Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 2:06 PM GMT on July 29, 2006
The tropical wave near 9N, 46W, about 1000 miles east of the southernmost Lesser Antilles Islands, continues to show signs that it may organize into a tropical depression. While we don't have a good recent QuikSCAT pass to look at, this morning's visible satellite imagery shows a pronounced surface circulation. However, the amount of heavy thunderstorm activity has greatly decreased since yesterday, due to the ingestion of dry Saharan air from the north (Figure 1). Dry air will continue to be a problem for the wave the farther north it goes. The forecast tracks from the latest computer models (Figure 2) show a mostly westward track, but the wave is expected to slowly gain latitude, putting it more into the influence of this very dry air from Africa.
Figure 1. Saharan air layer (SAL) analysis from 8am EDT Sat Jul 29. Dry, dust laden air from the Sahara desert is coded orange, and lies just north of the tropical wave we're tracking. Image credit: University of Wisconsin CIMSS and NOAA's Hurricane Research Division.
In the wave's favor, it is under only 5-10 knots of wind shear, and this shear is forecast to remain weak over the next two days. As the wave approaches the Lesser Antilles Islands on Monday, shear is forecast to increase again. Sea surface temperatures are favorable--28 C, about 2 degrees C above the threshold of 26 C needed for tropical storm formation.
The long term outlook for the system is not favorable, due to the large amount of dry air to its north and the increasing wind shear likely to affect the system once it nears the islands. In addition, the wave is expected to cross into the southeast Caribbean. This region is climatologically unfavorable for tropical cyclones, because the land mass of South America cuts off the inflow of moist air from the south, replacing it with drier continental air.
None of the computer models develop this wave into a tropical storm.
Figure 2. Preliminary model tracks for the tropical wave approaching the Lesser Antilles Islands.
Tropical wave south of Puerto Rico
An area of heavy thunderstorms associated with a westward-moving tropical wave has increased in coverage just south of Puerto Rico and the Dominican Republic this morning. Wind shear over this disturbance is marginal for development, 10-20 knots, so some slow development is possible over the next few days. This disturbance probably has more potential than the other wave, due to the presence of much moister air in the Caribbean.
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