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Tomas gradually strengthening; 14 dead in St. Lucia from the storm

By: Dr. Jeff Masters, 2:58 PM GMT on November 02, 2010

The islands of St. Lucia, Barbados, and St. Vincent and the Grenadines continue to assess damage and clean up after Hurricane Tomas pounded the Lesser Antilles as a strengthening Category 1 hurricane with 90 - 95 mph winds on Saturday. St. Lucia was hardest hit, with fourteen people dead, many more missing, and damage estimated at $100 million--about 10% of the nation's GDP. Damage on neighboring St. Vincent was estimated at $62 million, which is 4% of that nation's GDP. The storm damaged 1,200 houses, and the northern half of the island, where most of the crops are, was badly hit, with no banana trees left standing and the plantain crop wiped out. Banana production employs 60% of the workforce on St. Vincent, and accounts for more than 50% of their exports. Also hard-hit was Barbados, where damage estimates are at $55 million, 1.5% of the nation's GDP. Tomas may be the most damaging storm to affect the island since Hurricane Janet of 1955. The havoc wreaked by Tomas in the Lesser Antilles makes is likely that the name Tomas will be retired from the list of active hurricane names in the Atlantic.

Figure 1. Torrential rains from Tomas triggered massive flooding on St. Lucia that destroyed several bridges and severely damaged roads. Image credit: St. Lucia Star.

Tomas gradually strengthening
Satellite loops of Tomas show a considerably more organized storm than yesterday, with a modest but increasing amount of heavy thunderstorm activity. However, low-level spiral bands are limited, and upper-level outflow is weak, and Tomas is not in danger of building an eyewall today. Curacao radar shows that the echoes from Tomas are disorganized, with no spiral banding apparent. Wind shear has declined to a moderate 10 -15 knots and the atmosphere in the Caribbean has moistened over the past day, allowing Tomas to re-organize. A hurricane hunter aircraft is on its way to Tomas this morning, and will have a better estimate of the storm's strength by early this afternoon.

Figure 2. Curacao radar at 10:07am EDT on Tuesday, November 2, 2010, showed a large area of rain associated with Tomas over the central Caribbean, but these echoes were poorly organized.

Track forecast for Tomas
The ridge of high pressure pushing Tomas to the west has weakened, allowing Tomas to slow down slightly to a forward speed of 10 mph this morning. This speed will decrease further to 5 mph tonight, as a trough of low pressure approaches the eastern U.S. and breaks down the ridge. By Wednesday, the trough to Tomas' north should be able to pull the storm to the northwest. Tomas' outer spiral bands will bring heavy rains to southwestern Haiti and eastern Jamaica beginning on Thursday night. The computer models have come into better agreement that Tomas will turn more to the north-northeast by Friday, with Haiti or Jamaica the most likely landfall locations. NHC is giving Port-au-Prince, Haiti, a 50% chance of receiving tropical storm force winds, and a 6% chance of hurricane force winds. These odds are 45% and 7%, respectively for Kingston, Jamaica, and 23% and 3% for Santo Domingo in the Dominican Republic.

Tomas may stall
The models are increasingly suggesting the once Tomas begins moving to the north-northeast, the trough pulling the storm that direction will lift out, stranding Tomas in a region of weak steering currents. Tomas may then wander and dump heavy rains for several days, Saturday through Monday. Given recent model trends, I believe this is likely, but the exact location where Tomas might be stranded is uncertain. The NOGAPS model gives a nightmare scenario for Haiti, with Tomas remaining stationary just off the coast from Port-Au-Prince as a hurricane for many days. The UKMET stalls Tomas over the Turk and Caicos Islands, while the GFS, GFDL, and ECMWF models predict Tomas will stall several hundred miles north of Hispaniola and drift eastwards. It's reasonable to go with the model consensus and predict Tomas will pass over western Haiti and stall far enough north of the nation so that heavy rains will not linger over Hispaniola for many days. The uncertainties in the track forecast are greater than usual, though.

Intensity forecast for Tomas
Wind shear as diagnosed by the SHIPS model has dropped to the moderate range, 10 - 15 knots, and is predicted to stay low to moderate, 5 - 15 knots, for the remainder of the week. The relaxation of shear should allow Tomas to continue to re-organize over the next few days. Aiding this process will be an increasingly moist atmosphere. Dry air has decreased significantly over the past 24 hours, as seen on water vapor satellite imagery, and the models predict a very moist atmosphere will surround Tomas for the remainder of the week. With SSTs at a record warm 29.5°C and a very high ocean heat content, there is a substantial danger that Tomas will undergo a period of rapid intensification once it rebuilds its inner core and establishes an eyewall. This is not likely to happen today, but could occur as early as Wednesday night. This may give Tomas sufficient time to intensify into a major hurricane before landfall in Haiti or Jamaica, as predicted by the GFDL model. The most reasonable intensity forecast at this point is to call for a landfall on Friday at Category 2 strength, but Tomas could easily be anywhere from Category 1 to Category 3 hurricane strength on Friday. NHC is giving Tomas a 19% chance of reaching Category 3+ strength; I believe these odds are higher, 40%. With the atmosphere expected to be very moist, it is likely that Tomas will dump very heavy rains of 4 - 8 inches over much of Haiti, even if Tomas strikes as a tropical storm. Rains of this magnitude are capable of causing heavy loss of life due to extreme floods running down Haiti's deforested mountain slopes. Portlight.org is preparing to send their mobile kitchen with enough food to feed 500 people per day, if Tomas continues on its current forecast path.

Figure 3. Plot of all Category 1 and stronger hurricanes to pass within 50 miles of Barbados since reliable record keeping began in 1851. Image credit: NOAA Coastal Services Center.

Barbados hurricane history
Tomas is the strongest hurricane to affect Barbados since Category 3 Hurricane Allen of 1980, which passed just north of the island. Allen did $4 million in damage, compared to Tomas' $55 million. The deadliest hurricane in Barbados history was the Category 5 Great Hurricane of 1780, which killed approximately 4500 people on the island, and leveled every building, including the stone governor's mansion. The Great Hurricane of 1780 was also the Atlantic's deadliest hurricane of all-time, with 22,000 fatalities, mostly in the Lesser Antilles Islands.

Next update
I'll have an update Wednesday morning.

Jeff Masters


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