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Little change to TD 15; new Caribbean distubance gathering strength

By: Dr. Jeff Masters, 9:10 PM GMT on October 13, 2008

Tropical Depression Fifteen continues to move little as it spins over the Eastern Caribbean, a few hundred miles south of the eastern tip of the Dominican Republic. An Air Force Hurricane Hunter aircraft has just arrived, and found a 1001 mb central pressure at 5:47 pm EDT, and top surface winds of 35-40 mph. Eastern Caribbean buoy 42059 is in the heavy thunderstorm region to the east of TD 15's center, and recorded sustained winds of 20-25 mph this afternoon. Satellite loops show the amount of heavy thunderstorm activity has decreased some this afternoon. However, the heaviest thunderstorms now lie very close to the center of circulation, a sign the storm is getting more organized. Wind shear has fallen to a moderate 10-15 knots over TD 15, allowing this increased organization to occur. Radar from Puerto Rico and the Netherlands Antilles shows that the rain area is poorly organized, with no spiral rain bands evident. Dominican Republic radar was down this afternoon.

Figure 1. Current satellite image of TD 15.

The track forecast for TD 15
The storm is expected to move little through Tuesday afternoon. An upper-level trough of low pressure is forecast to position itself to the north of Puerto Rico by Tuesday afternoon, and the counter-clockwise flow of air around this trough should draw TD 15 to the northeast across Puerto Rico, the Virgin Islands, or the eastern Dominican Republic on Wednesday or Thursday. There are considerable timing variations between the models. The fastest model is the GFDL, which predicts a Wednesday morning landfall in Puerto Rico. The slower NOGAPS and UKMET models predict a Thursday morning landfall. If a Wednesday landfall occurs, it would probably be as a tropical storm with 50-60 mph winds. A landfall delayed until Thursday would allow a stronger storm to develop, and TD 15 could be a Category 1 hurricane in this case. Up to five inches of rain has already fallen over the Virgin Islands and eastern Puerto Rico (Figure 2), but the heaviest rains have retreated to the south for the time being. Heavy rains are likely to develop over these islands again on Tuesday, and move into the eastern Dominican Republic and the northern Lesser Antilles Islands. Additional rain amounts of 5-10 inches are likely over some of these regions. Over Puerto Rico, isolated rain amounts in excess of 20 inches are possible before the storm clears the islands by Thursday. It currently appears that Haiti will only get 1-2 inches of rain from TD 15.

Figure 2. Current radar-estimated rainfall from TD 15.

The intensity forecast for TD 15
Wind shear is expected to remain in the moderate 10-20 knot range over the next three days, and waters will remain warm, 29°C. This should allow TD 15 to intensify into a tropical storm by Tuesday. The HWRF and GFDL models both intensify TD 15 into a Category 1 hurricane before it hits Puerto Rico on Wednesday. This seems overly aggressive, given the moderate 10-20 knots of wind shear expected. I'm expecting Thursday will be the earliest that TD 15 will become a hurricane.

Links to follow
Puerto Rico radar
Eastern Caribbean buoy 42059
San Juan, Puerto Rico weather

Disturbance 99L off the coast of Nicaragua
An area of disturbed weather (99L) has formed in the Southwest Caribbean, off the coast of Nicaragua. An ASCAT pass from 11:28 am EDT showed a circulation center developing near 14N 83W, about 100 miles offshore from Puerto Cabeza, Nicaragua. The pressure there was 1005 mb and falling at 3 pm EDT today. The region is under low wind shear, 5-10 knots. Satellite loops show a moderate area of heavy thunderstorms that is increasing in areal coverage and intensity. Heavy rain has moved into extreme northeast Nicaragua and northeastern Honduras.

The forecast for 99L
Most of the models forecast development of a tropical depression in this region by Wednesday. The system is expected to track northwest, just off the coast of Central America, until Tuesday afternoon. On Tuesday, a ridge of high pressure is expected to build in, forcing 99L to the west. It currently appears that the center of 99L will remain over water, which should allow the storm to intensify into a tropical storm by Wednesday. Wind shear is forecast to remain low, 5-10 knots, for the rest of the week. As long as the center remains over water more than 50 miles from land and does not stall out, intensification should occur. The system will likely bring 5-10 inches of rain to northern Honduras tonight through Thursday, potentially causing flash flooding and destructive mudslides. Heavy rains may also spread over the Cayman Islands on Tuesday afternoon. It is 40% likely that the counterclockwise flow of air around the storm will grow large and strong enough to tap the Pacific Ocean as a source of moisture, triggering very heavy rains of 10-15 inches along the Pacific coasts of northern Costa Rica and Nicaragua Tuesday through Thursday. Rains of this magnitude are capable of causing severe flooding and life-threatening mudslides. By Thursday, 99L could be as far west as Belize (as forecast by the GFDL model), or wandering erratically in the Western Caribbean (as forecast by the NOGAPS model). Heavy rains will likely begin affecting Belize, northern Guatemala, and Mexico's Yucatan Peninsula beginning on Wednesday. If the storm is large enough, if will also bring heavy rains to the Pacific coasts of Honduras and El Salvador beginning on Wednesday. No models are currently forecasting a threat to the U.S. in the coming seven days from 99L.

Figure 3. Current satellite image of 99L.

Nana and the child of Nana
Tropical Depression Nana, over the middle Atlantic Ocean, is being torn apart by wind shear of 30-40 knots. A small vortex near 13N, 42W (90L) that was part of the original disturbance that developed into Nana, has developed its own cluster of heavy thunderstorms. The circulation of this "child of Nana" is apparent on this morning's ASCAT pass, which noted top winds of 25 mph on the south side. The child of Nana is south of the region of high wind shear affecting Nana, in a region where the shear is only 5-10 knots. Nana should be able to pull its child northwestward into the higher shear region on Tuesday, but this may be enough time for the child of Nana to develop into a tropical depression.

Figure 4. Current satellite image of 90L, the Child of Nana.

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 5. These are the supplies that were shipped.

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.

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.