Jeff Masters update Just in91L could become a tropical depression today
A strong tropical wave near 12.5N 44W, 1100 miles east of the Lesser Antilles Islands, is headed west-northwest at 10 mph. This system (91L)
has the potential to develop into a tropical depression today, and has the potential to become a large and dangerous major hurricane next week. This morning's QuikSCAT pass captured only the western side of the circulation, but did have some believable winds of 30-35 knots (35-40 mph). Last night's QuikSCAT pass showed an elliptical but better-defined circulation. The circulation looked more circular and better-defined in a pass from the European ASCAT satellite a few hours after that. A circular motion of winds around the center is much more conducive for strengthening than an elliptical circulation. Satellite loops show considerable improvement in organization has occurred since yesterday--a large area of thunderstorms that reach high in the atmosphere (as evidenced by very cold cloud tops on infrared satellite imagery) has formed near the circulation center. An upper-level outflow channel has opened to the south, and some low-level spiral banding is now apparent, particularly to the north. These spiral bands are also apparent on the latest microwave satellite imagery (Figure 1). Wind shear has dropped below 10 knots, and with warm 28 C waters underneath, 91L should become a tropical depression later today.
Shear is forecast to remain below 10 knots for the next 3-4 days, which should allow 91L the opportunity to grow to a Category 1 hurricane or stronger before it reaches the Lesser Antilles Islands. However, the SHIPS, GFDL, and HWRF intensity models all keep 91L below hurricane strength for the next five days. I think this is unrealistic, given the favorable conditions for strengthening present. The big question concerns the track--there is a good chance that 91L will miss the Lesser Antilles, as the current steering currents favor a more northwesterly track for the storm over the coming days. The system has already moved north of Barbados' latitude, and the southern Lesser Antilles Islands are unlikely to receive a direct hit from 91L. Most of the models indicate a west-northwest motion near 10-15 mph, which would bring 91L to the northern islands Monday. The U.S. East Coast may be at risk from this storm ten or so days from now, but it is far too early to speculate on the chance of this occurring, or what region might be most at risk.
The NOAA Hurricane Hunters may fly a research mission into 91L Thursday evening.
I'll have an update later today if either system develops into a tropical depression.
Jeff Masters
