Last night the “Potential Tropical Cyclone” lived up to its potential and became organized enough for the National Hurricane Center to upgrade it to a tropical depression. For the latest, as always your best source for the official word are the Key Messages regarding Tropical Depression Four (en Español: Mensajes Claves). Here is what the storm looks like as of 6am, along with the current (as of 6am) watch and warning areas. Note two things: first, areas of Florida are now under a hurricane watch, and second, expect watches to start appearing on the east coast of Florida and Georgia later today in anticipation of the storm exiting the coast and returning to the sea. However … read on for the full story, it’s complicated.
First lets look at the potential impacts on the official forecast track. NHC expects the system to become a tropical storm tonight as it exits Cuba, so it will get the name “Debby,” and should pass just west of the Florida Keys, off shore from Tampa, and make landfall in the Big Bend area as a strong tropical storm:
On this track, The Big Bend of Florida (aka South Georgia) will see a strong tropical storm, maybe minimal hurricane. In Florida that’s hazardous, take the advice of your local emergency management and take appropriate precautions to prepare for a hurricane. The Keys and west coast (Tampa/St. Pete north) will get side-swiped, probably just gusty winds and rain from the fringes, although as you know the embedded thunderstorms in these “feeder bands” can be strong. Minor wobbles left and right will mean the difference between a hit and miss by these.
As noted yesterday, intensity at Florida landfall depends on how far out into the Gulf the storm arcs. The farther out it goes, the more time (and energy from the fetid waters of the Gulf) is available for the storm to intensify. Right now NHC has the storm at just below hurricane strength at landfall. At the moment, all of the models show landfall in the Big Bend, but what happens after that they differ. So let’s take a look at the problem facing the forecasters in Miami. Here are the latest primary track models:
Notice how many of these scenarios hit Georgia and, like many who drive the roads of our beloved state, wander aimlessly and confused about the countryside doing sudden stops and U turns with little warning (such as the European model and the new HFSA and HFSB models)? Another big factor in the evolution of TD4/”Debby” is if it stays within the influence of trough currently over the US Southeast and continues northeast, as the GFS model indicates, or the trough moves on and the storm gets bogged down over the swamps of the Okefenokee and meanders. That will make a huge difference on impacts here in Georgia and the South Carolina Lowcountry.
Tropical cyclones need warm moist water to survive. The longer they stay inland the more they decay. However, a large enough system sitting over south Georgia can draw in warm moist air from both the Gulf and Atlantic. So it can survive as an organized system for quite a while and dump huge amounts of breezy rain. That’s the biggest concern about TD4/”Debby” at the moment, along with some potential for a scattered tornado or two as typically happens with decaying tropical systems over land.
At the moment impacts on the GA/SC coast heavily depend on which of these two scenarios comes to play. If the GFS scenario, then the inland areas will get some gusty winds and rain over the course of a day, with the worst right on the coast (near tropical storm winds). If the ECM scenario, then the storm stalls: winds will be lighter, but longer lasting (couple of days), and more rain and impacts further inland. A major unknown at this point are the onshore winds and timing with respect to the tides. We should have a better handle on that tomorrow.
So far, GFS has had a better handle on this storm. But, as the stock market weasels say, past performance is not necessarily indicative of future results. So we will have to watch and see. Once the storm gets past Cuba tonight maybe things will converge on a more consistent solution.
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Thank you Enki
Great summary. First time user of your info. Thanks.