12 September 2018

Florence will likely impact the Carolinas from Thursday morning through Monday

We are another day closer to Florence's landfall, and the trends in model guidance are actually even worse for more people. As of Wednesday morning, Florence is still a Category 4 hurricane and very little stands in its way to weaken it. It is centered 540 miles southeast of Wilmington NC and moving quickly to the west-northwest at 17 mph.

For 2-3 days, models have been hinting at a dramatic slowdown as the storm approaches the coast, perhaps stalling for days, or meandering. The bad news is that trend is solidifying. Florence is still expected to reach the coast (central NC to central SC) early Friday morning as a major hurricane, but could actually drift southward along the coast, delivering a destructive storm surge to hundreds of miles of coastline, as well as feet of rainfall at the coast and inland.

Taking a look at the most recent model guidance, we can see how this stall and southward drift is looking more certain.  It's extremely important to note that this is not a dramatic shift in NHC's "cone of uncertainty".  The entire South Carolina coast has been within the 66% probability cone since Saturday morning (with the exception of a brief window from Monday morning through early Tuesday morning when the southern part of SC wasn't in it)... but now the Georgia coast is also inside the cone while the NC Outer Banks is not. Note that hurricane warnings still extend up to Virginia -- the cone does not imply anything about impacts, which will be extensive.

In terms of storm surge and rainfall, a drifting track like this would be worse than a direct hit and inland push. Elevated water levels and coastal flooding are ALREADY being observed along the mid-Atlantic coast up to Virginia and it will only get worse. As of Wednesday morning, the track and intensity forecast supports a 9-13-foot storm surge between Cape Fear and Cape Lookout. Although the peak storm surge is extremely sensitive to the exact track, several feet of storm surge will impact the coastline for hundreds of miles away from the center.

Elsewhere in the Atlantic, Helene has turned northward is and is weakening over cooler water... now a Category 1 hurricane. Isaac is still a tropical storm and it appears that it may not ever regain hurricane status, which is great news for the Leeward Islands. It's expected to cross the Leewards on Thursday as a weakening tropical storm.

There is still some interest in the disturbance heading into the Gulf of Mexico from the western Caribbean. It's poorly organized and messy now, but could develop slightly this weekend as it heads for Texas. At this point, there does not seem to be any reason for concern, but it will bring gusty winds and heavy rain there regardless of development. If named, it would be Tropical Storm Joyce.

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