Figure 2. Satellite image of 94L (region 1) and 95L (region 2) as depicted by NHC's Graphical Tropical Weather Outlook.

The next blog? I'm not sure, since I'm traveling today, but it will be either late this afternoon or Saturday by 10:30 am EDT.

Jeff Masters

Updated: 1:52 PM GMT on August 22, 2008

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Fay Center Stuck Onshore

By: JeffMasters, 3:27 PM GMT on August 20, 2008

Forecasting the intensity of Fay continues to be difficult. Last night, Fay moved very slowly, and at times was almost stationary. The result was that Fay's center never emerged fully out into the Atlantic. Instead, Fay continued to weaken as a storm normal should over land. Winds have fallen, central pressure is closer to tropical depression strength, and there is no eye structure left.

The current ragged look is best demonstrated on radar:




Figure 1. Melbourne NEXRAD Base Reflectivity.

The Forecast
Fay may finally move over water later today for a short time, which could allow for some strengthening. Land interaction and the fact that the center isn't fully over water both are restricting chances for intensification. Right now rain bands seem to have intensified slightly on the southern side of the storm, though. Chances of hurricane strength are very limited (<10 %). Fay is the "Joker" after all, so stay tuned. Shortly after moving off Cape Canaveral, Fay will drift back inland on a northwest or westerly track. Heavy rain will continue to be the biggest threat to Florida, with highest amounts hitting the northern third of the state, especially near the Atlantic Coast.



Elsewhere in the Tropics
Invest 94L has limited thunderstorm activity this morning. Dry air, and a future full of wind shear gives 94L a smaller and smaller chance of developing as the days pass.

Updated: 1:50 PM GMT on August 22, 2008

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"The Joker" (Fay) intensifies over land!

By: JeffMasters, 8:10 PM GMT on August 19, 2008

Tropical Storm Fay (AKA "The Joker") is pulling a trick that may be unprecedented--significantly intensifying over land, developing a full eyewall. The radar and satellite images of Fay this afternoon (Figures 1 and 2) show a much better-organized storm than the Fay that made landfall this morning. Fay now has a symmetric appearance with a full eyewall, and the winds near the center were sustained at 60 mph this afternoon at Lake Okeechobee. These winds are higher than anything measured at landfall this morning. Remarkably, the pressure has fallen over 10 mb since landfall, and I can't ever recall seeing such a large pressure fall while a storm was over land. Hurricane Andrew of 1992 crossed South Florida and did not weaken significantly, but "The Joker" has significantly intensified. It does happen sometimes that the increased friction over land can briefly act to intensify a hurricane vortex, but this effect is short-lived, once the storm is cut off from its oceanic moisture source. To have a storm intensify over land and maintain that increased intensity while over land for 12 hours is hard to explain. The only thing I can think is that recent rains in Florida have formed large areas of standing water that the storm is feeding off of. Fay is also probably pulling moisture from Lake Okeechobee. Anyone want to write a Ph.D. thesis on this case? Wow.


Figure 1. What's wrong with this picture? Radar image of Fay over Lake Okeechobee.


Figure 2. What's wrong with this picture, too? Satellite image of Fay over Lake Okeechobee. Fay has a distinct eye on both satellite imagery and radar.

Where will Fay go next?
The computer models are now in better agreement that Fay will emerge over the Atlantic Ocean on Wednesday, and re-intensify. Given the remarkable ability of Fay to intensify over land, I am more of a believer that Fay could become a hurricane over the Atlantic, as forecast by the GFDL and HWRF models. However, the SHIPS intensity model keeps Fay below hurricane strength, and the very slow motion of the storm while over the ocean will likely stir up cold water from the depths, significantly hampering intensification. Wind shear is expected to be 10-20 knots Thursday and Friday, which should prevent rapid intensification, but allow slow to modest intensification. After stalling out off the coast, all of the models agree that a ridge of high pressure will build back in, forcing Fay to the west over northern Florida or southern Georgia. I continue to support a 60% chance that this turn will occur far enough south that Fay will emerge into the northern Gulf of Mexico early next week, and re-intensify.

Links to follow
Wundermap for Central Florida

Elsewhere in the tropics
A tropical wave about 100 0miles west-southwest of the Cape Verde Islands has changed little today. This system (94L) shows signs of rotation on visible satellite imagery. Wind shear is a modest 10 knots over this disturbance, and is expected to remain about 10 knots through Wednesday. The storm is over warm 28°C waters. Given these moderately favorable conditions, NHC is giving this system a medium (20%-50% chance) of developing into a tropical depression by Thursday afternoon. The storm is expected to track west-northwest and be near the northern Lesser Antilles Islands by Friday night. The wind shear forecast has been flip-flopping from low to high and back to low again over the past two days, so I will cautiously forecast some slow development over the next few days, assuming that the current forecast of 10-15 knots will hold over the next 3-5 days. There is a large area of dry air and Saharan dust to the northwest of the storm, as seen on water vapor satellite imagery. This dry air is already interfering with development, and likely will continue to do so over the next three days. The GFDL and HWRF develop 94L into a weak tropical storm.

The next blog will be Wednesday, and may not be until the afternoon. I may have one of the other wunderground meteorologists fill in for me for tomorrow's first blog, then jump in Wednesday evening if Fay looks dangerous.

Jeff Masters

Updated: 8:17 PM GMT on August 19, 2008

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Fay hits Southwest Florida, but never makes it to hurricane strength

By: JeffMasters, 1:04 PM GMT on August 19, 2008

Tropical Storm Fay plowed ashore just south of Naples, Florida at 5 am EDT, as a tropical storm with 60 mph winds. The radar image of Fay at landfall (Figure 1) shows that the storm was never able to form a complete eyewall, which limited it's ability to intensify. Wind shear was the primary reason Fay never got its act together.

Fay is headed north-northeast along the Florida Peninsula, but was still generating sustained winds of 42 mph at the coast at Naples 7 am EDT. The highest winds observed at Naples during landfall were sustained at 46 mph. Key West got Fay's brunt yesterday--winds reached 60 mph shortly before the center passed over around 5 pm.


Figure 1. Radar image of Fay at landfall.


Figure 2. Latest radar-estimated precipitation for Fay.

Impact on Florida
The main concern in Florida will be Fay's rains. Radar precipitation estimates (Figure 2) show that Fay has dumped 8-10 inches of rain over a large swath of Palm Beach County. Four to eight inches were common over the rest of South Florida. These heavy rains will spread northward into Central Florida today, and into northern Florida on Wednesday. One concern I have is that Fay may stall out over northern Florida with its center just offshore Wednesday through Friday, potentially dumping large amounts of flooding rains. However, Fay's rains should be beneficial for Central Florida, where Lake Okeechobee is three feet below normal, and water shortages are a problem.

Where will Fay go next?
The computer models continue to disagree on the long-term path of Fay. Most of the models now agree that the trough of low pressure pulling the storm to the north will not be strong enough to finish the job. Fay will probably be left behind by the trough, and forced westward by a ridge of high pressure expected to build in. The official NHC forecast keeps Fay over Florida and turns the storm to the west over southern Georgia. However, most of the models expect this turn to occur further south, and have been trending further south in recent runs. I give Fay a 60% chance surviving its traverse over Florida, then turning back to the west over the northern Gulf of Mexico by Saturday. This would allow the storm to regenerate, before potentially making landfall again along the northern Gulf Coast between New Orleans and the Florida Panhandle early next week. The GFDL has the rather unnerving solution of pushing Fay out into the Atlantic Wednesday, intensifying it to a strong Category 2 hurricane that then hits the Georgia/Florida border region Friday night. The HWRF is similar, but foresees that Fay will only be a 50-55 mph tropical storm when it hits Georgia. Given the rather high levels of wind shear (15-20 knots) predicted to be over Fay late this week, plus Fay's recent inability to build an eyewall in the 24 hours it had over the Florida Straits, I am discounting the GFDL model's solution of a hurricane hitting Georgia. However, Fay may push out to into the Atlantic far enough to regenerate some on Thursday or Friday, before it is forced back west again over northern Florida or southern Georgia.

Links to follow
Wundermap for Central Florida
Naples, FL weather
Melbourne, FL radar

Elsewhere in the tropics
A tropical wave near 13N 38W, about 900 miles west-southwest of the Cape Verde Islands, has lost most of its heavy thunderstorm activity this morning. This system (94L) shows signs of rotation on visible satellite imagery. Wind shear is a modest 10 knots over this disturbance, and is expected to remain about 10 knots through Wednesday. The storm is over warm 28°C waters. Given these moderately favorable conditions, NHC is giving this system a medium (20%-50% chance) of developing into a tropical depression by Thursday morning. The storm is expected to track west-northwest and be near the northern Lesser Antilles Islands by Friday night. Wind shear is forecast to increase to 20-30 knots over the storm Thursday-Saturday, which should halt any development. There is a large area of dry air and Saharan dust to the northwest of the storm, as seen on water vapor satellite imagery. This dry air is already interfering with development, and likely will continue to do so over the next three days. The GFDL and HWRF develop 94L into a weak tropical storm, but none to the other models develop it. I don't expect this storm will be a problem for the Lesser Antilles.

The next blog will be Wednesday, and may not be until the afternoon. I'm probably going to take the day off and let one of the other wunderground meteorologists fill in for me. It's time to take a break from "The Joker"!

Jeff Masters

Updated: 1:30 PM GMT on August 19, 2008

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Fay pounds the Florida Keys, begins to form an eyewall

By: JeffMasters, 7:36 PM GMT on August 18, 2008

Tropical Storm Fay is pounding the Florida Keys with high winds and heavy rain, as it churns north-northwestward towards a landfall in Southwest Florida. Winds at the Sand Point Buoy near Key West were sustained at 56 mph last hour, and the center of Fay passed over Key West at about 3:30 pm EDT. Sombrero Key buoy, at an elevation of 159 feet, reported sustained winds of 60 mph last hour. The latest Hurricane Hunter missions have found a large areas of 50-60 mph winds in the heaviest thunderstorms. However, the pressure has not fallen significantly, and was 1001 mb at 3pm.

From wunderground member keeywester:

Wow...Fay is really hitting us hard right now (1:52 p.m.) I'm guessing we are sustained at close to 50 with higher gusts out of the ENE. I have water spilling in under my east facing door on the third floor. We are in a really intense band and visibility is really low. Just thought I'd give you a quick update.

Dan Hogberg in Key West (Sunset Marina Condos)


Indeed, radar estimated rainfall (Figure 1) shows that about 5 inches of rain fell in Key West in just a few hours this afternoon. Long-range radar out of Key West shows that Fay is beginning to build an eyewall, which is about 40% complete. Fay's spiral bands are steadily organizing and growing more plentiful. Visible satellite loops show a lack of heavy thunderstorm activity and upper-level outflow on Fay's west side, where wind shear from an upper-level low to the west is interfering with the storm. Wind shear of 10-15 knots is expected to continue over Fay until the storm makes landfall on the southwest coast of Florida. This amount of wind shear usually means only modest intensification can occur, and Fay is unlikely to make it to Category 2 strength (96 mph or greater).


Figure 1. Latest radar-estimated precipitation for Fay.

Impact on the Caribbean islands
Fay dumped up to 8 inches of rain in southern Haiti, the southern Dominican Republic, and central Cuba, according to satellite estimates. The resulting flooding has probably killed at least 36 people. Thirty of those deaths came in Haiti when an overloaded bus attempted to cross a flooded river and flipped. Flooding from Fay also killed two people in Jamaica and four people in the Dominican Republic.

Impact on the west coast of Florida
The west coast of Florida will take the full brunt of Fay's fury early Tuesday morning. With the exception of the NOGAPS model, the computer models have finally come into agreement on a landfall, with their 8 am EDT runs. The consensus is that landfall will be in Southwest Florida, between Marco Island and Fort Myers. Marco Island is my guess of a landfall location given the trend of Fay to track to the right of the forecast. With Fay's expected track nearly parallel to the shore, though, a slight deviation in the path will result in a large change in where the eye comes ashore. Storm surge will be a major concern to the region just to the right of where the center comes ashore. For the Naples region, a Category 1 hurricane can generate storm surges of 4-8 feet (Figure 2). Both the GFDL and HWRF models are forecasting a Category 1 hurricane with winds of 80-90 mph at landfall. Fay does not have time to intensify into a Category 3 hurricane before a landfall in Southwest Florida. If Fay follows a track further north and makes landfall near Tampa Bay, as the NOGAPS model predicts, there is a 10% chance it couuld intensify into a Category 3 hurricane.

South Carolina? New Orleans? Where will Fay go next?
The computer models continue to show an unusual amount of disagreement about the longer term path of Fay. The official NHC forecast follows the GFDL and HWRF models, which takes Fay northwards through the Florida Peninsula. However, the latest runs of these models now predict Fay will emerge off the east coast of Florida, restrengthen a bit to a 60-70 mph tropical storm, then make landfall Wednesday along the Georgia/South Carolina coast. This solution assumes that the trough of low pressure turning Fay northward will be strong and enough and be moving slow enough to pull Fay all the way northwards into the U.S.

A weaker trough is predicted by the rest of the models, which foresee that Fay will stall over central Florida or the adjacent Atlantic Ocean on Wednesday. A ridge of high pressure will then build in, forcing Fay westward across the northern Gulf of Mexico. A second landfall in the Florida Panhandle or in Louisiana near New Orleans is then a possibility. Since more and more of the models are trending this way, I believe this solution has an equal chance of being correct. "The Joker" may be around to trouble us for another full week or longer.

Links to follow
Wundermap for the Florida Keys
Key West, FL weather
Naples, FL weather
Fort Myers, FL weather
Miami, FL weather


Figure 2. Worst-probable storm tide inundation (inundation from storm surge plus an adjustment for the mean high tide that occurs in the Naples area). The colors scale to various Category hurricanes--1, 2, 3, 4, and 5. The map was compiled using data from NOAA's SLOSH model. Image credit: floridadisaster.org.


Figure 3. Worst-probable storm tide inundation (inundation from storm surge plus adjustment for the mean high tide that occurs in the Fort Myers area). The colors scale to various Category hurricanes--1, 2, 3, 4, and 5. The map was compiled using data from NOAA's SLOSH model. Image credit: floridadisaster.org.

