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Tomas remains weak; St. Lucia hard-hit by the storm

By: Dr. Jeff Masters, 2:04 PM GMT on November 01, 2010

Hurricane Tomas dealt a punishing blow to the Caribbean island of St. Lucia on Saturday, with the neighboring islands of St. Vincent and Barbados also suffering heavy damage. St. Lucia received the full brunt of the northern eyewall of Tomas as it intensified, and the St. Lucia weather service reported that sustained winds of 90 - 95 mph affected the island. A state of emergency has been declared, and heavy flooding washed out many bridges and roads. Damage to structures is considerable, with many roofs gone, and damage reported to hospitals, schools, and businesses. The Minister of Communications for the island said yesterday that he had done an aerial assessment of the damage, and it was "worse that we could think of." The Caribbean Catastrophe Risk Insurance Facility estimated that they will pay out $8.5 million for Barbados, $3.2 million for Saint Lucia, and $1.1 million for St Vincent and the Grenadines.

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

Tomas remains weak
Satellite loops of Tomas show the classic signature of a tropical storm experiencing high wind shear, with a low-level swirl of clouds and the heavy thunderstorm activity all pushed to the east side of the circulation by strong upper-level winds out of the southwest. A few flare-ups of thunderstorms have occurred near the center of Tomas this morning, a sign that the storm is not going to die anytime soon. Wind shear is a high 20 knots due to strong upper level southwest winds, and these winds are driving dry air at mid-levels of the atmosphere into Tomas' west side, as seen on water vapor satellite imagery.

Figure 2. Morning satellite image of Tropical Storm Tomas.

Track forecast for Tomas
The ridge of high pressure pushing Tomas to the west-northwest will weaken by Tuesday, as a trough of low pressure approaches the eastern U.S. and breaks down the ridge. This will result in Tomas slowing from its current 14 mph forward speed to 5 mph by Tuesday afternoon. By Thursday, the trough to Tomas' north should be able to pull the storm to the north or north-northeast. The exact timing and location of this turn is still uncertain, but the computer models have come into better agreement that Haiti or Jamaica are the most likely targets of Tomas. NHC is giving Port-au-Prince, Haiti, a 40% chance of receiving tropical storm force winds, and a 5% chance of hurricane force winds. These odds are slightly lower for Kingston, Jamaica--29% and 4%, respectively.

However, the latest 2am EDT (06Z) run of the NOGAPS model calls into question whether or not the trough will be strong enough to pull Tomas northward into Haiti. The NOGAPS is keeping Tomas trapped in the Caribbean for the next seven days. The latest runs of the GFS and ECMWF models are also showing that the trough will not be strong enough to pull Tomas fully out to sea, and these models stall the storm just north of Haiti. So, the long-term fate of Tomas is now looking murkier, and the storm may still be in the Caribbean a week from now.

Intensity forecast for Tomas
Wind shear is forecast to remain in the high range today, near 20 knots, then decline to the moderate range, 10 - 20 knots, Tuesday through Wednesday. The relaxation of shear should allow Tomas to re-organize Tuesday and Wednesday, though it is unlikely the storm could rebuild its inner core and form and eyewall until Thursday at the earliest. Hampering this process will be the presence of a considerable amount of dry air to Tomas' west. The upper-level winds out of the southwest creating the shear over Tomas will drive this dry air into the core of the storm, slowing intensification. The waters are very warm in the Caribbean and these waters extend to great depth, so a period of rapid intensification just before landfall in Jamaica or Haiti is possible, particularly if the shear drops to the low range, as predicted by the latest SHIPS model forecast. The current south of due west motion of Tomas is putting the storm farther south in the Caribbean than originally expected, which will give the storm more time over water and more time to intensify. The intensity Tomas might have at landfall is thus highly uncertain, and a strength anywhere between a tropical storm and Category 3 hurricane would not be surprising. The HWRF model predicts Tomas will be a strong tropical storm with 70 mph winds on Friday at landfall, while the GFDL foresees Tomas will be a strengthening Category 3 hurricane as it bears down on Haiti Saturday morning. The HWRF model has been under-forecasting intensity this season, and I predict Tomas will be a strengthening Category 1 hurricane on Friday as it makes landfall in Jamaica or Haiti.

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

St. Lucia's hurricane history
Tomas is the strongest hurricane to affect St. Lucia since Category 1 Hurricane Dean of 2007 brought 90 mph winds to the island. Dean killed one person and did $6.4 million in damage--0.5% of the nation's GDP. The island's strongest hurricane since accurate records began in 1851 was Hurricane Allen of 1980, which struck as a Category 3 hurricane with 130 mph winds. Allen killed 18 people on St.Lucia, and caused catastrophic damage of $235 million dollars ($613 million 2010 dollars.) This was 177% of the nation's GDP that year. The deadliest hurricane in St. Lucia history was the Category 5 Great Hurricane of 1780, which killed approximately 700 people. The Great Hurricane of 1780 was 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 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.