Tropical Depression Six reorganizing

Tropical Depression Six is having trouble making up its mind where its center should be. The center it organized around is getting competition from another center of circulation about 400 miles to the southwest, as seen in the latest QuikSCAT image posted at 8:30pm EDT tonight (Figure 1). The large area of thunderstorms about 400 miles southwest of TD 6, formerly designated "Invest 98L", is still there, and is causing major difficulties for TD 6, which can't make up its mind which center to consolidate around. Until the depression resolves this conflict, you can throw all the model runs out the window--they will not be able to resolve the double circulation centers. TD 6 may well decide to organize around the old Invest 98L center to the southwest, which would pull the forecast tracks further south towards the Lesser Antilles Islands. Another wind card is the presence of the new tropical wave to the east of TD 6, near the Cape Verde Islands. This wave is close enough to TD 6 that it may be stealing some energy from the depression and inhibiting its intensification. Again, the models will not be able to deal with this interaction with another circulation nearby.

Despite this conflict, TD 6 has grown stronger since this morning--there are many more red 40 mph wind vectors visible in the evening QuikSCAT pass (Figure 1) compared to the morning pass (Figure 2). It will be interesting to see if NHC upgrades the system to Tropical Storm Florence at 11pm, or decides to wait to see which center TD 6 forms around.


Figure 1. QuikSCAT satellite winds from 5pm EDT Monday evening, September 4, 2006. Wind speed and direction are coded according to the standard station model, and are color coded (in knots) according to the color scale at the upper right (10 knots = 11.5 mph). Black winds barbs occur where there is rain, and one cannot trust the wind speeds measured in those areas. Tropical storm force winds (35 knots) are colored red, brown, or purple; one can see many red wind barbs in TD 6. Note the increased winds (orange barbs, 25-30 knots) around Invest 98L, compared to the morning QuikSCAT pass (below).


Figure 2. QuikSCAT satellite winds from 5am EDT Monday morning, September 4, 2006. One can see one red wind barb in TD 6.

Cape Verdes Islands tropical wave
The wave just west of the Cape Verdes Islands has a closed circulation, which can be seen on QuikSCAT imagery from this morning and this evening (Figures 1 and 2). The wave's winds have not increased between the two passes by the satellite, and TD 6 may be inhibiting the wave's development. The wave is under a modest 10 knots of wind shear, and has some potential for slow development over the next few days.

Next update
I'll have an update Tuesday morning.

Jeff Masters