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Posted at 11:01 AM ET, 06/25/2012

Tropical storm Debby drenching Florida as it crawls through northeast Gulf of Mexico

Tropical storm Debby’s forward motion over the northern Gulf of Mexico has nearly ground to a halt as it unleashes band after band of heavy rain over the state of Florida. The storm is going nowhere fast and could take until the end of the week to cross northern Florida.

With maximum sustained winds of 45 mph (gradual weakening has occurred), the storm is moving northeast at just 3 mph, positioned 75 miles south of Apalalachicaola, Florida. Tropical storm warnings stretch from Destin to Englewood, Florida.

Track forecast for Debby (National Hurricane Center)
Because of Debby’s slow movement, residents along the northern Gulf coast will continue to receive flooding rains and gusty winds to tropical storm strength for the next couple of days, at least. Up to 20-25 inches of rain is possible in spots in western and northern Florida.

Link: Current southeast U.S. radar

Already, isolated rainfall amounts of 1 foot have occurred (see map below) with widespread totals of 6-10 inches along and near Florida’s west coast (e.g. 10” in St. Petersburg and Tampa, and 9” in Gainesville) due to Debby’s proximity to land and its production of large, slowly moving clusters of convection well removed from the center.

3-day estimated rainfall totals over Florida (Saturday to Monday) (NOAA)
Though flooding will be a significant hazard across parts of the far Southeast, with the ground across much of Florida already saturated from recent heavy rains, some of Georgia’s soils should benefit from the expected rains. More than 5 inches are possible (see below) in the southern part of the state (right), very close to where severe drought conditions are currently in place (left).
Drought map of Southeast U.S. (left), Rainfall potential over next five days in Southeast U.S. (right) (U.S Drought Montior and NOAA HPC)

In addition (though subject to change based on Debby’s landfall intensity), a storm surge up to 3-5 feet is possible just east of this zone near Apalachee Bay (darkest blue shading below right). And finally, the threat for isolated, brief tornadoes will continue today in the strongest rainbands in northeast quadrant of the circulation. On Sunday, a tornado killed a woman in Venus, Florida.

Rip current and beach erosion forecast from NWS (left), and a map showing where the highest surge is likely (30% chance of reaching 3-5 feet in the darkest blue). (NOAA). (NOAA)
And, as with any land-falling tropical cyclone, rip currents and beach erosion will be significant hazards as well along parts of the northern Gulf Coast (above left).

Video overview of Debbie from Associated Press

Technical discussion

While a profilic rain-producer, recent satellite presentations and surface observations indicate that Debby is an unorganized tropical cyclone.

Recent infrared satellite picture (NOAA)
Deep convection (thunderstorm activity) has yet to persist near the center of circulation, which is located about 90 miles south of Apalachicola, Fl (the X in the image to the right). In fact, the biggest storms (encircled) are far removed from the inner region of Debby’s swirl, and located instead hundreds of miles to the east over the Florida peninsula and the adjacent Atlantic coastal waters.

The lack of organization is ostensibly due to the 20-knot westerly wind shear over the region (arrow) that has plagued Debby since her inception as well as ingestion of some dry air. Though the oceanic heat content in the Gulf is capable of supporting a major hurricane, enough shear is expected to persist until landfall to keep Debby at tropical storm strength.

The uncertainty in the track forecast has been unusually high with this system. Until very recently, there was a large and typically-reliable contingency of global weather models that turned Debby westward into Louisiana and/or Texas. Yet the GFS –despite its flip-flopping about the fierce Plains heat wave that is in fact coming this week (see last Tuesday’s post)- has all along stood as one of the few outliers in the suite of track forecasts. It insisted on a north or northeast movement from the Gulf to Florida. For now, the consensus has for the most part shifted toward the GFS solutions and thus given NHC enough confidence in its official forecast to bring the center of Debby to Florida’s panhandle. So with that, kudos so far goes to the GFS with Debby’s track.

With Debby’s fate still far from certain, we will have more updates as necessary.

By  |  11:01 AM ET, 06/25/2012

Categories:  Latest, U.S. Weather, Tropical Weather

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