Elsewhere in the tropics
A tropical wave near 13N 36W, about 700 miles west-southwest of the Cape Verde Islands, has gotten better organized this afternoon. This system (94L) shows signs of rotation on visible satellite imagery, and heavy thunderstorm activity has steadily increased near the center in recent hours. Wind shear is a low 5-10 knots over this disturbance, and is expected to drop below 5 knots Tuesday. The storm is over warm 28°C waters, and is in a moist environment, so continued development is likely. NHC is currently giving this system a medium (20%-50% chance) of developing into a tropical depression by Wednesday afternoon. This storm is expected to track west-northwest and be near the Lesser Antilles Islands on Saturday. Wind shear is forecast to remain below 15 knots for the next five days, which will favor development. However, there is a large area of dry air and Saharan dust to the northwest of the storm, as seen on water vapor satellite imagery, and this will probably begin to interfere with 94L's development by Wednesday. The GFDL, HWRF, and SHIPS models all develop 94L into a tropical storm that moves just north of the Lesser Antilles Islands by Saturday.

I'll have updates throughout the day.

Jeff Masters

Updated: 1:08 PM GMT on August 19, 2008

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Fay crosses into Florida Straits and intensifies

By: JeffMasters, 1:09 PM GMT on August 18, 2008

Tropical Storm Fay has moved off the coast of Cuba into the Florida Straits, and is already picking up strength. The latest Hurricane Hunter mission found a large area of 50-60 mph winds in the heavy thunderstorm activity over the Florida Straights, between 8-9 am EDT. Long-range radar out of Key West shows the center of Fay's rotation right on the north coast of Cuba, south-southeast of Key West. The radar signature of the storm shows plenty of spiral bands bringing heavy rain (Figure 1), but these bands are not well-organized yet. Visible satellite loops show that Fay has no eyewall yet, and it will not be until late tonight that the storm can build an eyewall and intensify to a Category 1 hurricane. The upper-level anticyclone that was on top of Fay is now to the south of the storm, which means that the clockwise circulation of air around this anticyclone is bringing about 10-15 knots of wind shear from the southwest over Fay. Wind shear of 10-15 knots is expected to continue over Fay until the storm makes landfall on the west coast of Florida. This amount of wind shear usually means only modest intensification can occur, and we should not see a repeat of Hurricane Charley's rapid intensification.


Figure 1. Latest radar-estimated precipitation for Fay.

Impact on the Florida Keys and South Florida
Winds are steadily increasing throughout the Keys, with sustained winds of 28 mph reported at Sombrero Key at 7 am EDT. The Wundermap for the Florida Keys is a good way to look at all the latest wind reports for the region. Wind gusts in excess of hurricane force are possible this afternoon in the Keys as the center of an intensifying Fay roars through the Upper Keys. Over the Miami metro area, winds will gust above tropical storm force, but it will be the rain, not the wind, that will cause the most trouble. Rainfall estimated by the Key West radar (Figure 1) is already at 5+ inches over the Florida Straights, and this heavy rain will be moving over South Florida today, causing extensive freshwater flooding. Southeast Florida is vulnerable to freshwater flooding damage--Hurricane Irene of 1999 dumped 10-20 inches of rain on the Miami area, causing $600 million in damage. Storm surge will not be a major concern in the Keys or Southeast Florida, with only a 2-3 foot storm surge likely in the Keys, and 1-2 feet in Southeast Florida.

Impact on the west coast of Florida
Fay's primary havoc will occur on the west coast of Florida, but it is too early to be confident where the storm might come ashore. Fay's expected track nearly parallel to the shore will mean that slight deviations in the path will result in large changes in where the eye comes ashore. Storm surge will be a major concern to the region just to the right of where the center comes ashore. As I discussed in great detail in last night's blog, the Tampa Bay area is highly vulnerable to storm surge, and can expect maximum surges from a Category 1 hurricane of 6-9 feet, with an additional 1-2 if the storm hits near high tide. Storm surge is also a major concern along all of Southwest Florida, including Fort Myers, Naples, and Venice. For the Naples region, a Category 1 hurricane can generate storm surges of 4-8 feet (Figure 2). Fort Myers is also highly vulnerable to storm surge (Figure 3).

The computer models continue to show and unusual amount of disagreement. The outliers in the 8 pm EDT runs continue to be the NOGAPS and UKMET models, which continue to forecast a landfall on the mid-Florida Panhandle or Big Bend area of Florida. In contrast, the 2 am EDT run of the GFS model is doing what the ECMWF model was doing yesterday--taking Fay across central Florida into the Atlantic, then bringing it back across the state from east to west, then into the Panhandle, resulting in a triple hit on Florida. Although these solutions are outliers, we cannot dismiss them. The latest 2 am EDT HWRF model run has moved considerably further south, bringing Fay ashore near Fort Myers as a 70-75 mph storm with a 977 mb pressure. The latest GFDL is almost identical, bringing a 75-80 mph Category 1 hurricane with a 977 mb pressure ashore near Fort Myers. Both models then anticipate a possible threat to South Carolina later in the week after Fay crosses Florida and emerges in the Atlantic. Right now, the best guess is that Fay will hit the coast between Naples and Sarasota, largely sparing the Tampa Bay region.

Links to follow
Wundermap for the Florida Keys
Key West, FL weather
Miami, Fl weather
Naples, FL weather


Figure 2. Worst-probable storm tide inundation (inundation from storm surge plus an adjustment for the mean high tide that occurs in the Naples area). The colors scale to various Category hurricanes--1, 2, 3, 4, and 5. The map was compiled using data from NOAA's SLOSH model. Image credit: floridadisaster.org.


Figure 3. Worst-probable storm tide inundation (inundation from storm surge plus adjustment for the mean high tide that occurs in the Fort Myers area). The colors scale to various Category hurricanes--1, 2, 3, 4, and 5. The map was compiled using data from NOAA's SLOSH model. Image credit: floridadisaster.org.

Elsewhere in the tropics
The UKMET model is suggesting development of a tropical wave currently located off the coast of Africa, about 600 miles west-southwest of the Cape Verde Islands. This system (94L) is expected to track west-northwest and be near the Lesser Antilles Islands on Saturday. Wind shear is a low 5-10 knots over this disturbance, and some slow development is likely over the next two days.

I'll have updates throughout the day.

Jeff Masters

Updated: 1:26 PM GMT on August 18, 2008

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Storm surge risk for Tampa Bay

By: JeffMasters, 1:21 AM GMT on August 18, 2008

One possible landfall location for Fay is at Tampa Bay. This is of major concern, because over 3 million people live the region, and it is highly vulnerable to storm surge-- particularly for a storm moving northeast or north-northeast at landfall, as Fay is likely to be moving. This vulnerability results from the long stretch of shallow Continental Shelf waters offshore, which allow large surges to pile up. A surge in deeper water can be dispersed down and out away from the hurricane. However, once that surge reaches a shallow, gently sloping shelf, it can no longer be dispersed away from the hurricane. Consequently, water piles up as it is driven ashore by the wind stresses of the hurricane. Even a Category 1 hurricane can create significant surges--up to 7' in Hillsborough County, 6' in Manatee County, 7' in Pinellas County, and 9' in Pasco County. An extreme Category 5 hurricane can create a storm surge of 28' in Hillsborough County. These storm surge heights are computed from NOAA's Sea, Lake and Overland Surges from Hurricane (SLOSH) numerical storm surge prediction model. The data reflects only still water saltwater flooding. Local processes, such as waves, rainfall and flooding from overflowing rivers, are usually included in observations of storm surge height, but are not surge and are not calculated by the SLOSH model.

When compared to observations, the surge computed by the SLOSH model is in error on average, by 1.4 feet, according to a study by Jarvinen and Lawrence (1985). The maximum errors in the study, which looked at 523 sites during 10 hurricanes, were -7.1 feet and +8.8 feet. About 79% of the errors were within 2 feet of the predicted value, though. Errors primarily came from three factors:

1) Maps that are outdated, which may result in inaccuracies of topography or bathymetry
2) Anomalous water heights which can affect the local sea level
3) Local processes, such as waves, astronomical tides, rainfall and flooding from overflowing rivers, which are not calculated by the SLOSH model

The SLOSH model is run hundreds of times, with different storm speeds, directions of motion, and landfall location. The maximum surge height for all of these runs are then compiled into a Maximum of Maximums (MOM), which is then adjusted upward by two feet, for the observed range between average low tide and high tide in the Tampa Bay area (storm surge plus this high tide adjustment is called storm tide). The resulting storm tide map showing the locations likely to be inundated for a mid-strength hurricane of Category 1, 2, 3, 4, 5 is shown in Figure 1.


Figure 1. Worst-probable storm tide inundation (inundation from storm surge plus a 2-foot adjustment for the mean high tide that occurs in the Tampa Bay area). The colors scale to various Category hurricanes--1, 2, 3, 4, and 5. The map was compiled using data from NOAA's SLOSH model. Image credit: Tampa Bay Regional Planning Council.

Individual SLOSH runs are also of interest, and these are usually plotted up to show the Maximum Envelope Of Waters (MEOW) for a particular hurricane category, forward speed, and direction of movement. These are not adjusted for high tide, so one should adjust these numbers upwards by 2 feet if the storm were to hit at high tide. The current NHC forecast puts Fay at its closest to Tampa about 2 pm Tuesday afternoon, which also happens to be the time of high tide at the entrance to Tampa Bay. The MEOWs for Tampa Bay for a Category 1 and 2 storm moving NNE at 15 mph are shown in Figures 2 and 3. If Fay hits Tampa, the storm is likely to be moving NNE, but at a slower forward speed. A slower moving storm typically brings a higher surge into bays and inlets, since the slower motion allows more time to pump water into these bays. For the case of Tampa Bay, though, the SLOSH model shows little difference in surge for a Category 2 storm moving at 12 mph versus a Category 2 storm moving at 15 mph.


Figure 2.Maximum storm surge in feet (no adjustment for high tide) expected in the Tampa Bay area for a Category 1 hurricane moving NNE at 15 mph. The map was compiled using data from NOAA's SLOSH model. Image credit: Tampa Bay Regional Planning Council.


Figure 3.Maximum storm surge in feet (no adjustment for high tide) expected in the Tampa Bay area for a Category 2 hurricane moving NNE at 15 mph. The map was compiled using data from NOAA's SLOSH model. Image credit: Tampa Bay Regional Planning Council.

The current official NHC forecast says Fay will miss Tampa Bay by about 100 miles on Tuesday afternoon. At Category 1 strength, the predicted storm surge is 3-5 feet in Tampa Bay.

I'll have an update on Fay by 9 am EDT Monday morning.

Jeff Masters

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Fay is not strengthing much yet

By: JeffMasters, 8:40 PM GMT on August 17, 2008

Tropical Storm Fay continues to look unimpressive as it tracks south of Cuba. Radar imagery from Punta del Este, Cuba shows that the low-level spiral bands are sparse and poorly organized. Visible satellite loops show that heavy thunderstorm activity only surrounds about 1/3 of the center of the storm. Fay is in a moderately favorable wind shear environment, with upper-level winds from the west creating about 10 knots of wind shear. Upper level outflow is well-established only to the north and east. The highest surface winds found between 2pm and 4pm EDT by the latest Hurricane Hunter flight were 47 mph. However, the pressure is falling, and stood at 1003 mb at 3:11 pm EDT.

The forecast for Fay
Fay is gradually building an eyewall this afternoon, but probably does not have time to complete this process before crossing Cuba. This means that the storm will have to start this process all over again Monday morning, delaying intensification longer than the models had predicted.

The computer models have come into better agreement, predicting that Fay will stay south of Cuba a bit longer and move further west than expected before turning northwest and crossing the island. This reduces the threat to Key West and Southwest Florida, but increases the threat to the Florida coast between Sarasota and the Florida Panhandle. This also increases the chances that Fay will hit Florida as a hurricane, since it will have more time over water.

The latest (8 am EDT) run of the GFDL model puts Fay ashore Monday night near Sarasota as a Category 1 hurricane with a 976 mb pressure and 80-85 mph winds. The HWRF model foresees a landfall on Tuesday morning further north, past Cedar Key, and makes Fay a strong Category 2 hurricane with a 945 mb pressure and 110 mph winds. Both of these forecasts are probably too intense, given Fay's current state of disorganization. Only the ECMWF model is currently forecasting a motion all the way across the Florida Peninsula and out into the open Atlantic. This model then foresees a triple hit on Florida--motion back across Florida from east to west, followed by a third Florida landfall int he Panhandle. The UKMET and NOGAPS models continue to show a threat to the Florida Panhandle.

If Fay hits the Sarasota/Tampa Bay region, these are the kind of probabilities for intensity I'm thinking:

Tropical storm: 50%
Category 1 Hurricane: 35%
Category 2 hurricane: 10%
Category 3+ Hurricane: 5%

For a landfall further north in the Panhandle, my probabilities are:

Tropical storm: 35%
Category 1 Hurricane: 35%
Category 2 Hurricane: 20%
Category 3+ Hurricane: 10%

Links to follow
Wundermap for Cuba and the Florida Keys
Punta del Este, Cuba radar
Key West, FL weather

Elsewhere in the tropics
Several of the reliable computer models are predicting development of a tropical wave currently located off the coast of Africa, just south of the Cape Verde Islands. This system is expected to track west-northwest and be near or just north of the Northern Lesser Antilles Islands 5-7 days from now.

I'll have an update Monday morning (or later this evening, if there's some significant development to report).

Jeff Masters

Updated: 9:17 PM GMT on August 17, 2008

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Fay slowly intensifying, but not expected to be another Hurricane Charley

By: JeffMasters, 3:22 PM GMT on August 17, 2008

Tropical Storm Fay is slowly growing better organized as it tracks south of Cuba, over the warmest waters of the Atlantic Ocean. Radar imagery from Pico San Juan, Cuba shows that the low-level spiral bands are not yet well-organized. Visible satellite loops show that Fay has only a small amount of heavy thunderstorm activity near its center. Fay is in a slightly less favorable wind shear environment, with upper-level winds from the west creating 10 knots of wind shear. Upper level outflow is well-established only to the north and east, and Fay does not look very impressive at this time.

Yesterday Fay claimed its first victims, when four people died in flooding on the Dominican Republic and Haiti. Satellite rainfall estimates suggest at least six inches of rain have fallen over portions of Hispaniola, with lesser amounts over Cuba and Jamaica. As Fay continues to organize today, heavy rain will increasingly affect Cuba, Jamaica, and the Cayman Islands, potentially causing serious flooding.

