Gustav hits Haiti and weakens

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


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

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

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

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

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

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

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


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

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

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

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


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

My next blog will be Wednesday morning.

Jeff Masters