Intensifying Gustav bears down on Haiti

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


Figure 1. Latest satellite image of Tropical Storm Gustav.

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

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

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

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

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

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

Links to follow
Wundermap for Haiti
Gran Piedra, Cuba radar

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

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


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

My next blog will be this afternoon.

Jeff Masters