Debby in the making

A very strong tropical wave moved off the coast of Africa Sunday. The NHC and Navy have a system where they officially designate a disturbance as an "invest"--something worth investigating. This storm is called 96L, and if there were hurricane hunters within range, they would fly the storm, as the system already appears to be a tropical depression. However, NHC typically hesitates to label these systems fresh off the coast of Africa depressions until they hold together for at least a day. Many such systems fall apart within their first day over water. The waters under the wave are 27-28 C, which is .5-1.5 degrees C above the 26.5 C threshold for tropical cyclone formation--not great, but good enough. Wind shear is a low 5-10 knots. There is a large area of dry air and Saharan dust to the wave's north, and this will likely be a major inhibiting factor for this wave once it moves north of 15 N latitude.


Figure 1. Visible satellite image at 8am EDT Mon Aug 21 from the European satellite. Image credit: Navy Research Lab.

The wave is impressive on satellite imagery this morning--low level spiral banding has formed, and there is a clear low-level rotation of the clouds. There is also some upper-level outflow developing on the east side. We don't have a recent QuikSCAT pass to judge the surface winds. The GFS, NOGAPS, and GFDL models develop the wave into a tropical storm; the UKMET does not. I expect that the model consensus is correct--this system will be Tropical Storm Debby by Wednesday. All the models predict that the storm will pass well north of the Lesser Antilles Islands. The storm may bring tropical storm conditions to the Cape Verde Islands Tuesday and Wednesday, as it moves west or west-northwest at 15 mph just south of the islands. Although it is too early to be certain, the Cape Verde Islands will probably be the only land areas the storm will affect. The preliminary model runs point to a track that will eventually recurve the storm out to sea before affecting any other land.

Sea surface temperatures cool to below the 26.5 C threshold tropical systems prefer north of the Cape Verdes Islands, so once the system moves north of about 15 N latutide, it may weaken. There is also a lot of dry air and Saharan dust to the north that will cause trouble for it. The GFDL model does intensify the storm to a Category 1 hurricane, then weakens it as the dry air and cooler SSTs take their toll.


Figure 2. Preliminary model tracks for the Atlantic disturbance 96L.

Elsewhere in the tropical Atlantic, there are no threat areas to discuss, and the computer models are not forecasting any development through Wednesday.

Jeff Masters