WunderBlog Archive » Category 6™

Category 6 has moved! See the latest from Dr. Jeff Masters and Bob Henson here.

Omar near hurricane strength; TD 16 slowly strengthening

By: Dr. Jeff Masters, 7:59 PM GMT on October 14, 2008

Tropical Storm Omar is steadily strengthening as it continues to lash northern Colombia, northern Venezuela, Aruba, Bonaire, and Curacao with heavy rains. The latest Hurricane Hunter center fix at 3:06 pm EDT found the pressure had dropped significantly, to 986 mb. The crew reported that heavy thunderstorms were beginning to wrap around the center, which had taken on a better-defined circular shape. Omar is beginning to form an eyewall, and could be a hurricane by morning. Indeed, surface winds reported by the Hurricane Hunters were 70 mph--very close to hurricane strength. Radar from the Netherlands Antilles shows that the rain has organized into spiral bands, but heavy thunderstorm activity is missing on the northwest side of the storm. Satellite loops show the amount of heavy thunderstorm activity continues to increase and organize. Wind shear has fallen to a moderate 10 knots, allowing this increased organization to occur.

Figure 1. Current satellite image of Omar.

The forecast for Omar
The storm is expected to turn northeastward tonight as an upper-level trough of low pressure digs far enough south to pull the storm towards the Virgin Islands and northern Lesser Antilles Islands. Wind shear is expected to remain in the moderate 10-15 knot range over the next three days, and waters will remain warm, 29°C. This should allow Omar to intensify into a hurricane before it moves through the Virgin Islands and northernmost Lesser Antilles Wednesday night. I expect Omar will remain near its current intensity until it completes building an eyewall, which it should happen between 12 am and 6 am EDT Wednesday. At that time, the potential for rapid intensification into a Category 3 hurricane exists. The latest SHIPS model is forecasting that Omar will rapidly intensify over the next 24 hours, arriving in the Virgin Islands Wednesday evening as a borderline Category 2 or 3 hurricane with 110-115 mph winds. The GFDL and HWRF are much less aggressive. They forecast higher shear affecting Omar, and 70-75 mph winds at landfall. Given Omar's current appearance and rather low pressure, landfall Wednesday evening as a Category 2 hurricane looks like a reasonable intensity forecast. I give Omar a 30% chance of being a major Category 3 hurricane at landfall.

Omar's storm surge
If Omar does become a hurricane, wind damage is likely to be the greatest threat from the storm. 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. 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 structure 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.

Heavy rains from an outer rain band are just south of Puerto Rico and the Virgin Islands, and will move into the islands over the next few hours. This rain should only last a few hours, and the bulk of Omar's rain will not arrive until Wednesday afternoon. Omar should bring heavy rains of 3-6 inches to the easternmost tip of the Dominican Republic on Wednesday, but the rest of Hispaniola should escape heavy rains. All of the Leeward Islands are at risk of total rainfall of 5-10 inches from Omar. Martinique radar shows heavy rains affecting all of the islands north of Martinique.

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, continues to slowly organize. The region is under low wind shear, 5-10 knots. Satellite loops show a modest amount of heavy thunderstorms spread out over a large portion of the Western Caribbean. Heavy rains are affecting extreme northeast Nicaragua and northeastern Honduras, and will be pushing into Belize and Mexico's Yucatan Peninsula this evening. So far, rainfall rates in the heaviest thunderstorms over land have been near 1/2 inch per hour (Figure 3). These rains will steadily increase in coverage and intensity as TD 16 continues to organize. This afternoon's Hurricane Hunter mission found 30-35 mph winds and a central pressure of 1004 mb.

The forecast for TD 16
Conditions are favorable for TD 16 to intensify into a tropical storm. The system is expected to track very coast to the coast of Honduras, and this proximity to land should limit intensification potential. A west-southwesterly motion into the northern coast of Honduras is not expected, since the latest set of model runs keeps TD 16 just offshore. However, it would not take much of a deviation in track for TD 16 to make landfall in northern Honduras. Wind shear is forecast to remain low, 5-10 knots, for the rest of the week. As long as the center remains over water within 50 miles of land, slow intensification should occur, and the GFDL and HWRF models are expecting TD 16 to make landfall Thursday morning in Belize as a 50-55 mph tropical storm. If TD 16 manages to move farther from land, more significant intensification could occur, but that doesn't appear likely at this time.

Links to follow
Puerto Lempira, Honduras weather

Figure 3. Estimated rainfall rates from TD 16 at 9:43 am EDT Tuesday, October 13, 2008. Image credit: Navy Research Lab, Monterey.

Nana and the child of Nana
Tropical Depression Nana has been torn apart by wind shear of 30-40 knots. Regeneration is unlikely due to the continued high wind shear expected along its path. The small area of disturbed weather south-southeast of Nana (90L), has also been disrupted by high wind shear, although latest satelite loops show a small amount of heavy thunderstorm activity still clinging to the east side. The shear should prevent either system from redeveloping.

Hurricane Ike relief efforts: Monday 10/13/08 update
From StormJunkie's blog today: Portlight.org is currently preparing the next push of supplies to head to the forgotten populations and disabilities community along the Texas Gulf Coast. Due to the exceptional pricing that U-haul has given us on trucks we have found the cheapest way to transport these goods is to continue to utilize our U-haul deal to transport these goods. We will be delivering many items to these outlying communities early to mid next week. Look for updates on this trip as it happens!

Some of the supplies that are being delivered include 50 manual wheel chairs that were specifically requested by the Houston Mayor of Disabilities. We have also received items from Coleman and Dick's Sporting goods which will be on the truck. A pallet of tents has also been donated by an anonymous person. Some quantity of socks has also been donated. As we receive and secure more items we will continue to update. In the mean time, if any have connections that may be able to help us acquire some of the following items; your timely help is greatly appreciated.

Figure 4. Chief Dickie Uzzle of the Bridge City Fire Department opening supplies sent by Portlight. Supplies were not only needed but also greatly appreciated.

Contributions to the portlight.org charity fund are fully tax-deductible, and will go to provide relief supplies for those smaller communities typically bypassed by the traditional relief efforts. More details can be found at StormJunkie's blog.

I'll have an update Wednesday 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.