The forecast for Fay
It's unusual for the computer models to be so divergent just 48 hours before expected landfall in the U.S. "The Joker" seems intent on keeping us guessing until the last day, and we really don't know where this storm is going to go. One heartening development: none of the three major intensity forecast models (GFDL, HWRF, and SHIPS) foresee Fay getting stronger than a Category 1 hurricane. This is in contrast to some of their runs late last week, which had Fay intensifying into a major hurricane. The new model runs are foreseeing a less favorable upper-level wind regime when Fay gets north of Cuba, with increased levels of wind shear on the northern side of the storm.

The latest (2 am EDT) model runs of the GFDL and GFS models are similar to each other, taking Fay to a landfall in Southwest Florida near Ft. Myers Monday night, then across Florida and northwards towards South Carolina. The GFDL predicts Fay will hit South Carolina as a Category 1 hurricane with 75 mph winds and a 981 mb pressure on Wednesday afternoon. The latest (8 pm EDT) run of the ECMWF model is similar.

In contrast, the latest (2 am EDT) run of the HWRF model takes Fay straight north into Tampa Tuesday morning, as a strong Category 1 storm with 90 mph winds and a 961 mb pressure. The latest UKMET model run is similar. The NOGAPS model continues to insist on a landfall near the Alabama coast on Wednesday, despite the sea turtle forecast I mentioned in yesterday's blog.


Figure 1. Track of Hurricane Charley, which struck Cuba as a Category 3 hurricane with 120 mph, then Southwest Florida as a Category 4 hurricane with 145 mph winds.

Comparing Fay to Hurricane Charley
Residents of Southwest Florida are nervously watching Fay as it approaches Cuba, recalling an August day four years ago when Hurricane Charley crossed Cuba at a similar location. Charley went on to become one of the most destructive hurricanes in history, wreaking $15 billion in damage after roaring ashore near Punta Gorda, Florida, as a Category 4 hurricane with 145 mph winds. Charley put on a remarkable and unexpected burst of rapid intensification just before landfall. Could the same thing happen with Fay? After all, the sea surface temperatures, total ocean heat content, and wind shear are similar for the two storms. And, Fay has consistently lived up to its knickname of "The Joker", with its unpredictable behavior.

Well, Fay is no Charley. By the time Charley crossed Cuba, it was already a well-developed Category 3 hurricane with 120 mph winds. Fay will be lucky to be a minimal Category 1 hurricane with 75 mph winds when it crosses Cuba. Charley's crossing of Cuba knocked its winds down by 10 mph, and we can expect a similar or greater loss of strength when Fay crosses Cuba, since it is a less organized storm than Charley was. This would make Fay a tropical storm with 65 mph winds as it moves past Key West towards Southwest Florida. It's a pretty tall order to ask a tropical storm that has not yet developed an organized eyewall to put on a major intensification burst to major hurricane status in just one day. Southwest Florida is not going to see a repeat of the Hurricane Charley experience. I give Fay just a 10% chance of making Category 3 status, and that would most likely occur if it avoids a Southwest Florida landfall, and is able to spend an extra day over water and hit the Florida Panhandle or middle Gulf Coast near Alabama. There is also a 10% chance Fay could make major hurricane status if it crosses Florida and re-organizes south of South Carolina later this week.

Links to follow
Wundermap for Cuba
Pico San Juan, Cuba radar
Key West, FL weather

Elsewhere in the tropics
Several of the reliable computer models are predicting development of a tropical wave currently located off the coast of Africa, just south of the Cape Verde Islands. This system is expected to track west-northwest and be near or just north of the Northern Lesser Antilles Islands about a week from now.

I'll have an update today by 4:30 pm EDT, when the latest set of model runs and new Hurricane Hunter data will be available.

Jeff Masters

Updated: 5:42 PM GMT on August 17, 2008

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Fay strengthening south of Cuba; and, the sea turtle forecast for Fay

By: JeffMasters, 12:51 AM GMT on August 17, 2008

Tropical Storm Fay is strengthening again, taking advantage of the deep, warm waters between Cuba and Jamaica. Though Fay's top winds are rated at just 45 mph, the storm has an increasingly impressive appearance on visible satellite loops. The circulation is expanding and intensifying, the upper-level outflow is expanding to the north and southeast, and low-level spiral bands are beginning to develop. Assuming Fay can dodge the southeast tip of Cuba without weakening, which appears likely to me, this should be a Category 1 hurricane by Sunday evening.


Figure 1. Satellite image of Fay at 7:40 pm EDT Saturday, 8/16/08. Image credit: NASA.

Where Fay might go then is still a matter of considerable disagreement among the models, with the GFDL eying Miami, the NOGAPS and UKMET the Florida Panhandle, and the HWRF, ECMWF, and GFS the west coast of Florida. One interesting forecast, relayed to me by a gentleman who lives by the coast in Magnolia, Alabama, concerns sea turtle nesting behavior along the Alabama coast. Here's the email I got before last year's hurricane season:

Dear Dr. Masters,

I live in Magnolia Springs, Alabama, just 15 miles from the direct path of Hurricane Ivan. I have a friend of mine who has been an in-shore charter boat captain in this area for 30 years. He has the answer to all the failed hurricane predictions we keep making. He told me that Sea Turtles have the answer. Every year sea turtles start nesting around the first of June, unless a hurricane comes that year. Well, storms usually come after June, right? So how could they know? Well he stated last year that I had nothing to worry about because the turtles were on point, even though hundreds of scientists had predicted the worst season in history. Well he made a believer out of me. 2006 nesting, 2005 no nesting, 2004 no nesting and so on. My memory is not that good. He said that when nesting, a hurricane will not hit within 200 miles of the area. Now, I'm no betting man, but I will put $1000 on this year that we won't get hit. How about it?

Thank you,

Trammell Henry


Well, Mr. Henry's sea captain friend was right last year, and I'm glad I didn't bet the $1000! I contacted Mr. Henry again this June to ask him what the sea turtles were forecasting for the 2008 hurricane season. The forecast: the sea turtles are nesting, so no hurricanes on the Alabama, Mississippi, or nearby Florida Panhandle coasts this year. Thus, the sea turtles think the most westerly path over the Florida Panhandle favored by the NOGAPS model will not happen. We shall see! It would be nice to have another reliable forecast method to use.

Links to follow
Wundermap for Cuba/Haiti
Gran Piedra, Cuba radar
Jamaica radar
Key West, FL weather

I'll have a full update Sunday by 12:30 pm EDT. I plan on comparing Fay to Hurricane Charley, the 2004 storm that rapidly intensified to a Category 4 hurricane just before hitting the west coast of Florida near Punta Gorda. Could a repeat of Charley happen with Fay?

Jeff Masters

Updated: 7:41 PM GMT on August 18, 2008

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Fay survives passage over Hispaniola, takes aim at Cuba

By: JeffMasters, 3:54 PM GMT on August 16, 2008

Tropical Storm Fay has completed a traverse of the length of the island of Hispaniola and survived intact. Although Fay's top winds are still rated at 45 mph, the storm did suffer from the passage, and the intensity and areal coverage of the storm's heavy thunderstorm activity are not as great as last night. However, the center is now over water, in the channel between Haiti and Cuba, and Fay is already beginning to show signs of regeneration. Visible satellite loops show an increase in heavy thunderstorm activity in all quadrants of the storm, and Haiti and the Dominican Republic have not see the last of Fay's wrath. Heavy rain is of particular concern today over Haiti, where any heavy rain is always a danger to cause major loss of life, due to the lack of vegetation on that nation's deforested mountainsides. Heavy rains will spread over Jamaica and eastern Cuba this afternoon, and over the Cayman Islands and central Cuba tonight. Damaging winds should not be a significant problem in Jamaica, Haiti, and the Cayman Islands, but are a concern in Cuba.

Fay may already have claimed its first victims. At least 23 people are dead due to a head-on collision between two buses at 8 am Friday near the town of La Romana in the eastern tip of the Dominican Republic. It was raining heavily at the time of the accident, due to the advancing rains of Fay. Fay's rains also washed out a bridge in the region and caused flooding in six provinces which displaced 2300 people and damaged 446 homes, according to DiarioLibre.com. About six inches of rain fell in a 12-hour period ending at 2 am EDT Saturday over the southeastern Dominican Republic, according to satellite estimates.


Figure 1. Geography of Cuba, with NHC forecast track overlaid. Image credit: Center for Genetic Engineering and Biotechnology (CIGB) of Havana, Cuba

The forecast for Fay
Wind shear is a low 10 knots over Fay, and is expected to remain below 10 knots for the remainder of Fay's life. An upper-level anticyclone has set up on top of Fay, allowing the air lifted from the surface by the storm's heavy thunderstorms to be efficiently spewed out to the sides, ventilating the storm and promoting even more intense thunderstorm activity. This favorable upper-level wind environment is expected to last at least through Sunday.

The latest (2 am EDT) model runs have shifted the track for Fay somewhat to the east, increasing the danger to the southwest coast of Florida near Ft. Myers. However, the models still show considerable spread, with one model (the NOGAPS) taking Fay to the Alabama coast, and several other foreseeing a strike near Miami. Fay will be passing near two areas of high mountains in Cuba (Figure 1). Fay may clip the rugged southeast tip of the island Sunday morning, then traverse the width of Cuba Sunday night over another region of high mountains. Both of these encounters may force sudden jumps in the center position as the storm reforms closer to its heaviest thunderstorm activity. As a result, the track forecast for Fay has a high level of uncertainty.

Fay's future strength when it hits the U.S. depends critically upon how much strengthening occurs today. Fay will be traversing the highest heat content waters in the entire Atlantic Ocean today, so there is the potential for the storm to reach Category 1 hurricane status before crossing Cuba Sunday night. If Fay does hit South Florida, the storm is likely to be a tropical storm or Category 1 hurricane, since it will not have enough time over water to reorganize much. A strike further up the coast will likely result in a stronger Fay at landfall, with a Category 3 storm not out of the question (20% chance).


Figure 1. Tropical Cyclone Heat Potential for Fay. Fay will be traversing over the highest heat content waters in the entire Atlantic today. Values of the Tropical Cyclone Heat Potential in excess of 80-90 are often associated with rapid intensification of hurricanes. Image credit: NOAA/CIRA/RAMMB.

Links to follow
Wundermap for Cuba/Haiti
Gran Piedra, Cuba radar
Jamaica radar
Key West, FL weather

I'll have an update Saturday night by 9 pm EDT, and present the sea turtle forecast for Fay's track.

Jeff Masters

Updated: 6:10 PM GMT on August 16, 2008

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"The Joker" transforms intoTropical Storm Fay

By: JeffMasters, 9:29 PM GMT on August 15, 2008

The true nature of the storm I've been calling "The Joker" has been exposed--"the Joker" is really Tropical Storm Fay, the sixth named storm of this busy 2008 Atlantic hurricane season. Fay moved ashore over the eastern Dominican Republic early this afternoon, after pounding Puerto Rico with heavy rains and high winds. I got this report from wunderground member isla2, who lives on the island of Vieques, just off the east coast of Puerto Rico:

I live on Vieques. I just wanted you to know that where I live mid-island (Monte Santo) we got NAILED last night by invest Fay. I cannot imagine it was not a named storm! I have been through a Cat #1 and this was just as bad. It seemed like around 10pm the constant lightning, thunder, sideways rain and strong winds began in earnest and hammered us till around 3 am. {My three dogs were glad to be inside - two vying to be closest to my bed and one hiding under the draped dining room table.} I was very glad I had put up our hurricane shutters!

I suspect that we were hit harder than the radar showed. If anyone had been here they would know that this was no mere Invest!!


Fay may not technically have been a tropical storm earlier today, but this trick of not having a closed surface circulation hid the fact that this storm had the same winds and rain as a tropical storm. Two Hurricane Hunter aircraft found sustained surface winds of 40-50 mph both north and south of the center of Fay this afternoon. Satellite intensity estimates at 2 pm today put Fay at tropical storm strength (40 mph). Despite the fact the center is over land, visible satellite loops show a well-organized system with a steadily increasing area of heavy thunderstorms. Upper-level outflow is fair on three sides, and spiral banding at the low levels is increasing. San Juan, PR radar shows the storm's rains have ended over Puerto Rico, but are hammering the eastern Dominican Republic.


Figure 1. Total estimated rainfall from Fay.

Links to follow
Wundermap for Hispaniola
Punta Cana, Dominican Republic radar

The forecast for Fay
Wind shear is a low 10 knots over Fay, and is expected to remain below 10 knots for the remainder of Fay's life. An upper-level anticyclone has set up on top of Fay, allowing the air lifted from the surface by the storm's heavy thunderstorms to be efficiently spewed out to the sides, ventilating the storm and promoting even more intense thunderstorm activity. This favorable upper-level wind environment is expected to last at least through Sunday.

The latest (8 am EDT) model runs have shifted the track for Fay considerably further west, maximizing the time Fay will spend over land. Fay will be passing over the high mountains of both Hispaniola and Cuba, which may force sudden jumps in the center position as the storm reforms closer to its heaviest thunderstorm activity. As a result, the track forecast for Fay has an unusually high level of uncertainty. "The Joker" will continue to keep us guessing!

If Fay does hit South Florida, the storm is likely to be a tropical storm or Category 1 hurricane, since it will not have enough time over water to reorganize much. I think the models are overdoing the intensification of Fay once it does pop off the coast of Cuba. We saw in 2006 that Ernesto popped off the coast of Cuba as a weak tropical storm, and took a full 36 hours to get its act together. If Fay misses South Florida and veers either to the east or west of the Peninsula, the storm could easily reach Category 2 status before a potential landfall either on the Gulf Coast or in North Carolina/South Carolina.

I'll have an update Saturday by noon EDT.

Jeff Masters

Updated: 3:59 AM GMT on August 16, 2008

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"The Joker" still not classified as a tropical depression

By: JeffMasters, 1:17 PM GMT on August 15, 2008

Well, I was right on when I dubbed the tropical disturbance 92L "The Joker" earlier this week. I don't recall ever seeing a system that looked this good on satellite imagery not technically qualify as a tropical depression. Last night's Hurricane Hunter mission did find that 92L had a vigorous circulation at mid levels of the atmosphere, but this circulation did not extend down to the surface. Even when the center of 92L was over northwestern Puerto Rico, I couldn't find any winds out of the west on the island, suggesting that 92L did not have a closed surface circulation. However, the latest QuikSCAT pass from 5:52 am EDT shows that 92L had a large area of west winds south of the island. However, this data was not enough for NHC to upgrade 92L to a tropical depression or tropical storm this morning, presumably because the QuikSCAT data was contaminated by rain and not deemed trustworthy.

