Florence still a weak tropical storm

Florence still looks pretty disorganized, with a large, sloppy center and some clumps of heavy thunderstorms on the east side. Wind shear of 10-20 knots due to upper-level westerly winds is causing much of this disorganization. Part of Florence's struggles are due to her inability to overcome her initial indecision on where her center of circulation should be. In any case, Florence remains a weak tropical storm today. Only slow intensification should happen today. This morning's QuikSCAT satellite pass found only 40 mph winds in Florence, although it did miss sampling the most intense portion of the storm.

Intensity forecast
The winds shear forecast is a bit more uncertain today. An upper-level low to the west of Florence that is creating the shear is forecast to move away, allowing Florence to intensify to a hurricane (and possibly a major hurricane) over the next four days. However, the speed with which this upper low may move off is uncertain, and a slower than expected movement way will keep significant winds shear over Florence. The disturbance about 800 miles to the east-southeast may also steal some energy from Florence over the next few days.

Track forecast
The computer models had a better-defined storm to track with their more recent runs, and should be reasonably reliable today. The disturbance "Invest 91L" about 800 miles to the east-southeast of Florence may still cause some trouble if it develops into a tropical depression, however. When two storms get within 13 degrees of arc of each other (900 miles), they can interact (the Fujiwhara effect), causing difficulties in the track and intensity forecasts.

The models have a very believable scenario where Florence moves north of the Lesser Antilles Islands, then turns northward in response to a trough of low pressure swinging across the Eastern U.S. four days from now. On this trajectory, Florence would only be a threat to Bermuda and perhaps the Maritime provinces of Canada. When one consults the map of historical paths of September tropical storms that have tracked near Florence's current position, we see that only one of these previous storms managed to hit the U.S. East Coast. I will be surprised (though not amazed) if Florence does manage to strike the U.S.

Florence's little brother
Tropical disturbance 91L, about 800 miles east-southeast of Florence, is a little less organized than yesterday. The disturbance's close proximity to its big sister is probably hampering its development. Some of the computer models predict that 91L will never escape the shadow of big sister, following her on a recurving path out to sea between Bermuda and the U.S. East Coast and never developing into a tropical storm. However, some of the models predict that when Florence gets pulled sharply north, this will open up enough separation between the two storms to allow 91L to split away and intensify.


Figure 1. Preliminary model tracks for Invest 91L, the tropical wave 800 miles to the ease-southeast of Florence.

Carolinas
A stalled cold front off the Carolina coast has spawned a low-pressure area with a clump of intense thunderstorms a few hundred miles off the Florida/Georgia coast. This low is expected to track north-northeast and pass near the Outer Banks of North Carolina Thursday morning. A second low may develop in a similar location on Thursday and pass by the Outer Banks on Friday morning. Neither of these lows have enough time to develop into tropical depressions.

Ioke
Ioke is finally gone! It turned into a powerful extratropical storm with 60 mph winds yesterday over the ocean waters east of Japan.

Next update
I'll have an update Thursday morning, unless there's something interesting to report on this afternoon.

Jeff Masters