Dr. Jeff Masters' WunderBlog

Noel stalls out; no change in strength

By: JeffMasters, 8:08 PM GMT on October 31, 2007

Tropical Storm Noel remains stuck in a region of weak steering currents, just offshore the coast of Cuba. It is common for a tropical cyclone to move slowly and erratically when a trough of low pressure is about to pick it up and scoot the storm out to sea. The latest (12Z, 8am EDT) model runs continue to unanimously insist that Noel will turn north and then north-northeast tonight, and I have no reason to doubt that will happen.

Visible satellite imagery shows that the low level center of Noel is exposed to view, thanks to wind shear of 20 knots. Strong upper level winds from the west are keeping all the heavy thunderstorm activity to the east of Noel's center. Noel will not be able to undergo more than slow intensification, due to this shear. The last three fixes from the Hurricane Hunters have shown a surface pressure of 995 or 996 mb, so Noel is not intensifying appreciably this afternoon. Peak surface winds remain in the 50 mph vicinity.

The latest GFDL and SHIPS intensity model runs bring Noel's winds up to 65-70 mph by Thursday afternoon, when the storm should be exiting the Bahamas. The HWRF model is less aggressive, forecasting winds of 55 mph at that time. Given that the current wind shear is already causing Noel trouble, and is not forecast to decrease, I expect top winds for Noel of 55-60 mph Thursday afternoon. Wind shear is expected to rise above 45 knots when Noel moves north of the northern Bahamas, so the storm should weaken then. By Friday, the models agree that Noel will transition to a powerful extratropical storm. Noel is expected to bring winds near hurricane force to Nova Scotia and Newfoundland on Saturday. Noel's remains may pass close enough to Nantucket and Cape Cod, Massachussetts to bring sustained winds of 50 mph, according to the latest run of the HWRF model. The GFDL forecasts winds closer to 40 mph. Other models take Noel farther from Cape Cod, and do not bring winds quite as high as the GFDL and HWRF.

Noel's death toll
Another 1-2 inches of rain has fallen today over hard-hit Hispaniola, where the death toll is at least 59. In the Domincan Republic, the death toll stands at 41, with another 38 missing. At least 6,000 buildings were damaged, and 10 bridges washed out. Most of the deaths were in the southern part of the country, just west of the capital of Santo Domingo. Up to 20 inches of rain fell in that region. Haiti suffered 18 deaths, but not as much damage as the Dominican Republic. Noel is the deadliest tropical cyclone to affect the Dominican Republic since Hurricane Georges hit Hispaniola in 1998, killing 380 Dominicans and causing over $1 billion in damage to the county.

Links to follow for Noel
Satellite loop
Camaguey, Cuba radar.
Long range radar out of Miami, FL
Nassau, Bahamas current weather
Google Maps interface, zoomed in on Nassau, Bahamas

Impact on Florida
There is no change to the forecast for Florida. Noel will pass east of the state as a weak but strengthening tropical storm. Winds will probably blow 20-30 mph with gusts to 40 mph along the coast of Florida on Thursday morning and afternoon, when Noel makes its closest approach. Florida will be on the dry side of Noel, thanks to upper level winds from the west that will be creating about 15-25 knots of wind shear over the storm. Expect occasional heavy rain showers with rain amounts totaling 1-3 inches if you live along the Southeast Florida coast. Most of Noel's heavy rains should stay offshore. The main hazard from Noel will be beach erosion, thanks to the 10-foot seas expected to pound area beaches.

I'll have an update Thursday morning.

Jeff Masters

Updated: 8:11 PM GMT on October 31, 2007

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Noel over water, headed north towards the Bahamas

By: JeffMasters, 1:50 PM GMT on October 31, 2007

Deadly Tropical Storm Noel has popped off the coast of Cuba and is headed north, according to the latest hurricane hunter data from 8:49am EDT this morning. The hurricane hunter data put Noel's center at 22.7 north, which is about 100 miles north of the coast of Cuba. Part of this northward motion was probably a relocation of the center underneath an impressive blow-up of thunderstorm activity visible on the latest satellite loops. These thunderstorms are generating rains of up to 1/2 inch per hour. Top winds found by the Hurricane Hunters were 40 mph, but these winds are expected to increase today as Noel re-organizes after its long stay over Cuba.

Hispaniola
The rains continue to fall over hard-hit Hispaniola, where the death toll is at least 43, with many more missing. Most of the deaths were in the southern part of the Dominican Republic just west of the capital of Santo Domingo, where up to 19 inches of rain has have fallen. An additional 1-3 inches of rain fell in the 24 hours ending at 2am EDT today (Figure 1), and an additional 1-4 inches is likely by Thursday morning. The rains should taper off Thursday as Noel pulls away from the island. Noel is the deadliest tropical cyclone to affect the Dominican Republic since Hurricane Georges hit Hispaniola in 1998, killing 380 Dominicans and causing over $1 billion in damage to the county.

Cuba and the Bahamas
Rains have also been heavy over the Bahamas, where some islands have received six inches of rain. Nearby ocean areas have gotten up to ten inches. Cuba has fared better, with maximum rainfall amounts less than six inches. The rains will continue into Thursday then taper off as Noel pulls away to the north.


Figure 1. Satellite estimated rainfall for the 24 hours ending at 2am EDT Wednesday, 10/31/07. Rainfall is in millimeters, and 1" = about 25 mm, so orange colors are 10" of rain. Noel dumped as much as 10 inches of rain over ocean areas in the Central Bahamas, and an additional three inches over hard-hit regions of the Dominican Republic that had already received 10-17 inches earlier this week. Image credit: Navy Research Lab, Monterey.

Links to follow for Noel
Satellite loop
Camaguey, Cuba radar.
Nassau, Bahamas current weather
Google Maps interface, zoomed in on Nassau, Bahamas

The forecast
Visible satellite from this morning suggests that the surface circulation and mid-level circulation are closer together, which will aid in intensification of Noel today as the storm pushes off the coast. Wind shear is 10-20 knots, which will allow some modest strengthening. By Thursday, wind shear is expected to increase to 25 knots, halting any intensification as a tropical storm. The latest computer model runs from 00Z and 06Z this morning all take Noel through the Bahamas, over or near Andros Island and Nassau. The SHIPS and GFDL intensity models forecast that Noel will have 60 mph top winds by Thursday afternoon, when the storm should be leaving the Bahamas. The HWRF model does not intensify Noel. I expect some intensification, given the better vertical alignment of the surface and mid-level centers of Noel, respectable 995 mb pressure just measured by the Hurricane Hunters, and the improved satellite appearance of the storm. A storm with 50-60 mph top winds on Thursday afternoon is a reasonable forecast. None of the models take Noel over South Florida, and the region most likely to suffer wind damage from Noel will be the Canadian Maritime provinces of Nova Scotia and Newfoundland. Noel is expected to transition to, or be absorbed by, an extratropical storm on Thursday. This extratropical storm will then intensify, potentially bringing sustained winds of 55-75 mph to the Canadian Maritimes on Saturday night and Sunday morning.

Impact on Florida
There is no change to the forecast for Florida. Noel will pass east of the state as a weak but strengthening tropical storm. Winds will probably blow 20-30 mph with gusts to 40 mph along the coast of Florida on Thursday morning, when Noel makes its closest approach. Florida will be on the dry side of Noel, thanks to upper level winds from the west that will be creating about 15-25 knots of wind shear over the storm. Expect occasional heavy rain showers with rain amounts totaling 1-3 inches if you live along the Southeast Florida coast. Most of Noel's heavy rains should stay offshore. The main hazard from Noel will be beach erosion, thanks to the 10-foot seas expected to pound area beaches.

I'll have an update this afternoon.

Jeff Masters

Updated: 1:54 PM GMT on October 31, 2007

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Noel chugging along over Cuba

By: JeffMasters, 8:18 PM GMT on October 30, 2007

Tropical Storm Noel is headed slightly north of due west across Cuba, and there are no signs that the storm will pop out over water today. Noel's heaviest rains are currently over the central Bahamas, where rain rates as high as 1/2 inch per hour are occurring. Satellite estimates put total rainfall amounts in the Bahamas at up to eight inches so far from Noel. Cuba has avoided Noel's heaviest rains, receiving no more than three inches.

Noel's heaviest rains have fallen over the southern coast of the Dominican Republic just west of the capital of Santo Domingo, where up to 17 inches have fallen (Figure 1). A nation-wide power outage knocked out power to all of the Dominican Republic for two hours yesterday, and flooding damage is reportedly heavy. Haiti has fared better, with peak rainfall totals of 7-10 inches in southeast Haiti near the Dominican Republic border. The sun has emerged in many locales on the island, but many areas in both Haiti and the Dominican Republic can expect to receive another 2-4 inches before the rains finally subside Wednesday night. These rains will cause very dangerous flash floods, particularly in Haiti, where deforestation has left only 1.4% of the original forest cover remaining. Media reports put the death toll so far in the Dominican Republic at 20, with 20 more missing, and this toll is almost certain to go higher. Noel is the deadliest tropical cyclone to affect the Dominican Republic since Hurricane Georges hit Hispaniola in 1998, killing 380 Dominicans and causing over $1 billion in damage to the county.


Figure 1. Satellite estimated rainfall for October 26-30, as estimated by the NASA TRMM satellite. At least 1-3 more inches of rain have fallen across Hispaniola since this image was created. Image credit: NASA.

Links to follow for Noel
Satellite loop
Camaguey, Cuba radar.
Google Maps interface, zoomed in on Canagua, Cuba

The forecast
Satellite loops show Noel's surface circulation is inland over Cuba, but the storm's mid-level center is spinning just off the coast, about 60 miles east-northeast of the surface center. How quickly these two centers can rejoin will determine how quickly Noel can re-intensify. The latest computer model runs from 12Z this morning are in good agreement that Noel will remain over Cuba until Wednesday morning, then pop off the coast and recurve sharply to the north, just offshore the coast of South Florida. None of the computer models show a landfall in South Florida. Passage over Cuba has severely weakened Noel, and it is looking very unlikely Noel will be stronger than a 55 mph tropical storm until it moves well past Florida and the Bahamas. The official NHC forecast still looks reasonable, with Noel passing 50-200 miles off the coast of South Florida Thursday morning as a weak but strengthening tropical storm, with top winds of 40-50 mph. Winds will probably be sustained at 20-30 mph with gusts to 40 mph along the coast of Florida on Thursday morning, when Noel makes its closest approach to Florida. Florida will be on the dry side of Noel, thanks to upper level winds from the west that will be creating about 20 knots of wind shear over the storm. Expect occasional heavy rain showers with rain amounts totaling 1-3 inches if you live along the Southeast Florida coast. Most of Noel's heavy rains should stay offshore.

I'll have an update Halloween morning.

Jeff Masters

Updated: 8:23 PM GMT on October 30, 2007

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Noel hits Cuba and weakens significantly

By: JeffMasters, 2:46 PM GMT on October 30, 2007

Tropical Storm Noel made landfall in eastern Cuba this morning, and has weakened significantly as a result. Top winds are now 45 mph near the center, but this morning's QuikSCAT pass also noted winds up to 50 mph about 150 miles to the north of Noel's center, in the central Bahamas. Top winds measured in Cuba this morning have been at Canagua, where the winds were 28 mph gusting to 39 mph at 8am EDT. Canagua is on the north central coast of Cuba, south of Andros Island in the Bahamas. Georgetown in the central Bahamas measured sustained winds of 27 mph gusting to 36 mph at 7am EDT. A good way to track current wind readings along Noel's path is to use our Google Maps interface, zoomed in on Canagua, Cuba.


Figure 1. Satellite estimated rainfall rate at 2:58 am EDT Tuesday 10/30/07 for Tropical Storm Noel. The heaviest rains of one inch per hour were observed in the central Bahamas. Heavy rains up to 1/2 inch per hour were still affecting Haiti and the Dominican Republic on the island of Hispaniola. Image credit: NOAA.

Noel's rains
Noel's main threat continues to be heavy rains. Rainfall rates of up to one inch per hour are affecting the central Bahamas today (Figure 1), and Haiti and the Dominican Republic are getting up to 1/2 inch per hour. The heaviest rains from Noel have fallen over the southern coast of the Dominican Republic near the capital of Santo Domingo, where over a foot of rain has fallen (Figure 2). A nation-wide power outage knocked out power to all of the Dominican Republic for two hours yesterday, and flooding damage is reportedly heavy. Haiti has fared better, with peak rainfall totals of 6-8 inches in regions near the Dominican Republic border. Both nations can expect to receive another 5-7 inches in isolated regions before the rains finally subside Wednesday night. These rains will cause very dangerous flash floods, particularly in Haiti, where deforestation has left only 1.4% of the original forest cover remaining. Media reports put the death toll so far in the Dominican Republic at 20, with 20 more missing, and this toll is almost certain to go higher. Noel is the deadliest tropical cyclone to affect the Dominican Republic since Hurricane Georges hit Hispaniola in 1998, killing 380 Dominicans and causing over $1 billion in damage to the county.


Figure 2. Satellite estimated rainfall for the week ending at 11pm EDT Monday 10/29/07. Image credit: NASA.

Links to follow for Noel
Satellite loop
Holguin, Cuba radar
Google Maps interface, zoomed in on Canagua, Cuba

The forecast
The latest computer model runs from 00Z and 06Z this morning had the luxury of using data from last night's flight of the NOAA jet. This usually produces track forecasts that are 20% better. The models are in good agreement that Noel will move west-northwest to northwest during the next 24 hours, then recurve sharply to the north, just offshore the coast of South Florida. The models are off to a bad start, because Noel is tracking almost due west this morning, something the models did not anticipate. This increases the chance that Noel will recurve father to the west and pass over South Florida. However, passage over Cuba is weakening Noel, and it is looking much less likely Noel will be able to attain Category 1 hurricane strength. I give Noel a 30% chance of making a direct hit on South Florida, with a 5% chance that such a strike will be as a Category 1 hurricane. The most likely scenario is that Noel will pass 50-200 miles off the coast of South Florida Halloween night through Thursday morning as a weak but strengthening tropical storm, with top winds of 40-50 mph. Winds will probably be sustained at 20-30 mph with gusts to 40 mph along the coast. Florida will be on the dry side of Noel, thanks to upper level winds from the west that will be creating about 20 knots of wind shear over the storm. Expect occasional heavy rain showers with rain amounts totaling 1-3 inches if you live along the Southeast Florida coast. Most of Noel's heavy rains should stay offshore.

I'll have an update late this afternoon.

Jeff Masters

Updated: 3:05 PM GMT on October 30, 2007

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Disorganized Noel still dumping heavy rains

By: JeffMasters, 8:59 PM GMT on October 29, 2007

Tropical Storm Noel's center is back over water this afternoon, near the eastern tip of Cuba. An encounter with the mountains of Hispaniola has left Noel very disorganized. Top winds from this afternoon's hurricane hunter flight were in the 45-50 mph range, and the pressure was a high 1001 mb. Noel's prodigious rains of over one inch per hour have slowed to 1/2 inch per hour, according to satellite estimates. Long range radar out of Puerto Rico shows heavy rains affecting the Dominican Republic and western Puerto Rico. These rains have already exceeded eight inches over a wide area of ocean to the east of Noel's center, according to rainfall estimates from the Puerto Rico radar. Rainfall amounts of 4-8 inches over southern Puerto Rico have triggered numerous flash floods and landslides. The Holguin, Cuba radar shows spiral bands of heavy rain affecting the eastern part of Cuba and the easternmost Bahama Islands.