Satellite intensity estimates at 2 am and 8am EDT today put 92L at tropical storm strength (40 mph), and visible satellite loops show a well-organized system with a steadily increasing area of heavy thunderstorms. Upper-level outflow is fair on three sides. San Juan, PR radar shows the center of 92L is in the Mona Passage, between Puerto Rico and the Dominican Republic. The rain areas look disorganized, with little evidence of spiral banding. Radar estimated precipitation (Figure 1) shows up to 5 inches of rain has fallen on some regions of the island, with much higher amounts over neighboring ocean areas.


Figure 1. Latest estimated rainfall from 92L.

The forecast for 92L
Wind shear is a mere 1 knot over 92L, as analyzed by the SHIPS model. An upper-level anticyclone has set up on top of 92L, allowing the air lifted from the surface by the storm's heavy thunderstorms to be efficiently spewed out to the sides, ventilating the storm and promoting even more intense thunderstorm activity. This favorable upper-level wind environment is expected to last at least through Sunday. Beyond then, increasing shear may be a problem for the storm.

The latest (2 am EDT) model runs all foresee a track for 92L very close to the north coast of the Dominican Republic and Haiti today through Sunday. Heavy rains will be the main threat to these places, with 4-8 inches likely. Isolated higher amounts of up to 12 inches may fall in the mountains, triggering life-threatening flash floods and mudslides.

In the longer term, an encounter with the rugged terrain of eastern Cuba is forecast by many of the models on Sunday. Passage over the rough terrain of Hispaniola and Cuba could severely disrupt or even destroy 92L. Once the storm finally emerges and stays over the warm waters of the Bahamas or Florida Straits, the models expect it to intensify quickly into a hurricane, possibly a major hurricane. The eventual strength is highly dependent on the track of 92L, with a longer track over water giving it a greater chance of becoming a hurricane. I think the models are overdoing the intensification of 92L once it does pop off the coast of Cuba. We saw in 2006 that Ernesto popped off the coast of Cuba as a weak tropical storm, and took a full 36 hours to get its act together. Still, 92L may have a much longer time over water than Ernesto had, and if the storm does spend 2 or 3 days over the warm waters of the Gulf Stream off the coast of Florida, the storm could easily intensify into a major hurricane, as the GFDL and HWRF models have been predicting for several days now.

The long range track of 92L is highly uncertain. Take your pick of 8 pm/2 am EDT model runs:

GFDL, HWRF: parallel to the east coast of Florida, 50-100 miles offshore, eventually threatening South Carolina
UKMET: Through South Florida
NOGAPS: Through the Florida Keys, into South Florida
ECMWF: Through the Bahamas and into North Carolina
GFS: Through the Bahamas, then west across central Florida in the Gulf of Mexico
Canadian: Across Cuba and through the Cayman Islands, then north in the Gulf of Mexico

No model is calling for "The Joker" to recurve out to sea, completely missing the U.S.

Links to follow
Puerto Rico radar
Wundermap for Puerto Rico
Puerto Rico weather

Elsewhere in the tropics
A tropical wave (93L), near 15N 49W, about 700 miles east of the Lesser Antilles Islands, has gotten better organized this morning. Wind shear has fallen to 10 knots, and is expected to remain below 10 knots the next two days. The system does have a closed circulation at the surface, as seen on last night's QuikSCAT pass. The surface circulation is also apparent on visible satellite loops, as well as a slow increase in heavy thunderstorm activity. NHC has given the reasonable forecast that there is a medium (20-50%) chance that this system will be a tropical depression by Sunday morning. Most of the computer models track this storm well north of the Lesser Antilles Islands, recurving out to sea.

Several of the reliable computer models forecast development of a new tropical wave coming off the coast of Africa about 1-3 days from now.

I'll have an update by 5 pm today, when the latest data from the Hurricane Hunters will be available. "The Joker" is certainly worthy of having a full set of advisories issued for it, given is possible impacts, even if it doesn't technically qualify as a tropical depression. For the future, perhaps NHC should consider an extension to their "Special Tropical Disturbance Statement" that includes a set of forecast tracks, marine forecast, discussion, etc.

Jeff Masters

Updated: 3:31 PM GMT on August 15, 2008

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Hurricane Hunters do not find a tropical depression

By: JeffMasters, 8:51 PM GMT on August 14, 2008

The Hurricane Hunters are inside the tropical disturbance 92L over the northern Lesser Antilles Islands, and are reporting winds of tropical depression strength (30-35 mph). However, the two aircraft in the storm have had difficulty finding a closed circulation at the surface, and the National Hurricane Center has decided not to upgrade 92L to Tropical Depression Six as of 5 pm EDT today:

Special tropical disturbance statement
5 PM EDT Thu Aug 14 2008

Data from NOAA and Air Force Reserve hurricane hunter aircraft...along with surface observations and satellite imagery...indicate that the low pressure area located over the Virgin Islands has not developed into a tropical depression. However...upper-level winds are becoming more favorable for development...and a tropical depression could form at any time during the next day or two as the system moves west-northwestward about 15 mph.


Water vapor satellite loops show that 92L has generated enough heavy thunderstorm activity to insulate it from the surrounding region of dry Saharan air. Water temperatures are a warm 28.5°C and wind shear is less than 10 knots. Visible satellite loops show a steadily increasing area of heavy thunderstorms. Upper-level outflow has appeared on the north and east sides. Martinique radar shows an impressive area of heavy rain, with low-level spiral bands developing on the east side.


Figure 1. Latest satellite image of 92L.

The forecast for 92L
Dry air will probably not be a problem for 92L any more. Wind shear is also not likely to be a problem--shear is forecast to remain below 10 knots the next five days, and may drop to near zero on Friday. Furthermore, an upper-level anticyclone is setting up on top of 92L. This will allow the air lifted from the surface by the storm's heavy thunderstorms to be efficiently spewed out to the sides, ventilating the storm and promoting even more intense thunderstorm activity.

I give 92L a high (>80% chance) of becoming a tropical storm by Friday afternoon. The latest (8 am EDT) model runs all foresee a track for 92L very close to Puerto Rico and the north coast of the Dominican Republic and Haiti Friday through Sunday. Heavy rains will be the main threat to these places, with 4-8 inches likely. Isolated higher amounts of up to 12 inches may fall in the mountains, triggering life-threatening flash floods and mudslides.

In the longer term, an encounter with the rugged terrain of eastern Cuba is forecast by most of the models for Sunday. Passage over the rough terrain of Hispaniola and Cuba could severely disrupt or even destroy 92L. If the storm survives, it could intensify quickly into a hurricane, possibly a major hurricane, once it emerges over the warm waters of the Bahamas or Florida Straits. The eventual strength is highly dependent on the track of 92L, with a longer track over water giving it a greater chance of becoming a hurricane. The long range track of 92L is highly uncertain. Take your pick of 8 am EDT model runs:

GFDL, HWRF: parallel to the east coast of Florida, 50-100 miles offshore
UKMET: Through South Florida into the Gulf of Mexico
NOGAPS: Through the Florida Keys, then northwest towards the Alabama/Mississippi coast
GFS: Through the Bahamas, then north towards North Carolina
Canadian: Across Cuba and through the Cayman Islands, then north in the Gulf of Mexico

Links to follow
Puerto Rico radar
href=http://www.wunderground.com/wundermap/?lat=18.12000084&lon=-65.77999878&zoom=7 target="_blank" >Wundermap for Puerto Rico
St. Thomas, Virgin Islands weather
St. Martin webcam
Puerto Rico weather

Elsewhere in the tropics
A tropical wave (93L) about 1000 miles east of the Lesser Antilles Islands is disorganized, and should not develop during the next 1-3 days. Several of the reliable computer models forecast development of a new tropical wave coming off the coast of Africa about 2-4 days from now.

I'll have an update when NHC designates 92L a tropical depression or tropical storm.

Jeff Masters

Updated: 1:09 AM GMT on August 15, 2008

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Hurrincane Hunters find 92L is at the threshold of becoming a tropical depression

By: JeffMasters, 8:08 PM GMT on August 14, 2008

The Hurricane Hunters are inside the tropical disturbance 92L over the northern Lesser Antilles Islands, and are reporting winds of tropical depression strength (30-35 mph). However, the two aircraft in the storm have had difficulty finding a closed circulation at the surface, and the National Hurricane Center has decided not to upgrade 92L to Tropical Depression Six as of 5 pm EDT today:

Special tropical disturbance statement
5 PM EDT Thu Aug 14 2008

Data from NOAA and Air Force Reserve hurricane hunter aircraft...along with surface observations and satellite imagery...indicate that the low pressure area located over the Virgin Islands has not developed into a tropical depression. However...upper-level winds are becoming more favorable for development...and a tropical depression could form at any time during the next day or two as the system moves west-northwestward about 15 mph.


Water vapor satellite loops show that 92L has generated enough heavy thunderstorm activity to insulate it from the surrounding region of dry Saharan air. Water temperatures are a warm 28.5°C and wind shear is less than 10 knots. Visible satellite loops show a steadily increasing area of heavy thunderstorms. Upper-level outflow has appeared on the north and east sides. Martinique radar shows an impressive area of heavy rain, with low-level spiral bands developing on the east side.


Figure 1. Latest satellite image of 92L.

The forecast for 92L
Dry air will probably not be a problem for 92L any more. Wind shear is also not likely to be a problem--shear is forecast to remain below 10 knots the next five days, and may drop to near zero on Friday. Furthermore, an upper-level anticyclone is setting up on top of 92L. This will allow the air lifted from the surface by the storm's heavy thunderstorms to be efficiently spewed out to the sides, ventilating the storm and promoting even more intense thunderstorm activity.

I give 92L a high (>80% chance) of becoming a tropical storm by Friday afternoon. The latest (8 am EDT) model runs all foresee a track for 92L very close to Puerto Rico and the north coast of the Dominican Republic and Haiti Friday through Sunday. Heavy rains will be the main threat to these places, with 4-8 inches likely. Isolated higher amounts of up to 12 inches may fall in the mountains, triggering life-threatening flash floods and mudslides.

In the longer term, an encounter with the rugged terrain of eastern Cuba is forecast by most of the models for Sunday. Passage over the rough terrain of Hispaniola and Cuba could severely disrupt or even destroy 92L. If the storm survives, it could intensify quickly into a hurricane, possibly a major hurricane, once it emerges over the warm waters of the Bahamas or Florida Straits. The eventual strength is highly dependent on the track of 92L, with a longer track over water giving it a greater chance of becoming a hurricane. The long range track of 92L is highly uncertain. Take your pick of 8 am EDT model runs:

GFDL, HWRF: parallel to the east coast of Florida, 50-100 miles offshore
UKMET: Through South Florida into the Gulf of Mexico
NOGAPS: Through the Florida Keys, then northwest towards the Alabama/Mississippi coast
GFS: Through the Bahamas, then north towards North Carolina
Canadian: Across Cuba and through the Cayman Islands, then north in the Gulf of Mexico

Links to follow
Martinique radar

Wundermap for the northern Lesser Antilles Islands

St. Thomas, Virgin Islands weather

St. Martin webcam

Elsewhere in the tropics
A tropical wave (93L) about 1000 miles east of the Lesser Antilles Islands is disorganized, and should not develop during the next 1-3 days. Several of the reliable computer models forecast development of a new tropical wave coming off the coast of Africa about 2-4 days from now.

I'll have an update when NHC designates 92L a tropical depression or tropical storm.

Jeff Masters

Updated: 8:49 PM GMT on August 14, 2008

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Dangerous disturbance 92L poised to become a tropical depression

By: JeffMasters, 2:06 PM GMT on August 14, 2008

The ominous music is rising once more, and The Joker (also known as disturbance 92L) appears poised to develop into a tropical depression today. This storm has the potential to become a hurricane that will affect the Bahama Islands and Florida early next week.

The storm fell apart yesterday, thanks to an infusion of dry air from the large region of Saharan air that continues to surround it. However, as each day goes by, 92L is moistening its environment to insulate itself from the destructive influences of this dry air. The storm is evaporating large amounts of water vapor from the warm 28-29°C waters below, then condensing this vapor inside intermittent bursts of heavy thunderstorm activity. Water vapor satellite loops show that the moistened area has steadily expanded, and is now large enough to insulate 92L from the surrounding dry air--as long as there is little wind shear. A strong jet of wind from the side could carry dry air into the core of 92L and disrupt it. However, wind shear is less than 10 knots, and this does not appear likely to happen. Visible satellite loops show some rotation at low levels, and a steadily increasing area of heavy thunderstorms. Upper-level outflow is beginning to appear to the north side. This morning's 6:20 am EDT QuikSCAT pass showed a pronounced wind shift associated with 92L, but no closed surface circulation. Martinique radar shows an impressive area of heavy rain, with a spiral band developing on the east side.


Figure 1. Latest satellite image of 92L.

The forecast for 92L
It currently appears that dry air will not be able to stop 92L from developing into a tropical depression later today or Friday. Wind shear is also not likely to be a problem. Wind shear is forecast to remain below 10 knots the next two days. There is the possibility that by Sunday, an upper-level anticyclone will set up on top of 92L. This would allow the air lifted from the surface by the storm's heavy thunderstorms to be efficiently vented out to the sides, ventilating it, promoting even more intense thunderstorm activity. The main bug-a-boo for 92L will be a possible encounter with the high mountains of Hispaniola and eastern Cuba. After passing Puerto Rico on Friday, 92L is expected to move along the north coast of the Dominican Republic on Saturday. The high mountains of the island may disrupt or even destroy the storm. However, most of the computer models are predicting that The Joker will survive this encounter, be turned northwestward by an approaching trough of low pressure on Sunday, and move into the Bahama Islands. Once over the deep, warm waters of the Bahamas, 92L could easily intensify into a hurricane. This is the solution of the latest (2 am EDT) runs of the GFDL, HWRF, and SHIPS models. By Tuesday, the storm could be very near the east coast of Florida. The long-range fate of The Joker is difficult to guess. The possibilities range from the ECMWF forecast of a turn to the north with a threat to North Carolina, to the Canadian model's prediction of passage along the length of Cuba, followed by emergence into the Gulf of Mexico.

