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Virgin Islands brace for Hurricane Omar

By: Dr. Jeff Masters, 2:24 PM GMT on October 15, 2008

Hurricane Omar is on the move, headed for an encounter with the Virgin Islands and northern Lesser Antilles late tonight and early Thursday morning. Omar is passing very close to Buoy 42059, which measured sustained winds of 58 mph gusting to 72, and 17 foot waves, at 8:50 am EDT. The storm continues to pound the ABC Islands of Aruba, Bonaire, and Curacao with heavy rains. Rainfall amounts of 2-4 inches have been observed over the past three days in these islands, and Netherlands Antilles radar shows that more rain is on the way from Omar's southern spiral band. Rain will be ending early this evening in the ABC Islands, and no significant flooding or damage is likely there.

The latest Hurricane Hunter center fix at 7:30 am EDT found that the pressure has remained nearly constant since yesterday afternoon, 984 mb. The 35-mile diameter eyewall had a gap in it on the south side, and it is apparent that Omar is still struggling with wind shear. Shear was analyzed at 20 knots this morning, which is on the high side of where hurricanes are still able to intensify. Satellite loops show that low level spiral bands and upper level outflow are not present on Omar's west side, due to strong upper level winds from the west creating wind shear on that side of the storm. Outflow and spiral banding are impressive on the other three sides of the storm. A hint of an eye is apparent on visible satellite images. The eye of Omar is now visible on long range radar out of Puerto Rico, and this will be a good way to track the storm until the next Hurricane Hunter flight arrives at 2 pm EDT this afternoon.

Figure 1. Current satellite image of Omar.

The forecast for Omar
The models are tightly clustered along a path that would take Omar through the Virgin Islands late tonight. However, the east coast of Puerto Rico and the islands farther east, such as St. Martin/St. Maartin and Anguilla, are still in the cone of uncertainty, and could get a direct hit. Our main intensity models--the GFDL, HWRF, and SHIPS--all forecast that Omar will be a Category 1 hurricane with winds of 75-85 mph as it passes through the islands. Wind shear is expected to be in the moderate to high range, 15-25 knots, over the next two days, which should allow some modest intensification. I give Omar a 30% chance of reaching Category 2 strength before landfall early Thursday morning, and a 10% chance of being a major Category 3 hurricane.

Omar's storm surge
Wind damage is likely to be the greatest threat from Omar. Storm surge is usually not a problem in the Virgin Islands and northern Lesser Antilles, since these islands are surrounded by deep water, and the surge tends to flow around the islands, rather than be forced up onto the islands. As seen in Figure 2, the maximum storm tide from a mid-strength Category 3 hurricane with 120-125 mph winds is generally in the 3-4 foot range in the Virgin Islands. St. Thomas and Anedega Islands can get a slightly higher surge of 5-6 feet, due to their convex shape facing southwest, which will tend to trap the surge from a northeastward moving hurricane. Since Omar is expected to be a weaker Category 1 or 2 hurricane, maximum storm surge heights of 1-2 feet are expected in the Virgin Islands. The down side of having deep water close to shore is that the waves will be high, and the Virgin Islands and northern Lesser Antilles Islands can expect considerable coastal erosion and damage to coastal structures due to high battering waves.

Figure 2. Expected maximum storm tide (storm surge plus a correction for the storm hitting at high tide) for a Category 3 hurricane moving northeast at 12 mph through the Virgin Islands. This is NOT the surge expected from a particular storm, but rather the maximum computed storm tide (Maximum Envelope of Waters, or MEOW) from eleven different simulated hurricanes (with tracks shown in black with arrows). The simulations were done using NOAA's SLOSH model.

Links to follow
Puerto Rico radar
Eastern Caribbean buoy 42059
San Juan, Puerto Rico weather
St. Croix, Virgin Islands weather
Martinique radar

Tropical Depression 16 off the coast of Honduras
Tropical Depression 16, near the Honduras/Nicaragua border, remains a disorganized heavy rainmaker. Satellite estimates suggest TD 16 has already dumped up to six inches of rain on northern Honduras and Nicaragua, and more heavy rains are on the way for those countries, plus Belize, northern Guatemala, and southern portions of Mexico's Yucatan Peninsula. Satellite loops show that TD 16 is a very large but poorly organized storm, with several far-flung spiral bands that are spreading heavy thunderstorms over a wide area of the Western Caribbean and Central America. The center of circulation is broad and difficult to pin down, but appears to be over the water, within 50 miles of the coast of Honduras.

The forecast for TD 16
The system is expected to track very close to the coast of Honduras, and this proximity to land should limit intensification potential. None of the models are calling for TD 16 to become any stronger than a 40 mph tropical storm. Given the storm's current disorganization, I doubt that it will ever become a tropical storm. However, the depression is going to bring potentially dangerous amounts of rainfall capable of causing flash flooding and mudslides.

Hurricane Ike relief efforts
There continues to be an urgent need for relief supplies in the wake of Hurricane Ike. I recommend contributions to the portlight.org charity fund, formed by wunderground memebrs to serve the needs of those often bypassed by traditional relief efforts. Contributions are fully tax-deductible, and more details can be found at StormJunkie's blog.

I'll have an update this afternoon.

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.