Flooding on Hispaniola
Noel has increased it's forward speed, but heavy rains will continue to affect Hispaniola for two more days, and the flooding situation will be extremely serious on the island. Satellite loops show very vigorous thunderstorms reaching high into the atmosphere continue to stream over Hispaniola. These thunderstorms dumped about 175 mm (7 inches) of rain in the past 24 hours near the capital of the Dominican Republic, Santo Domingo (Figure 1).

So far, Haiti has escaped the worst of Noel's heavy rains, giving hope that a repeat of the floods triggered by Hurricane Jeanne in 2004 might be avoided. Jeanne passed just north of Haiti as a tropical storm, and dumped about 13 inches of rain over the northern mountains. The resulting floods killed over 3,000 people. However, satellite images show a large region of disturbed weather to the southeast of Hispaniola associated with Noel, and Noel's counter-clockwise circulation will pull heavy rains over Hispaniola for the next two days. I expect that some regions of Haiti will receive over 12 inches of rain from Noel.


Figure 1. Satellite estimates of rain for the 24-hour period ending at 8 am EDT Monday, 10/29/07. Note the pink "bulls-eye" at upper right of the image over Hispaniola, indicating heavy rain of about 175 mm (7 inches) fell over the southern Dominican Republic. Image credit: Navy Research Lab, Monterey.

The track forecast
The latest computer model runs from 12Z this morning are in better agreement, but have shifted Noel's point of recurvature closer to South Florida. The models all forecast that Noel will follow a northwest or west-northwest track for the next 2-3 days parallel to Cuba, through the Bahamas. One key question is, what will be the timing and strength of a trough of low pressure forecast to move off the U.S. East Coast Thursday? A slower arrival of this trough will allow Noel to penetrate farther west. The other key question is, how strong will Noel be then? A weaker Noel that does not extend as high into the atmosphere is likely to make it further west. The ECMWF model forecasts that Noel will be a very shallow and weak tropical storm which will not recurve until it reaches South Florida. The HWRF is similar, forecasting a 45 mph tropical storm that will recurve about 50-100 miles off the coast of Miami. A stronger Noel will extend higher in the atmosphere, and will recurve sooner. The GFDL model forecasts Noel will strengthen into a Category 1 hurricane, and recurves Noel the farthest east, in the central Bahamas. The other models are in between.

The intensity forecast
Passage over the mountainous terrain of Haiti has severely disrupted Noel, and it will likely not start intensifying significantly until Tuesday afternoon. Wind shear is about 10-20 knots, and is expected to remain in that range over the next three days. This will allow some strengthening of Noel if its center can avoid moving over Cuba. I give Noel a 30% chance of reaching hurricane strength. After three days, wind shear is expected to increase above 20 knots, and Noel should weaken.

In summary, if Noel does make it all the way to South Florida, it will probably be as a weak tropical storm. If Noel strengthens significantly, it is likely to recurve sooner. It could be a windy trick-or-treat time in Miami Wednesday night.

The NOAA jet is scheduled to make its first flight this evening, so we should have a set of high-quality model runs to look at first thing Tuesday morning, when I'll post my next update.

Jeff Masters

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Noel dumping torrential rains on the Dominican Republic

By: JeffMasters, 2:28 PM GMT on October 29, 2007

Tropical Storm Noel hit Haiti this morning just south of the capital of Port-Au-Prince, dumping prodigious rains of over one inch per hour over some regions of the island of Hispaniola. The storm's slow forward speed means that heavy rains will affect the island for several more days. Long range radar out of Puerto Rico shows heavy rains affecting the Dominican Republic. These rains have already exceeded eight inches over a wide area of ocean to the east of Noel's center, according to rainfall estimates from the Puerto Rico radar. Rainfall amounts of 4-7 inches over southern Puerto Rico have triggered numerous flash floods and landslides.

This morning's QuikSCAT pass showed top winds of about 50 mph over a small region north of Hispaniola. Wind and storm surge damage should be minimal on Hispaniola from Noel.

The Dominican Republic
The worst of the rains for Puerto Rico are now over, but the flooding situation on Hispaniola today will be extremely serious, particularly in the Dominican Republic. Satellite loops show very vigorous thunderstorms reaching high into the atmosphere roiling over Hispaniola. Early this morning, these thunderstorms dumped about 150 mm (6 inches) of rain in just six hours in a region southwest of the capital of the Dominican Republic, Santo Domingo (Figure 1). Santo Domingo reported a visibility of zero at 2am local time during this heavy rain. Rainfall amount of about 12 inches have fallen over the Dominican Republic's southernmost point, the Barahona Peninsula, according to satellite estimates. The region's only airport weather station stopped transmitting data at 8pm last night.

Haiti
So far, Haiti has escaped the worst of Noel's heavy rains, giving hope that a repeat of the floods triggered by Hurricane Jeanne in 2004 might be avoided. Jeanne passed just north of Haiti as a tropical storm, and dumped about 13 inches of rain over the northern mountains. The resulting floods killed over 3,000 people. However, satellite images show that a large region of disturbed weather to the southeast of Hispaniola associated with Noel, and Noel's counter-clockwise circulation will pull heavy rains over Hispaniola for the next two days. I still expect that some regions of Haiti will receive over 12 inches of rain from Noel.


Figure 1. Satellite estimates of rain for the 6-hour period ending at 5 am EDT Monday, 10/29/07. Note the red "bulls-eye" at upper right of the image over Hispaniola, indicating heavy rain of about 150 mm (6 inches) fell in just six hours. Image credit: Navy Research Lab, Monterey.

The track forecast
Passage over the rugged terrain of Haiti has severely disrupted Noel, and satellite imagery suggests that the center of the storm is now trying to reform just north of Hispaniola. The latest computer model runs from 00Z and 06Z this morning continue to show a wide range of solutions for Noel's path. All of the models forecast that Noel's current north-northwest motion will continue today, in response to the counter-clockwise flow of air around an upper level low to the west of Noel. This upper low is forecast to weaken over the next few days, allowing a ridge of high pressure to build in, which will force Noel to the northwest or west-northwest. The GFS, NOGAPS, GFDL, and HWRF models all take Noel through the Bahamas, to the north of Cuba. The UKMET and ECMWF take Noel to the south of Cuba, close to its coast. This is unlikely, since Noel's center is trying to reform to the north of Hispaniola. The key question is the timing and strength of a trough of low pressure forecast to move off the U.S. East Coast Thursday. A slower arrival of this trough will allow Noel to penetrate farther west into the western Bahamas. The NOGAPS and GFS models foresee that Noel will reach a point between 100-300 miles east of South Florida before recurving out to sea. The HWRF and GFDL recurve Noel much further to the east. The GFDL doesn't take Noel very far west at all, predicting that the storm will graze the eastern Bahamas, then accelerate to the northeast and threaten Bermuda as a strong tropical storm on Friday. Given that Noel appears to be taking a big jump to the north and reforming north of Haiti this morning, I would expect that the official NHC forecast is the correct one, and Noel will recurve before reaching the western Bahamas.

The intensity forecast
Noel's intensity will be controlled by its interaction with the land masses of Hispaniola and Cuba over the next day. Passage over the mountainous terrain of Haiti has severely disrupted Noel, and any intensification over the next day should be slow. Wind shear is about 10-20 knots today, and is expected to remain in that range over the next two days. This will allow some slow strengthening of Noel if its center can remain over water. I give Noel a 40% chance of reaching hurricane strength at some point. After two days, wind shear is expected to increase above 20 knots, and Noel should weaken.

I'll have an update late this afternoon.

Jeff Masters

Updated: 5:55 PM GMT on October 29, 2007

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Tropical Storm Noel dumping huge rains

By: JeffMasters, 1:08 AM GMT on October 29, 2007

Tropical Storm Noel continues to represent a serious rainfall threat to the Dominican Republic and Haiti due to the storm's very slow motion. Noel has essentially stalled out tonight, and is dumping very heavy rains over the southernmost tip of the Dominican Republic--the Barahona Peninsula. Most of Noel's heaviest rains are still offshore, but these rains will move inland over the island of Hispaniola tonight, and pound the island for at least the next two days. This will result in an extremely dangerous flooding situation in the southern portion of both Haiti and the Dominican Republic, due to the high mountains that will enhance Noel's rains. Long range radar out of Puerto Rico shows heavy rains affecting Puerto Rico and the Dominican Republic, and these rains have already exceeded eight inches over a wide area of ocean to the east of Noel's center (Figure 1). Tonight's weather discussion from the National Weather Service in San Juan, Puerto Rico, called the situation on that island "an increasingly more dangerous and life-threatening event for many areas." Many flash floods and mudslides have been reported on the island, and with at least 24 more hours of flooding rains expected there, the island can expect millions of dollars in flooding damage from Noel.

The flooding situation on Hispaniola will be far worse. Satellite loops show very vigorous thunderstorms reaching high into the atmosphere have developed on the storm's northeast side. These thunderstorms will trigger rains of up to 1-2 inches per hour when they move over Hispaniola Monday. With Noel moving very slowly and expected to bring heavy rains to the island for at least two days, a flooding situation as dangerous as occurred in 2004 with Hurricane Jeanne may result. Jeanne passed just north of Haiti as a tropical depression, dumping about 13 inches of rain over the northern mountains. The resulting floods killed over 3,000 people.

Wind shear has fallen to 10-20 knots this evening, and may continue to fall. This may allow Noel to intensify into a hurricane Monday. However, wind shear is higher the further north Noel gets, and passage over the rugged southern Peninsula of Haiti may prevent the storm from reaching hurricane strength.


Figure 1. Latest precipitation estimate from the Puerto Rico radar.

The latest models runs still do not give us a lot of confidence that we know where Noel will go or how strong it will get. The key feature controlling Noel's path is a trough of low pressure forecast to move off the U.S. East Coast four days from now. If this trough is strong enough, and Noel is large enough and far enough north, the trough will force Noel across Cuba, into the Bahamas, then northeastward out to sea. Exactly where this recurvature will occur is problematic, with the GFDL and HWRF predicting this will occur over the western Bahamas, and the GFS predicting a path close to Miami in the Eastern Bahamas. In either scenario, it is unlikely that Noel would affect South Florida as anything stronger than a Category 1 hurricane, since the storm must get through Cuba first, and deal with higher wind shear to the north of Cuba. If the storm makes a direct hit on South Florida this week, it would likely be as a tropical storm.

The models have been trending more northerly with their solutions, and it now appears unlikely that Noel will make it into the Western Caribbean past the Cayman Islands. The Bahamas, Cuba, Jamaica, the Cayman Islands, South Florida, and Hispaniola are the places that may receive major impacts from Noel.

I'll have an update Monday morning.

Jeff Masters

Updated: 1:11 AM GMT on October 29, 2007

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TD 16 near tropical storm strength

By: JeffMasters, 4:39 PM GMT on October 28, 2007

Tropical Depression 16 continues to get more organized today as it chugs slowly west-northwest at 6 mph. Long range radar out of Puerto Rico shows heavy rains affecting Puerto Rico and the Dominican Republic, and these rains are expected to spread over Haiti today. Satellite loops show very vigorous thunderstorms reaching high into the atmosphere have developed on the storm's northeast side. A recent microwave satellite image (Figure 1) shows rain rates of up to 1 inch per hour in these thunderstorms. Wind shear has fallen to 15-20 knots this morning, and is expected to fall below 15 knots later today. This should allow TD 16 to continue to develop, and it is probably already Tropical Storm Noel. The Hurricane Hunters are in the storm now, and will let us know more later this afternoon. The first flight of the NOAA jet is Monday night.


Figure 1. Microwave satellite image of TD 16 taken at 6:12 am EDT Sunday , 10/28/07. Rainfall rates over 1" per hour (orange colors) were observed to the northeast of the center of TD 16's circulation. Image credit: Navy Research Lab, Monterey.

This is a slow moving system that will dump very dangerous amounts of rain along its path. Of particular concern is the 8 to 12 inches of rain, with possible accumulations of up to 20 inches, forecast to fall over Haiti. Rains of this magnitude have killed thousands of people on Haiti in the recent past. Most recently, Hurricane Jeanne passed just north of Haiti as a tropical depression in 2004, and dumped about 13 inches of rain over the northern mountains. The resulting floods killed over 3,000 people. Heavy rains of up to 18 inches associated with a tropical disturbance in May 2004 also killed thousands.

The latest 00Z and 06Z models runs are similar to previous runs, and do not give us a lot of confidence that we know where TD 16 will go or how strong it will get. The key feature controlling TD 16's path is a trough of low pressure forecast to move off the U.S. East Coast four days from now. If this trough is strong enough, and TD 16 is large enough and far enough north, the trough will force TD 16 across Cuba, into the Bahamas, then northeastward out to sea. Exactly where this recurvature will occur is problematic, with the GFDL and HWRF predicting this will occur over the western Bahamas, and the GFS predicting a path close to Miami in the Eastern Bahamas. In either scenario, it is unlikely that TD 16 would affect South Florida as anything stronger than a Category 1 hurricane, since the storm must get through Cuba first, and deal with higher wind shear to the north of Cuba. If the storm makes a direct hit on South Florida this week, it would likely be as a tropical storm.

Another possibility, preferred by the NOGAPS and ECMWF models, is that TD 16 will stay south of Cuba and make it all the way to the extreme Western Caribbean near Mexico's Yucatan. The trough of low pressure forecast to move off the U.S. East Coast four days from now might not be strong enough to turn TD 16 northeastward in this case, and the storm could stay trapped in the Western Caribbean for many days. This scenario might allow TD 16 to intensify into a hurricane, and possibly a major hurricane, since wind shear is predicted to be light over the area, and the ocean heat content is high. This is the scenario I have been favoring, but this is a low-confidence forecast.

I'll have an update Monday morning, and perhaps late tonight.

Jeff Masters

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Tropical Depression 16 forms

By: JeffMasters, 3:19 AM GMT on October 28, 2007

The surface low pressure system about 150 miles south of the Haiti/Dominican Republic border has gained enough organization to be upgraded to Tropical Depression 16. Long range radar of of Puerto Rico shows bands of heavy rain continuing to affect the region. Satellite loops show most of the heavy thunderstorm activity is to the north and east of the low's center of circulation, but these thunderstorms have shown some impressive development tonight. Wind shear has fallen to 15-20 knots tonight, and is expected to fall below 15 knots on Sunday. This should allow TD 16 to develop into a tropical storm on Sunday.


Figure 1. Latest satellite rainfall estimate for TD 16.

This is a slow moving system that will dump very dangerous amounts of rain along its path. The system will continue to bring heavy rains and the threat of flash flooding and mudslides to Puerto Rico through Sunday night. Heavy rains of up to eight inches have already fallen in southeast Puerto Rico and the Virgin Islands (Figure 1). Heavy rains will also affect the Dominican Republic and Haiti Sunday and Monday, and are likely to trigger life-threatening flash floods in Haiti.

This afternoon's 18Z (2 pm EDT) major intensity forecast models--the GFDL, HWRF, and SHIPS models--all agree that TD 16 will intensify into a hurricane. The 18Z GFDL, HWRF, and UKMET models predict TD 16 will move northwesterly across Hispaniola, then into the Bahamas on Tuesday, and intensify into a Category 1 hurricane over the central Bahamas on Tuesday. A trough of low pressure would then swing TD 16 northeastwards out to sea. This forecast track seems unreasonable, as TD 16 has headed more to the west today than these models predicted.