The National Hurricane Center is giving 92L a high (>50% chance) of becoming a tropical depression by Saturday morning. Heavy thunderstorms from 92L have already spread over the northern Lesser Antilles Islands, and the bulk of the storm will hit Puerto Rico on Friday. The storm will probably will not have time to grow beyond a 60 mph tropical storm by Saturday, when it could bring heavy rains of 4-8 inches to the northern Dominican Republic. These rains should spread to northern Haiti by Sunday, where heavy rain is always a threat to cause significant loss of life. The Hurricane Hunters are scheduled to investigate 92L this afternoon, and I'll have a new blog once they've had time to collect a few hours of data.

Links to follow
Martinique radar

Wundermap for the northern Lesser Antilles Islands

St. Thomas, Virgin Islands weather

Elsewhere in the tropics
A tropical wave (93L) about 1200 miles east of the Lesser Antilles Islands is disorganized, and should not develop during the next 1-3 days. Several of the reliable computer models forecast development of a new tropical wave coming off the coast of Africa about 2-4 days from now.

I'll have an update this afternoon.

Jeff Masters

Updated: 2:07 PM GMT on August 14, 2008

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Major steering current shift coming; update on Lesser Antilles disturbance 92L

By: JeffMasters, 7:28 PM GMT on August 13, 2008

There is little new to report on 92L, the tropical disturbance now just 200 miles east of the Antigua in the northern Lesser Antilles Islands. Heavy thunderstorm activity remains limited, and dry air surrounding the storm continues to interfere with it. The latest crop of 12Z (8 am EDT) computer model runs are similar to the last set of runs, except the new runs are becoming more definitive about a threat to the Bahamas on Saturday and Sunday, and the east coast of Florida or Florida Keys on Monday. The National Hurricane Center continues to give a medium (20-50% chance) that 92L will be a tropical depression by Friday afternoon. I give a 20% chance that 92L will be a hurricane by Monday. An Air Force and a NOAA Hurricane Hunter aircraft are scheduled to investigate 92L simultaneously Thursday afternoon.


Figure 1. Latest satellite image of 92L.

Major shift in steering currents coming
As I discussed in last week's blog on steering currents, the hurricane steering pattern for all of July and the first two weeks of August over the North Atlantic has predominantly acted to recurve hurricanes out to sea. The jet stream has been "stuck" in a standing wave pattern, where it dips southward over the East Coast of the U.S., creating a trough of low pressure capable of recurving tropical storms once they get north of the Caribbean Sea (20° latitude). This pattern is in contrast to the steering pattern that set up in 2004 and 2005, when a ridge of high pressure set got stuck over the Eastern U.S. A ridge in this location does not allow hurricanes to recurve, and the U.S. took a terrific battering those years.

This year's steering pattern is about to make a major shift towards the steering pattern observed in 2004 and 2005. According to recent 500 millibar (mb) upper-air forecasts from the GFS model. and ECMWF model, the trough of low pressure over the U.S. East Coast will be replaced by a ridge of high pressure 7-10 days from now. As a result, the surface Bermuda High will extend far to the west over the Eastern U.S. This pattern will mean that fewer hurricanes will be recurving beginning a week from now, and the threat to the U.S. Gulf Coast will increase. Conversely, the threat to Bermuda and the Northeast U.S. will diminish.

There is no way of telling how long this new steering pattern might stay in place. It could last only a few days, or remain in place for several months.


Figure 2. Upper air charts showing the height where a pressure of 500 mb is forecast to be found this Saturday (top) and next Saturday (bottom). The forecast was made at 8 pm EDT Tuesday August 12 by the ECMWF model. Note that the axes where dominant troughs and ridges are found are predicted to reverse over this 1-week timespan, with a ridge of high pressure setting up over the eastern half of the U.S. This upper-air pattern is conducive to more hurricane activity along the Gulf of Mexico coast.

Links to follow
Martinique radar
Wundermap for the northern Lesser Antilles Islands
St. Thomas, Virgin Islands.

I'll have an update Thursday morning.

Jeff Masters

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Lesser Antilles disturbance fizzles, but may still cause trouble

By: JeffMasters, 2:26 PM GMT on August 13, 2008

A tropical wave about 300 miles east of the Lesser Antilles Islands (92L) fell apart last night, but is beginning to increase in organization today. We don't have a QuikSCAT pass this morning to look at, but visible satellite loops show some rotation at low levels (though not a full closed circulation), and one clump of heavy thunderstorms near the center of rotation. The storm has no low-level spiral bands or upper-level outflow, and is in a very fragile state. Moderate wind shear of 15-20 knots or a major injection of dry air could doom the system. Right now, it doesn't appear that wind shear will be high enough to destroy 92L--shear is only 5 knots, and expected to remain less than 10 knots for the next 5 days. Water vapor satellite loops show that a large area of dry air and Saharan dust surrounds 92L on all sides, and this dry air was responsible for 92L's severe degradation last night and early this morning. Dry air remains 92L's greatest enemy.

The forecast for 92L
Watching the the model forecasts for 92L over the past three days has, for me, been akin to watching the latest Batman movie, The Dark Knight. As the Joker prepares for one of his deadly pranks, the music rises in pitch and volume, and the audience nervously waits to see what terrible mayhem the Joker has planned next. Like music in the movie, the reliable GFDL model forecasts of 92L the past three days have risen in pitch and volume. The GFLD has been forecasting successively stronger hurricanes each day, culminating in yesterday afternoon's run predicting a Category 3 hurricane plowing through the Bahama Islands towards Florida this weekend. Well, our Batman--dry air--has come to the rescue this time, significantly disrupting 92L. However, it remains to be seen if the Joker--92L--has one more trick up its sleeve. The GFDL model is still calling for 92L to develop into a borderline Category 1 hurricane by early next week, as is the latest run of the SHIPS intensity model. The other models are less gung-ho, and most of the models foresee that 92L will come close enough to the high mountains of the Dominican Republic to cause the storm trouble.

The National Hurricane Center is giving 92L a medium (20-50% chance) of becoming a tropical depression by Friday morning. A few showers from 92L have already begun to spread over the northern Lesser Antilles Islands this morning, and the bulk of the storm will spread over the islands tonight and Thursday. By Friday, 92L will be affecting Puerto Rico and the eastern Dominican Republic. Haiti, the Bahamas, and eastern Cuba can expect heavy rains Saturday or Sunday, and 92L could affect Florida early next week. The Hurricane Hunter mission for today was canceled, but a new mission is scheduled for Thursday if the storm overcomes its dry air problems.

Links to follow
Martinique radar
Wundermap for the northern Lesser Antilles Islands

Disturbance 93L off the coast of Africa
A tropical wave (93L) about 700 miles west of the Cape Verde Islands has gotten better organized since yesterday Visible satellite loops show a small clump of heavy thunderstorms near the swirling center of circulation, but no low-level spiral bands or upper-level outflow yet. A large area of stratocumulus clouds is visible to the northwest, indicating the presence of cool, dry, stable air. This stable air is inhibiting development of 93L. Wind shear has been a risen to 10-15 knots today. Water vapor satellite imagery shows dry Saharan air to the northwest.

The forecast for 93L
Water temperatures have cooled to a marginal 26.5°, but will slowly warm as 93L continues westward. Wind shear is forecast to remain below 20 knots through Friday afternoon, then increase to 30 knots. The National Hurricane Center is giving 93L a medium (20-50% chance) that it will be a tropical depression by Friday morning. I believe that this is too high, and 93L has a less than 20% chance of becoming a tropical depression this week. There is too much stable dry air to overcome between now and Friday, followed by high wind shear late in the week.

Elsewhere in the tropics
Several of the reliable computer models forecast development of a new tropical wave coming off the coast of Africa about 3-5 days from now.

I'll have an update this afternoon between 4pm and 5 pm EDT.
Jeff Masters

Updated: 2:35 PM GMT on August 13, 2008

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Hurricane Hunters find 92L remains disorganized

By: JeffMasters, 8:26 PM GMT on August 12, 2008

A tropical wave about 550 miles east of the Lesser Antilles Islands (92L) has changed little in organization today. Between 2-4:30 pm EDT, today's Air Force Hurricane Hunter mission could not find a surface circulation. The highest surface winds measured were 32 mph, and the lowest surface pressure was 1008 mb. As noted on this morning's blog, 92L has an elongated 600-mile long "convergence zone" along which the winds moved in opposite directions, rather than a closed surface circulation. However, visible satellite loops show that a circulation has developed at mid levels of the atmosphere (about 18,000 feet in altitude), and this circulation will probably be able to migrate down to the surface by Wednesday morning. Heavy thunderstorm activity has decreased some today, but modest upper-level outflow continues to the north, visible as cirrus clouds streaming away from the center. Low-level spiral bands are not present, but appear to be beginning to emerge again to the north. Water vapor satellite loops show that a large area of dry air and Saharan dust surrounds 92L on three sides, and this dry air may periodically interfere with 92L's organization over the next two days.


Figure 2. Latest satellite image of 92L.

The forecast for 92L
Water temperatures are a warm 28°C and forecast to remain above 28°C the next five days. Wind shear is a modest 10-15 knots, and is forecast to remain 5-15 knots for the next five days. The models are split on whether 92L will develop or not. The 8 am EDT GFDL model is the most aggressive, intensifying 92L to a tropical storm that passes just north of the northernmost Lesser Antilles Islands on Thursday, then intensifies into a Category 2 hurricane that plows into the Bahama Islands on Saturday. In contrast, the 8 am EDT HWRF model develops 92L into a minimal 40 mph tropical storm on Saturday, when it enters the easternmost Bahama Islands. The SHIPS intensity model is in between, forecasting a 70-75 mph storm by Saturday. The National Hurricane Center is giving 92L a high (>50% chance) of becoming a tropical depression by Thursday afternoon. Given the moderately favorable wind shear and water temperatures, 92L will probably become a tropical depression by Wednesday night, and has about a 20% chance of reaching hurricane strength by Saturday. In its current disorganized state, 92L will be difficult for the computer models for make an accurate track forecast for, since we don't know where along the long axis of converging winds that the center will form at. We can expect the northern Lesser Antilles to get some heavy weather on Thursday, and these conditions may spread to Puerto Rico late Thursday into Friday. 92L may represent a threat to the U.S. East Coast early next week. The Hurricane Hunters are scheduled to pay another visit to 92L at 2 am EDT Wednesday. Beginning Wednesday afternoon, there will be both a NOAA P-3 and an Air Force C-130 Hurricane Hunter aircraft in the storm, as the P-3s collect detailed radar data to aid in research efforts to improve hurricane computer forecast models.

Disturbance 93L off the coast of Africa
A tropical wave near 13N 31W (93L), about 400 miles west-southwest of the Cape Verde Islands, has continued to show a decrease in heavy thunderstorm activity. Visible satellite loops show a large swirling circulation, but little heavy thunderstorm activity remaining. A large area of stratocumulus clouds is visible to the northwest, indicating the presence of cool, dry, stable air. This stable air is inhibiting development of 93L. Wind shear has been a low 5-10 knots today. Water vapor satellite imagery shows dry Saharan air to the northwest.


Figure 1. Latest satellite image of 93L.

The forecast for 93L
Water temperatures are a warm 28°, and forecast to cool below 27°C Thursday. Wind shear is forecast to remain below 10 knots through Thursday afternoon, then increase to 15-25 knots Friday through Saturday. The National Hurricane Center is giving 93L a medium (20-50% chance) of becoming a tropical depression by Thursday afternoon. I believe that 93L has a less than 20% chance of becoming a tropical depression this week. There is too much stable dry air to overcome between now and Thursday, followed by higher wind shear later in the week.

Elsewhere in the tropics
Several of the reliable computer models forecast development of a new tropical wave coming off the coast of Africa about 5-6 days from now.

I'll have an update Wednesday morning.
Jeff Masters

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Disturbance 92L continues towards Lesser Antilles Islands

By: JeffMasters, 1:52 PM GMT on August 12, 2008

A tropical wave about 650 miles east of the Lesser Antilles Islands (92L) continues be a threat to turn into a tropical depression as it moves west-northwest at 10-15 mph. This morning's QuikSCAT pass (Figure 1) showed that 92L did not have a closed circulation, but rather an elongated 600-mile long "convergence zone" along which the winds moved in opposite directions. Visible satellite loops have shown an increase in heavy thunderstorm activity and organization since yesterday. Surface spiral bands have formed to the west and north, and there is now upper-level outflow to the north visible as cirrus clouds streaming away from the center. Water vapor satellite loops show that a large area of dry air and Saharan dust surrounds 92L on three sides, and this dry air may periodically interfere with 92L's organization over the next two days.


Figure 1. QuikSCAT image from the morning of Tuesday August 12, 2008. Two sweeps of the QuikSCAT satellite are shown. The data on the right swath was taken at 7:48 GMT (3:48 am EDT), and shows the center of circulation of disturbance 93L at the far right of the image. The colors of the wind barbs show winds of 10-20 knots circulating around the center. After the QuikSCAT satellite scanned 93L, it passed all the way around the planet and returned over the Atlantic again at 9:29 GMT (5:29 am EDT), when it scanned the region over disturbance 92L. Note the elongated region with winds moving in opposite directions where 92L is. This disturbance does not have a closed center of circulation. Also note the presence of some bad data along the right edge of the data swath; bad data is common at the edges of QuikSCAT swaths. Image credit: NOAA/NESDIS/ORA.


Figure 2. Latest satellite image of 92L.

