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Tropical Depression 15 forms; new Caribbean disturance a threat to develop

By: Dr. Jeff Masters, 2:58 PM GMT on October 13, 2008

Tropical Depression Fifteen has formed this morning, and is already bringing flooding rains to the Virgin Islands and Puerto Rico. This morning's QuikSCAT pass showed a well-defined closed surface circulation, but did not scan the eastern portion of the storm, where the heaviest thunderstorms are. Eastern Caribbean buoy 42059 is in the heavy thunderstorm region to the east of 98L's center, and winds there were sustained at 20-26 mph this morning. Satellite loops show an impressive amount of heavy thunderstorm activity that is increasing in intensity and coverage. Wind shear is a high 20 knots over the disturbance, due to strong upper level winds out of the west. These strong winds are keeping 98L's heavy thunderstorms pushed over to the east side of the center of circulation. The storm is a little too far from Puerto Rico radar to see rotation of the rain. The rain area is poorly organized, with no spiral rain bands evident. However, radar from the Netherlands Antilles does show spiral bands beginning to develop on the south side of the depression. Dominican Republic radar was down this morning.

Figure 1. Current satellite image of TD 15. Image credit: NOAA.

Figure 2. Current radar-estimated rainfall from 98L.

The track forecast for TD 15
The storm is drifting slowly northwestward, and this motion is expected to continue through Tuesday. A upper-level trough of low pressure is forecast to position itself to the north of Puerto Rico by Tuesday. The counter-clockwise flow of air around this trough should draw TD 15 to the northeast across Puerto Rico or the eastern Dominican Republic on Wednesday. Up to five inches of rain has already fallen over the Virgin Islands and eastern Puerto Rico (Figure 2), and additional heavy rains of 5-10 inches are likely over these islands through tonight. Heavy rains of 5-10 inches per day will likely spread to the eastern Dominican Republic and the northern Lesser Antilles Islands by Tuesday morning, and continue through Wednesday night. 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 1-3 inches of rain from TD 15.

The intensity forecast for TD 15
Wind shear is expected to fall to the moderate 10-20 knot range over the next three days, and waters will remain warm, 29°C. This should allow 98L to intensify into a tropical storm by Tuesday. The HWRF and GFDL models both intensify TD 15 into a hurricane before it hits Puerto Rico on Wednesday. This seems overly aggressive, given the moderate 10-20 knots of wind shear expected. The SHIPS model forecast and official NHC forecast of a strong tropical storm hitting the island on Wednesday is more reasonable. A Hurricane Hunter aircraft will investigate the storm this afternoon.

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

Tropical Storm Nana, the fourteenth named storm of this busy Atlantic hurricane season, formed yesterday over the middle Atlantic Ocean. Nana is one of those "blink and you'll miss it" storms, as high wind shear of 30 knots is in the process of tearing the storm apart. The UKMET model did a nice job forecasting the development of this storm, up to a week in advance.

New disturbance in the Southwest Caribbean (99L)
An area of disturbed weather (99L) has formed in the Southwest Caribbean, off the coast of Nicaragua. The region is under a low to moderate amount of wind shear, 5-15 knots. The NOGAPS model forecasts the development of a tropical depression in this region by Thursday. The model predicts the storm will move northwest and threaten Honduras, the Cayman Islands, the Yucatan Peninsula, and western Cuba by early next week. I put the odds a tropical depression forming in this region by Wednesday in the moderate range (20-50%).

Saturday's update on Hurricane Ike relief efforts
I got this very nice email Friday, giving appreciation for all those who helped out through the portlight.org Hurricane Ike charity effort:

I just wanted to express my sincere gratitude, on behalf of the City of Houston, the Mayor, and our community partners over at TIRR/Memorial Hermann, for all of your involvement in bringing medical supplies and equipment together to help Texans with disabilities affected by Hurricane Ike. I can't tell you how much we all appreciate the fact that you so quickly mobilized and leveraged such a tremendous amount of support to bring these needed items to Houston and other Texas cities.

An email simply doesn't do justice to the generous spirit and initiative that you, Paul Timmons, and your partners took to make this happen, nor to our gratitude. However, I just want you to know that we think about you all in appreciation every single day over here, and there are many who are directly benefiting from your generosity.

By the way, Paul Timmons mentioned that it looks like another shipment of 30-50 wheelchairs can be sent over here. All I can say is wow! Thank you for not forgetting about us, and for realizing that we still have a lot of needs here that we are trying to meet - even 4 weeks after the hurricane.

Again, please accept my sincere thanks. I hope that you have a wonderful weekend!

Michelle Colvard, MPH
Executive Director
Mayor's Office for People with Disabilities

Contributions to this highly worthy 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 this afternoon, between 3pm-4:30pm EDT.
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.