The ECMWF and GFS models predict TD 16 will track west-northwest along the length of Cuba, then pass within 50 miles of Miami on Thursday before recurving northeastwards out to sea. These models do not intensify TD 16 into a hurricane, due to the amount of time the storm spends over the mountainous terrain of Cuba. This is a reasonable forecast, should TD 16 track over Cuba for a long distance.

I believe the 12Z forecast of the NOGAPS model, which predicts a more southerly track into the Western Caribbean, just south of Cuba, is the most reasonable one. This track would favor TD 16 developing into a hurricane, possibly a major hurricane, since the heat content of the waters in the Western Caribbean is high, and the wind shear will be lower further to the south.

The area of disturbed area of weather in the extreme Western Caribbean, just east of the Yucatan Peninsula now appears to be too insignificant to affect the path or intensity of TD 16 very much.

I'll have an update Sunday morning.

Jeff Masters

Updated: 3:24 AM GMT on October 28, 2007

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A Halloween hurricane?

By: JeffMasters, 10:11 PM GMT on October 27, 2007

A surface low pressure system (90L), near 16N 71W, about 150 miles south of the Haiti/Dominican Republic border, is moving west to west-northwest at about 10 mph. Long range radar of of Puerto Rico shows bands of heavy rain continuing to affect the region. Satellite loops show most of the heavy thunderstorm activity is to the east of the low's center of circulation, but these thunderstorms have gotten more organized in the past few hours, and a more circular center has developed. Wind shear has fallen to 20-25 knots this afternoon, and is expected to fall below 15 knots on Sunday. This should allow 90L to develop into a tropical depression on Sunday.


Figure 1. Latest satellite rainfall estimate of 90L.

The system will continue to bring heavy rains and the threat of flash flooding and mudslides to Puerto Rico through Sunday night. Heavy rains of up to 6 inches have already fallen in southeast Puerto Rico and the Virgin Islands (Figure 1). Heavy rains will also affect the Dominican Republic and Haiti Sunday and Monday, and are likely to trigger life-threatening flash floods in Haiti.

This morning's 12Z (8am EDT) computer model forecasts have made a major change: the three major intensity forecast models--the GFDL, HWRF, and SHIPS models--all agree that 90L will intensify into a hurricane. The GFDL and HWRF predict 90L will move northwesterly across Haiti and western Cuba, and into the Bahamas on Tuesday, and intensify into a hurricane over the central Bahamas on Tuesday. A trough of low pressure would then swing 90L northeastwards out to sea. This forecast track seems unreasonable, as 90L has headed more to the west today than these models predicted.

The ECMWF and GFS models predict 90L will track along the length of Cuba early next week, then pass within 50 miles of Miami on Thursday before recurving northeastwards out to sea. These models do not intensify 90L into a hurricane, due to the amount of time the storm spends over the mountainous terrain of Cuba. This is a reasonable forecast, should 90L track over Cuba for a long distance.

I believe the forecasts of the UKMET and NOGAPS models, which predict a more southerly track into the Western Caribbean, just south of Cuba, are the most reasonable ones. This track would favor 90L developing into a hurricane, possibly a major hurricane, since the heat content of the waters in the Western Caribbean is high.

One possible wild card is the disturbed area of weather that has formed in the extreme Western Caribbean, just east of the Yucatan Peninsula. This disturbance is currently very disorganized, but is under only 10 knots of wind shear. If it starts to develop early next week, it could alter the path and development of 90L.

I am expecting 90L to develop into a tropical storm by Monday, and a into hurricane later in week, if the system does not track directly along the length of Cuba. The eastern 2/3 of Cuba, Jamaica, and the Cayman Islands should anticipate the possibility of a tropical storm affecting them on Monday and Tuesday. Later in the week, these regions, plus western Cuba, South Florida, the Bahamas, and Mexico's Yucatan, are at risk of a hurricane.

Jeff Masters

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Disturbance 90L continues towards the Western Caribbean

By: JeffMasters, 12:31 PM GMT on October 27, 2007

A surface low pressure system (90L) moved over Puerto Rico Friday, and is now centered about 175 miles south of Santo Domingo in the Dominican Republic. The system has maintained its spin and some respectable heavy thunderstorm activity in the face of some hostile wind shear of 30-35 over the past day. This wind shear has fallen to 20-30 knots this morning, which is still too high to allow development today. Long range radar of of Puerto Rico shows isolated bands of heavy rain that are not well-organized continue to affect the region. Satellite loops show most of the heavy thunderstorm activity is to the east of the low's center of circulation, and the high wind shear is keeping this thunderstorm activity disorganized. This morning's QuikSCAT pass showed a well-formed circulation with top winds of about 30 mph to the north of the center.


Figure 1. Lastest satellite rainfall estimate of 90L.

The system is headed west at about 10 mph, and will continue to bring heavy rains and the threat of flash flooding and mudslides to Puerto Rico today. Heavy rains of up to 4 inches have already fallen in southest Puerto Rico and the Virgin Islands so far from this storm. (Figure 1). Heavy rains may also affect the Dominican Republic and Haiti today and Sunday.

The computer models forecasts this morning are similar to yesterday's runs, although some of the guidance has shifted further northwards, implying more of threat to Cuba, the Bahamas, and South Florida. It is possible 90L could intensify into a tropical depression as early as Sunday, since wind shear will fall below 20 knots by then. The Hurricane Hunters are on call to fly Sunday afternoon, if necessary. The UKMET, ECMWF, and NOGAPS models predict that 90L will develop into a tropical storm by Monday as it moves slowly into the Western Caribbean, just south of Cuba. This track would favor 90L developing into a hurricane late next week. NOGAPS indicates 90L might turn northwards over South Florida late next week, but the other two models keep 90L trapped in the Western Caribbean. The GFS model keeps wind shear 15-25 knots through the period, and does not develop 90L. The HWRF and GFDL models suggest 90L may move down the length of Cuba then into the Bahamas and recurve to the northeast, missing South Florida. This sort of significant interaction with land would keep 90L from developing into a hurricane, and these models predict just a weak tropical storm will form.

One possible wild card is the disturbed area of weather that has formed in the extreme Western Caribbean, just east of the Yucatan Peninsula. This disturbance is currently very disorganized, but is under only 10 knots of wind shear. If it starts to develop early next week, it could alter the path and development of 90L.

Residents and visitors to the Western Caribbean should continue to anticipate the possibility of a hurricane forming in the Western Caribbean by late next week.

Jeff Masters

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California fires and global warming; 90L lashes Puerto Rico

By: JeffMasters, 2:38 PM GMT on October 26, 2007

A surface low pressure system (90L) moved over Puerto Rico this morning, and is now centered just west of the island. The surface low is entangled with an upper-level low pressure system that is bringing about 30 knots of wind shear, so no development is likely today. Long range radar of of Puerto Rico shows isolated bands of heavy rain that are not well-organized. Satellite loops show most of the heavy thunderstorm activity is to the east of the low's center of circulation, and the high wind shear is keeping this thunderstorm activity disorganized. This morning's QuikSCAT pass showed a large, vigorous circulation. Top winds were about 30 mph to the north of the center, and 90L is close to tropical depression status.


Figure 1. Latest satellite rainfall estimate of 90L.

The surface low is separating from the upper level low today, and will move west-southwest at about 10 mph. This will bring heavy rains and the threat of flash flooding and mudslides to Puerto Rico. Heavy rains of 2-4 inches in just two hours hit the Virgin Islands this morning (Figure 1), prompting flash flood warnings there. Heavy rains also hit many of the islands of the northern Lesser Antilles. Rain amounts as high as 3-5 inches are expected today over eastern Puerto Rico. Several mudslides have already been reported on the island.

The action shifts to the Dominican Republic on Saturday and Haiti on Sunday, as 90L tracks just south of the island of Hispaniola. These nations can expect rains of 3-6 inches, which could trigger life-threatening flash floods and mudslides. It is possible 90L could intensify into a tropical depression on Sunday, as wind shear will slowly fall to 20 knots. The Hurricane Hunters are on call to fly Sunday afternoon, if necessary. On Sunday, 90L will be approaching Jamaica, and the ECMWF and NOGAPS models predict that wind shear will drop to 10-20 knots. These models develop 90L into at least a strong tropical storm as it moves slowly into the Western Caribbean. The GFS model keeps wind shear 20-30 knots through the period, and does not develop 90L. The HWRF model also does not develop 90L. The GFDL is not keen on developing the system either, but does suggest that a weak tropical storm may form a week from now. I believe the most reasonable solution is the NOGAPS and ECMWF solution, and 90L will intensify into hurricane in the Western Caribbean late next week. The long-term path of such a storm is very uncertain, with the NOGAPS and ECMWF suggesting a track north into the Gulf of Mexico to threaten the U.S., and the GFDL predicting 90L will get trapped in the Western Caribbean and perform a counter-clockwise loop. If you have travel plans that take you to Jamaica or the Cayman Islands Sunday through Tuesday, or Cancun/Cozumel/Western Cuba Tuesday through Saturday next week, be prepared for the possibility of disruptions.

California's smoke
The worst of the air pollution hazard from California's fires has now passed. The smoke has thinned some, as seen on satellite images (Figure 2). The smoke made it yesterday to Fresno, in California's Central Valley, and is moving northward into Nevada and northwest Arizona today. Most of this smoke is aloft at altitudes of about 15,000 feet, but some mixing down to the surface has occurred, thanks to an upper-level low pressure system. Increases in particulate matter pollution due to smoke are expected to affect Las Vegas this weekend (Figure 1). However, the smoke will be dilute enough to keep pollution levels in the Moderate range--below the federal air quality standard.


Figure 2. Visible satellite image at 11:15 am PDT Thursday October 25, showing thinning smoke over the Pacific Ocean and much of California. Low stratus clouds are visible over the ocean, and these clouds have moved ashore into Los Angeles and San Diego this morning, triggering Dense Fog Advisories. Image credit: NASA and EPA.

Were the California fires worsened by global warming?
Dr. Ricky Rood points out in his latest wunderblog that the California fires were mostly a land-use and land-management issue. In a previous blog, he had this to say about the link between climate change and Western U.S. fires:

We do know that drought and floods, heat waves and cold snaps are all part of nature. Like the problem of urban heat waves, we have an event that already exists, and there should be a change associated with global warming. I have already mentioned that some studies have attributed the pinyon pine die off in the U.S. Southwest to the fact that the temperature in the recent drought years is higher than in previous droughts. Therefore, ground water is reduced; there is more stress on the plants. (And perhaps it is really the warmer nighttime temperatures that matter?)

There have also been papers which make a compelling argument that wild fires in the western U.S. are increasing in intensity and duration. In the paper of Westerling et al. (Science, 2006), the conclusion is drawn that this is directly related to snow melt occurring earlier in the year, a hotter and drier forest, and hence, a longer burning season. Plus they isolate the impact to be at mid-elevations in the Rockies, and hence, relatively free of land-use changes. While many newspapers reported that this work showed an increase of wild fires due to climate change, I quote directly from their paper: "Whether the changes observed in western hydroclimate and wildfire are the result of greenhouse gas-induced global warming or only an unusual natural fluctuation is beyond the scope of this work".


Jeff Masters

Climate Change

Updated: 8:01 PM GMT on August 16, 2011

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Disturbance 90L brings heavy rain to Puerto Rico; major pollution in California

By: JeffMasters, 2:05 PM GMT on October 25, 2007

A surface low pressure system (90L) just east of Puerto Rico is moving to the west at 5-10 mph. This low is entangled with an upper-level low pressure system that is bringing about 30 knots of wind shear, so no development is likely today. Long range radar out of Puerto Rico shows bands of heavy rain hitting the islands and surrounding waters, but these bands are not well-organized. Satellite loops show most of the heavy thunderstorm activity is to the northeast of the low's center of circulation, and high wind shear is keeping this thunderstorm activity disorganized. This morning's QuikSCAT pass missed 90L.


Figure 1. Latest satellite image of 90L. Image credit: NOAA.

The surface low is expected to separate from the upper level low tonight and move west-southwest or southwest across Puerto Rico, bringing the threat of heavy rain and flooding to the island. Recent rains have left large areas of interior and western Puerto Rico at or near saturation, and mudslides were reported yesterday in Utuado. Flooding problems and mudslides are likely across Puerto Rico and the Virgin Islands Friday and Saturday, and this is shaping up to be the most significant rain event of the fall wet season thus far for Puerto Rico.

On Friday and Saturday, 90L will move westward along or just south of Hispaniola, bringing heavy rain, flash flooding, and possible mudslides to that island. The ECMWF and NOGAPS models predict that wind shear will drop to 10-20 knots by Friday night, which may allow some slow development of the disturbance if the center remains over water. The GFS model keeps wind shear 20-30 knots through the period, and does not develop 90L. By Saturday, the storm will near Jamaica, and the ECMWF and NOGAPS models predict a tropical depression will form by Sunday. Steering currents grow weak on Monday, and the storm could stall out for many days in the Western Caribbean, in an area of low wind shear and high heat content waters. This may allow the system to intensify into a hurricane. The eventual fate of this system is highly uncertain, since steering currents will be so weak. Those of you planning to travel to the Western Caribbean next week should keep a close watch on this system. NHC has not scheduled any flights into 90L yet.

It is possible that wind shear and passage over the islands will sufficiently disrupt this disturbance so that it does not develop. However, surface pressures have fallen 2-3 mb over most of the Western Caribbean the past two days, and there is a good chance that a new area of disturbed weather will develop. One such disturbance formed yesterday south of Haiti, but has since dissipated. Unsettled rainy weather can be expected to affect much of the Western Caribbean over the coming week.

California's fires
Surface maps show that the high pressure system that brought this week's strong Santa Ana winds to Southern California has now moved east and is over Colorado. The Santa Ana winds have ceased over California, and only light winds with afternoon sea breezes are expected today and for the next seven days. No precipitation is expected for at least the next week. The combination of light winds and lack of rain will make for a serious air pollution hazard in the region, until the fires are extinguished. There is still plenty of smoke over the ocean waters (Figure 2) that will get blown back over land by afternoon sea breezes over the next week.


Figure 2. Visible satellite image at 1:45 pm PDT Wednesday October 24, showing a huge area of smoke over the Pacific Ocean. Some of this smoke is being blown northward (top of image), and is expected to move over northern California and northern Nevada over next two days. This northward-moving smoke is being lifted by the flow around an approaching low pressure system, and is not expected to affect air quality near the surface. However, the smoke just offshore San Diego and Los Angeles will remain near the surface, and some of it will be pushed back over land by afternoon sea breezes. Image credit: NASA.

Jeff Masters

Air and Water Pollution Fire

Updated: 9:35 PM GMT on July 13, 2011

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Santa Ana winds ease; tropical wave nearing Puerto Rico could develop

By: JeffMasters, 2:24 PM GMT on October 24, 2007

A surface low pressure area has developed near 17.5N, 60.5W, about 150 miles east-northeast of the northernmost Lesser Antilles Islands. This morning's QuickSCAT pass showed an elongated circulation that is not well organized. Top winds were about 25 mph to the northeast of the circulation center. Winds in the northern Lesser Antilles Islands were less than 10 mph this morning, but pressures have been falling for the past two days. Satellite loops show little heavy thunderstorm activity--less than yesterday. However, this activity appears to be increasing just north of the circulation center this morning.