The forecast for 92L
Water temperatures are a warm 28°C and forecast to remain above 28°C the next five days. Wind shear is 5-10 knots, and is forecast to remain below 10 knots for the next two days, then increase to 15-20 knots Thursday night through Saturday, thanks to strong upper-level winds from the northwest. The models are split on whether 92L will develop or not. The 2 am EDT GFDL model is the most aggressive, intensifying 92L to a tropical storm that passes just north of the northernmost Lesser Antilles Islands on Thursday, then intensifies into a Category 1 hurricane that plows into the Bahama Islands on Saturday. In contrast, the 2 am EDT HWRF model has 92L at tropical depression strength on Saturday, when it enters the Bahama Islands. The National Hurricane Center is giving 92L a high (>50% chance) of becoming a tropical depression by Thursday morning. Given the continued trend in visible, infrared, and water vapor satellite imagery, 92L will probably become a tropical depression by Wednesday night. A big question is how the storm will adjust to formation of a closed center of circulation; it will lose much of the heavy thunderstorm activity currently along its long axis of converging winds, and this will ultimately affect the track of the storm. In its current disorganized state, 92L will be difficult for the computer models for make an accurate track forecast for, since we don't know where along the long axis of converging winds that the center will form at. We can expect the northern Lesser Antilles to get some heavy weather on Thursday, and these conditions may spread to Puerto Rico late Thursday into Friday. 92L may represent a threat to the U.S. East Coast early next week. The Hurricane Hunters are scheduled to pay their first visit to 92L on this afternoon.

Disturbance 93L off the coast of Africa
A tropical wave near 13N 31W (93L), about 400 miles west-southwest of the Cape Verde Islands, has shown a decrease in heavy thunderstorm activity since yesterday. Wind shear has fallen to a modest 10 knots over the disturbance. Water vapor satellite imagery shows that 93L is embedded in a large area of moist air, with some dry air and Saharan dust to its north.


Figure 1. Latest satellite image of 93L.

The forecast for 93L
Water temperatures are a warm 28° and forecast remain near 28°C for the next five days. Wind shear is forecast to remain 10-15 knots through Thursday morning, then drop below 5 knots. This should allow 93L to develop into a tropical depression by Friday. Dry air may begin to be a problem for 93L beginning on Thursday, as it works its way a bit further to the north where a dry Saharan Air Layer (SAL) exists. The National Hurricane Center is giving 93L a medium (20-50% chance) that it will be a tropical depression by Thursday morning. This storm could threaten the Lesser Antilles 6-7 days from now, but may also recurve out to sea.

Elsewhere in the tropics
Several of the reliable computer models forecast development of a new tropical wave coming off the coast of Africa about 7 days from now.

I'll have an update between 3-5 pm EDT today.
Jeff Masters

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August Hurricane Outlook Part III: Wind Shear and African Dust; update on 92L and 93L

By: JeffMasters, 8:40 PM GMT on August 11, 2008

Wind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation. Wind shear below 12 knots is very conducive for tropical storm formation. High wind shear acts to tear a storm apart.


Figure 1. Departure of wind shear from average for the 31 days ending August 9, 2008. Image credit: NOAA. Units are in meters per second; multiply by two to convert (approximately) to knots.

Wind shear during the past 31 days over the Main Development Region (MDR), between 10 and 20° N, extending from the coast of Africa to Central America, is shown in Figure 1. Recall that the MDR is where 85% of all major hurricanes form, and 60% of all tropical storms and weaker hurricanes. Shear has been above average over the Caribbean (blue colors), peaking at 4-6 m/s above average over the Western Caribbean. Wind shear has been below average over most of the rest of the MDR. Overall, wind shear has been a little below average so far this hurricane season over the tropical Atlantic.

However, the GFS model is predicting that very low levels of wind shear will affect much of the Main Development Region (MDR) during the coming two weeks (Figure 2). In particular, the Caribbean will see some of the lowest levels of wind shear it has seen all season. The long-range wind shear forecast from NOAA's CFS model (Figure 3) foresees wind shear values of 2-4 m/s (4-8 kt) below average across most of the Main Development Region during the peak months of hurricane season.


Figure 2. GFS model wind shear forecast for Tuesday, August 19, 2008. The forecast was made on Monday, August 11, at 2am EDT. Wind shear is the difference in wind between 200 mb (roughly 40,000 feet in altitude) and 850 mb (roughly 5,000 feet in altitude), in meters per second. Multiply this number by two to get the approximate wind shear in knots. In most circumstances, wind shear below 12 knots (6 m/s, the lighter red colors) is conducive for tropical storm formation. Note the very low wind shear, less than 10 knots, forecast for most of the Atlantic Main Development Region (white box) a week from now.


Figure 3. Forecast wind shear anomaly from NOAA's CFS model for the peak months of hurricane season, August, September, and October 2008. Wind shear is forecast to be 2-4 m/s (4-8 kt) below average across most of the Main Development Region (MDR) for Atlantic hurricanes. Image credit:NOAA's Climate Prediction Center.

If these wind shear forecasts come true, we are in for a very active main portion of hurricane season, since wind shear is typically the single most important factor inhiibiting tropical storm formation.

Dry air and African dust
June and July are the peak months for dust coming off the coast of Africa, and dust activity begins to subside in August. The Sahel region of Africa has seen three straight years of average to above-average rains, which should result in soil stabilization and fewer dust outbreaks than average this year. So far this August, dust activity has (subjectively) appeared near average. Dust levels are very difficult to forecast, as they depend upon levels of soil moisture. Measurements of soil moisture in western Africa are almost non-existent, so the models must guess how dry the soil is. There are several groups that produce short-term forecasts (3-5 days) of Tropical Atlantic dust levels, including the Tel-Aviv University and the U.S. Navy (Figure 4). I don't find these forecasts particularly useful for forecasting tropical storm development, and I don't know how accurate they are. They may be valuable to residents of the region who want to know when they might be impacted by African dust.


Figure 4. Forecast dust concentrations (in micrograms per cubic meter) from the U.S. Navy's NAAPS model. The model was initialized at 8 am EDT Monday August 11, 2008. The model is predicting low levels of African dust over the eastern Atlantic at week's end.

Update on disturbance 92L approaching Lesser Antilles
A tropical wave about 800 miles east of the Lesser Antilles Islands (92L) has had its center reform today farther to the north. This location is difficult to pinpoint (I put it at 13.5N 48W at 4 pm EDT), but is farther away from the dry air to its west. The center was forced to re-form after dry air disrupted the original circulation early this morning. Thanks to the center re-location, dry air is troubling 92L much less, and visible satellite loops show a steady increase in heavy thunderstorm activity and organization, and surface spiral bands have formed to the west and north. Water vapor satellite loops show that a large area of dry air and Saharan dust lies to the west of 92L's center, and this dry air does not appear to be getting drawn into the storm's center any more.


Figure 1. Latest satellite image of 92L.

The forecast for 92L
Water temperatures are a warm 28.2°C and forecast to increase to 28.8°C five days from now. Wind shear is forecast to remain below 10 knots for the next four days, then increase to 10-15 knots five days from now. This environment is favorable for intensification. The models are split on whether 92L will develop or not. The 2 am EDT GFDL model is the most aggressive, intensifying 92L to a 55 mph tropical storm that passes just north of the northernmost Lesser Antilles Islands on Thursday. In contrast, the 2 am EDT HWRF model does not develop 92L at all. The National Hurricane Center is giving 92L a medium (20-50% chance) of becoming a tropical depression by Wednesday afternoon. This is a step down from their forecast last night of a high (>50%) chance of developing, and reflects the uncertainty that 92L will be able to get organized in the face of significant dry air to its west. However, given the recent trend in visible, infrared, and water vapor satellite imagery, I believe 92L has sufficiently insulated itself from the dry air to the west, and is destined to become a tropical depression by Tuesday night or Wednesday. Residents of and visitors to the northern Lesser Antilles should anticipate the possibility of a tropical depression or tropical storm arriving in the islands as early as Wednesday afternoon (though Thursday morning is more likely). The southern islands are not likely to be affected. Puerto Rico could be affected by Thursday, the Dominican Republic by Friday, and the Bahamas by Saturday. The U.S. East Coast could be affected by this system early next week, although it is too early to assess whether the system may end up recurving out to sea or not. The Hurricane Hunters are scheduled to pay their first visit to 92L on Tuesday afternoon.

Disturbance 93L off the coast of Africa
A tropical wave near 11N 29W (93L), about 400 miles southwest of the Cape Verde Islands, has shown a steady increase in heavy thunderstorm activity today, although there is almost no heavy thunderstorm activity near the center of circulation. Wind shear is a high 20 knots over the disturbance. Water vapor satellite imagery shows that 93L is embedded in a large area of moist air, with some dry air and Saharan dust to its north.


Figure 1. Latest satellite image of 93L.

The forecast for 93L
Water temperatures are a warm 28° and forecast remain near 28°C for the next five days. Wind shear is forecast to drop to 5-10 knots by Tuesday morning, and remain below 10 knots through Friday. Dry air may begin to be a problem for 93L beginning on Wednesday, as it works its way a bit further to the north where a dry Saharan Air Layer (SAL) exists. Odds are, 93L will be able to develop into a tropical depression by Wednesday. The GFDL, HWRF, and SHIPS intensity models all predict 93L will be a hurricane by Saturday. The National Hurricane Center is giving 93L a medium (20-50% chance) that it will be a tropical depression by Wednesday afternoon. This storm could threaten the Lesser Antilles 6-7 days from now, but preliminary indications are that it will pass to the north of the islands.

Elsewhere in the tropics
Several of the reliable computer models forecast development of a new tropical wave coming off the coast of Africa about 7 days from now.

Tune in tomorrow!

Jeff Masters

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Disturbances 92L, 93L a threat to the Lesser Antilles Islands

By: JeffMasters, 2:35 PM GMT on August 11, 2008

A tropical wave near 12N 49W, about 800 miles east of the Lesser Antilles Islands (92L), has changed little since Sunday. A QuikSCAT pass at 5:12 pm EDT last night revealed a large, elongated circulation, with top winds of 30 mph. This morning's QuikSCAT pass missed 92L. Wind shear is a low 5-10 knots over the disturbance. Satellite loops show a limited amount of heavy thunderstorm activity that has decreased slightly since Sunday. Water vapor satellite loops show that 92L is at the edge of a large area of dry air and Saharan dust to its west and north, and this dry air has been drawn into the center of circulation, disrupting the storm.


Figure 1. Latest satellite image of 92L.

The forecast for 92L
Water temperatures are a warm 28.4°C and forecast to increase to 28.8°C three days from now. Wind shear is forecast to remain below 10 knots for the next five days. The environment appears favorable for intensification, except for the dry air to the west. Most of the models are indicating 92L will have trouble developing. The GFDL model is the most aggressive, intensifying 92L to a 55 mph tropical storm that passes just north of the northernmost Lesser Antilles Islands on Thursday. In contrast, the HWRF model does not develop 92L at all. The National Hurricane Center is giving 92L a medium (20-50% chance) of becoming a tropical depression by Wednesday morning. This is a step down from their forecast last night of a high (>50%) chance of developing, and reflects the uncertainty that 92L will be able to get organized in the face of significant dry air to its west. Nevertheless, residents of and visitors to the northern Lesser Antilles should anticipate the possibility of a tropical depression or minimal tropical storm arriving in the islands as early as Wednesday (though Thursday is more likely). The southern islands are less likely to be affected.

Disturbance 93L off the coast of Africa
A tropical wave near 11N 29W (93L), about 300 miles west-southwest of the Cape Verde Islands, has developed a modest amount of heavy thunderstorm activity overnight. A QuikSCAT pass at 2:34 am EDT this morning showed a closed circulation, with top winds of 30 mph. Wind shear is a high 20 knots over the disturbance. Water vapor satellite imagery shows that 93L is embedded in a large area of moist air, with some dry air and Saharan dust to its north.


Figure 1. Latest satellite image of 93L.

The forecast for 93L
Water temperatures are a warm 28° and forecast remain near 28°C for the next five days. Wind shear is forecast to drop below 10 knots by Tuesday night, and remain below 10 knots through Friday. Dry air may begin to be a problem for 93L beginning on Wednesday, as it works its way a bit further to the north where a dry Saharan Air Layer (SAL) exists. Odds are, 93L will be able to develop into a tropical depression by Friday. Most of the computer models are developing 93L into a tropical storm by early next week, and put it on a track to skirt the northernmost Lesser Antilles Islands. The National Hurricane Center is giving 93L a medium (20-50% chance) that it will be a tropical depression by Wednesday morning. Residents of and visitors to the Lesser Antilles should watch this system, as it has the potential to bring tropical storm conditions to the islands early next week.

Elsewhere in the tropics
Several of the reliable computer models forecast development of a new tropical wave coming off the coast of Africa about 7 days from now.

This afternoon, I'll post an analysis of the expected wind shear and dry air over the tropical Atlantic during the coming week.

Jeff Masters

Updated: 2:59 PM GMT on August 11, 2008

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Lesser Antilles need to watch two new disturbances

By: JeffMasters, 2:21 PM GMT on August 10, 2008

A tropical wave near 11N 43W, midway between Africa and the Lesser Antilles Islands (92L), has developed a modest amount of heavy thunderstorm activity overnight. A QuikSCAT pass at 6:21 am EDT this morning revealed a large, elongated circulation. Wind shear is a low 5-10 knots over the disturbance. Satellite loops show a limited amount of heavy thunderstorm activity that is slowly increasing. Water vapor satellite loops show that 92L is at the edge of a large area of dry air and Saharan dust to its west and north, which will slow development.


Figure 1. Latest satellite image of 92L.

The forecast for 92L
Water temperatures are a toasty 28° and forecast to increase to 29°C three days from now. Wind shear is forecast to remain below 10 knots for the next five days. The environment appears favorable for intensification, except for the dry air to the west. Circulations of the sloppy nature that 92L has usually take at least two days to get organized and form a tropical depression, particularly when there is dry air nearby to interfere. The SHIPS intensity model brings 92L to hurricane strength in just 3 days, which is way over-aggressive. The UKMET, GFS, and ECMWF models are more reasonable, bringing 92L to weak tropical storm strength by the time the disturbance crosses into the northern Lesser Antilles Islands on Wednesday or Thursday. The National Hurricane Center is giving 92L a medium (20-50% chance) that 92L will be a tropical depression by Tuesday morning. Residents and visitors to the Lesser Antilles should prepare for the possibility of tropical storm conditions arriving as early as Wednesday.

Elsewhere in the tropics
Most of the reliable computer models are also developing another tropical wave, just off the coast of Africa. This system is also expected to move westward, potentially threatening the Lesser Antilles Islands early next week.

Jeff Masters

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Updated hurricane season predictions from CSU, NOAA, and TSR

By: JeffMasters, 2:27 PM GMT on August 09, 2008

The tropical Atlantic is quiet and there are no threat areas to discuss today. Three of the four reliable computer models predict that some development could occur between Wednesday and Saturday between Africa and the Lesser Antilles Islands.

