Debby mid-day Sunday (4 Aug) update: two questions.

NHC’s 11am forecast update – Key Messages regarding Tropical Storm Debby (en Español: Mensajes Claves) – raises two questions. NHC has increased their landfall intensity forecast a bit to 80 knots, not much below Category Two strength (starts at 83 knots or 96mph). They are on the high side of the guidance and seem to be a bit nervous about the potential for the storm to gain more strength than originally estimated. There is certainly plenty of energy in the Gulf, the question remains if the storm stays far enough from the Florida peninsula (avoiding land interactions) and develops a well defined core. It seems to be trying to do that, as seen on satellite and radar …

Another subtle but potentially significant change is in the trajectory over Georgia, now staying just inland rather than the center moving out over water. Here is the 11am track and watch/warning areas:

The damage swath doesn’t look too different, although there are potentially important nuances:

The amount of rain and how much you get from a tropical cyclone depend on the time over land, how much of the circulation remains over water, and where you are. All of these are complex interactions. The latest estimate on the NHC site hasn’t changed from this morning (a 20″ swath from Ft. Steward to just south of Charleston) but on this track some of the models suggest that rain total might be somewhat less by about 25% (15″ in Savannah vs 20″).

So in summary are two significant questions raised by the latest forecast, questions that have been with us for a couple of days but unfortunately still are not clearly answered. The first, as noted above, is landfall intensity. That will matter for the immediate damage on the Florida coastline and for a 100 or so miles inland as the winds decay back to tropical storm strength. But the bigger question (especially for those who live in Georgia and South Carolina like Enki) is how far inland and how organized the core remains, and how much it slows down.

Unfortunately, at this point, we will have to wait and see, and hope for some clarity by tomorrow morning. Meanwhile, despite the uncertainty, with respect to impacts and timing not much to add to this morning’s analysis.

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