Wind shear has dropped to 10-20 knots in the region, and some slow development of this system is possible as it moves to the west at about 5 mph. This westward motion is forecast to bring the low over Puerto Rico Thursday night and Friday morning, then to a point between Haiti and Jamaica on Saturday. All of these islands can expect heavy rains during passage of this system. Both the ECMWF and NOGAPS models develop the disturbance into a tropical depression on Saturday near the southern coast of Haiti. The GFS models does not, because of high wind shear. All of the models forecast falling pressures and low wind shear over the Western Caribbean late this week and early next week, and it would not be a surprise to see a tropical depression form in the region. Today is the 2nd anniversary of Wilma's strike on South Florida, so powerful hurricanes are still a concern at this point in the season. Wunderblogger Mike Theiss has posted a blog today recounting his experiences in the eye of Wilma.

Heavy thunderstorm activity has increased in the central Caribbean south of Haiti this morning. There is some rotation evident at mid levels of the atmosphere in satellite loops, but QuikSCAT showed no evidence of a surface circulation in this morning's pass. This disturbance will need to be watched for development. Activity in the western Caribbean between the Yucatan Peninsula and Cuba is associated with a cold front.

California's fires
Surface maps show a high pressure system centered over Nevada and Utah. This high is weakening and moving eastward away from California. The clockwise flow of air around this high is still driving northeasterly Santa Ana winds over Southern California, but these winds are much weaker than yesterday. By Thursday, the Santa Ana winds will be gone, to be replaced by a weak flow of moist air off the ocean. The new weather pattern will bring increased humidity, cooler temperatures, and lighter winds, which should allow firefighters to gain the upper hand. The long range forecast shows light winds for Southern California for most of the coming week, but no rain, and not as much onshore ocean winds as firefighters would like to see.


Figure 1. Visible satellite images from Monday and Tuesday with satellite-derived particulate air pollution levels overlaid. Image credit: NASA and EPA.

Air quality
Air quality due to particulate matter continues to be awful in Southern California (Figure 1). Exceedances of the Federal air quality standards by more than a factor of two have occurred the past four days in both Los Angeles and San Diego. The air pollution problem is expected to linger for several days after the fires are out. The onshore winds that are expected to form will recirculate smoke that is over the ocean back over land. As seen in a vertical cross section of the smoke taken by NASA's Calipso satellite Monday (Figure 2), smoke from the fires has stayed confined to the lowest 1 km (1,000 meters) of the atmosphere over the ocean regions southwest of San Diego. The cold waters of the California Current creates stable air above it that resists moving upwards, keeping the pollution trapped near the surface. However, some of the smoke is expected to rotate clockwise along the California coast, moving back over the U.S. over the northern half of California Thursday and Friday. Trajectory forecasts indicate that this smoke will be lifted as it circulates back over the U.S, thanks to the lifting motion associated with the low pressure system approaching the Pacific Northwest. The smoke should color sunsets over much of Northern California and northern Nevada over the next few days, but should not cause significant pollution problems at the surface, since most of the smoke will be aloft.


Figure 2. Altitude of smoke as measured by NASA's Calipso satellite at 3:07 am PDT Monday Oct 22. Calipso uses a LIDAR (LIght Detection And Ranging) to detect particles suspended in the atmosphere. Lidar is like radar, except that it uses a visible light beam instead of a radar-wavelength beam. Image credit: NASA.

The University of Wisconsin's CIMSS group has another excellent blog on yesterday's fires, complete with satellite animations and detailed analysis.

San Francisco Climate Challenge
Today is the last day for San Francisco residents to sign up for the San Francisco Climate Challenge, an innovative contest designed to encourage residents to reduce energy usage. The content offers prizes up to $5000 for those residents able to reduce their energy consumption the most over the coming month. For more information, see the new Weather Underground climate page at http://www.wunderground.com/climate/.

Ricky Rood has posted a blog this morning on the ongoing Georgia/Southeast U.S. drought.

Jeff Masters

Updated: 4:49 PM GMT on October 24, 2007

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California fires fueled by record drought

By: JeffMasters, 1:40 PM GMT on October 23, 2007

An area of disturbed weather has developed near the northern Lesser Antilles Islands, due to a westward-moving tropical wave interacting with an upper-level low. Wind shear is currently too high (30 knots) to allow development of this disturbance. However, the past two runs of the NOGAPS model have predicted that wind shear will fall enough by Friday to allow a tropical depression to form near the western Bahamas. This storm is predicted to move westward across eastern Cuba and into the Western Caribbean. The GFS model does not go along with this scenario, but hints at a weak system developing and moving northeast out sea over the middle Atlantic. The UKMET and ECMWF models do not develop a Bahamas storm, either, but do show a large region of low pressure with low wind shear developing over the Western Caribbean later this week. It would not be a surprise to see a tropical depression develop in the Western Caribbean late this week or early next week.

Southern California's fire storm
Surface maps show a strong high pressure system centered over Nevada and Utah. The clockwise flow of air around this high is driving strong northeasterly winds over Southern California. As the air spills down the mountain passes into coastal San Diego and Los Angeles, gravity helps accelerate the winds. The air compresses and warms as it descends, due to the higher pressures found at sea level. This creates a very hot, low-humidity wind--the dangerous Santa Ana wind. At 1:37 pm PDT yesterday, the humidity in downtown Los Angeles was 8%. Some wind reports Monday afternoon in Southern California, showing the strength of the Santa Ana winds:

Los Angeles County peak wind gusts
------------------------------------------------- --
Leo Carrillo Beach... ... ... ... ... ... ... Northeast 44 mph.
Van Nuys... ... ... ... ... ... ... ... ..... North 39 mph.
Tonner Canyon... ... ... ... ... ... ... .... Northeast 46 mph.
Malibu Canyon... ... ... ... ... ... ... .... Northeast 41 mph.
Malibu Hills... ... ... ... ... ... ... ..... North 54 mph.
Newhall... ... ... ... ... ... ... ... ... .. Northeast 38 mph.
Newhall Pass... ... ... ... ... ... ... ..... North 72 mph.
Saugus... ... ... ... ... ... ... ... ... ... North 60 mph.
Del Valle... ... ... ... ... ... ... ... .... Northeast 50 mph.
Acton... ... ... ... ... ... ... ... ... .... Northeast 45 mph.
Camp Nine... ... ... ... ... ... ... ... .... North 71 mph.
Chilao... ... ... ... ... ... ... ... ... ... Northeast 55 mph.
Mill Creek... ... ... ... ... ... ... ... ... North 41 mph.
Sandberg... ... ... ... ... ... ... ... ..... East 40 mph.
Warm Springs... ... ... ... ... ... ... ..... East 62 mph.
Whitaker Peak... ... ... ... ... ... ... .... North 48 mph.
Lake Palmdale... ... ... ... ... ... ... .... Northeast 46 mph.
Poppy Park... ... ... ... ... ... ... ... ... East 43 mph.
Saddleback Butte... ... ... ... ... ... ..... East 33 mph.
Lancaster... ... ... ... ... ... ... ... .... East 31 mph.

Ventura County peak wind gusts
--------------------------------------------
Oxnard... ... ... ... ... ... ... ... ... ... East 41 mph.
Camarillo... ... ... ... ... ... ... ... .... Northeast 51 mph.
Point Mugu... ... ... ... ... ... ... ... ... Northeast 43 mph.
Piru... ... ... ... ... ... ... ... ... ..... North 49 mph.
Simi Valley... ... ... ... ... ... ... ... .. Northeast 41 mph.
Thousand Oaks... ... ... ... ... ... ... .... North 42 mph.
Laguna Peak... ... ... ... ... ... ... ... .. Northwest 62 mph.

Fourteen large wildfires have developed spanning seven counties in California since Saturday, thanks to the Santa Ana winds, and the prevailing extreme drought conditions. Rainfall in San Diego has been a mere 2.6 inches thus far in 2007, 32% of normal. The July 1, 2006 through June 30, 2007 period was the 4th driest on record in San Diego. In Los Angeles, it was the driest year since record keeping began 130 years ago. Downtown Los Angeles has recorded just 3.37" of rain thus far in 2007--only 28% of normal.

Air quality
Not surprisingly, air quality due to particulate matter has been awful in Southern California (Figure 1). Exceedances of the Federal air quality standards by more than a factor of two have occurred the past three days in both Los Angeles and San Diego. Hightened particulate pollution is strongly correlated with increased death rates, particulary in vulnerable populations, such as those with heart conditions, athsma, or other lung diseases. Everyone should avoid any outdoor exertion; people with respiratory or heart disease, the elderly, and children should remain indoors. Keep your windows and doors closed unless it is extremely hot inside. In these cases, seek alternate shelter. Run your air conditioner if you have one. Keep the fresh air intake closed and the filter clean to prevent bringing additional smoke inside. EPA's Airnow website has more information.


Figure 1. Visible satellite images taken midday on Saturday, Sunday, and Monday. Overlaid on the images is the peak daily particle pollution Air Quality Index (AQI) levels due to smoke. Poor air quality became more widespread across the region as the number and size of the fires increased from day to day. Smoke is visible as white or bluish-white streaks. Blowing dust (brown streaks), reducing visibility under three miles, is also apparent on the images. Image credit: NASA and EPA.

The forecast
The warm, dry, and windy weather will continue today, bringing a repeat of yesterday's extremely dangerous fire situation. A trough of low pressure is expected to move into northern California Wednesday morning, weakening the high pressure system driving the Santa Ana winds. By Thursday, the Santa Ana winds will be gone, and fire fighters will be able to gain the upper hand.

This may only be the first of several serious fire situations in Southern California in the coming months. Santa Ana winds conditions commonly develop during the October through March period, and the extreme drought conditions in Southern California are not going to improve until at least December, when the winter rainy season typically starts. The 3-month precipitation forecast from NOAA's Climate Prediction Center calls for a 33% chance of below-average rainfall over Southern California for the coming winter.

The University of Wisconsin's CIMSS group has a more detailed blog with many weather maps and satellite animations of the Southern California fires.

San Francisco's Climate Challenge
San Francisco residents have a different and more positive kind of challenge this week. An innovative contest designed to encourage residents to reduce energy usage is being launched, with a sign-up deadline of Wednesday. The content offers prizes up to $5000 for those residents able to reduce their energy consumption the most over the coming month. For more information, see the new Weather Underground climate page: at http://www.wunderground.com/climate/. The new page will track current climate trends each month, and feature stories on new research and programs in the climate change field every few weeks. We'll also add a full set of information on the science of climate change over the coming months. The goal is to have a web site that keeps track of the most important issues in climate change.

Jeff Masters

Drought

Updated: 9:27 PM GMT on August 16, 2011

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Spectacular tornado photos from Michigan

By: JeffMasters, 1:22 PM GMT on October 22, 2007

The tropical Atlantic is quiet and none of the reliable computer models forecast formation of the tropical cyclone in the Atlantic over the next five days. However, some of the models are predicting a subtropical storm could form in the Mediterranean Sea by Thursday this week.

Last week's tornado outbreak was one of the largest ever recorded in October, bringing as least 78 twisters to the U.S. beginning October 17, 2007 and continuing into the early hours of October 19. Hardest hit was Michigan, which recorded three deaths and 11 tornadoes.

Since I live in Michigan, friends and family frequently send me photos of weather events in the state. A rare gem landed in my inbox Friday. The photo (below) was undoubtedly the finest one I've ever seen of a Michigan tornado. The tornado formed at about 5 pm EDT just southwest of Black Lake, Michigan, in the northern Lower Peninsula. The twister was rated an EF1 with winds up to 94 mph. It destroyed one barn just north of M-68, leaving a sporadic damage path up through Black Lake about 1/8 mile wide and 10 miles long. When the tornado crossed over Black Lake, Nathan Krinsky took these spectacular pictures from the back deck of his home. The sun was setting at 5:25 pm EDT when the photos were taken, and if you look closely, you can see a rainbow, thanks to the spray kicked up by the tornado.




Figure 1. Black Lake tornado of October 18, 2007 (top) and storm-relative velocity (bottom) from the Doppler radar in Gaylord, Michigan just before the Black Lake tornado crossed over the lake. The characteristic signature of a tornado is evident just southwest of the lake, with an area of strong winds blowing towards the radar (blue colors) right next to an area of strong winds blowing away from the radar (red colors). A classic hook echo is visible in the radar reflectivity animation (330 Kb) of the storm.

Alas, I did not see the Black Lake tornado. I've never seen a tornado, and it has always been a dream of mine to see and photograph a beautiful tornado--literally. In a recurrent dream I've had at least 50 times since I was a boy, I see and attempt to photograph a spectacular tornado. The dreams have two common features:

1) I'm always ABSOLUTELY SURE it is not a dream, and I really am seeing the tornado this time.

2) Something always happens to prevent me from photographing the tornado. Usually, I can't get to my camera, the camera is out of film, or I have go wait in line at the store to buy film while the tornado passes by. My favorite frustration was when the tornado got too close, and tore off the screen door of my house. The latch of the door snagged my camera strap and yanked the camera out of my hands just as I snapped the picture. I watched forlornly as the camera and screen door spiraled up into the tornado, forever lost. In the last few versions of the dream, I haven't even bothered to try to photograph the storm, since I know it is a futile cause. I'll just have to enjoy the photos taken by such lucky people as Nathan Krinsky!

Jeff Masters

Tornado

Updated: 6:42 PM GMT on October 01, 2012

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Tornadoes kill 3 in Michigan...

By: JeffMasters, 2:10 PM GMT on October 19, 2007

More tornadoes hit the U.S. on Thursday and early Friday morning, causing damage and injuries in Michigan, Indiana, Kentucky, and Florida. A 29-year old man was killed in Kalkasaka County, Michigan, when a tornado destroyed his home. Two other people were killed in Williamston, Michigan, after a tornado destroyed their lakefront home. In Nappanee, Indiana, five people were injured and 20 homes destroyed by a tornado at 10:30 pm. At least eight people were injured in western Kentucky from a series of tornadoes that raked the area, and a tornado hit downtown Pensacola, Florida, flipping cars and damaging the town's main shopping mall. A tornado that hit near Paris, Missouri, killing two people just after midnight on Thursday, was rated as an EF-2 storm with top winds of 135 mph.

The storm system that spawned this week's severe weather has moved over the Eastern U.S., and there is a chance of severe weather today from Florida northwards to New England, according to the latest severe weather outlook from the Storm Prediction Center. Only isloated tornadoes are likely today, as the atmosphere is not nearly as unstable as it was Wednesday and Thursday.

"Medicane" (Medepression?) hits Spain
A tropical storm-like system swept over the island of Majorca in the Mediterranean on Wednesday, triggering flooding that killed two people. The storm then made landfall on the Mediterranean coast of Spain yesterday morning near the city of Murcia. The satellite presentation of the storm at landfall (Figure 1) showed well-formed spiral bands and a cloud-free center. Murcia, Spain reported sustained winds of 30 mph, gusting to 45 mph, at 14 GMT Thursday. A personal weather station in Santa Pola recorded sustained winds of 40 mph, gusting to 45 mph, and 0.68 inches of rain during passage of the storm. We have a number of other personal weather stations in the region, but none reported higher winds, or a pressure lower than 1013 mb. Radar from the Spanish Instituto Nacional de Meteorologia (Figure 2) showed some well-organized banding. The UKMET model did not indicate the storm had a warm core, so this was likely not a true tropical depression. Sea surface temperatures were about 23° C (about 1° C warmer than normal) under the storm, which is quite a bit colder than the 26.5° C usually associated with tropical storm formation. The satellite presentation suggests that the storm was probably generating a shallow warm core near the surface, and was getting some of its energy from release of latent heat--the same energy source that powers tropical cyclones. Yesterday's "Medepression" was probably a hybrid tropical/extratropical storm, and was predominantly non-tropical.


