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Tomas' rains reach Haiti and Jamaica

By: Dr. Jeff Masters, 2:11 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 and the eastern tip of Jamaica. It appears at this time that the most dangerous flooding rains of 5 - 10 inches will be confined to the southwestern peninsula 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 areal coverage and intensity of the thunderstorms has grown this morning. An Air Force hurricane hunter is in Tomas this morning, and reported top surface winds of 52 mph at 9am EDT this morning with their SFMR instrument. Rainfall amounts as observed by the F-16 polar orbiting satellite at 8:05am were 0.5 - 1.0" per hour in a 100-mile wide region near the center of Tomas (Figure 1.)

Figure 1. Rainfall rate for Tomas as observed by the F-16 polar orbiting satellite at 8:05am 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 north-northwestward at 6 mph, and this forward speed will gradually increase to 10 mph late tonight. Heavy rains from Tomas will spread over all of Haiti's southwestern peninsula and eastern Jamaica this afternoon, then move into the rest of Haiti and western portions of the Dominican Republic by tonight. Satellite-based estimates of current rainfall rates from Tomas (Figure 2) yield predictions of 4 - 6 inches of rain falling over an 18-hour period near the core of Tomas, and we can expect that the heaviest rains from Tomas will fall over Haiti's southwest peninsula, and accumulate to 5 - 10 inches. According to this rainfall forecast, precipitation amounts over Haiti's earthquake zone will be much lower, perhaps just 2 - 4 inches. However, 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. The latest rainfall forecast from the GFDL model (Figure 3) agrees that it will primarily be Haiti's southwest peninsula that will experience dangerous rains in the 4 - 8 inch range, and that Haiti's earthquake zone is likely to see lower amounts of 1 - 4 inches. Rains of 1 - 4 inches in the earthquake zone are still capable of causing life-threatening flooding. Rains of similar magnitude killed 12 people there last weekend, and the soils are still saturated. However, potentially catastrophic flooding from Tomas is likely to be limited to Haiti's southwest peninsula, and we are not likely to see a massive flooding catastrophe killing hundreds of people in Haiti's earthquake zone. There will probably be some isolated regions of eastern Haiti and the western Dominican Republic that will see heavier rains of 4 - 8 inches; hopefully, the earthquake zone of Haiti will avoid being one of these spots of higher rainfall. I expect general rainfall amounts of 2 - 4 inches over eastern Jamaica and the western Dominican Republic from Tomas, with higher rainfall amounts of 3 - 6 inches likely over eastern Cuba, the Turks and Caicos Islands, southeastern Bahamas, and eastern tip of Cuba.

Figure 2. Predicted rain amounts for the 18-hour period ending at 8pm EDT Thursday, November 4, 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 2am EDT (06Z) 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, 20 - 30 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 distort the circulation of Tomas into a more oval shape and make the circulation tilt with height. Recent satellite imagery suggests that the circulation of Tomas is becoming more circular and vertically aligned, though, and Tomas may be able to intensify into a minimal Category 1 hurricane before land interaction with Haiti disrupts the storm. Wind shear as predicted by the SHIPS model will remain in the moderate range, 10 - 15 knots, which should allow some modest intensification through Friday afternoon. NHC is giving Tomas a 41% chance of reaching hurricane strength by Friday afternoon.

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 later today.

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.