American (GFS) model simulation of Florence overnight Tuesday. (VentuSky.com)

(This article, originally posted at 11:30 a.m., was updated at 5:15 p.m. and 11:05 p.m. based on new model forecast information and data from the National Hurricane Center.)

The once-powerful Florence took a hit Thursday, but its life as a major hurricane is probably not over just yet, and it has East Coast residents on the edge of their seats.

The tropical storm is predicted to re-intensify to a hurricane by Sunday, and odds have substantially increased that it will have direct effects on the East Coast starting between Wednesday and Friday next week anywhere between Florida and southern New England.

Environmental conditions in a few days favor “significant intensification,” the National Hurricane Center wrote Friday evening. It predicts Florence will reach Category 4 hurricane strength by next Tuesday night.

“[T]he extended-range risk to the United States keeps rising,” the Hurricane Center concluded.

At 11 p.m. on Friday, Tropical Storm Florence was centered about 1,700 miles due east of Miami and tracking toward the west at 8 mph. It has peak sustained winds of 60 mph, down slightly (5 mph) from earlier in the day.

Florence wilted some Thursday as it plowed into the subtropical jet stream. The strong wind shear disrupted the thunderstorms at its core and caused the storm’s peak winds to fall from 130 mph to 70 mph in 30 hours.

The weakening nudged the storm on more of a westerly course toward the United States. Weaker storms tend to be guided more by shallow winds, while stronger storms can be steered by winds at higher altitudes. As soon as Florence’s structure fell apart Thursday, it started on a more threatening track toward the East Coast rather than getting drawn to the north.

The great majority of simulations from the American (blue lines in the graphic below) and the European (red lines in the graphic below) modeling systems predict the storm to make landfall along the East Coast. Only a few simulations (mainly from the American model) predict the storm could slow down as it nears the U.S. and meander offshore before turning back out to sea.


Group of simulations from American (blue) and European (red) computer models from Friday. Each color strand represents a different model simulation with slight tweaks to initial conditions. Note that the strands are clustered together where the forecast track is most confident, but they diverge where the course of the storm is less certain. The thick bold red line is the average of all of the European model simulations, while the blue is the average of all the American model simulations. The thinner bold line is the main or operational simulation from each model. (StormVistaWxModels.com)

Significantly, the average track of the American model simulations, which had suggested the storm was most likely to stay well out to sea, has trended closer to the Mid-Atlantic coast with time. The average European model simulation has shifted its track from just off the Mid-Atlantic coast to inland over the Carolinas. Both models are increasingly threatening.


Average of the American and European model simulations have shifted west, closer to the coast or inland, with time. (StormVistaWxModels.com)

Assuming the storm does come ashore, it is still too soon to know exactly where. Current model simulations favor the Southeast or Mid-Atlantic, but the Northeast is not out of the woods.

Because of the uncertainty in the storm’s final destination, it is premature to discuss exactly what hazards it will inflict, where and when. But it is reasonably likely some coastal areas will deal with a damaging wind threat, flooding rain and a substantial storm surge — which is a rise in ocean water above normally dry land. Some areas further inland could also contend with damaging winds and flooding rain.

Even if Florence avoids a direct hit on the East Coast, it will likely come very close, resulting in dangerous surf, beach erosion and the potential for coastal flooding.

Despite weakening on Wednesday, Florence has already started to rebound as the wind shear relaxes. From Saturday on, Florence will be over the warmest ocean it has been since it formed eight days ago.

A pool of anomalously warm water over the western Atlantic will provide ample fuel for another round of intensification. By next Tuesday night, the National Hurricane Center predicts Florence will attain Category 4 intensity with 130 mph sustained winds.


Florence’s forecast track overlaid on a map of current sea surface temperature. (UW/CIMSS)

If Florence does hit or make it close to the East Coast, it would be a first for a storm at its present location.  No tropical storm or hurricane has ever gotten close to the United States when it was anywhere near Florence’s current position.

But the unusually strong high pressure system that is predicted to park itself north of the storm seems as if it has the potential to push Florence on an unprecedented and potentially perilous course.

Now is the time to begin thinking about hurricane preparedness if you live along or near the coast.

Do you have a plan if the storm targets your area? Do you know where you would go if evacuations are required? Do you have an emergency kit with at least three days’ worth of critical supplies? Do you have trees that could fall on your house during a windstorm? Trim them now. Declutter drains and gutters.

Two tropical depressions form in the eastern tropical Atlantic

Elsewhere, two tropical waves in the far eastern Atlantic have been upgraded to tropical depression numbers 8 and 9.  The westernmost depression, number 9, is the one to most closely monitor, as models keep it at low latitudes, potentially reaching the Lesser Antilles by Thursday as a hurricane. If these depressions become tropical storms, which they likely will, they will be named Helene and Isaac.


(National Hurricane Center)