Figure 1. Satellite image from NOAA-17 polar orbiting satellite at 10:37 GMT 10/18/07. Image credit: U.S. Navy.


Figure 2. Radar image at 6:20 GMT for the Mediterranean coast of Spain. Image credit: Spanish Instituto Nacional de Meteorologia (INM).

Warm-cored hybrid storms have been reported in the Mediterranean Sea before, and there is a large body of scientific literature published on the subject (see below). These storms can become quite severe and cause considerable damage. However, there is no system in place to name these storms, and the National Hurricane Center is not responsible for issuing warnings in the Mediterranean Sea. There are quite a few "Medicanes" in past years that would have earned names as subtropical storms had NHC been responsible for warnings in the Mediterranean Sea. There is concern that global warming may raise sea surface temperatures enough in the Mediterannean later this century to allow full-fledged hurricanes to form and threaten the densely populated cities that dot the coast.

Some of the scientific literature discussing hybrid storms in the Mediterrean Sea:

Emmanuel, K., 2005, "Genesis and maintenance of Mediterranean hurricanes", Adv. Geosci., 2, 217-220.

Lagouvardos K., V. Kotroni, S. Nickovic, D. Jovic, and G. Kallos, 1999: "Observations and model simulations of a winter sub-synoptic vortex over the Central Mediterranean", Meteorol. Appl., 6, 371-383.

Mayengon, R., 1984, "Warm core cyclones in the Mediterranean", Mariners Weather Log, 28: 6?9.

Pytharoulis, I., G.C. Craig and S. P. Ballard, 2000, "The hurricane-like Mediterranean cyclone of January 1995", Meteorol. Appl., 7, 261-279.

Rasmussen, E. A., and Turner J., 2003: Polar Lows, Cambridge Press. 214-219

Rasmussen, E. & Zick, C., 1987, "A subsynoptic vortex over the Mediterranean with some resemblance to polar lows", Tellus, 39A: 408-425.

Reale, O., and R. Atlas, 2001, "Tropical Cyclone-Like Vortices in the Extratropics: Observational Evidence and Synoptic Analysis", Weather and Forecasting, 16, No. 1, pp. 7-34.

Reale, O. ,1998, "Dynamics and classification of two sub-synoptic scale "Hurricane-like" vortices over the Mediterranean Sea", Annales Geophysicae Part II: Hydrology, Oceans & Atmosphere (Supplement II to Volume 16), EGS, C634.

How to search for strongest winds from a storm
A good way to search for the strongest winds from a storm in our personal weather station data is to load a google map for the region of interest:

http://www.wunderground.com/stationmaps/gmap.asp? zip=00000&wmo=08360

Then, click on the station plot for stations of interest. The history page will then pop up, allowing one to see plots and tabular data for today beginning at midnight local time for the station. Airport weather data and conditions from U.S. buoys are also available on the same google map. Use the search box at upper right to change the location the map is centered on.

Tropical update
The tropical Atlantic is quiet today. The GFS model is predicting formation of a tropical cyclone on Tuesday about 800 miles east of the Lesser Antilles Islands. None of the other models are going along with this forecast. If there are no major developments to report this weekend, I may not update this blog until Monday.

Jeff Masters

Tornado

Updated: 10:07 PM GMT on October 24, 2011

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Tornado kills two in Missouri; more tornadoes on tap for today

By: JeffMasters, 12:53 PM GMT on October 18, 2007

The tropical Atlantic is quiet today, and none of the reliable models are predicting tropical cyclone formation over the next five days.

At approximately 12:15 am CDT this morning, a tornado near Paris, Missouri, killed two people when it ripped apart their mobile home and tossed them 400 feet away. At least four other tornadoes hit Missouri yesterday, and two tornadoes touched down in Texas, and one each in Louisiana and Mississippi. The driver of a Petal Water & Sewer service truck was hospitalized in Mississippi after strong winds picked up his truck and tossed it across I-59.

Another severe weather outbreak is expected today in the U.S., from the Mississippi Valley northward through the Tennessee Valley and the Great Lakes. Tornado Watches have already been posted, and today's severe weather has the potential to generate a few strong, long-track tornadoes. You can follow the outbreak today on our new interactive tornado map, which will post the tornado damage reports as they are received. The new feature also allows one to plot all the historical tornado activity back to 1950 for any region in the U.S. If you take a wunderphoto of a tornadic storm or tornado damage, and click on the "tornado" type of image flag when uploading it, our software will attempt to match your photo to the storm report for that tornado. These photos will then be available when you click on a storm report on the interactive tornado page. One of the storm reports for Missouri yesterday has several wunderphotos of the thunderstorm that spawned the tornado available, thanks to wunderphotographer Paleohebrew.

Jeff Masters

Tornado

Updated: 10:06 PM GMT on October 24, 2011

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Quiet in the tropics; major severe weather outbreak in the U.S.

By: JeffMasters, 1:11 PM GMT on October 17, 2007

A low pressure system (99L) over the Gulf of Mexico has moved ashore this morning near the Texas/Louisiana border. Lake Charles, LA long-range radar shows some heavy rain showers are moving ashore, but flooding problems are not expected.

A low pressure system has developed along the coast of Southeast Florida this morning. Long range radar out of Melbourne shows a little bit of organized banding of the associated rain showers. However, this system is headed northeastward out to sea. Wind shear is 20-30 knots over the low, and will increase, so development into a tropical depression is not expected.

Severe weather outbreak today and Thursday
A major severe weather outbreak is likely today over the Midwestern U.S., due to an intensifying low pressure system that is drawing in plenty of tropical moisture from the Gulf of Mexico (thanks in part to tropical disturbance 99L). Tornado and Severe Thunderstorm Watches have already been posted, and today's severe weather has the potential to generate strong, long-track tornadoes. The activity shifts to Chicago and the Great Lakes region tomorrow. Follow the action on our new interactive tornado map, which will post the tornado damage reports as they are received. The new feature also allows one to plot all the historical tornado activity back to 1950 for any region in the U.S.



Jeff Masters

Tornado

Updated: 10:07 PM GMT on October 24, 2011

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Outlook for the remainder of hurricane season

By: JeffMasters, 2:46 PM GMT on October 16, 2007

A low pressure system (99L) over the Gulf of Mexico, about 500 miles south of the Texas/Louisiana border, is headed to the north at about 10-15 mph. QuikSCAT showed top winds of about 30 mph near this low. Satellite loops show that the thunderstorm activity has increased slightly this morning near the low, but remains disorganized. Wind shear is about 15 knots, and is expected to increase to 30 knots by the time 99L crosses the Louisiana coast Thursday morning. The shear should be high enough to prevent 99L from forming into a tropical depression. However, the Hurricane Hunters are on call to fly the storm Wednesday afternoon, if necessary.

The storm and an associated trough of low pressure extending northward to the Gulf Coast will bring heavy rains to the northern Gulf Coast today through Thursday, and should bring welcome rains to the drought-parched Southeast U.S. on Thursday and Friday.

Outlook for the remainder of hurricane season
Atlantic tropical cyclone activity finishes its peak phase in mid-October, and takes a major downturn during the final half of October (Figure 1). Since the current active hurricane period began in 1995, the last half of October through the end of hurricane season has given birth to an average of 1.9 named storms, 0.8 hurricanes, and 0.3 intense hurricanes. These numbers are about double the long-term climatological averages for the past 100 years. So, we're probably not done with the season yet, since this is not an El Nino year--El Nino years typically bring higher wind shear to the Atlantic, and an early end to hurricane season.

Climatology of major hurricanes
Let's examine the possibilities of getting a major hurricane this year, since those are the storms we care most about. Since 1960, there have been ten hurricanes that have existed as major Category 3 or higher storms after October 15. Six of these have occurred since 1995: Wilma of 2005 (Cat 4, Mexico; Cat 3, SW Florida), Beta of 2005 (Cat 3, Nicaragua), Michelle of 2001 (Cat 4, Cuba), Lenny of 1999 (Cat 4, northern Lesser Antilles), Mitch of 1998 (Cat 5, Honduras), and Lili of 1996 (Bahamas, Category 3). The other four were Joan of 1988 (Cat 4, Nicaragua), Kate of 1985 (Cat 3, Gulf of Mexico), Ella of 1962 (Cat 3, west of Bermuda), and Hattie of 1961 (Cat 4, Belize). Wilma of 2005 was the only major hurricane since 1960 to hit the U.S. after October 15. The highest risk region for late season major hurricanes is the Western Caribbean, along the coasts of Nicaragua, Honduras, Belize, Mexico, and Cuba. So, we can say with high confidence that most of the U.S. coast can relax. Only the west coast of Florida, Florida Keys, and South Florida need to still be concerned about the possibility of a major hurricane. The Lesser Antilles Islands, Puerto Rico, and Hispaniola are also at low risk for a major hurricane the remainder of the season.


Figure 1. Atlantic hurricane season activity over the past 100 years.

October storms tend to form both from tropical waves that come off the coast of Africa, and from the remains of old fronts that push off the coast of the U.S. As we can see from the track plot of all last half of October storms (Figure 2), there is a lot of activity during the period, but relatively few storms form out near the African coast. The water temperatures off the coast of Africa are starting to cool and be marginal for hurricane formation, and wind shear is starting to pick up in its normal fall cycle.

The jet stream is now more active and extends further south, which brings higher levels of wind shear to the Atlantic. The more active jet stream also acts to recurve storms more quickly. Any system penetrating north of about 20 degrees north latitude we can expect to recurve quickly to the north and northeast this late in the season.


Figure 2. Tracks of all tropical storms and hurricanes since 1851 that formed October 16-31.

Sea Surface Temperatures
Sea Surface Temperatures (SSTs) have been about 0.5 ° C above average over the Caribbean the past few weeks (Figure 3). This is the primary formation area for October storms. Note also the tongue of colder than average SSTs extending out into the Pacific Ocean from the coast of South America. This is the signature of a moderate strength La Nina event.


Figure 3. Sea Surface Temperature (SST) departure from average for the first portion of October. Image credit: NOAA.

Wind shear
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 by tearing a storm apart. Wind shear 10 knots and lower is very conducive for tropical storm formation.

Despite the presence of a La Nina event the past month--which is supposed to bring lower than average wind shear to the tropical Atlantic--wind shear the past month has been near average (Figure 4). The latest two-week wind shear forecast from the GFS model predicts near-average wind shear for the last half of October.


Figure 4. Wind shear departure from average for the 31 days ending October 13. Near average levels of wind shear were observed over the primary hurricane formation regions of the tropical Atlantic, Caribbean, and Gulf of Mexico. Wind shear is the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude) in meters per second (multiply by two to get the approximate wind shear in knots).

Summary
We've gotten very lucky this hurricane season since the departure of Hurricane Felix in early September. We had a record eight named storms form in September, yet we had only four hurricane days that month. Wind shear has been strategically high at the right time and right place, and storms have tended to form too close to land to develop. A good case in point is the current storm 99L. Had that system formed just 200 miles further east, it would have spent five days meandering over the Western Caribbean instead of over the Yucatan Peninsula, and could have easily grown into a Category 4 or 5 hurricane. There are still several low-shear periods ahead for the Western Caribbean this hurricane season, and I expect our luck may not hold for one of these periods. There is still one hurricane likely to form this season, possibly a major hurricane. The GFS model is predicting a large area of low shear will develop over the Caribbean around October 30, and I expect further low shear periods may occur through the first half of November.

I thank Margie Kieper for helping out the major hurricane seasonal stats.

Jeff Masters


Updated: 3:25 PM GMT on October 16, 2007

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Two unimpressive Atlantic disturbances to watch

By: JeffMasters, 1:32 PM GMT on October 15, 2007

A low pressure system (99L) that tried to organize into a tropical depression Sunday in the Western Caribbean moved inland Sunday evening over the Yucatan Peninsula before being able to do so. The system is headed west-northwest at about 10-15 mph, and should emerge over the Gulf of Mexico tonight. This morning's QuikSCAT pass showed top winds of about 25 mph to the northwest of 99L in the Gulf of Mexico. Satellite loops show only limited heavy thunderstorm activity at present. Wind shear is about 10-15 knots, and is expected to remain 10-20 knots through Tuesday afternoon. This may allow 99L to organize into a tropical depression by Tuesday night. A trough of low pressure is expected to pull 99L northwards by Tuesday night, and a landfall in Texas on Wednesday is a likely scenario. Since the storm may be moving almost parallel to the coast, it is difficult to say where in Texas landfall might occur. The trough will also bring increased levels of wind shear to 99L Tuesday night as it approaches Texas, and there is also some very dry air over the Gulf of Mexico for the storm to contend with. These factors should keep 99L from getting any stronger than a 45 mph tropical storm. I don't expect the system will become a tropical depression, and NHC does not think highly enough of 99L to bother putting the Hurricane Hunters on call to fly the storm on Tuesday. The primary threat from 99L will be heavy rain.

On Sunday afternoon, 99L gave us a lesson on why it is difficult to predict the track and intensity of a system trying to get organized into a tropical depression. The storm initially attempted to form in the extreme southwestern Caribbean (Figure 1). However, strong upper level winds from the northeast were creating about 15 knots of wind shear here. When a new burst of heavy thunderstorms developed underneath an upper level anticyclone with very low wind shear (Figure 2), the center of 99L reformed there, jumping over 100 miles to the north-northwest.


Figure 1. Satellite image of 99L Sunday afternoon, showing the old center in the southwest corner of the Caribbean Sea, which strong wind shear due to upper-level northeasterly winds had exposed to view. A new center formed to the north-northwest of the old center. Image credit: NASA/MSFC.


Figure 2. Wind shear map of 99L Sunday afternoon, showing that wind shear was 15 knots over the old center in the southwest corner of the Caribbean Sea. The new center developed to the north-northwest of the old center, in a region where wind shear was only 5 knots. Image credit: University of Wisconsin CIMSS group.

Tropical wave approaching Nicaragua
A tropical wave (98L) in the southern Caribbean will move ashore over Nicaragua and Honduras later today. This morning's QuikSCAT pass showed no circulation and top winds of only 25 mph. The system is under about 25 knots of wind shear, and no development is expected before the system makes landfall. The center will remain over land the next two days, spreading heavy rains across Nicaragua and Honduras today, then into Mexico, Guatemala, and Belize on Tuesday.

I'll have an update Tuesday morning, and present my hurricane season outlook for the remainder of October.

Jeff Masters

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Two Caribbean disturbances to watch

By: JeffMasters, 2:14 PM GMT on October 14, 2007

A persistent low pressure system extending from the Central American nations of Guatemala, El Salvador, and Belize northeastward over Jamaica, the Cayman Islands, Cuba, Haiti, and the Bahamas continues to dump heavy rain over much of the region. Heavy rains of at least five inches in northern Haiti triggered floods that killed at least 47 people Friday and left over 20,000 homeless. The flooding was worst in Cabaret, Haiti, near the north coast. No new rains fell Saturday, and further flooding is not expected. Heavy rain will continue to affect central and western Cuba today.

Connected to this deadly rain-making low pressure system is the large "sleeping giant" low pressure system that has been spinning over the Yucatan region the past week. This low continues to spin in the extreme southwest corner of the Western Caribbean, where Belize, Guatemala, and Honduras meet. Last night's QuikSCAT pass showed an elongated, poorly formed circulation, which will make development into a tropical depression today unlikely. None of the models develop the system. It is forecast to move northwest over the Yucatan Peninsula on Monday, emerging into the Gulf of Mexico on Tuesday. At that point, a trough of low pressure may be able to pull it towards a landfall near the Texas/Mexico border by Thursday. The trough may not be strong enough to turn the storm northwards, though, and it may come ashore in Mainland Mexico near Veracruz.

