Tropical Depression Four whips the Cape Verde Islands

Tropical Depression Four is making what is likely to be its closest pass to any land areas during its life--a brush of the Cape Verde Islands off the coast of Africa. The storm should get no closer than about 100 miles from the islands today, and its outer rain bands could bring 2-4 inches of rain and winds gusts to 50 mph to these islands today. The only station that reports hourly observations in these islands is Sal, which has seen the winds increase to a steady 17 mph so far this morning. Winds measured from the QuikSCAT satellite in a 4am EDT pass this morning were about 35 mph in the islands.

TD Four is a favorable environment for intensification, and should be Tropical Storm Debby later today. Wind shear is a low 5-10 knots. There is a large area of dry air and Saharan dust to the wave's north, but this is far enough away that it shouldn't inhibit intensification over the next day or two. The GFDL intensifies the storm up to a strong Category 1 hurricane by Saturday, and the official NHC forecast is close behind.

Unless you live in the Cape Verde Islands, this storm is unlikely to affect you. As we can see from the plot of historical storm tracks of August tropical depressions that formed near the location of the current storm (Figure 1), none of these storms that have started out as far north as this storm have affected any land areas other than the Cape Verde Islands. The GFS and GFDL models have the storm recurving northwards well east of Bermuda early next week, as a trough of low pressure picks up the storm. However, it is too early to have high confidence in this forecast. The UKMET, NOGAPS, and Canadian models all predict that this trough of low pressure will miss TD 4, which will continue on a more westerly path and possibly threaten Bermuda. The models have not done a great job with the track of this storm, which was 70 miles further south than the models predicted it would be last night.


Figure 1. Historical tracks of August tropical depressions that tracked near the current position of TD 4.

Elsewhere in the tropical Atlantic
An area of thunderstorms in the western Caribbean west of Jamaica is drifting northwestwards towards the Yucatan Channel and Gulf of Mexico. Wind shear is a high 20 knots over this area, and upper level winds are not favorable for development. The shear is expected to drop over the Gulf by Thursday, so we'll have to keep an eye on this system later this week. None of the computer models develop anything in the Gulf or Caribbean this week, though. The long-range GFS model is no longer predicting any new tropical storms forming anywhere in the Atlantic in the next 10 days.

I'll have an update on this system when it becomes Debby.
Jeff Masters