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Little change to Tomas as it bears down on Haiti

By: Dr. Jeff Masters, 7:44 PM GMT on November 04, 2010

Tropical Storm Tomas is headed north towards Haiti, and the northernmost spiral bands of the storm have already reached the tip of Haiti's southwestern peninsula, the eastern tip of Jamaica, and eastern Cuba. It appears at this time that the most dangerous flooding rains of 5 - 10 inches will be confined to the southwestern and northwestern peninsulas of Haiti, and that the earthquake zone where 1.3 million people live in makeshift shelters and tents will experience lesser rains that will cause serious but not catastrophic flooding. Satellite loops of Tomas show an average-sized tropical storm with a modest amount of heavy thunderstorm activity and low-level spiral bands. Upper-level outflow is good to the north, fair to the east, and poor elsewhere. Wind shear as diagnosed by the University of Wisconsin CIMSS group is a moderate 10 - 20 knots. The shear is due to strong upper-level winds out of the southwest, and the shear is keeping most of Tomas' heavy thunderstorms pushed over to the northeast side of the storm. The low-level center of circulation has been exposed to view most of the afternoon, the tell-tale sign of a tropical storm struggling with wind shear. The areal coverage and intensity of Tomas' thunderstorms has continued to grow this afternoon, but Tomas' winds and pressure have remained about the same. A 2:31pm center fix by an Air Force hurricane hunter aircraft found a central pressure of 999 mb--several millibars higher than early this morning. Top surface winds of 57 mph were seen with their SFMR instrument. Rainfall amounts as observed by the Metopa polar orbiting satellite at 10:23am were 0.5 - 1.0" per hour in a 100-mile wide region near the center of Tomas (Figure 1.) One odd aspect of Tomas is that the area of low pressure 200 - 300 miles southwest of Tomas has gotten better defined with an steady increase in heavy thunderstorm activity this afternoon. This movement of energy to the southwest is probably responsible for the 3 mb rise in Tomas' pressure over the past three hours. It is possible, but not likely, that Tomas' center could relocate 200 miles to the south-southwest later tonight, resulting in a 12 hour longer period of rainfall for Haiti, eastern Cuba, the Dominican Republic, and Jamaica.

Figure 1. Rainfall rate for Tomas as observed by the Metopa polar orbiting satellite at 10:23am EDT Thursday, November 4, 2010. Heaviest rainfall rates in excess of 0.5 inches per hour (green colors) were confined to a 100-mile wide area near the core of Tomas. Image credit: Navy Research Lab, Monterey.

Track and rainfall forecast for Tomas
A trough of low pressure over the Eastern U.S. is drawing Tomas northward at 7 mph, and this forward speed will gradually increase to 10 mph late tonight. Heavy rains from Tomas will spread over all of Haiti, western portions of the Dominican Republic, and eastern Cuba by tonight. Satellite-based estimates of current rainfall rates from Tomas (Figure 2) yield predictions of 2 - 4 inches of rain falling over an 18-hour period near the core of Tomas. This forecast only uses the current intensity of the storm to come up with a rainfall forecast, and if Tomas intensifies today, rainfall amounts will be higher. Tomas has an elongated band of heavy rains that extend all the way to South America, so I expect that the southwest and northwest peninsulas of Haiti will receive heavy rains until Saturday night or Sunday morning. Taking these factors into account, plus the current track forecast, I expect that the heaviest rains from Tomas will fall over these two peninsulas, and accumulate to 5 - 10 inches. Precipitation amounts over Haiti's earthquake zone and the western Dominican Republic will be 3 - 5 inches, with some isolated areas receiving up to 8 inches. The most severe flooding problems from Tomas will probably be in Haiti's southwestern Peninsula, and in the city of Gonaives in northwest Haiti, where most of the 3,000 deaths from Hurricane Jeanne's rains in 2004 occurred. The NHC rainfall forecast for 1 to 3 inches over Jamaica, 3 to 5 inches over eastern Cuba, the southeastern Bahamas, and the Turks and Caicos Islands looks good.

Figure 2. Predicted rain amounts for the 18-hour period ending at 2am EDT Friday, November 5, 2010, as forecast using satellite-derived measurements of precipitation rates. Image credit: NOAA/NESDIS.

Figure 3. Predicted cumulative rainfall from Tomas as predicted by the 8am EDT (12Z) Thursday, November 4, 2010 run of the GFDL model. The model predicts no rainfall amounts in excess of 8 inches (yellow colors) for Haiti. Image credit: Morris Bender, NOAA/GFDL.

The computer models now agree that the trough of low pressure pulling Tomas northward will be strong enough to pull Tomas well north of the Bahamas, and the storm will not stall out near Hispaniola for many days as was being predicted by many of the models yesterday. Wind shear will rise to the high range, 35 - 60 knots, by Sunday, resulting in a steady weakening of Tomas.

Intensity forecast for Tomas
Tomas continues to have difficulty disentangling itself from an area of low pressure over the southwest Caribbean, and this low is acting to steal moisture from Tomas and distort its circulation. As Tomas pulls away from this low, these effects will lessen, and Tomas may be able to intensify into a minimal Category 1 hurricane before reaching the Turks and Caicos Islands and southeastern Bahamas. Wind shear as predicted by the SHIPS model will remain in the low to moderate range, 5 - 15 knots, through Saturday afternoon, which should allow for some modest intensification. NHC is giving Tomas a 43% chance of reaching hurricane strength by Friday evening.

Comparison with Hurricane Ernesto of 2006
Tomas's current position and intensity are similar to that of Tropical Storm Ernesto of 2006. Ernesto was a small to moderate-sized tropical storm with 50 mph winds when it hit the southwestern tip of Haiti on August 27. According to Wikipedia, In Haiti, the storm caused heavy rainfall of over 11 inches (300 mm) and strong winds, causing flooding and destroying 13 homes on the island of La Gonave. In Port-au-Prince, rainfall severely damaged a bridge, isolating the southern portion of the region. Across the country, 59 homes were damaged, of which six destroyed, and a total of five deaths were reported..

Tomas is a larger and wetter storm than Ernesto , and I expect that Tomas' rains in Haiti will be about 25% greater than Ernesto's. Since Haiti is more vulnerable to disasters because of the floods of 2008 and the 2010 earthquake, the rains from Tomas will cause more death and destruction than Ernesto did, but I don't expect a catastrophe with many hundreds of deaths in Haiti's earthquake zone. Such a catastrophe is possible in southwest or northwest Haiti, however.

Figure 4. Track of Hurricane Ernesto in 2006.

Organizations Active in Haitian Relief Efforts:
Portlight disaster relief
Lambi Fund of Haiti
Haiti Hope Fund
Catholic Relief Services of Haiti

Next update
I'll have an update Friday morning.

Yesterday's post on Haiti's hurricane history is now a permanent link in the "Articles of interest" section on our Tropical & Hurricane web page.

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.