Even if the sleeping giant does not intensify into a tropical depression, this low could be a dangerous storm for Central America, bringing heavy rains of up to five inches to Belize, Guatemala, northwest Honduras, and Mexico's Yucatan over the the next three days. These heavy rains may also affect the Pacific coast regions of El Salvador, Mexico, and Guatemala, as the counter-clockwise flow of air around the low sucks in air from the Pacific Ocean.


Figure 1. Satellite estimated rainfall for the 24 hours ending at 2 am EDT Sunday.

New Caribbean disturbance
A tropical wave (98L) in the southern Caribbean is headed west towards Nicaragua and Honduras. This morning's QuikSCAT pass showed a fairly well-formed circulation. The system is under about 25 knots of wind shear, so any development today will be slow. Shear is expected to drop to 15 knots by Monday morning, which may allow for a better chance of development.

I'll have an update Monday.

Jeff Masters

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Flooding kills 45 in Haiti; sleeping giant begins to awaken

By: JeffMasters, 3:04 PM GMT on October 13, 2007

A persistent low pressure system extending from the Central American nations of Guatemala, El Salvador, and Belize northeastward over Jamaica, the Cayman Islands, Cuba, Haiti, and the Bahamas continues to dump heavy rain over much of the region. Heavy rains of at least five inches in northern Haiti triggered floods that killed at least 45 people Friday. The flooding was worst in Cabaret, Haiti, near the north coast. Over 6,000 people fled their homes, and entire neighborhoods were submerged. Thankfully, the heaviest rains have ended in Haiti, and the worst of the flooding is likely past.

Schools were canceled Friday in much of Jamaica due to flooding, and flood waters damaged over 1,000 homes in eastern Cuba. Vista Alegre in Santiago de Cuba reported 12 inches (306 mm) of rain in just 24 hours yesterday. Heavy rain will continue to plague Cuba today. These rains should shift more to the central and eastern part of the island, allowing the hard-hit eastern portion a chance to dry out. Additional rain amounts of five inches are likely today over Cuba.

Connected to this deadly rain-making low pressure system is the large "sleeping giant" low pressure system that has been spinning over the Yucatan region the past week. The sleeping giant has moved to the extreme southwest corner of the Western Caribbean, where Belize, Guatemala, and Honduras meet. Now that the storm can pull in tropical moisture from the ocean, it should begin to awaken. Its close proximity to land will probably prevent a tropical depression from forming today, but most of the models are predicting a slow drift to the north through Sunday, which could give the storm enough clearance from the coast to intensify into a tropical depression on Sunday or Monday. However, most of the models predict the storm will track more northwesterly across the Yucatan Peninsula Sunday night and Monday, which would halt any development. Some of the models are predicting that the storm could turn north once it reaches the Gulf of Mexico and affect the U.S. late next week.

Even if the sleeping giant does not intensify into a tropical depression, this is a dangerous storm for Central America. The storm will probably bring heavy rains in excess of five inches to Belize, Guatemala, northwest Honduras, and Mexico's Yucatan over the the next three days. These heavy rains may also affect the Pacific coast regions of El Salvador, Mexico, Honduras, and Guatemala, as the counter-clockwise flow of air around the low sucks in air from the Pacific Ocean. These rains may cause life-threatening flash floods and mudslides in mountainous areas. Flooding problems have already been reported in El Salvador and Nicaragua near the Pacific coast, where up to eight inches of rain (203 mm) have fallen in the past 24 hours (Figure 1).


Figure 1. Satellite estimated rainfall for the 24 hours ending at 2 am EDT Saturday. Image credit: Navy Research Lab Monterey.

I'll have an update Sunday by 10 am EDT.

Jeff Masters

Updated: 3:26 PM GMT on October 13, 2007

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Fabled Northwest Passage begins to re-freeze

By: JeffMasters, 2:29 PM GMT on October 12, 2007

This summer's dramatic loss of Arctic sea ice peaked on September 15, and the polar ice cap is finally beginning to re-freeze, according to a press release issued by the National Snow and Ice Data Center on October 1. Extent of the September polar sea ice fell 39%, compared to the 1979-2000 average. To put this loss in perspective, in one year we lost as much ice as we lost during the previous 28 years. Summertime Arctic sea ice is now at 50% of what it was in the 1950s (Figure 1). One may look at at graph and wonder, but what about sea ice loss in other seasons? It hasn't been nearly so severe. True, but it is the summer ice we care most about, since summer is when the thick, multi-year ice melts, which can then precondition the Arctic for much greater ice loss in future years. As sea ice melts in response to rising temperatures, more of the dark ocean is exposed, allowing it to absorb more of the sun's energy. This further increases air temperatures, ocean temperatures, and ice melt in a process know as the "ice-albedo feedback" (albedo means how much sunlight a surface reflects). There is an excellent chance that the summer of 2007 will be remembered as the "tipping point" for Arctic sea ice, when an irreversible ice-albedo feedback process firmly established itself.



Figure 1. Arctic sea ice extent since 1900, as estimated from satellite and ship reports compiled by Walsh and Chapman (2001). Image credit: University of Illinois cryosphere group.

Northwest Passage opens for the first time in recorded history
Long before the Panama and Suez Canals made commercial trading between the Atlantic and Pacific Oceans economically feasible, ships made the long and perilous trip around the African and South America continents. Explorers, traders, and world leaders looking for faster and less dangerous shipping routes to far-away areas of the world have long eyed two routes through the ice-choked Arctic Ocean--the fabled Northwest Passage, through the cold Arctic waters north of Canada, and the Northeast Passage, extending along the northern coast of Russia. The first recorded attempt to find and sail the Northwest Passage was in 1497, and ended in failure. The thick ice choking the waterways thwarted all attempts at passage for the next four centuries. Finally, in 1905, Roald Amundsen completed the first successful navigation of the Northwest Passage. It took his ship two-and-a-half years to navigate through narrow passages of open water, and his ship spent two cold, dark winters locked in the ice during the feat. More recently, icebreakers and ice-strengthened ships have on occasion battered their way through the ice-blocked route.



Figure 2. The Northwest Passage shipping route (red line) and Northeast Passage (green line) superimposed on an ice coverage map from August 22, 2007. The Northwest Passage was ice-free and navigable for 36 days between August 14 and September 18, 2007. The Northeast Passage was blocked by a narrow strip of ice most of the summer. Image credit: National Snow and Ice Data Center.

Times are changing. In 2001, the Bering Strait, a key portion of both the Northwest and Northeast Passages, was completely ice free. This was followed in 2005 by record-breaking sea-ice melt in the Arctic, leading to the first ever recorded opening of the Northeast Passage. The fabled Northwest Passage remained closed in 2005. Arctic ice recovered a bit in 2006, and both passages remained closed. But the unprecedented melting during the summer of 2007 saw the Northwest Passage become ice-free and navigable along its entire length without the need for an icebreaker as of August 14, 2007. Remarkably, the Northwest Passage remained ice-free for 36 days, finally refreezing over a small section on September 19. The Northeast Passage was blocked by a narrow strip of ice all summer. However, this strip of ice thinned to just 30% coverage on September 25 and 26, making the Northeast Passage passable for ordinary ships on those days.

When is the last time the Northwest Passage was open?
We can be sure the Northwest Passage was never open from 1900 on, as we have detailed ice edge records from ships. It is very unlikely the Passage was open between 1497 and 1900, since this was a cold period in the northern latitudes known as "The Little Ice Age". Ships periodically attempted the Passage and were foiled during this period, and the native Inuit people have no historical tales of the Passage being navigable at any time in the past.

A good candidate for the last previous opening of the Northwest Passage was the period 5,000-7,000 years ago, when the Earth's orbital variations brought more sunlight to the Arctic in summer than at present. Prior to that, the Passage was probably open during the last inter-glacial period, 120,000 years ago. Temperatures then were 2-3 degrees Centigrade higher than present-day temperatures, and sea levels were 4-6 meters higher.

Final thoughts
If we have reached the tipping point for Arctic ice, what are the implications? I'll discuss this more in a future blog. Sea ice is very complicated, and it is not a sure thing that we have reached the tipping point. For more on the complexities of sea ice, read wunderblogger Dr. Ricky Rood's latest blog.

NASA has posted a beautiful satellite image of the Arctic ice cap at the September 15 2007 minimum, showing the open water of the Northwest Passage.

I thank Edalin Michael of the University of Michigan's School of Natural Resources and Robert Grumbine of NOAA's Sea Ice Group for their contributions to this blog.

References
Walsh, J.E and W.L.Chapman, 2001, "Twentieth-century sea ice variations from observational data", Annals of Glaciology, 33, Number 1, January 2001 , pp. 444-448.

Jeff Masters

Climate Change Sea Ice

Updated: 8:01 PM GMT on August 16, 2011

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A few minor threat areas for the Atlantic

By: JeffMasters, 2:54 PM GMT on October 11, 2007

A surface circulation has developed near 30N, 53W, about 800 miles east-southeast of Bermuda. This system has been labeled Invest 96L by NHC this morning. Some impressive heavy thunderstorms have built on the northeast side of the low. Wind shear is a marginal 15-20 knots, and is expected to rise to 25-35 knots tonight, so 96L has only about 12 hours to become a tropical depression before wind shear tears it apart.

The "sleeping giant" Yucatan low
A large low pressure system that moved inland over Mexico's Yucatan Peninsula Monday continues to bring heavy rain to Mexico, Belize, and Guatemala. This low was labeled "94L" by NHC, but is no longer being tracked. The low crossed the Yucatan Wednesday and reached the edge of the Gulf of Mexico, where it began pulling in Gulf moisture and firing up some heavy thunderstorms. However, the low turned south before it could fully emerge into the Gulf, inland just west of the Guatemala/Mexico border. If it emerges into the Pacific Ocean, it may intensify enough to trigger additional heavy rains over the Pacific shores of Guatemela and Mexico. The low won't have enough time over water to develop into a tropical depression, I expect. Steering currents are such that the low will perform a loop back into the Western Caribbean near Belize on Saturday. If the low still has some spin at that time, it could develop into a tropical depression and bring another round of heavy rain to the Yucatan Peninsula.


Figure 1. Today's lineup of tropical disturbances to watch.

Elsewhere in the tropics
A surface trough of low pressure is bringing disorganized heavy thunderstorm activity to Jamaica, Cuba, and the Bahama Islands. Some of the computer models are forecasting that a low pressure system will develop in the Bahamas on Friday, then move rapidly northeastward to Bermuda. This is likely to be an extratropical storm, but could bring wind gusts of 40 mph and heavy rain to Bermuda on Saturday.

Jeff Masters

Updated: 4:54 PM GMT on October 11, 2007

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A sleeping giant: 94L

By: JeffMasters, 1:00 PM GMT on October 10, 2007

A large low pressure system (94L) has moved inland over Mexico's Yucatan Peninsula, and is bringing heavy rain to Mexico, Belize, and Guatemala. These heavy rains can be viewed on Cancun radar. Satellite loops and the steering flow product from the University of Wisconsin CIMSS group (Figure 1) show the large size of this circulation nicely. This is an unusually large and deep low pressure system, and it will take several days for it to spin down. Most of the computer models are indicating that the center will remain over land and 94L will eventually die. However, the GFDL model continues to show the possibility that the center of 94L will drift back into the Western Caribbean, or into the Southern Gulf of Mexico, allowing 94L to intensify into a tropical storm. Considering the huge amount of atmosphere 94L has put into motion, it would not be a surprise to see some of that spin still remaining 3-4 days from now, and we will have to watch this system until that spin is gone. Wind shear is about 10 knots, and is expected to remain 10 knots or below for the next 2-3 days over the Yucatan.


Figure 1. Average steering flow at low levels in the atmosphere (between 700 mb and 850 mb) as computed by University of Wisconsin's CIMSS group. The arrows show the counter-clockwise flow of air around the low pressure system 94L over Mexico's Yucatan Peninsula.

Elsewhere in the tropics
Most of the computer models are forecasting that a low pressure system will develop in the Bahamas along an old cold front on Thursday or Friday, then move rapidly northeastward to Bermuda. This is likely to be an extratropical storm, but could bring wind gusts of 40 mph and heavy rain to Bermuda on Friday or Saturday.

Jeff Masters

Updated: 1:01 PM GMT on October 10, 2007

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Western Caribbean disturbance soaking the Yucatan; record October heat in the U.S.

By: JeffMasters, 1:28 PM GMT on October 09, 2007

A large low pressure system (94L) over the Western Caribbean continues to be a threat to develop into a tropical depression. This morning's QuikSCAT pass showed a well-defined surface circulation, with top winds of 30 mph. Satellite loops show this surface circulation is centered less than 100 miles off the Mexican coast near Chetumal. Concentrated thunderstorm activity is mostly absent near the center of circulation, but Cancun radar shows heavy rain showers in bands removed from the center moving ashore over the Yucatan. Wind shear remains below 10 knots, but given the very large amount of atmosphere 94L is trying to spin up, it may not have time to form into a tropical depression before the center moves ashore. The center should move ashore in the Yucatan between Chetumal and Cozumel later tonight or Wednesday. All hurricane hunter flights into 94L have been canceled.

Steering currents are weak in the Western Caribbean, but a slow motion to the west or northwest is expected for the next 1-3 days, which will keep the storm over the Yucatan. Heavy rains may cause flooding problems in Belize, Mexico's Yucatan, and northeast Guatemala over the next five days. Some models, such as the GFDL, predict that 94L will perform a counter-clockwise loop over the Yucatan and re-emerge into the Western Caribbean 3-4 days from now. Another round of drenching rains might then ensue over Belize, Guatemala, and Mexico's Yucatan as the storm re-intensifies. It is also possible 94L could emerge into the Gulf of Mexico or Pacific Ocean, and re-intensify in those locations.

There is a strong trough of low pressure forecast to swing across the U.S. this week, which could turn 94L northwards into Western Cuba, the Florida Keys, or Southwest Florida. The HWRF is the only of our reliable computer models forecasting such a turn, and I don't expect it to happen.

Record heat, record cold
Unprecedented heat cooked the eastern half of the U.S. during the first week of October--but record cold temperatures affected some regions of the West, as well. On Sunday October 7, and Monday October 8, hundreds of daily high temperature records were set, and many stations recorded their hottest October temperature ever (or their highest temperature so late in the year). Among these hottest ever October records:

Bluefield, VA 88
Beckley, WV 86
Memphis, TN 95
New York City, NY (Kennedy Airport) 90
South Bend, IN 89
Fort Wayne, IN 89
Indianapolis, IN 91
Jackson, KY 88
London, KY 92
Detroit, MI 90
Alpena, MI 90
Saginaw, MI 90

The heat brought an early end to the Chicago Marathon on Sunday, which was halted after the 88-degree heat sent 49 runners to the hospital. Emergency vehicles made 300 calls along the race course.


Figure 1. Departure of maximum temperature from average for Monday, October 7. Image credit: National Climatic Data Center.

