Florence near hurricane strength

Florence has begun the long-expected intensification phase that was forecast for the past week, much to the relief of hurricane forecasts who worried that their basic understanding of hurricane intensification processes was flawed--but much to the dismay of residents of Bermuda. Bermuda is under a tropical storm warning today, and this will almost certainly be upgraded to a hurricane warning tonight. Florence managed to get rid of its large lopsided shape that was inhibiting organization, and adopt a more a symmetric cloud pattern conducive for development. Satellite imagery this morning shows good outflow to the north and east, but no eye yet. Satellite intensity estimates already put Florence at hurricane strength, but the NHC is waiting until the Hurricane Hunters arrive in the storm at 2pm EDT to verify hurricane force winds exist before upgrading Florence to a hurricane. The forecast track of Florence puts the storm over or just west of Bermuda on Monday, and that island is bracing for its worst weather day since Hurricane Fabian of 2003 hit the island as a Category 3 hurricane with 115 mph winds.

Florence has a very large swath of tropical storm force winds that have been blowing for many days over a huge stretch of ocean. These factors, when combined with the storm's expected intensification into a Category 2 hurricane, will create very high ocean swells that will impact the entire Atlantic coast from the Lesser Antilles to Canada. Five to ten foot seas will be common in many nearshore areas, and the wave height forecast from the global wave model run by the National Weather Service predicts wave heights of 15-20 feet offshore the mid-Atlantic and New England coasts by Tuesday. Bermuda can expects waves of 15-25 feet on top of a 6-8 foot storm surge on Monday when the center of Florence passes.


Figure 1. Forecast wave heights for Monday night at 8pm EDT, from the Global Wave Model of the NWS.

Elsewhere in the tropics
An area of disturbed weather 900 miles east-southeast of Florence has a pronounced surface spin one can see on visible satellite imagery, and was declared "Invest 93L" last night by NHC. This disturbance is under 40 knots of vertical wind shear from its big sister, Florence, and has a limited chance of survival. If it does survive, it is likely to follow its sister northward then northeastward, out to sea. There are no other threat areas to discuss. The computer models forecast a new development off the coast of Africa by the middle of next week, but anything developing in this region is likely to recurve out to sea.


Figure 2. Preliminary model tracks for Invest 93L, 900 miles east-southeast of Florence.

I'll have an update Sunday morning by 10:15am EDT.

Jeff Masters