Hurricane Florence poses an extreme threat to Southeast, Mid-Atlantic
Hurricane Florence was tracking toward the East Coast on Sunday with invariability rarely seen in storms several days away from landfall. While forecasters were careful to cite “high uncertainty” and “low model confidence” last week, their tone changed after watching the storm’s eventual path barely shift from what they had considered to be the worst-case scenario.
On Sunday evening, the National Hurricane Center was forecasting Florence to become a strong Category 4 just prior to making landfall somewhere on the Southeast or Mid-Atlantic coast Thursday.
With each passing flight into the eye of the storm and every new forecast from the global weather models, it is increasingly unlikely Florence will turn out to sea and spare the Eastern Seaboard from potentially devastating storm surge, flooding and wind. There’s even some indication the hurricane will slow or stall out over the Mid-Atlantic this week, which could lead to a disastrous amount of rain.
“There is an increasing risk of two life-threatening impacts from Florence: storm surge at the coast and freshwater flooding from a prolonged heavy rainfall event inland,” the National Hurricane Center wrote Sunday. Storm surge is the rise in ocean water above normally dry land at the coast, which can inundate homes, roads and businesses.
South Carolina, North Carolina and Virginia have declared states of emergency to position money and resources for the storm.
As of 5 p.m. Sunday, Florence was crawling across the North Atlantic Ocean at just 7 mph. Its peak winds were 85 mph, making it a Category 1 hurricane. Over the coming days, its forward motion is expected to increase as it turns to the west-northwest, and its intensity is expected to rapidly increase. But then the storm is forecast to slow as it approaches a likely landfall.
The environment ahead of the storm for the next four days favors at least a Category 3 hurricane landfall on the Southeast coast by week’s end. The storm will pass over warmer-than-normal ocean water, which will provide fuel. And there will be little wind shear to disrupt the thunderstorm development at its core. It is not out of the question that Florence could become a Category 5 hurricane at some point.
The Hurricane Center predicts it will reach Category 4 intensity by Tuesday, with maximum winds of 150 mph prior to landfall. Residents along the Georgia, South Carolina and North Carolina coasts should continue preparations for a major hurricane landfall and have a plan should they be required to evacuate.
In these coastal areas, heavy surf and elevated water levels arrive by Wednesday morning, and rainfall could begin by Thursday morning.
Tropical-storm-force winds could reach the coastline as early as Wednesday night, at which point all outdoor preparations should be completed. Extremely dangerous hurricane-force winds could batter coastal locations Thursday into Friday. Hurricane to tropical-storm-force winds could extend inland depending on the storm’s exact track.
Of particular concern is the rainfall potential in the Mid-Atlantic. Models have come into agreement that a northward turn before reaching the United States is unlikely and that a building high-pressure zone north of the storm will cause it to slow or stall once it reaches the coast or shortly thereafter.
Florence could sit over some part of the Mid-Atlantic for several days, similar to what Harvey did last year over eastern Texas. It has the potential to dump unthinkable amounts of rain over a large area in the Mid-Atlantic and perhaps into the Northeast. Rainfall could begin Friday or Saturday and continue into the following week. Where exactly the zone of heaviest rain sets up is a big uncertainty. It could reasonably occur anywhere between the mountains and the coast.
In the simulations, the European model focuses the heaviest rainfall in north central North Carolina and southwest Virginia — up to one to two feet or more, falling between Wednesday night and Monday night. The new American model is not as wet but shows widespread amounts of 5 to 10 inches or more up the Interstate 95 corridor in North Carolina through Washington, D.C., and Baltimore.
This region will be particularly susceptible to flooding because of much above-normal rainfall in the region since May. In addition, because the ground is likely to be saturated, trees will be vulnerable in the strong winds.
Parts of the Mid-Atlantic, especially from Virginia to Pennsylvania, have seen 150 to 300 percent of their normal rainfall since May.
Further north into Delmarva, New Jersey, Pennsylvania and New York, coastal and inland areas also should be preparing for significant storm effects.
Where the storm makes landfall has implications on where the strongest winds and biggest rise in water at the coast occurs, but strong winds and extreme rainfall could occur at great distances from the landfall location. Keeping this in mind, here is the likelihood of landfall at different locations based on our evaluation of model data:
—60 percent in the Carolinas.
—15 percent between Virginia and Southern New England.
—15 percent offshore.
—10 percent between North Florida and Georgia.
Even in the unlikely event that the storm center remains just offshore, it will almost certainly come close enough to bring dangerous wind and flooding to coastal areas. Inland areas may be somewhat spared in this scenario.
If a major hurricane (Category 3 or higher) does make landfall along the Southeast coast, the rarity of such an event is relevant. Since 1851, only 10 major hurricanes have done so, and the most recent was Fran in 1996, 22 years ago. Hugo in 1989 was the one before that and was a Category 4 at landfall. No hurricane has made landfall as a Category 5 in this region on record.
There are probably many people in the Southeast and Mid-Atlantic who have not experienced a storm of the potential magnitude of Florence.
Elsewhere in the tropics
There’s also Tropical Storm Helene south of the Cape Verde islands and Tropical Storm Isaac west of Helene.
Helene is nearly a hurricane, and tropical storm warnings and hurricane watches cover the southern Cape Verde islands. It is expected to intensify, possibly becoming a Category 2 hurricane by Tuesday but then turn toward the north into the open central Atlantic, where it will quickly find itself over cool water.
Isaac, on the other hand, looks like trouble. It is forecast to remain in the deep tropics and reach the Lesser Antilles on Thursday as a Category 1 or 2 hurricane. Currently 1,500 miles away, it could pass over the same islands that got devastated by Category 5 Hurricane Maria last year on Sept. 19.
After crossing the Lesser Antilles, Isaac should continue its westward track through the Caribbean Sea but gradually weaken in the face of strong wind shear. We have time to watch this one, but the immediate concern is in the Lesser Antilles.