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Posted at 01:30 PM ET, 07/23/2010

Tropical storm Bonnie over Florida

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The tropical disturbance we have been monitoring the past several days (see last blog entry) officially became tropical storm (TS) Bonnie late yesterday. The National Hurricane Center (NHC) upgraded this system to tropical storm status based on recent evidence of a closed circulation at the surface with winds of 40 mph.

As of noon Friday, Bonnie's center was crossing the south Florida shoreline, just a few miles south of Miami. Recent data from Fowey Rocks weather station, which is located 8 miles SSE of Key Biscayne, reported sustained winds of 39 mph with a gust to 56 mph at 10m elevation. These readings are at the lowest end of the tropical-storm strength scale.

Showers and storms are right now rolling across all of south Florida in association with Bonnie's still poorly-defined rain bands. Inland, where frictional effects of the land will slow down the winds measured at Fowey Rocks, wind gusts in the 30-40 mph are confined only to the heaviest rains, with only light breezes otherwise.

Keep reading for more, including possible impacts on the mid-Atlantic...

Present indications suggest that Bonnie may have a hard time intensifying much once over the Gulf of Mexico, probably because of the unfavorable wind shear (turning of wind speed/direction with height) pattern it will continue to encounter during the next couple of days. A strong upper-level cyclone (the clockwise circulation seen over the central Gulf) is expected to move westward in tandem with Bonnie, at least for the next day or so, all the while imparting a relatively strong wind shear over the tropical cyclone. In addition to creating windy conditions aloft, the proximity of the upper vortex should keep relatively dry air close by, as shown by the dark shading just to the west and north of Bonnie's primary thunderstorm cluster. The adjacent dry air will give these thunderstorms an opportunity to develop strong downdrafts that will tend to prohibit further spinup of its circulation

Though the shear is expected to relax a bit later tomorrow, as the upper-level feature gradually moves away from Bonnie, it should remain at TS strength all the way to the bayous of eastern Louisiana ... where the track models have picked as the most likely landfall location.

Assuming Bonnie's primary circulation remains small and relatively weak, storm surge will not be a significant concern. And though heavy showers are expected along and very close to Bonnie's track, widespread heavy rains are not likely with the storm's encroachment on the northern Gulf Coast.

Once the storm moves inland early on Sunday, whatever is left of it will dissipate quickly and evolve into a small rainmaker over the lower Tennessee Valley. By Sunday night, the remnant circulation should get absorbed by a weak cold that will stretch from eastern Kansas to the mid-Atlantic coast. That front will push whatever is left of Bonnie toward the mid-Atlantic states. The merger of these two features should somewhat enhance the rainfall the front would otherwise produce.

By  |  01:30 PM ET, 07/23/2010

Categories:  Tropical Weather

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