Temperatures have averaged over 30 degrees above normal in some regions of the country the past week (Figure 1). On the other side of the country, though, record daily low temperatures have been recorded at a few locations in Arizona and Utah this week. The jet stream is to blame for the record heat and record cold--a sharp kink in the jet has put a persistent trough of low pressure over the Western U.S. and a ridge of high pressure over the Eastern U.S. Beginning today, this kink is expected to straighten out some, resulting in far fewer temperature extremes across the country.

Jeff Masters

Updated: 2:50 PM GMT on October 09, 2007

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Tropical depression likely by Tuesday in Western Caribbean

By: JeffMasters, 8:45 PM GMT on October 08, 2007

A vigorous surface circulation has developed in association with a broad area of low pressure over the Western Caribbean (94L). A pass from the European ASCAT satellite at 11:02 am EDT showed that the surface circulation had gotten much less elongated, compared to this morning's QuikSCAT pass. Top winds were only about 10 mph in the 11 am ASCAT pass, but have no doubt increased since then. Satellite loops show a very large surface circulation covering the entire Western Caribbean, with a concentrated area of thunderstorms forming near the center of circulation. Surface pressures remain very low over the entire Western Caribbean. Wind shear is less than 10 knots, and is expected to remain less than 10 knots through Thursday. It is likely that this system will form into a tropical depression on Tuesday, despite the very large amount of atmosphere it is trying to spin up.

Steering currents are weak in the Western Caribbean, but most of the models show a slow motion to the west or northwest that will take 94L over Belize or Mexico's Yucatan Peninsula by Thursday. It is unlikely 94L will have time to become a hurricane before it moves over the Yucatan, and the main threat from the system will be heavy rain. These rains may cause significant flooding problems in Belize and Mexico's Yucatan. Heavy rains may also affect northeast Guatemala, but should not cause significant flooding.

There is a strong trough of low pressure forecast to swing across the U.S. this week, which could turn 94L northwards into Western Cuba, the Florida Keys, or Southwest Florida. However, this is unlikely, since none of the reliable computer models are forecasting such a turn.

It is likely that 94L will eventually emerge into the Gulf of Mexico or Western Caribbean after spending a few days over the Yucatan. When it does so, it will probably be intact enough to re-strengthen, since it is such a large system. Its long-term fate it highly uncertain, as the steering currents are weak and the storm's intensity will be controlled by interaction with land.

Elsewhere in the tropics
A nearly stationary tropical disturbance a few hundred miles north of Puerto Rico is generating a large area of thunderstorms. The region is under 15-20 knots of wind shear, due to strong upper level winds from the west. These winds are blowing the heavy thunderstorm activity downwind, to the east of the elongated surface circulation apparent on visible satellite imagery. Wind shear is expected to remain 15-30 knots over the region over the next three days. The high shear should discourage any significant development.

I'll have an update in the morning.

Jeff Masters

Updated: 8:56 PM GMT on October 08, 2007

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Western Caribbean disturbance a threat to develop

By: JeffMasters, 2:16 PM GMT on October 08, 2007

Heavy thunderstorm activity continues to flare up in association with a broad area of low pressure over the Western Caribbean (94L). This morning's QuikSCAT pass showed a 300 mile-long line of converging surface winds north of the Honduras coast (Figure 1), but not a closed circulation. Top winds were about 25 mph. Satellite loops show fewer heavy thunderstorms than yesterday, but it appears that this activity is starting to get organized. Some low-level banding of the thunderstorm activity is occurring over the Cayman Islands, to the northeast side of the center of low pressure. Surface pressures remain very low over the entire Western Caribbean, but have not fallen since Saturday. Wind shear is about 10 knots, and is expected to remain 10 knots or below through Wednesday. The low surface pressures, light wind shear, and warm ocean waters are all very favorable for formation of a tropical depression. The Hurricane Hunter flights scheduled for this afternoon and tonight were canceled, and have been rescheduled for Tuesday. I expect a tropical depression will form in the next 1-3 days, most likely on Tuesday.

Steering currents are weak in the Western Caribbean, and most of the computer models forecast that 94L will wander erratically for a week or longer in the region. A slow motion to the west or northwest is predicted for the next three days, which may bring the storm over the Yucatan Peninsula late this week. There is a strong trough of low pressure forecast to swing across the U.S. this week, which could pull 94L northwards across Western Cuba, the Florida Keys, or Southwest Florida, as forecast by the HWRF model. However, 94L would have to form quickly and grow large to "feel" the influence of this trough, and I estimate there is only a 30% chance that the trough will be able to pull 94L northwards over Florida.

Residents of the Cayman Islands, Cuba, Mexico's Yucatan Peninsula, Belize, northern Honduras, and Jamaica can expect occasional heavy rain squalls over the next 1-3 days from this storm. This activity could spread into the Florida Keys by Wednesday.


Figure 1. High-resolution (12.5 km) QuikSCAT pass from 7:34 am EDT Monday October 8, 2007. A 300 mile-long line of converging winds is apparent in association with disturbance 94L. Image credit: NOAA/NESDIS/ORA.

Elsewhere in the tropics
A tropical disturbance a few hundred miles north of Puerto Rico is generating a large area of thunderstorms. QuikSCAT data from this morning shows an elongated circulation near 25N 66W, and top winds of 25 mph. The region is under 20 knots of wind shear, and wind shear is expected to remain 20-30 knots over the region over the next three days. The high shear should discourage any significant development.

I'll have an update Tuesday morning.

Jeff Masters

Updated: 2:51 PM GMT on October 08, 2007

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Western Caribbean disturbance slowly organizing

By: JeffMasters, 4:06 PM GMT on October 07, 2007

Thunderstorm activity has increased over the Western Caribbean a few hundred miles north of the northeast coast of Honduras. This area has been labeled "Invest 94L" by NHC. This morning's QuikSCAT pass showed a sharp wind shift in the region, but not a closed circulation. Satellite loops show a steady increase in heavy thunderstorm activity, but the cloud pattern has no organization yet. Surface pressures over the entire Western Caribbean, from Cancun to Cuba, to the Cayman Islands, and south and west to Honduras and Belize have shown a large drop over the past two days. It is uncommon for pressures to fall over this large of an area during hurricane season. Wind shear is about 10 knots, and is expected to remain 10 knots or below through Tuesday. The low surface pressures, light wind shear, and warm ocean waters should allow a tropical depression to form by Tuesday at the latest. The Hurricane Hunters are scheduled to investigate the system Monday afternoon.

Steering currents are weak in the Western Caribbean, and any storm that does form may remain for a week over the high-heat content waters of the region. In that case, we can expect a hurricane a week from now. However, some of the models indicate a slow motion northwestward later this week, bringing the system over Belize or Mexico's Yucatan, before it would have a chance to intensify into a hurricane. The GFDL model predicts 94L will hit near Cozumel later this week as a tropical storm, then be forced south-westward deep into the southwestern Gulf of Mexico. There is a trough of low pressure swinging across the U.S. later this week that may be strong enough to pull 94L northwards. Western Cuba, the west coast of Florida, the Florida Keys, and the Bahamas would be at risk in this scenario. The HWRF model is the only model showing this scenario.

In the shorter term, residents of northern Honduras can expect heavy rains beginning Monday. These heavy rains will likely spread to Belize on Tuesday and Mexico's Yucatan coast by Wednesday.


Figure 1. Pressure trace at the buoy in the Western Caribbean at 20N 85W, 120 miles east of Cozumel. A steady pressure fall the past 2-3 days is apparent. Superimposed on this falling trend is an oscillation due to the pressure wave that affects all tropical stations when the rising sun makes the air expand at sunrise. Image credit: National Data Buoy Center.

Elsewhere in the tropics
A tropical wave (91L) about 500 miles east of the Lesser Antilles Islands has grown disorganized. Satellite loops show that strong upper level winds from the northwest have removed all the heavy thunderstorm activity from the center of circulation. Wind shear is expected to remain too high to allow 91L to develop. The storm is headed northwest, and is expected to recurve out to sea without affecting any land areas. A tropical disturbance a few hundred miles north of Puerto Rico (93L) has become disorganized, and is no longer a threat to develop. The system is heading slowly northwest.

Typhoon Krosa smashes into Taiwan
Typhoon Krosa made landfall on the northern tip of Taiwan yesterday as a Category 3 storm with 115 mph winds, according to Taiwan's Central Weather Bureau (CBW). Krosa, which is the Cambodian word for a species of crane, killed five people on Taiwan, and knocked out power to 700,000. The storm was apparently deflected by high terrain of the island, and rolled in a semi-circle path along the northern shore of Taiwan before heading northwest to a landfall in Mainland China as a tropical storm. Over 1 million people were evacuated in China in advance of the storm.

I'll have an update Monday morning.

Jeff Masters

Updated: 4:17 PM GMT on October 07, 2007

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Atlantic tropical update; Category 4 Typhoon Krosa slams Taiwan

By: JeffMasters, 2:56 PM GMT on October 06, 2007

A tropical wave (91L) near 14N 51W, 650 miles east of the Lesser Antilles Islands, has become better organized today. Satellite loops show that 91L has developed a surface circulation and some heavy thunderstorm activity on the southeast side of the center. The storm may be close to tropical depression status, but wind shear of 20-30 knots over the system should keep further development very slow. By Sunday evening, wind shear should drop below 15 knots, and 91L may be able to develop further. The storm is headed northwest, and is expected to recurve out to sea without affecting any land areas.


Figure 1. Microwave satellite image of 91L showing some heavy thunderstorm activity on the southeast side of the storm. There is some hint of low-level spiral banding beginning to develop on the the lower left of the image, on the storm's south and southwest side. Image credit: Navy NRL Monterey.

Puerto Rico disturbance
A tropical disturbance (93L) about 200 miles north of Puerto Rico is kicking up some disorganized heavy thunderstorm activity that is visible on long range Puerto Rico radar. This morning's QuikSCAT pass showed very little in the way of a wind shift or strong winds. This disturbance is under 10-20 knots of wind shear, and may show some development over the next few days as it moves slowly north or northwest. Most of the models expect 93L to recurve out to sea without affecting any land areas. However, steering currents are weak in the region, and recurvature is not a sure thing. The GFDL models predicts 93L could become a weak tropical storm 5 days from now, but none of the other models develop the storm. The Hurricane Hunters are on call to investigate this disturbance Monday, if needed.

Western Caribbean disturbed weather
Limited and disorganized thunderstorm activity is present over the Western Caribbean. This area has been labeled "Invest 94L" by NHC this afternoon. Most of the computer models continue to show the possibility of a tropical depression forming here in the next 2-7 days, and moving over the Yucatan Peninsula. Given the current lack of activity there now, nothing is likely to develop today, but the Hurricane Hunters are on call to fly Sunday, if needed. Upper level winds over the Western Caribbean and Gulf of Mexico are forecast to be favorable for tropical storm development over the coming week, if a disturbance with some spin at low levels can get going.

Super Typhoon Krosa smashes into Taiwan
Typhoon Krosa, a formidable Category 4 storm with 145 mph winds, is making landfall in the northern portion of Taiwan. Latest Taiwan radar shows the storm well. Krosa has already killed at least two people and brought rains of 16 inches to portions of Taiwan. Rainfall may exceed one meter (39 inches) in some mountainous regions of the island. Krosa is the Cambodian word for crane.


Figure 2. Radar image of Krosa as it hit northern Taiwan at 14:30 GMT Oct 6, 2007. Image credit: Central Weather Bureau of Taiwan.

I'll have an update Sunday morning.

Jeff Masters

Updated: 7:01 PM GMT on October 06, 2007

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Atlantic disturbances not much of a threat today

By: JeffMasters, 2:04 PM GMT on October 05, 2007

There's really not much new to report in the Atlantic today. A low pressure system over the northern Gulf of Mexico, (90L), continues to be unimpressive. Satellite loops show a small amount of heavy thunderstorm activity, to the northeast of the center. This activity is visible on Lake Charles long range radar. Water vapor satellite loops show plenty of dry air surrounding 90L, which should keep any development of the storm slow. The Hurricane Hunter aircraft scheduled to investigate 90L today was canceled. With 90L expected to make landfall Friday night near the Texas/Lousiana border, I don't expect the storm will become a tropical depression. However, it may be able to develop enough heavy thunderstorm activity today to cause some isolated flooding problems in southern Louisiana and northeast Texas.


Figure 1. Today's line up of tropical disturbances to watch.

Disturbance 92L east of the Bahamas
An area of heavy thunderstorm activity associated with a surface trough of low pressure (92L) near 22.4N 73W, in the southwestern Bahama Islands, has diminished. Wind shear of 20-30 knots has blown away nearly all of the heavy thunderstorm activity near the surface low pressure system, which has now reformed about 200 miles to the south of where it was last night. This morning's QuikSCAT pass showed some westerly surface winds over the southern Bahamas, which would aid in the formation of a new surface circulation under the heavy thunderstorms there. However, wind shear is still 20-25 knots today, and I don't expect any development until 92L can move into the Western Caribbean where wind shear is lower. This may occur Sunday. Another possibility is that a surface circulation just offshore of the Florida Keys, visible as a swirl of low clouds on this morning's visible satellite loop, could begin to develop. The computer models still indicate the possibility of a tropical storm forming in the Western Caribbean or southern Gulf of Mexico 2-7 days from now, but are much less insistent upon it. Upper air conditions are expected to be very favorable for tropical storm formation over the Western Caribbean and Gulf of Mexico for the next seven days, but nothing will form if we don't get a disturbance with some spin to it entering the region. A Hurricane Hunter aircraft is on call to investigate the Western Caribbean on Sunday, if necessary.

Disturbance 91L between Africa and the Lesser Antilles
A tropical wave (91L) near 12N, 49W, 900 miles east of the Lesser Antilles Islands, has become very disorganized. Wind shear is a high 20-30 knots over the system, and is expected to be 15-35 knots for the next five days. I don't expect 91L to develop.

Super Typhoon Krosa takes aim at Taiwan
Super Typhoon Krosa, a Category 4 storm with 150 mph winds, is bearing down on the northern portion of Taiwan. Latest Taiwan radar shows the eye of this huge storm approaching the island. Krosa is expected to hit Taiwan as a Category 3 typhoon, then brush the coast of Mainland China near Shanghai early next week as a Category 1 storm.


Figure 2. Radar image of Krosa from the Central Weather Bureau of Taiwan.

I'll have an update Saturday morning.

Jeff Masters

Updated: 3:25 PM GMT on October 05, 2007

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Not much change to the three Atlantic disturbances

By: JeffMasters, 2:02 PM GMT on October 04, 2007

A low pressure system over the southern Gulf of Mexico (90L) continues to be unimpressive. This morning's QuikSCAT pass shows this circulation nicely, with top winds of 30 mph south of the Louisiana coast. Satellite loops show only a small amount of heavy thunderstorm activity, to the northeast of the center. This activity is now visible on New Orleans long range radar. Water vapor satellite loops show a large upper-level low pressure system also covers the entire Gulf of Mexico, with embedded swirls. This upper level low has a cold core and is wrapping plenty of dry air into 90L. These factors, plus the very large size of the surface circulation of 90L, will keep any development of the storm slow. With 90L expected to make landfall Friday near the Texas/Lousiana border, I don't expect 90L will become a tropical storm. However, I do expect it will develop enough heavy thunderstorm activity today to potentially become a tropical depression and cause some isolated flooding problems in southern Louisiana tonight through Friday afternoon. The Hurricane Hunter aircraft scheduled to investigate 90L this afternoon was canceled, and has been rescheduled for Friday afternoon.


Figure 1. Today's line up of tropical disturbances to watch.

