As of early Wednesday morning, Hurricane Florence had weakened slightly with sustained winds of 130mph, but this is of little consequence as the track forecast now shows a dangerous stalling out near the coast, or just onshore by late Thursday or Friday morning. This will exacerbate already extremely heightened concerns about inland flooding due to torrential rainfall from Florence.
Hurricanes produce three major kinds of threats: storm surge, damaging winds, and inland flooding from rainfall. Most hurricanes produce a combination of the above with varying severity, but rarely does a hurricane present all three threats at an extreme level. Florence is such a hurricane, with what the National Hurricane Center characterizes as a “life-threatening” storm surge for portions of the North and South Carolina coast and “damaging” winds for these same coastal areas. (See warning areas.)
However, probably the biggest concern with Florence is inland flooding, especially as the storm is now likely to become nearly stationary along the Carolina coast and then slowly trudge inland. Adding to these concerns is a new twist the forecast models are indicating will happen—a southwestward jog toward Georgia.
This should only amplify rainfall totals as the storm’s core remains offshore, with access to warm Atlantic waters to regenerate rain-making bands of precipitation. Texas saw a similar scenario in 2017, when Hurricane Harvey’s center moved just inland, and its counter-clockwise rotation allowed the storm to continually pull moisture inland from the Gulf of Mexico.
The southwest jog
As recently as Monday morning, hurricane models were forecasting a fairly conventional landfall for Florence, bringing the storm to the coast before bringing it slowly inland on a generally northwestward track. However, recent trends in the forecast models now indicate a rare—if not unprecedented for an Atlantic hurricane this far north and west—turn to the southwest before Florence resumes its west-northwest motion.
This change in track can be discerned from the ensemble forecast from the European model initialized at 8am Monday (12:00 UTC) on the left in the image below, and the European model initialized at 8pm Tuesday (00:00 UTC Wednesday). Ensemble forecasts capture the uncertainty in a forecast by initializing a computer model with slightly different starting conditions. It is striking that nearly all of the ensemble members now show a southwestern jog, which raises confidence in this forecast.
So what is causing this? A ridge of high pressure situated over the Central and Eastern United States is forecast to be a little stronger than anticipated, and this feature should force Florence to move around its periphery. As Florence trances the edge of this high pressure system, by later Friday or so, the atmospheric steering currents essentially break down.
Normally, when hurricanes get above 30 degrees North in latitude, they get pulled poleward by the westerly pattern in the atmosphere between the tropics and poles. However, because the aforementioned high pressure will be blocking a northward movement by Florence, this rapid turn to the north and eventually northeast will not happen for some time.
Major flooding event
All of this means that, in addition to very strong winds and storm surge beginning Thursday along the Carolina coasts, the southeastern United States will have to prepare for a major, and perhaps historic, flooding event.
It really is impossible to say where a “bullseye” will occur in terms of rainfall due to the uncertain storm motion, but coastal areas of the Carolinas likely face the greatest threat. Forecast modeling indicates broad areas may receive 10 to 30 inches of rain, with higher localized totals of 30 to 40 inches over the next week or so.
It is worth noting that both the European and US Global Forecast System models forecast isolated totals of more than 40 inches. We cannot rule such extremes out should Florence stall out near the coast, or just inland. Needless to say, with soils already sodden in the southeast, this would present a dire scenario for inland flooding.