Debby FL landfall and outlook for GA/SC/NC

Debby is making landfall in the “Big Bend” of Florida this morning as a Category One hurricane. Not to minimize the impacts there (and across the rest of Florida as strong thunderstorms and a tornado or two sweep across the state, as can be seen in this radar composite), unusually the main focus now is on what happens next as the storm decays and stalls near the GA/SC coast tomorrow.

As a reminder for the official word, here are links to the National Hurricane Center’s Key Messages regarding Hurricane Debby (en Español: Mensajes Claves). That is a great product and a good “one stop” catch up on the latest. The impact swath map as of 5am this morning (Monday) hasn’t changed a lot at first glance, although there is an important nuance discussed below.

The Florida landfall should be pretty typical for a Category One storm. There will be mostly limited (unless it’s your house of course) wind damage to roofs, some coastal storm surge impacts, power outages, but should not be a wide spread disaster. Trees shedding large limbs and even coming down in Florida and South Georgia will be a problem and cause a lot of the damage. Again this is not to be dismissive because people will suffer, especially vulnerable structures like mobile homes of which there are many in this area, but this an 80mph storm.

The storm will then travel across Georgia. Inland (Waycross) will see strong thunderstorms and gusty winds, north Florida (Jacksonville) and the South Georgia coast (Brunswick to around Darien) will probably see low end tropical storm conditions right on the coast throughout the day today and tonight until on the south side of the storm, when things should begin to improve.

As has been discussed at length the last week, the biggest potential problem is that Debby is forecast to basically stall as it reaches the Georgia coast. The reason for this is pretty straightforward. Storms are pushed around by wind currents in the middle part of the atmosphere. Right now for the storm, that was blowing to the north and today will turn northeast, steering the storm to the GA/SC border and just offshore by mid day tomorrow. Then the steering currents stop. So the storm stops moving, and just wobbles around, until the next pattern in the middle atmosphere picks it up and, in the latest NHC scenario, starts it moving north again.

As for wind, it shouldn’t be too bad. More than likely Debby will have decayed to a minimal tropical storm by the time it reaches the coast mid-day Tuesday. There is always the risk of tornadoes in a decaying tropical system after landfall, everyone in north Florida, Georgia, and South Carolina should have their weather radios on and be alert for them, but this isn’t a “Wizard of Oz” kind of tornado outbreak situation. Generally these are weaker and short lived, but still something to be aware of. Gusty winds within thunderstorms are a given. So that means scattered power outages, and the now water logged trees will be shedding limbs so there will be damage from that.

The problem is that the storm looks to be in a perfect position to pump a LOT of rain into the coast to the north and west of wherever the storm center slows down and stalls. Right now, Debby is forecast to spend almost three days loitering between Savannah and Charleston. Here is the National Weather Services rainfall forecast, and it’s epic:

Yes, the forecast is again back well over 20″ of rain, but fortunately for Savannah and the Georgia coast that swath has shifted to the north. That’s still bad for Frogmore (and its suburb of Beaufort), Hilton Head and points inland to Allendale, and potentially catastrophic for the Charleston area.

Keep in mind that this isn’t storm surge – this is rising water from heavy rain, so the at risk areas are not the coastal marshes and shoreline, but further inland in urbanized areas and near creeks and other low lying areas.

This isn’t to say the coast of Georgia from Fort Stewart to Savannah are entirely off the hook – we will probably at least experience a rain event similar to the one in July where we got 16″ of rain in a week. So expect flooding in all the same places. But if NHC is right, it looks like the Savannah area will be on the southern fringes of the worst of the flooding. And that’s the twist. Literally. Take a look at the track forecasts:

The situation is still extremely uncertain. For example, the primary GFS model run takes the storm offshore near Brunswick, back across south Georgia into Alabama, then ejects the remains to the northeast and back over Georgia and South Carolina. That puts the high rainfall blob squarely over the coast from Ft. Stewart to Beaufort with Savannah in the middle, with bad flooding as far south as Brunswick. The rest of the models look like the proverbial “squashed spider.” Looking at day two of the GFS forecast it maps out like this (rain in mm, recall 1″ = 25.4mm, so 10″ = 254mm):

The NHC forecast is an attempt to make sense of all that mess. A warning here: your local TV station or other source may be using a rain forecast derived from only one model such as GFS, so it might show the rain blob in a different place than the official forecast. Hopefully they will explain that …

The bottom line is that somebody is (probably) going to get a lot of rain. We will just have to watch and see who gets it. Will more than likely do an update in the mid-afternoon.

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