The updated August 5 hurricane season forecast from Colorado State University
The forecast team at Colorado State University (CSU) led by Phil Klotzbach and Bill Gray issued their August forecast of Atlantic hurricane activity on Tuesday. The new CSU forecast calls for seasonal totals of 17 named storms, 9 hurricanes, 5 intense hurricanes, and an ACE index of 175. The ACE index is a more intelligent way to assess total activity for a season, and is computed by summing the squares of the estimated maximum sustained wind speed of each named storm (when its wind speed is 39 mph or higher), at six-hour intervals. The resulting number (divided by 10,000 to make it more manageable), is called the Accumulated Cyclone Energy (ACE). An average hurricane season has 10 named storms, 6 hurricanes, 2 intense hurricanes, and an ACE index of 96. So far this season, we have had 5 named storms, 2 hurricanes, 1 intense hurricane, and 39 ACE units, so the CSU team is expecting 13 more named storms, 7 hurricanes, 4 intense hurricanes, and 136 more ACE units. They give a 67% chance that an intense hurricane will hit the U.S. (52% is the historical average). They say that conditions this year are most similar to 1926, 1961, 1996, 1998, and 2000. These seasons had 6, 7, 6, 3, and 3 intense hurricanes, respectively. The CSU team's August numbers are an increase over what they predicted in their June 3 forecast, which called for 15 named storms, 8 hurricanes, and 4 intense hurricanes. The CSU team notes that is is pretty unlikely that we will see an El Niño event this hurricane season, as we are just two week from the beginning of the peak part of the season, and there is no indication that El Niño conditions are developing. El Niño events substantially reduce hurricane activity, due to the increased levels of wind shear they bring to the tropical Atlantic.

The updated August 7 hurricane season forecast from NOAA
NOAA released their updated Atlantic hurricane season forecast Thursday, and they also increased their numbers. NOAA is calling for an 85% probability of an above-normal hurricane season, a 10% chance of a near-normal season, and a 5% chance of a below-normal season. Their May forecast called for only a 65% chance of an above-normal hurricane season. The August forecast calls for a total of 14-18 named storms, 7-10 hurricanes, 3-6 major hurricanes, and an ACE index of 135-220 this season. Since we've already had 5 named storms, 2 hurricanes, and 1 intense hurricane, this means we can expect an additional 9-13 named storms, 5-8 hurricanes, and 2-5 major hurricanes between August 7th and November 30th.

The updated August 5 hurricane season forecast from Tropical Storm Risk, Inc.
Tropical Storm Risk, Inc. (TSR) released their August 5 update this week, and are calling for a 95% chance that 2008 will be in the top 1/3 of years historically. The latest TSR forecast calls for 18.2 named storms, 9.7 hurricanes, 4.5 intense hurricanes, and an ACE index of 191. This is an increase from their June numbers of 14.4 named storms, 7.7 hurricane, 3.4 intense hurricanes, and an ACE index of 131. TSR says that the expected very light trade winds over the tropical Atlantic for the remainder of hurricane season are the main reason for their boosted forecast. Light trade winds allow the ocean to heat up more, due to decreased evaporative cooling, and reduced mixing up of cold water from the deep ocean. Light trade winds also contribute to increased spin in the atmosphere at low levels, allow incipent tropical storms to spin up more readily.

Exceptional July activity portends an active main part of hurricane season
As seen in the plot of typical hurricane season activity (Figure 1), only about 10% of a typical season's activity occurs by August 8. However, this year has been unusually active. We've already had nearly 50% of the typical activity of an entire season. An average season has 10 named storms, 6 hurricanes, 2 intense hurricanes, and an ACE index of 96. We've had 5 named storms, 2 hurricanes, 1 intense hurricane, and an ACE index of 39 so far in 2008. This year's activity (prior to August 1) ranked 4th all-time (since 1851) for the amount of Accumulated Cyclone Energy (ACE). Only 2005, 1916, and 1933 had more. All three of these years had at least five major hurricanes and ACE values of at least 175.

A big part of the reason that CSU, NOAA, and TSR have bumped up their August forecast numbers is because of the extremely high levels of early-season tropical storm activity. High levels of activity in the deep tropics in June or July are a strong indication that the rest of hurricane season will be above average in activity, according to Goldenberg (2000). Goldenberg found that 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 were at least average, and often above average. Hurricane Bertha formed in this region in 2008, so according to Goldenberg's research, we are due for an above average hurricane season.


Figure 1. Average Atlantic hurricane and tropical storm activity through the year. Typically, only about 10% of a season's activity occurs by August 8.

Summary: expect a very active year
While I doubt we'll see anything like 2005's record hurricane season this year, I do think that the very active July we had portends that 2008 will be one of the twenty most active hurricane seasons on record. I expect four major hurricanes, with at least one of these hitting the U.S. The fact that the tropics are very quiet at present is the proverbial "calm before the storm". As seen in Figure 1, hurricane season activity takes a big jump around August 18, and I expect the Atlantic will start getting very active around that date.

References
Goldenberg, S.B., 2000: "Intraseasonal predictability of Atlantic basin hurricane activity" Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, pp.59-60

My next update will be Monday, unless there's some significant development Sunday.
Jeff Masters

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August hurricane outlook, part II: steering currents

By: JeffMasters, 3:50 PM GMT on August 08, 2008

The tropical Atlantic is quiet and there are no threat areas to discuss today. Several of the models predict that some development could occur Monday off the North Carolina coast, at the tail end of an old cold front. Any such development would likely move northeastward out to sea, and would probably not have enough time to develop into a tropical depression. The UKMET model predicts formation of a tropical depression off the coast of Africa about 5-6 days from now, but no other models are predicting this.

August Hurricane Outlook Part II: Steering Currents
The hurricane steering pattern for all of July and the first week of August over the North Atlantic has predominantly acted to recurve hurricanes out to sea. The jet stream has been "stuck" in a standing wave pattern, where it dips southward over the East Coast of the U.S., creating a trough of low pressure capable of recurving tropical storms once they get north of the Caribbean Sea (20° latitude). Current long-range forecasts from the GFS and ECMWF models indicate that this pattern will last for at at least another week, and possibly longer. This pattern is in contrast to the steering pattern that set up in 2004 and 2005, when a ridge of high pressure set got stuck over the Eastern U.S. A ridge in this location does not allow hurricanes to recurve, and the U.S. took a terrific battering those years. There is currently no indication that a repeat of the 2004-2005 steering current pattern will occur in 2008.

One often hears about how the Bermuda High acts to steer hurricanes, but this semi-permanent surface low-pressure system is really a reflection of what is going on in the upper atmosphere, at the level of the jet stream winds. The tool I like to use to study steering currents is the 500 millibar (mb) upper-air forecast from the latest run of the GFS model. (One can also use the 500 mb forecast from the ECMWF model, but this forecast only goes out 10 days, compared to the 16 days of the GFS forecast). A pressure of 500 mb is found at about 18,000 feet altitude, where the jet stream is active. Plotted on these 500 mb maps are lines showing how high above sea level one finds a pressure of 500 mb. Where a U-shaped bend occurs, a trough of low pressure is present. Any tropical cyclones that get far enough north to "feel" the westerly winds of the jet stream embedded in the trough will recurve to the north and northeast. Conversely, an upside-down "U" in the 500 mb height lines reveals the presence of a ridge of high pressure. Ridges force tropical cyclones to move westward (in the Northern Hemisphere). A ridge of high pressure aloft always accompanies the surface Bermuda High, so one can talk about either one when discussing steering currents. As seen in Figure 1, a trough of low pressure was present this morning over the U.S. East Coast, with ridges of high pressure over the mid-Atlantic Ocean (over the surface Bermuda High), and over the Midwest U.S. Under this steering pattern, any hurricanes in the mid-Atlantic Ocean north of about 20° latitude would be recurved by the East Coast trough. This type of steering pattern was in place during Hurricane Bertha, which recurved out to sea very close to Bermuda. One can pull up a loop going out a full 16 days of the 500 mb forecast and watch the evolution of the trough/ridge pattern to see how the steering currents might change.


Figure 1. GFS model forecast of heights of the 500 mb surface above sea level (white lines) for 2pm EDT today. The colors show how much counter-clockwise spin is present (vorticity). High vorticity is associated with storms.

Uncertainty in the steering current forecast
The GFS model has been indicating for the past several days that the trough of low pressure entrenched over the Eastern U.S. will get unstuck 10-16 days from now, and replaced by a series of weak, fast-moving troughs and ridges. To get an idea of the uncertainty in this steering pattern forecast, a good tool to use is the Global Ensemble Forecast System (GEFS) charts. The GEFS charts show runs of the GFS model done using 20 slightly different initial conditions. This creates an "ensemble" of 20 possible forecasts. By examining how these 20 different forecasts diverge with time, one can get an idea of how confident one should be of major steering current changes forecast by the GFS model.

These 20 forecast solutions are plotted as a series of colored lines that trace out the height (in decameters, or tens of meters) above sea level where a certain pressure is found. It turns out that the southern edge of the jet stream is currently found at a 500 mb height of about 582 decameters (5820 meters). Go to the NOAA/NCEP Model Analyses and Forecasts web site, click on the link for the GEFS model, then select to plot up the "500mb 540/582 Hgt Contours". The loop takes a while to load, but gives one the best idea of how the steering currents might evolve. The 20 forecasts all lie close to each other the first few days of the forecast, then begin to diverge at later times. By the end of two weeks, you'll see why these are called "spaghetti plots" (Figure 2). You'll also see why I'm stretching a bit when I say we can forecast steering patterns out to two weeks. The plots of where the jet stream might be 16 days from now are all over the place. Still, one can often get the general picture of whether a dominant trough or ridge will set up over the U.S. East Coast 16 days from now.



Figure 2. Forecast of the location where the 500 mb pressure surface will be at a height of 582 decameters (5280 meters) above sea level. This height marks the approximate southern boundary of the jet stream. Top image: the forecast for 2pm EDT today. Bottom map: the forecast for 16 days from now. The 20 different lines correspond to 20 different runs of the GFS model with slightly different initial conditions. The runs were all initialized at 06 GMT (2am EDT) August 8. Image credit: NOAA/NCEP.

For this morning's GEFS run, we see that 16 days from now the 20 ensemble members cannot agree at all on whether there will be a trough of low pressure over the Eastern U.S. This is an indication that the GFS model expects the dominant East Coast trough to break down later in the forecast period, and be replaced by a series of weak troughs and ridges moving across the U.S. The exact timing of these fast-moving troughs and ridges is difficult to pinpoint 16 days in advance, resulting in a big spread in the "spaghetti". This pattern favors less recurvature of approaching tropical storms and hurricanes. How believable is this forecast? Well, I've found that the GFS model is often over-eager to break down a "stuck" jet stream pattern that has been entrenched for many weeks. The latest ECMWF model runs do not support the GFS prediction of a weaker trough over the Eastern U.S. ten days from now. I believe the jet stream will remain stuck in its current configuration for two more weeks. This pattern should act to recurve most of the dangerous "Cape Verdes" type hurricanes that form off the coast of Africa and penetrate north of 20° latitude. However, the GFS often has the right idea about a major pattern change, but jumps the gun a bit. I believe it is likely that after two weeks, the East Coast trough will weaken as the GFS is hinting at, and a steering pattern less favorable for recurvature will set up. It's too early to speculate what might happen in September during the peak of hurricane season.

Since I went on rather long about the steering current forecast, I'll save the discussion of wind shear and African dust for Saturday or Monday. I'll have an update Saturday morning.

Jeff Masters

Updated: 3:51 PM GMT on August 08, 2008

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August hurricane outlook, part I: SSTs

By: JeffMasters, 3:28 PM GMT on August 07, 2008

The tropical Atlantic is quiet and there are no threat areas to discuss today. The UKMET model foresees that a tropical depression could form Sunday off the Carolina coast, at the tail end of an old cold front. Any such development would likely move northeastward out to sea. The GFS model predicts formation of a tropical depression off the coast of Africa about 6-7 days from now. The other two reliable models, the NOGAPS and ECMWF, do not predict any tropical storm formation in the coming week.

August Sea Surface Temperature Outlook
Eighty-five percent of all major hurricanes form in the Main Development Region (MDR) of the Atlantic, from the coast of Africa to the coast of Central America, between 10° and 20° latitude. This region also spawns 60% of all weaker hurricanes and tropical storms. Sea Surface Temperatures (SSTs) in the MDR are near average over the Caribbean, and 0.5-2.0°C above average between the Lesser Antilles Islands and Africa (Figure 1). SSTs are well below the record levels observed in 2005 and 2006, when they were up to 2°C above average over large portions of the Main Development Region. Still, the very warm waters between the Lesser Antilles and Africa are cause for concern, and have already fueled near-record levels of July activity. The SST anomalies over most of the MDR have been increasing in recent weeks, which is not good news. SSTs are near average in the Gulf of Mexico due to the passage of Hurricane Dolly and Tropical Storm Edouard, which stirred up cool waters from the depths. SST anomalies should return to above average levels in the Gulf of Mexico in coming weeks as the surface waters recover from the passage of these storms.


Figure 1. Departure of SST from average for August 4, 2008. Image credit: NOAA.

Total heat content of the ocean
Hurricanes generally require sea surface temperatures (SSTs) of at least 80° F (26.5° C) to exist, and the hotter the water, the better. Hurricanes also like to have these warm ocean waters extend to a depth of several hundred feet, since the winds of a hurricane generate ocean turbulence that stirs up colder water from the depths to the surface. Hurricane that pass over a region of ocean with very deep warm waters can intensify explosively; this happened with Katrina, Rita, and Wilma in 2005. A good way to monitor this total oceanic heat is with the Tropical Cyclone Heat Potential (TCHP) imagery prepared daily by NOAA using satellite measurements of the height of the ocean surface. Hotter water expands, creating a higher water surface that the satellite can measure.

