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Tropical Storm Andrea Pelts Florida; Threatens Flooding

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The 2013 Atlantic Hurricane Season is off to a rapid start, with Tropical Storm Andrea making landfall on Thursday afternoon in Florida's Big Bend area between Tampa and Tallahassee, with maximum sustained winds at landfall of 65 mph. It is forecast to lose its tropical characteristics, becoming more like a typical windswept rainstorm, as it moves through northern Florida and into Southeast Georgia, and then up the East Coast through Saturday.

The biggest threat the storm poses is very heavy rainfall, and flash flood watches were in effect from Florida northward to Maine through Sunday.

Satellite image of Tropical Storm Andrea as of Thursday morning, as it approached northern Florida.
Credit: NWS.

Rainfall amounts in some parts of Florida were forecast to reach 8 inches or more, with a wide swath of 2 to 4 inch rains spreading up the eastern seaboard. The rains in Florida came on top of heavy rain that fell during the month of May. For example, West Palm Beach received nearly a foot more rainfall than it normally does for the month.

In addition, Tropical Storm Andrea is predicted to bring storm-surge flooding to portions of Florida's west coast, including vulnerable parts of Tampa Bay, which Climate Central named as the most vulnerable U.S. city to a major hurricane, largely due to its susceptibility to coastal flooding.

While this storm's storm surge impacts will pale in comparison to a worst-case scenario, the National Weather Service is predicting that  the combined storm surge and astronomical tide will raise waters up to 4 feet above mean sea level within coastal areas of Tampa Bay, "resulting in worst-case flood inundation of 2 to 4 feet above ground level somewhere within the surge zone." In a statement on Thursday morning, the NWS forecast office in Tampa said the areas at greatest risk for coastal flooding include Cedar Key, Yankeetown, and areas along the Levy County coast, with the biggest threat lasting from late Thursday morning through early Friday morning. 

Helpful Links Twitter Accounts to Follow
    National Hurricane Center Andrea Info
    NWS Tampa Bay 
    NWS Tallahassee 
    Weather.gov 
    Hurricane Safety Tips
      @NHC_Surge         @capitalweather
      @NHCDirector       @TWCBreaking
      @CraigatFEMA
      @ClimateCentral
      @afreedma                    

Accoring to the NHC's storm-surge unit, flooding of up to 5 feet above ground is possible in Florida's Big Bend area with the high tide on Thursday afternoon. The unit tweeted a link to real-time storm-surge observations, and noted that a storm surge of 2 to 4 feet above ground level may extend northward from Tampa Bay to Apalachicola.

Long-term sea level rise from global warming and other causes escalates the risk of damaging storm surge flooding in the Tampa-St. Petersburg area. Climate Central calculated the 100-year flood height in this area is 6.5 feet above the high tide line. Using Climate Central’s Surging Seas sea level rise mapping tool, it becomes clear that there are about 125,000 people currently living below this flood level. In St. Petersburg alone, there are more than 45,000 homes that lie below 6 feet in elevation, and would likely be vulnerable to a storm surge of that magnitude or greater.

Projected three-day rainfall todals, showing heavy rains from Tropical Storm Andrea streaming up the East Coast from Florida to Maine.
Credit: NOAA/WPC.

According to Climate Central’s research, sea level rise is escalating the threat of damaging storm surge flooding in the Tampa area. The odds that a flood exceeding 6.5 feet would occur in Tampa before 2030 are about 14 percent without global warming, but those odds increase to 20 percent with the effects of global warming-related sea level rise factored in. Fortunately, Tropical Storm Andrea's storm surge is not predicted to reach that height.

As of Thursday morning, Tropical Storm Andrea had peak sustained winds of 60 mph, and little additional strengthening is forecast prior to landfal, according to the National Hurricane Center. In a discussion on Thursday morning, the NHC said: "Andrea is probably near its peak intensity since strong shear should halt any significant intensification before landfall." The Hurricane Center is expecting the storm to transition into an extratropical storm, more closely related to a nor'easter than a tropical system, during the next 36 hours.

Forecasters said they do not intend to issue tropical storm warnings north of Virginia, and that any hazardous wind threats for the Mid-Atlantic and New England will be handled by local National Weather Service offices.

Probability of storm surge exceeding two feet above ground level.
Credit: NOAA/NHC. 

Heavy rainfall will be one of the biggest threats from the storm, with widespread amounts of 2 to 4 inches likely, and isolated 8 to 10 inch amounts possible in parts of Florida. 

The forecast gets a bit more tricky as the storm interacts with a front that will be draped along the East Coast, which will act as a funnel for the storm's moisture. That tropical air could result in widespread heavy rainfall — on the order of 2 to 4 inches or more — from Georgia to Maine as the storm, or what is left of it, moves northeast through the weekend, the NWS said. The rainfall, coming on top of recent heavy rains, could create river, small stream, and urban flooding along the eastern seaboard.

Also, as with any landfalling tropical storm or hurricane, the atmospheric spin created by the storm may result in tornadoes, with tornado watches in effect on Thursday for parts of the Florida peninsula. The NWS was already investigating several possible tornado touchdowns from Tampa to West Palm Beach as of Thursday morning.

Typically, though, tornadoes spawned by tropical storms and hurricanes are relatively weak, and not the EF-5 monsters that have recently roamed the Plains States.

The storm does not pose a significant wind or coastal flooding threat for areas impacted by Hurricane Sandy, although tropical storm force winds between 39 mph and 73 mph are still possible.

Editor's Note: This story was updated on Thursday evening to reflect that the storm has come ashore and begun weakening.

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