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A strengthening Gustav takes aim at Haiti

By: Dr. Jeff Masters, 8:14 PM GMT on August 25, 2008

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

Figure 1. Latest satellite image of Tropical Storm Gustav.

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

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

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

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

My next blog will be Tuesday morning.

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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