Disturbance 92L northeast of the Bahamas
Of greater concern is an area of heavy thunderstorm activity associated with a surface trough of low pressure (92L) near 26N 73W, just northeast of the Bahama Islands. Last night's QuikSCAT pass showed a 200-mile long southeast-northwest oriented zone of converging winds just northeast of the Bahamas, but no surface circulation. Satellite loops show a large area of disorganized thunderstorm activity that is not getting better organized. This disturbance is under about 15-30 knots of wind shear, and development today will be very slow. Wind shear is expected to fall to 10-15 knots beginning Friday afternoon, which may allow 92L to develop into a tropical depression as early as Saturday. The computer models expect a strong ridge of high pressure will force 92L slowly west-southwest over the Bahamas Friday and Saturday, then over the Florida Keys and western Cuba Saturday and Sunday. The storm will probably bring heavy rains to the western Bahamas, South Florida, and the Florida Keys beginning on Saturday. By Monday and Tuesday, the system is expected to be over the Western Caribbean or southern Gulf of Mexico near the Yucatan Peninsula. The GFDL and SHIPS models predict 92L will be a strong tropical storm with 55-65 mph winds on Tuesday. With an upper-level anticyclone with light wind shear expected to set up over the disturbance, the potential exists for a hurricane to form--if 92L can avoid the Yucatan.

The long term fate of the storm is highly uncertain. The GFS model predicts the ridge of high pressure ridge forcing it west will intensify, pushing 92L southwestward into Mexico. There is, however, a strong trough of low pressure expected to swing across the central U.S. next week and become a cut-off low. This system may be able to swing 92L northwards into the U.S. Gulf Coast, as the Canadian model is predicting. The Hurricane Hunters were scheduled to investigate 92L this afternoon, but this flight was canceled and has been rescheduled for Friday afternoon.

Disturbance 91L between Africa and the Lesser Antilles
A tropical wave (91L) near 10N, 45W, midway between Africa and the Lesser Antilles Islands, has become less organized since yesterday. Satellite loops show scattered disorganized heavy thunderstorm activity. The disturbance is headed west-northwest at 10-15 mph.

Wind shear is about 15 knots over the wave, which may allow for some slow development today. However, beginning tonight, wind shear is expect to increase to 20 knots, and will remain 20-35 knots through Monday. This should prevent further development. None of the computer models develop 91L.

Typhoon Krosa takes aim at Taiwan
After a slower than usual September, things have heated up in the Western Pacific this week. Typhoon Krosa, a Category 4 storm with 135 mph winds, is forecast to pass close to the northern tip of Taiwan Saturday, then brush the coast of Mainland China near Shanghai early next week.

I'll have an update Friday morning at the latest.

Jeff Masters

Updated: 2:32 PM GMT on October 04, 2007

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Three Atlantic disturbances to watch

By: JeffMasters, 2:40 PM GMT on October 03, 2007

A low pressure system over the southern Gulf of Mexico (90L) has developed a large surface circulation covering most of the Gulf of Mexico, but is not a threat to develop rapidly. This morning's QuikSCAT pass shows this circulation nicely, with top winds of 35 mph to the southeast of the center. Satellite loops show that 90L's heavy thunderstorm activity has decreased since yesterday. Water vapor satellite loops show a large upper-level low pressure system also covers the entire Gulf of Mexico, with two embedded swirls. This upper level low has a cold core and is wrapping plenty of dry air into 90L. These factors, plus the very large size of the surface circulation of 90L, will keep any development of the storm slow. A Hurricane Hunter aircraft is scheduled to investigate the system Thursday afternoon, if necessary. Today's flight was canceled.

The latest computer model runs continue to point to a landfall Thursday night or Friday morning near the Louisiana/Texas border. I don't see 90L becoming a hurricane, and I give equal chances of 90L arriving at the coast as a tropical disturbance, tropical depression, or tropical storm.


Figure 1. Today's line up of tropical disturbances to watch.

Disturbance 92L east of the Bahamas
Of greater concern to me is an area of heavy thunderstorm activity associated with a surface trough of low pressure (92L) that has developed just east of the Bahama Islands. This morning's QuikSCAT pass showed an east-west oriented zone of converging winds at 27N between 69W and 72W, but no surface circulation. Satellite loops show a large area of disorganized thunderstorm activity that is not getting better organized. This disturbance is under about 10-15 knots of wind shear. Wind shear is expected to remain ten knots or less over 92L for the next five days. The computer models expect 92L will move slowly west-southwest over the Bahamas, then the Florida Straits or Cuba during the next three days. By Saturday, the GFS and ECMWF models predict a tropical depression could form in the Western Caribbean or southern Gulf of Mexico. The NOGAPS and UKMET model forecast that development Monday is more likely. These are the highest heat content waters in the Atlantic, and with a upper-level anticyclone with light wind shear expected to set up over the disturbance, the potential exists for a hurricane to form from 92L next week. The Hurricane Hunters are scheduled to investigate 92L Thursday afternoon.

Disturbance 91L between Africa and the Lesser Antilles
A tropical wave (91L) near 9N, 41W, midway between Africa and the Lesser Antilles Islands, has gotten better organized this morning. This morning's QuikSCAT pass showed a wind shift associated with the wave, but no closed circulation. Satellite loops show a modest area of heavy thunderstorm activity that is not well-organized. The disturbance is headed west at 10-15 mph, and is expected to take a more west-northwesterly track Thursday.

Wind shear is about 10 knots over the wave, and is forecast to remain below 15 knots until Thursday night. This may allow for some slow development. However, beginning Thursday night, wind shear is expect to increase and remain 20-30 knots through Sunday. This should prevent further development.

I'll have an update Thursday morning at the latest.

Jeff Masters

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Little change to 90L; first half of October hurricane outlook

By: JeffMasters, 7:09 PM GMT on October 02, 2007

A low pressure system over the southeastern Gulf of Mexico (90L) has changed little today. The buoy 262 nm south of Panama City, FL had winds of 36 mph gusting to 45 mph this morning, but these winds have fallen some this afternoon. Satellite loops show some disorganized heavy thunderstorm activity that is not increasing in intensity. This system has the potential to become a subtropical depression by Wednesday, and a Hurricane Hunter aircraft is scheduled to investigate the system Wednesday afternoon. This afternoon's model runs continue to point to a landfall Thursday or Friday in Louisiana or Texas. I don't see this storm becoming a hurricane, due to the large amount of dry air to overcome, plus the extended amount of time it will take to transition from a subtropical to a tropical storm. The GFDL model probably has the right idea, bringing 90L ashore in Texas Friday with top winds near 40-45 mph.

First half of October hurricane outlook
In the first half of October, Atlantic tropical cyclone activity remains high, and is in the final two weeks of its peak phase. Since the current active hurricane period began in 1995, the first half of October has given birth to an average of 1.9 named storms, 0.75 hurricanes, and 0.3 intense hurricanes. For October through December, these figures are 3.8 named storms, 1.7 hurricanes, and 0.75 intense hurricanes. These numbers are nearly double the long-term climatological averages for the past 100 years. The final seasonal forecast from the Phil Klotzbach/Dr. Bill Gray team at Colorado State University, issued today, calls for four named storms, two hurricanes, and one intense hurricane for the remainder of this year.

October storms form from tropical waves that come off the coast of Africa, and from the remains of old fronts that push off the coast of the U.S. As we can see from the track plot of all first half of October storms (Figure 1), there is a lot of activity during the period, but relatively few storms form out near the African coast. The water temperatures off the coast of Africa are starting to cool and be marginal for hurricane formation, and wind shear is starting to pick up in its normal fall cycle.


Figure 1. Tracks of all tropical storms and hurricanes since 1851 that formed October 1-15. There are very few storms forming off the coast of Africa during this period.

Sea Surface Temperatures
Sea Surface Temperatures (SSTs) have been about 0.5 °C above average over the Caribbean and Gulf of Mexico the past few weeks (Figure 2). This is the primary formation area for October storms. Note also the tongue of colder than average SSTs extending out into the Pacific Ocean from the coast of South America. This is the signature of a moderate strength La Niña event.


Figure 2. Sea Surface Temperature (SST) departure from average for the end of September. Image credit: NOAA.

Tropical Cyclone Heat Potential
It's not just the SSTs that are important for hurricanes, it's also the total amount of heat in the ocean to a depth of about 150 meters. Hurricanes stir up water from down deep due to their high winds, so a shallow layer of warm water isn't as beneficial to a hurricane as a deep one. The Tropical Cyclone Heat Potential (TCHP, Figure 3) is a measure of this total heat content. A high TCHP over 80 is very beneficial to rapid intensification. There are similar levels of heat energy available in the Western Caribbean and Gulf of Mexico this year compared to the devastating Hurricane Season of 2005. Recall that Wilma, the most intense hurricane on record, formed on October 15 of that year.


Figure 3. Tropical Cyclone Heat Potential (TCHP) for September 30 2005 (top) and September 30 2007 (bottom). TCHP is a measure of the total heat energy available in the ocean. Image credit: NOAA/AOML.

Wind shear
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 by tearing a storm apart. Wind shear 10 knots and lower is very conducive for tropical storm formation.

Despite the presence of a La Niña event the past month--which is supposed to bring lower than average wind shear to the tropical Atlantic--wind shear the past two weeks has been above average (Figure 4). The latest two-week wind shear forecast from the GFS model predicts near-average wind shear for the first half of October.


Figure 4. Wind shear departure from average for the 11 days ending September 29. Above average levels of wind shear were observed over the Caribbean and Gulf of Mexico. Wind shear is the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude) in meters per second (multiply by two to get the approximate wind shear in knots).

Steering currents
The steering current pattern for the last half of September has been dominated by a strong ridge of high pressure over the Eastern U.S. This ridge is expected to remain in place until at least October 12, and will reduce the tendency of storms to recurve out to sea. Long range forecasts from the GFS and ECMWF models foresee the ridge may break down beginning October 12, but it is too early to be confident of this.

Summary
I predict two named Atlantic storms will form in the first half of October. One of these storms will probably be 90L. Several models, including the GFS, have been hinting at formation of a tropical depression in the Bahamas, Western Caribbean, or Gulf of Mexico sometime in the next 4-7 days. The upper air environment is forecast to be favorable for intensification over the Gulf of Mexico next week, with low wind shear.

Jeff Masters


Updated: 10:13 PM GMT on October 02, 2007

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Gulf of Mexico disturbance 90L slowly getting organized

By: JeffMasters, 2:05 PM GMT on October 02, 2007

Heavy thunderstorms are on the increase over the southeastern Gulf of Mexico, several hundred miles west of Key West, in association with an upper-level low pressure system that is now generating an area of low pressure at the surface. This system is being referred to as "Invest 90L" by NHC. The buoy 262 nm south of Panama City, FL had winds of 36 mph gusting to 45 mph this morning, and there were ship reports this morning of winds of 25-33 knots (29-38 mph) in the eastern Gulf of Mexico. This morning's QuikSCAT pass showed a large area of 30-35 mph winds over the eastern Gulf of Mexico. Satellite loops show a steady increase in heavy thunderstorm activity, but the activity is not well-organized at present.

A surface area of low pressure was over the western Bahamas last night, several hundred miles east of the Gulf of Mexico upper level low. This Bahamas surface low was the initial suspect area we were watching (labeled "Invest 90L" by NHC). This surface low moved westward overnight, and is now underneath the upper low. This is a situation very much like we saw with Tropical Depression Ten last month, which also formed from an upper level low over the Gulf of Mexico. Like TD 10, 90L will probably start off as a subtropical depression. There will be a warm core to the storm at the surface, but the upper low has a cold core aloft. This will make the storm subtropical in nature--a system has to have a warm core all the way from the surface to the upper atmosphere to be truly tropical. 90L will gradually warm up its entire core and become tropical, but this will probably take two days. It took two days for TD 10 to become fully tropical, and it ran out of time to intensify when it moved ashore into the Florida Panhandle as a tropical depression. Until a subtropical system establishes a fully warm core, it cannot undergo rapid intensification. With a landfall expected Thursday or Friday in Louisiana or Texas, this does not give 90L much time to strengthen. There is plenty of warm water in the Gulf of Mexico to aid intensification, but hampering intensification will be dry, continental air from North America that is being pulled southward over the Gulf of Mexico by the counter-clockwise circulation around the upper low. This dry air is readily apparent on water vapor satellite loops (the brown colors). The most likely scenario is that 90L will make landfall as a tropical storm. The Hurricane Hunter mission scheduled for today has been canceled, since 90L is not organized enough to warrant a flight. The mission has been rescheduled for Wednesday afternoon.

I'll have an update by 4 pm EDT this afternoon, and post my first half of October outlook for hurricane season.

Jeff Masters

Updated: 2:17 PM GMT on October 02, 2007

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Bahamas tropical disturbance a threat to the Gulf of Mexico

By: JeffMasters, 1:04 PM GMT on October 01, 2007

Heavy thunderstorms are firing up over South Florida and the nearby waters, thanks to an upper-level low pressure system interacting with an old front. Long range radar out of Miami shows that this activity is disorganized. This morning's QuikSCAT pass showed some rotation in the surface winds over the Central Bahamas, but with wind shear 20-30 knots over the region, no development is likely today. Most of the computer models forecast that wind shear will fall, and a tropical or subtropical depression will form by Wednesday near the Florida Keys, eastern Gulf of Mexico, or western Cuba. There is a strong upper-level low pressure system just southwest of Florida (Figure 1), and water vapor satellite loops show that this low is pulling plenty of dry, continental air from North America southward over the Gulf of Mexico. The upper low is expected to move southwestward. This is a situation very similar to the one that spawned Subtropical Depression Ten in September, off the Gulf Coast of Florida. In that case, the subtropical depression formed right underneath the upper level low, making for a very slow transition phase to a tropical system. It took two days for Subtropical Depression Ten to become Tropical Depression Ten, and the storm ran out of time to intensify into a tropical storm before moving ashore over the Florida Panhandle. This time, the GFS model is supporting formation of a subtropical depression to the northeast of the upper low. This situation would potentially allow a faster conversion of the subtropical storm to a tropical storm. The UKMET and Canadian model predict that a fully tropical storm will form, instead. I think a subtropical storm is more likely.

Any storm that forms is forecast to move west-northwestward across the Gulf of Mexico, pushed by a strong ridge of high pressure expected to build in. An upper-level anticyclone aloft is expected to develop as well, providing an environment favorable for intensification. However, intensification will be slowed by the presence of all the dry air dragged into the Gulf of Mexico by the upper low, and by the transition of the storm from subtropical to tropical. The models project a landfall in Texas or Louisiana on Friday or Saturday. The Hurricane Hunters are on call to fly into this system Tuesday afternoon, if necessary.


Figure 1. This morning's water vapor satellite image. The counter-clockwise flow of air around the upper southwest of Florida is dragging plenty of dry, continental air (brown colors) from North America over the Gulf of Mexico. Heavy thunderstorm activity (blue and orange colors) is over South Florida.

Karen's remains not expected to develop
Karen's remains continue to generate a large area of disturbed weather a few hundred miles east of the northern Lesser Antilles Islands (Figure 1). This morning's QuikSCAT pass showed some rotation still exists near the surface, with winds up to 35 mph. Wind shear is about 20 knots over Karen's remains, and wind shear is expected to remain too high to allow re-development of Karen over the next several days. None of the reliable computer models resurrect Karen.

Melissa
Tropical Storm Melissa is dead, torn up by wind shear.

I'll have an update Tuesday, and post my first half of October outlook for hurricane season.

Jeff Masters

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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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