Let's look at the TCHP data from August 5 this year, and compare it to the record-breaking Hurricane Season of 2005 (Figures 2 and 3). The units of measurement are in kilojoules per square centimeter, and any value greater than 20 kJ/cm**2 (a medium blue color) is high enough to support a Category 1 hurricane. A TCHP greater than 80-90 kJ/cm**2 (orange color) is often associated with rapid intensification of a hurricane. The TCHP images from the two years are quite similar. There was more heat available in the Western Caribbean in 2005, but this year's THCP is higher in the Bahama Islands. An orange bullseye in the Gulf of Mexico in 2005 marks the Loop Current Eddy that broke off in July of that year, and helped fuel Katrina and Rita to record intensities. There is a similar bullseye visible in this year's image, due to a Loop Current Eddy that broke off in July. This eddy is not quite as big or as hot as the 2005 eddy, but is still capable of fueling rapid intensification of any hurricanes that might pass over it. There is also an eddy in the western Gulf of Mexico that broke off in April, but this eddy is not quite as warm, since it broke off before the peak heat of summer. It is unusual to have two eddies break off just three months apart; usually there is a lag of at least six months betweens eddies. The presence of two Loop Current Eddies in the Gulf of Mexico this hurricane season provides an unusual amount of warm water to great depth that can help fuel rapid intensification of any hurricanes that cross the Gulf. Overall, the total heat content of the ocean in the Atlantic Main Development Region and Gulf of Mexico this year is not much less than during the record breaking Hurricane Season of 2005. Thus, we can expect an unusually high number of intense hurricanes this year, assuming that wind shear and dry air are average or below average.


Figure 2. Tropical Cyclone Heat Potential (TCHP) for August 5, 2005.


Figure 3. Tropical Cyclone Heat Potential (TCHP) for August 5, 2008.

What are SSTs likely to do in August?
The latest long range forecast from the NOAA CFS seasonal forecast models shows a continuation of above average SSTs through the remainder of hurricane season. The latest 15-day forecast from the GFS model predicts a weaker than average Bermuda High, which will drive weaker trade winds than normal. Weaker trade winds result in less evaporative cooling of the ocean, and less mixing of cool water from the depths, resulting in higher SSTs. An additional factor affecting SSTs is the amount of African dust coming off the Sahara Desert. Higher dust levels than usual will block more sunlight than usual, resulting in cooler SSTs than average. So far, the August Saharan dust levels appear near average. It is difficult to predict how these dust levels might change later in the month.

I'll summarize wind shear, steering currents, and dust levels in Friday's blog.

Jeff Masters

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Edouard brings welcome rains to Texas

By: JeffMasters, 1:22 PM GMT on August 06, 2008

The remains of Edouard continue to soak Texas, providing much-needed rain to regions under severe to exceptional drought. The heaviest rains fell near the coast in Baytown, where up to six inches were recorded (Figure 1). Additional rains of 1-2 inches are expected in West Texas today. Edouard has caused no significant flooding, and no reports of major damage have been received. The storm did knock out power to up to 37,000 people in Southeast Texas at the height of the storm. Overall, Edouard was more of a blessing than a bane, and Texas could use another tropical storm like this one to help with the drought. Rainfall has been only 15% of normal since May 1 in some regions of the state.


Figure 1. Radar-estimated rainfall from Tropical Storm Edouard.

Some rainfall amounts from Edouard, as of 1 am CDT this morning:

...TEXAS...

BAYTOWN EOC 6.48
SHELDON: SAN JACINTO RIVER BANANA BEND 5.99
PASADENA: BIG ISLAND SLOUGH 5.75
BAYTOWN: CEDAR BAYOU 5.27
BAYTOWN: GOOSE CREEK 5.08
PASADENA: WILLOW SPRING 5.08
SHOREACRES: TAYLOR BAYOU 4.73
LA PORTE: LITTLE CEDAR BAYOU 4.57
SHELDON: CARPENTERS BAYOU 4.49
HUNTSVILLE 3.32
HOUSTON (IAH) 2.81
HOUSTON (HOU) 1.99

...LOUISIANA...

LAKE CHARLES 1.87
SALT POINT 20SSW 1.28

Elsewhere in the tropics
There are no threat areas to discuss in the tropical Atlantic. Two of our four reliable computer models predict the possible formation of a tropical depression off the coast of North Carolina along an old cold front on Saturday or Sunday. Such a storm would likely move northeastward out to sea.

Now that I'm back from vacation and ready for hurricane season's long haul, I'll be posting lots of extra material on hurricanes in addition to my daily updates. This begins tomorrow with the Atlantic hurricane season outlook for the next two weeks. I want to thank Bryan Woods for ably filling in for me while I was gone!

Jeff Masters

Updated: 1:27 PM GMT on August 06, 2008

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Edouard makes landfall

By: JeffMasters, 2:42 PM GMT on August 05, 2008

Edouard has now made landfall along the upper Texas coast at the McFaddin National Wildlife Refuge. Edouard never did make it to hurricane intensity and came ashore as a strong tropical storm with maximum sustained winds of 65 mph.

20080805.1345.goes12.x.ir1km.05LEDOUARD.55kts-996mb-296N-942W.100pc.jpg
Edouard - Infrared satellite


Edouard - Watches and Warnings

The largest threat now from Edouard is inland flooding. There is currently some very heavy rain over the Houston area. Fortunately the rain in moving quickly at 14 mph so it won't add up too much. It looks like Edouard will slowly continue to weaken and bring beneficial rains to agricultural areas of Texas in the short term.

at200805_radar.gif
Radar loop

Cheers,
Bryan Woods

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Edouard intensifying

By: JeffMasters, 6:44 PM GMT on August 04, 2008

Tropical Storm Edouard is intensifying as it approaches landfall Tuesday morning along the Texas/Louisiana border. Visible satellite loops show Edouard's heavy thunderstorm activity has increased significantly in the past few hours, and is now starting to wrap all the way around the storm. Long range radar out of Lake Charles, Louisiana also shows an increase in the organization and intensity of the radar echoes. Edouard is over ocean waters of about 29°C, which extend to a moderate depth. The Tropical Cyclone Heat Potential, a measure of the total energy available for rapid intensification, is about 30 kJ/cm^2, which is less than the value of 80 typically associated with rapid intensification. Wind shear has fallen from about 10 knots last night to 5 knots this afternoon, which has allowed the storm to organize. A new hurricane hunter plane entered the storm at about 2pm EDT, and so far has measured top winds of 50 knots (57 mph) at the surface with its SFMR instrument.


Figure 1. Latest satellite image of Edouard.

The forecast
The shear is forecast to remain below 5 knots through Monday night, but the depth of the warm waters get shallower, with the Tropical Cyclone Heat Potential decreasing to about 10 kJ/cm^2 Monday night. Taken together, these factors should allow Edouard to intensify to a least a 60-70 mph tropical storm. The latest 12Z (8am EDT) GFDL model brings Edouard to a 75 mph Category 1 hurricane by landfall, with the strongest winds occurring near the Texas/Louisiana border. Other intensity models, such as the SHIPS and HWRF models, predict Edouard will remain a tropical storm at landfall. Our skill predicting intensity changes is poor, but at present at appears that Edouard will be a strong tropical storm or weak Category 1 hurricane at landfall. Wind damage and heavy rain are the main damage threats from Edouard. However, these rains will also greatly benefit the region, which is under severe to extreme drought.

I'll have another blog on Edouard Tuesday morning.

Jeff Masters

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Edouard moving toward Texas

By: JeffMasters, 1:57 PM GMT on August 04, 2008

The two areas of interest in the central Atlantic have both quieted down over the past 24 hours and now all eyes have turned to the Gulf. As we mentioned yesterday, there is a developing storm sitting just south of Louisiana. The circulation is still not very strong and convection is relatively unorganized, but its proximity to the coast is worrisome. Maximum sustained winds are at 50 mph and the pressure is still a relatively high 1002 mb.

20080804.1245.goes12.x.ir1km.05LEDOUARD.45kts-1002mb-281N-892W.100pc.jpg
Edouard - Infrared satellite


Edouard - Watches and Warnings

There is strong consensus among the models that Edouard is slowly continue to move westward and make landfall somewhere just to the southwest of the Galveston and Houston area early tomorrow morning. While the models do agree well on Edouard's track, they have no handle on his expected intensity.

Click for full size model imagery:


Edouard - Track Forecast



Edouard - Wind Speed Forecast




Edouard fortunately has not gotten his act together and there is no sign that a rapid strengthening is in the cards. However, interests in the upper Texas coast should consider Edouard to be a Category 1 hurricane at landfall. While Edouard is not a strong storm yet, I see nothing limiting Edouard other than time. Sheer is low and waters are warm. Convection has become better organized this morning as also seen on the radar, but overnight the storm has weakened too. I expect Edouard to come ashore as a strong tropical storm, but not to make hurricane strength. The bottom line is that Edouard's intensity is a big question, but take him seriously.


at200805_radar.gifRadar loop

Cheers,
Bryan Woods

Updated: 4:11 PM GMT on August 04, 2008

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Edouard springs to life, threatens Texas/Louisiana

By: JeffMasters, 6:06 AM GMT on August 04, 2008

Well, today was the day one of the co-founders of Weather Underground, Chris Schwerzler, decided to get married. While I highly complement him on his choice of brides, and his choice of wedding days, weather-wise--the weather here in Sonoma California was beautifully clear and 75 degrees--his choice of wedding days was less advantageous weather-wise as far as the tropics are concerned. So, I'm taking a break from the reception to take a look at Edouard.


Figure 1. Latest satellite image of Edouard.

Tropical Storm Edouard has not changed much in appearance over the past 12 hours, as seen on infrared satellite loops. The heavy thunderstorm activity is limited to the south side of the storm, thanks to about 10 knots of wind shear from upper-level winds blowing from the north. Edouard is over ocean waters of about 29°C, which extend to a moderate depth. The Tropical Cyclone Heat Potential, a measure of the total energy available for rapid intensification, is about 40 kJ/cm^2, which is less than the value of 80 typically associated with rapid intensification. These conditions are very similar to those Dolly had, when it intensified into a borderline Category 1/2 hurricane. A new hurricane hunter plane entered the storm at about 2am EDT, and so far has measured top winds of 36 knots (42 mph) at the surface with its SFMR instrument.

The forecast
The shear is forecast to decrease to near zero by Monday night, and sea surface temperatures will increase by about 0.5°C. The depth of the warm waters get shallower, with the Tropical Cyclone Heat Potential decreasing to about 20 kJ/cm^2 Monday night. Taken together, these factors should allow Edouard to intensify, although it may take 24 hours or so for the storm to organize an eyewall, as occurred with Dolly. The latest 00Z (8pm EDT) GFDL and SHIPS intensity models both bring Edouard to a borderline tropical storm/Category 1 hurricane by landfall, with the strongest winds occurring near the Texas/Louisiana border. The 00Z HWRF model, though, predicts that Edouard will not get its act together at all, and will be a tropical depression at landfall. The latest GFS model is also not very enthusiastic about Edouard.

My best guess is that Edouard will be a tropical storm with 60-70 mph winds when it comes ashore Tuesday morning. It would be a big surprise if the storm made it to Category 2 strength, since it has a limited time before it comes ashore, and is so poorly organized at present. Heavy rain is the main damage threat from Edouard.

I'll have another blog on Edouard Monday afternoon, and Bryan Woods may also chip in Monday morning.

Jeff Masters

Updated: 6:08 AM GMT on August 04, 2008

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Three areas of interest in the Atlantic -- one near the Gulf Coast

By: JeffMasters, 6:40 AM GMT on August 03, 2008

We now have three areas to keep an eye on in the Atlantic. Two of the systems pose no immediate threat to land, but there is one system quite close to the Gulf coast.

two_atl.gif
Map of areas of interest

The first system, Invest 90L has been slowly moving across the central Atlantic but continues to support little thunderstorm activity. Conditions with this tropical wave remain less than favorable for development. No storm formation in expected over the next couple of days, but we will continue to monitor the wave for signs of organization.

20080803.0515.goes12.x.ir1km.90LINVEST.25kts-1010mb-115N-320W.77pc.jpg
Invest 90L - Infrared satellite

There is also a low pressure center located northwest of Invest 90L that is continuing to move across the central Atlantic. Invest 99L is a closed surface low that still lacks organized convection. The surface circulation makes development of a tropical system much more likely than in the case with a tropical wave. However, Invest 99L is approaching more hostile environmental conditions as time progresses. The system is leaving the vicinity of the upper-level anticyclone that has been favoring development.

20080803.0515.goes12.x.ir1km.99LINVEST.30kts-1010mb-185N-465W.100pc.jpg
Invest 99L - Visible satellite

tgwind.gif
200 mb streamlines (credit: WSI)

Closer to home, there is an area of thunderstorms associated with a trough in the northern Gulf of Mexico. Convection is still fairly limited with this system, but its proximity to land innately raises concern. Surface pressures are already falling in the area and the system does seem to be undergoing limited development. If this system does develop, it is unlikely to have time to become very strong. However, any tropical system impacting land can lead to heavy damage from inland flooding. Accordingly, a hurricane hunter has been scheduled to investigate the system later today.

20080803.0515.goes12.x.ir1km.91LINVEST.20kts-1009mb-295N-870W.100pc.jpg
Invest 91L - Infrared satellite

This system is expect to drift slowly westward and will reach the central Texas coast in a few days.

at200891_model.gif
Invest 91L - Weather models

Cheers,
Bryan Woods

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New Invest 99L to watch in the central Atlantic

By: JeffMasters, 7:22 PM GMT on August 01, 2008

Welcome to August! Out in the middle of the Atlantic there is a new area of interest to watch. The low pressure system is expected to move slowly westward and toward the increasingly warm waters of the western Atlantic. While there is no immediate threat of development, a gradually strengthening is possible.

two_atl.gif
Map of area of interest

There is currently little to no deep convection associated with this system. It is mainly for this reason that no immediate development is expected. It'll be at least 48 hours before we could possibly see any real action here.

20080801.1845.goes12.x.ir1km.99LINVEST.25kts-1008mb-184N-354W.97pc.jpg
Invest 99L - Infrared satellite

However, the surface circulation looks very well defined. Visible satellite imagery is showing a clear closed circulation. With a closed circulation already established, this could lead to rapid intensification of a developing system approaching warm waters. I expect that if a system does develop, it will be very broad which could limit its ultimate peak intensity.

20080801.1845.goes12.x.vis1km.99LINVEST.25kts-1008mb-184N-354W.100pc.jpg
Invest 99L - Visible satellite

A gradual track to the west is expected over the next couple of days. The dynamical models all seem to be in agreement in preventing immediate recurvature of the system.

Cheers,
Bryan Woods

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About

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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