Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 2:44 PM GMT on August 27, 2008
Tropical Storm Gustav continues to pound Haiti with torrential rains and tropical storm-force winds, as the storm slowly lumbers westward towards Cuba. The storm has killed at least five people on Haiti, according to early news reports. Satellite rainfall estimates show that the southern peninsula of Haiti has probably received 4-8 inches of rain, and an additional 2-4 inches are likely over the worst-hit regions before Gustav finally moves away. Yesterday, Gustav plowed into Haiti as a Category 1 hurricane with 90 mph winds. The encounter with Haiti's high mountains has weakened Gustav to a tropical storm. The latest center fix from the Hurricane Hunters at 7:03 am EDT put Gustav's central pressure at 999 mb, and the aircraft was not able to find surface winds above 50 mph. The Hurricane Hunters have left Gustav, and a new airplane is due in the storm this afternoon around 2 pm EDT.
Figure 1. Current satellite image of Gustav.
Visible satellite loops and radar from Gran Piedra, Cuba show that Gustav is not well-organized, and the eyewall has mostly collapsed. However, the appearance of the storm on both radar and satellite is slowly improving.
The track forecast for Gustav
The latest 6Z (2 am EDT) model runs are in good agreement that Gustav will turn west and pass through the narrow channel between Cuba and Jamaica after leaving Haiti. Gustav is small enough that the Kingston, Jamaica airport on the south side of the island may be able to stay open Thursday as the storm passes to the north. However, the Montego Bay airport on the north side of the island is likely to close. Heavy rain will be the primary threat to Jamaica and eastern Cuba.
The trough of low pressure that was steering Gustav northwest has moved off to the east, allowing a ridge of high pressure to build in. This ridge of high pressure will force Gustav west. There is now better agreement in the longer term track of Gustav. All of the models foresee a turn to the north over western Cuba, followed by a landfall on the central U.S. Gulf Coast Sunday or Monday. The final landfall location of Gustav depends on the strength and speed of a trough of low pressure forecast to move across the Midwest U.S. late this week. While it currently appears that Louisiana is the most likely target, keep in mind that if you're in the cone of uncertainty, you're not safe. Final landfall of Gustav could occur anywhere from Texas to the Florida Panhandle. The latest GFDL model predicts landfall near New Orleans on Sunday night as a Category 3 hurricane. The HWRF model picks Mississippi on Monday morning, as a Category 3 hurricane. The NOGAPS prefers the Florida Panhandle on Sunday night, and the ECMWF targets central Louisiana on Monday morning.
The intensity forecast for Gustav
One key question controlling Gustav's short-term intensity is how close it will stay to land. Gustav will not be able to intensify much until it clears the southwest Peninsula of Haiti, which should occur late this afternoon. If Gustav passes close to the rugged southeastern tip of Cuba, this will also impede its intensification. However, all the models are forecasting that Gustav will be able to thread the narrow gap between Cuba and Jamaica, allowing Gustav to intensify back to hurricane status on Thursday. Gustav is currently under moderate wind shear (10-15 knots). This shear is expected to remain in the low to moderate range (0-15 knots) for the remainder of the week. By Friday, as Gustav approaches the Cayman Islands, the storm will be underneath an upper level anticyclone, and over the highest heat content waters of the Atlantic. The HWRF and GFDL models respond by intensifying Gustav to a Category 3 hurricane as it passes through the Caymans. I think it is plausible that Gustav could intensify further, to Category 4 strength, before hitting the Caymans. If you have plans to be on the northern Cayman Islands--Cayman Brac or Little Cayman Island--on Friday, be prepared to be stuck there for several days, as Gustav may heavily damage these islands. Grand Cayman Island is also at risk--the GFDL model predicts Gustav will pass very close to Grand Cayman on Friday afternoon.
Gustav will lose intensity when it crosses the western tip of Cuba--perhaps by 25 mph or so--but should easily regain that lost strength within 12-24 hours. Gustav will likely be a major Category 3 or 4 hurricane in the Gulf of Mexico, and make landfall in the U.S. as a major hurricane.
