Tropical Storm Isaac, expected to become a hurricane, pointed toward Louisiana Monday and people in three states were told to evacuate.
The storm had New Orleans in the center of its path, officials said.
Governors of Louisiana, Alabama and Mississippi declared states of emergency and thousands of people were ordered to evacuate.
Authorities said Florida had avoided the worst of Isaac but was still feeling its effects. On Sunday, the Republican National Committee announced the first day of party's convention in Tampa would be canceled.
The National Hurricane Center declared a hurricane warning for an area stretching from east of Morgan City, La., to Destin, Fla. That arc includes New Orleans, Lake Pontchartrain and Lake Maurepas.
Alabama Gov. Robert Bentley ordered mandatory evacuations of coastal and low-lying areas to begin at 8 a.m. , CNN reported.
At 5 a.m., the storm was about 180 miles southwest of Fort Myers, Fla., and 405 miles southeast of the mouth of the Mississippi River. It was moving west-northwest at 14 mph.
The hurricane center said the center of Isaac would move over the eastern Gulf of Mexico Monday and approach the northern Gulf Coast Tuesday, by which time the storm is expected to be a Category 2 hurricane with winds 96 to110 mph, Accuweather.com reported.
Tropical storm-force winds extend 240 miles out from the center of the storm.
South Florida was under a tornado watch until 9 a.m. Monday and a flood watch throughout the evening, The Miami Herald reported.
More than 18,000 residents in Miami-Dade, Broward and Palm Beach counties on the state's east coast were without power Sunday night.
Hundreds of flights had been canceled at South Florida airports.
Along Florida's west coast, emergency shelters opened in Pinellas and Hillsborough counties surrounding Tampa Bay the Tampa Bay Times reported.
The storm was predicted to drop 2 to 4 inches of rain in the Tampa Bay area by Monday night but Florida Gov. Rick Scott said he was most concerned about how it might affect northern sections of the state, which were still waterlogged from Tropical Storm Debby.
Wednesday is the seventh anniversary of Hurricane Katrina, which struck the Gulf Coast with 145 mph winds, devastating a a region that included New Orleans.
More than 1,700 people were killed and hundreds of thousands of others were displaced by Katrina, which was a Category 3 hurricane. Its storm surge -- a 29-foot wall of water -- was the highest ever measured in the United States.
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