Dangerous disturbance 92L poised to become a tropical depression

The ominous music is rising once more, and The Joker (also known as disturbance 92L) appears poised to develop into a tropical depression today. This storm has the potential to become a hurricane that will affect the Bahama Islands and Florida early next week.

The storm fell apart yesterday, thanks to an infusion of dry air from the large region of Saharan air that continues to surround it. However, as each day goes by, 92L is moistening its environment to insulate itself from the destructive influences of this dry air. The storm is evaporating large amounts of water vapor from the warm 28-29°C waters below, then condensing this vapor inside intermittent bursts of heavy thunderstorm activity. Water vapor satellite loops show that the moistened area has steadily expanded, and is now large enough to insulate 92L from the surrounding dry air--as long as there is little wind shear. A strong jet of wind from the side could carry dry air into the core of 92L and disrupt it. However, wind shear is less than 10 knots, and this does not appear likely to happen. Visible satellite loops show some rotation at low levels, and a steadily increasing area of heavy thunderstorms. Upper-level outflow is beginning to appear to the north side. This morning's 6:20 am EDT QuikSCAT pass showed a pronounced wind shift associated with 92L, but no closed surface circulation. Martinique radar shows an impressive area of heavy rain, with a spiral band developing on the east side.


Figure 1. Latest satellite image of 92L.

The forecast for 92L
It currently appears that dry air will not be able to stop 92L from developing into a tropical depression later today or Friday. Wind shear is also not likely to be a problem. Wind shear is forecast to remain below 10 knots the next two days. There is the possibility that by Sunday, an upper-level anticyclone will set up on top of 92L. This would allow the air lifted from the surface by the storm's heavy thunderstorms to be efficiently vented out to the sides, ventilating it, promoting even more intense thunderstorm activity. The main bug-a-boo for 92L will be a possible encounter with the high mountains of Hispaniola and eastern Cuba. After passing Puerto Rico on Friday, 92L is expected to move along the north coast of the Dominican Republic on Saturday. The high mountains of the island may disrupt or even destroy the storm. However, most of the computer models are predicting that The Joker will survive this encounter, be turned northwestward by an approaching trough of low pressure on Sunday, and move into the Bahama Islands. Once over the deep, warm waters of the Bahamas, 92L could easily intensify into a hurricane. This is the solution of the latest (2 am EDT) runs of the GFDL, HWRF, and SHIPS models. By Tuesday, the storm could be very near the east coast of Florida. The long-range fate of The Joker is difficult to guess. The possibilities range from the ECMWF forecast of a turn to the north with a threat to North Carolina, to the Canadian model's prediction of passage along the length of Cuba, followed by emergence into the Gulf of Mexico.

The National Hurricane Center is giving 92L a high (>50% chance) of becoming a tropical depression by Saturday morning. Heavy thunderstorms from 92L have already spread over the northern Lesser Antilles Islands, and the bulk of the storm will hit Puerto Rico on Friday. The storm will probably will not have time to grow beyond a 60 mph tropical storm by Saturday, when it could bring heavy rains of 4-8 inches to the northern Dominican Republic. These rains should spread to northern Haiti by Sunday, where heavy rain is always a threat to cause significant loss of life. The Hurricane Hunters are scheduled to investigate 92L this afternoon, and I'll have a new blog once they've had time to collect a few hours of data.

Links to follow
Martinique radar

Wundermap for the northern Lesser Antilles Islands

St. Thomas, Virgin Islands weather

Elsewhere in the tropics
A tropical wave (93L) about 1200 miles east of the Lesser Antilles Islands is disorganized, and should not develop during the next 1-3 days. Several of the reliable computer models forecast development of a new tropical wave coming off the coast of Africa about 2-4 days from now.

I'll have an update this afternoon.

Jeff Masters