Puerto Rico disturbance near tropical depression status

Tropical disturbance 93L continues to slowly organize, and will probably be a tropical depression by tonight or Monday. Visible satellite loops show that heavy thunderstorm activity continues to increase, and upper-level outflow is now visible on the north side of 93L. There is as yet no evidence of a closed surface circulation. This morning's QuikSCAT pass showed a sharp wind shift, but no closed circulation. Top winds observed by QuikSCAT were in the 30-35 mph range. Puerto Rico radar shows a large area of heavy rain advancing on the island, but there is no evidence of rotation or spiral bands beginning to form yet. Wind shear has fallen to the moderate level, about 10-15 knots.


Figure 1. Current satellite image of 93L.

Expect heavy rains of 4-8" to affect Puerto Rico and the Virgin Islands Sunday through Monday. Heavy rains will also spread over eastern portions of the Dominican Republic Sunday night into Monday, potentially causing life-threatening flash floods and mudslides in mountainous regions. Since most of 93L's heavy thunderstorm activity is on its east side, it currently appears that Haiti and the Bahamas will escape dangerous heavy rains from this storm.

The intensity forecast
Wind shear is forecast to remain 10-20 knots over the next five days, and four of the six reliable forecast models predict that 93L will develop into a tropical depression by Tuesday. The GFDL and HWRF models predict 93L will strengthen into a hurricane by Friday. However, there will be high wind shear very close to 93L for the next five days, and the storm may struggle at times with this high shear. Water temperatures are a warm 29.5°C and ocean heat content will be moderate to high over the next five days. The NHC is giving 93L a high (>50% chance) of developing into a tropical depression by Tuesday. A Hurricane Hunter aircraft will investigate 93L this afternoon.

The track forecast
The models agree on a general north-northwesterly motion for 93L over the next 3-4 days, which would bring the storm just west of Bermuda. A major complicating factor in the long-range track forecast is the expected development of an extratropical Nor'easter storm off the coast of South Carolina on Thursday. The Nor'easter could bring hostile wind shear over 93L, weakening it into a subtropical storm. The Nor'easter might then recurve out to sea, drawing 93L behind it. This is the solution of the latest 06Z (2 am EDT) GFS model run. Alternatively, the two storms may rotate cyclonically around a common center (the Fujiwhara effect), sending the Nor'easter west-southwestward into the Southeast U.S., and 93L northwestwards towards North Carolina. This is the solution of the 00Z (8 pm EDT) GFDL model. The NOGAPS model predicts that the Nor'easter will not develop at all, and instead 93L will absorb the energy that would have gone into creating the Nor'easter. This would convert 93L into a hybrid subtropical storm that would affect the coast of North and South Carolina late this week with sustained winds in the 50-60 mph range. I don't have a good feel for what will happen in this complicated situation, but it currently appears that coastal North and South Carolina can expect tropical storm force winds from either an extratropical or tropical storm beginning on Friday. It is possible that 93L may impact the mid-Atlantic or New England regions early next week.

Links to follow
Puerto Rico radar
San Juan, Puerto Rico weather

Elsewhere in the tropics
A tropical wave with some limited heavy thunderstorm activity is a few hundred miles south of the Cape Verde Islands off the coast of Africa. The GFS and NOGAPS models continue to predict that this system will develop into a tropical depression by the middle of the week.

Hurricane Ike relief efforts
A group of wunderground bloggers have done some amazing work to gather, purchase, and deliver relief supplies to victims of Hurricane Ike. So far, the group (spearheaded by Presslord, StormJunkie, and Patrap) have raised over $12,000 and sent three truckloads of supplies, with another truck on the way, plus several air freight shipments. For photos of the effort, plus links to donate to the cause, visit stormjunkie's blog.


Figure 2. Paul Timmons (AKA presslord), with one of the trucks full of Hurricane Ike relief supplies. Image credit: StormJunkie.

I'll have an update later today.

Jeff Masters