Gustav's intensification potential in the Gulf of Mexico
As we saw in 2005 with Katrina and Rita, the large amounts of deep, warm water brought into the Gulf of Mexico by the Loop Current can help intensify hurricanes to Category 5 intensity. As explained in my Loop Current tutorial, the Loop Current is an ocean current that transports warm Caribbean water through the Yucatan Channel between Cuba and Mexico. The current flows northward into the Gulf of Mexico, then loops southeastward through the Florida Keys. The Loop Current commonly bulges out in the northern Gulf of Mexico and sometimes will shed a clockwise rotating ring of warm water that separates from the main current. This ring of warm water slowly drifts west-southwestward towards Texas or Mexico at about 3-5 km per day. This feature is called a "Loop Current Ring", "Loop Current Eddy", or "Warm Core Ring", and can provide a key source of energy to fuel rapid intensification of hurricanes that cross the Gulf. The Loop Current itself can also fuel rapid intensification, such as happened with Hurricane Charley in 2004. When a Loop Current Eddy breaks off in the Gulf of Mexico at the height of hurricane season, it can lead to a dangerous situation where a vast reservoir of energy is available to any hurricane that might cross over. This occurred in 2005, when a Loop Current Eddy separated in July, just before Hurricane Katrina passed over and "bombed" into a Category 5 hurricane. The eddy remained in the Gulf and slowly drifted westward during September. Hurricane Rita passed over the same Loop Current Eddy three weeks after Katrina, and also explosively deepened to a Category 5 storm.
This year, we had another Loop Current Eddy break off in July. This eddy is now positioned due south of New Orleans (Figure 2), and this eddy has similar levels of heat energy to the 2005 eddy that powered Katrina and Rita. Should Gustav pass over or just to the left of this eddy, we can expect the storm to significantly intensify. There is also a weaker eddy present in the western Gulf; this eddy broke off from the Loop Current in April, and is much cooler then the eddy that broke off in July. Should Gustav pass over the April eddy, it shouldn't make much difference.
A NOAA Hurricane Hunter research aircraft is scheduled to drop a network of 20-30 specialized buoys (AXBTs) in the Gulf of Mexico Thursday to provide precise measurements of ocean temperatures in order to aid intensification forecasts for Gustav.
Figure 2.Tropical Cyclone Heat Potential (TCHP) for the Gulf of Mexico on August 24, 2008. TCHP values in excess of 80 kJ/cm^2 (yellow colors) have been found to promote rapid intensification of hurricanes. The Loop Current is shown by the arrows at lower right, and begins in the Caribbean, flows north through the Yucatan Channel into the Gulf of Mexico, then loops back to the south and turns eastward over the Florida Keys. Two Loop Current Eddies have broken off from the Loop Current this year--one in April, and one in July. These eddies have drifted slowly westward, and still maintain heat from the Loop Current. Image credit: NOAA/AOML.
Links to follow
Wundermap for Cuba/Haiti/Jamaica
Gran Piedra, Cuba radar
Figure 3. QuikSCAT image from 5:43 am EDT Wednesday August 27, 2008 clearly shows the surface circulation associated with disturbance 95L (marked with an "L"). The black wind barbs indicated "flagged" regions where rain is falling and the wind estimates cannot be trusted. Highest unflagged winds were 20-25 knots (24-28 mph). Image credit: NOAA.
Disturbance 95L east-northeast of Puerto Rico
A tropical disturbance (95L) near 20N 58W, a few hundred miles east-northeast of Puerto Rico, has a closed surface circulation, as seen on this morning's QuikSCAT pass (Figure 3). This system could develop and be a problem for Bermuda by Monday, and the U.S. East Coast late next week. Visible satellite loops show that wind shear continues to play havoc with this system--strong upper-level winds from an upper level low pressure system to the west are pushing 95L's heavy thunderstorms to the east side of the center of circulation. Wind shear is a high 20 knots over 95L today, but is forecast to decrease below 5 knots on Thursday and remain below 15 knots for most of the remainder of the week. Dry air should not be a problem for 95L. NHC is giving 95L a medium (20%-50% chance) of developing into a tropical depression by Friday. Most of the models develop 95L, foreseeing a northwest track and a threat to Bermuda 4-7 days from now. The latest (2 am EDT) GFDL model run develops 95L into a strong Category 1 hurricane that passes very close to Bermuda on Monday. The HWRF model is more aggressive, predicting a Category 3 hurricane by Saturday, that then weakens to a Category 1 hurricane by the time it nears Bermuda on Monday. In the longer range, it appears that 95L will stall in the Bermuda area and move slowly, as steering currents collapse early next week. By the end of next week, the storm may scoot northward towards Canada (as predicted by the GFS model), or head west-southwestward into the Bahamas and Florida (as predicted by the ECMWF model). It is too early to guess which of these solutions is more likely.
Elsewhere in the tropics
Most of the computer models forecast the development of one additional tropical wave between Africa and the Lesser Antilles Islands during the coming week. The first candidate is a system a few hundred miles west-southwest of the Cape Verde Islands. Wind shear is a low 5-10 knots in the region, and forecast to remain in the low to moderate range for the next few days. None of the models are developing this region, and NHC is giving this system a low (<20% chance) of developing into a tropical depression by Friday. The models predict this system will be near the northern Lesser Antilles Islands Monday or Tuesday.
My next blog will be this afternoon.
P.S., a little humor from The Onion might be what is needed for some. Check out this morning's video, Hurricane Bound For Texas Slowed By Large Land Mass To The South.
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