Details on Tropical Depression 6

Discussion: A close look at Tropical Depression 6 indicates a system which is developing. Any newborn tropical system with a “tail” of convection extending into the deep tropics is likely to intensify due to the entrainment of lower latitude high water content air mass. Convection appears to be on all sides of T.D. 6, strongest in the right-front quadrant relative to forward motion. TD 6 is moving across 84F water which is certainly sufficient for development.

Right now, TD 6 is in a light shear zone. However, a 35-40 knot shear zone is northwest of the system. Additionally, an upper trough is located over Western Cuba. The upper trough and attendant shear axis must weaken for TD 6 to flourish in those longitudes.

The SST is favorable for development of this system particularly in the eastern Gulf of Mexico where support (85F) for a minimal hurricane is present. TD 6 (eventually Tropical Storm Fred) is forecast to move through the Hispaniola Mountain area…the “graveyard” for tropical cyclones tomorrow night. If Fred can survive that trek, this system should regain strength slowly moving north of Cuba Thursday.

Tropical cyclone models indicate a likely forecast track which misses to the north the high terrain of Hispaniola which, if correct allows the storm to more easily maintain an inner core (Fig. 1).

Tropical cyclone model tracks also keep this system north of Ciba crossing warm water which should easily allow re-strengthening. In the Gulf of Mexico, the upper ocean heat increases and further strengthening is likely possibly to hurricane strength as indicated by several models (Fig. 2).

Learning point: In recent years there has been reluctance by NOAA/NHC to forecast intensification in the 3-5 day forecast if the tropical cyclone was expected to move over very warm water. Additionally, short-range forecasts have been slow to recognize warm plumes of ocean water which leads to rapid intensification. The 85F water surface in the eastern Gulf of Mexico easily supports strengthening to a hurricane late this week/weekend if Fred reaches that zone.

Fig. 1: Tropical cyclone model tracks for T.D. 6.

Fig. 2: Tropical cyclone model intensity for T.D. 6.