Hurrincane Hunters find 92L is at the threshold of becoming a tropical depression

The Hurricane Hunters are inside the tropical disturbance 92L over the northern Lesser Antilles Islands, and are reporting winds of tropical depression strength (30-35 mph). However, the two aircraft in the storm have had difficulty finding a closed circulation at the surface, and the National Hurricane Center has decided not to upgrade 92L to Tropical Depression Six as of 5 pm EDT today:

Special tropical disturbance statement
5 PM EDT Thu Aug 14 2008

Data from NOAA and Air Force Reserve hurricane hunter aircraft...along with surface observations and satellite imagery...indicate that the low pressure area located over the Virgin Islands has not developed into a tropical depression. However...upper-level winds are becoming more favorable for development...and a tropical depression could form at any time during the next day or two as the system moves west-northwestward about 15 mph.

Water vapor satellite loops show that 92L has generated enough heavy thunderstorm activity to insulate it from the surrounding region of dry Saharan air. Water temperatures are a warm 28.5°C and wind shear is less than 10 knots. Visible satellite loops show a steadily increasing area of heavy thunderstorms. Upper-level outflow has appeared on the north and east sides. Martinique radar shows an impressive area of heavy rain, with low-level spiral bands developing on the east side.


Figure 1. Latest satellite image of 92L.

The forecast for 92L
Dry air will probably not be a problem for 92L any more. Wind shear is also not likely to be a problem--shear is forecast to remain below 10 knots the next five days, and may drop to near zero on Friday. Furthermore, an upper-level anticyclone is setting up on top of 92L. This will allow the air lifted from the surface by the storm's heavy thunderstorms to be efficiently spewed out to the sides, ventilating the storm and promoting even more intense thunderstorm activity.

I give 92L a high (>80% chance) of becoming a tropical storm by Friday afternoon. The latest (8 am EDT) model runs all foresee a track for 92L very close to Puerto Rico and the north coast of the Dominican Republic and Haiti Friday through Sunday. Heavy rains will be the main threat to these places, with 4-8 inches likely. Isolated higher amounts of up to 12 inches may fall in the mountains, triggering life-threatening flash floods and mudslides.

In the longer term, an encounter with the rugged terrain of eastern Cuba is forecast by most of the models for Sunday. Passage over the rough terrain of Hispaniola and Cuba could severely disrupt or even destroy 92L. If the storm survives, it could intensify quickly into a hurricane, possibly a major hurricane, once it emerges over the warm waters of the Bahamas or Florida Straits. The eventual strength is highly dependent on the track of 92L, with a longer track over water giving it a greater chance of becoming a hurricane. The long range track of 92L is highly uncertain. Take your pick of 8 am EDT model runs:

GFDL, HWRF: parallel to the east coast of Florida, 50-100 miles offshore
UKMET: Through South Florida into the Gulf of Mexico
NOGAPS: Through the Florida Keys, then northwest towards the Alabama/Mississippi coast
GFS: Through the Bahamas, then north towards North Carolina
Canadian: Across Cuba and through the Cayman Islands, then north in the Gulf of Mexico

Links to follow
Martinique radar

Wundermap for the northern Lesser Antilles Islands

St. Thomas, Virgin Islands weather

St. Martin webcam

Elsewhere in the tropics
A tropical wave (93L) about 1000 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 when NHC designates 92L a tropical depression or tropical storm.

Jeff Masters