posted by Dr. Jeff Masters, today 19 Sept:
Heavy thunderstorm activity is increasing over the Lesser Antilles Islands today in association with tropical disturbance 93L. Visible satellite loops indicate that a closed circulation may be developing at middle levels of the atmosphere, near 14N 64W. Additional slow development appears likely, and I expect 93L's mid-level circulation will work its way down to the surface by Sunday. Slowing down this process will be the presence 15-20 knots of wind shear, which is marginal for development. Wind shear is forecast by the GFS model to remain 15-20 knots for the next five days, but other models are forecasting wind shear in the moderate range, 10-15 knots. The NHC is now giving 93L a medium (20%-50% chance) of developing into a tropical depression by Sunday, and I put these odds a bit higher, in the 30%-60% range. None of the reliable computer models develop 93L, and it is possible that given its current rather fragile state, high wind shear will prevent development. I give a 60% chance that 93L will eventually develop into a tropical depression.
Most of the heavy rains from 93L are well to the east of the center, and expect heavy rains of 3-6" to affect Puerto Rico Saturday through Sunday. By Sunday, heavy rain will spread to the Dominican Republic and Haiti, potentially causing life-threatening flash floods and mudslides. Steering patterns will very likely prevent 93L from recurving out to sea, and I expect the storm will affect Jamaica, Cuba, and possibly the southeastern Bahamas beginning on Monday night. It is uncertain at this time whether 93L will retain its current west-northwest motion and cross into the Bahamas (as forecast by the HWRF model), or be forced more to the west on Sunday, and remain in the Caribbean (as forecast by the Canadian model).