A tropical storm warning was in effect Friday for southern US Gulf states from Louisiana to Florida, the National Hurricane Center said, dpa reported.
The storm was in the Gulf of Mexico about 375 kilometres south-west of the mouth of the Mississippi River, which flows into the Gulf south of New Orleans. Its winds were measured at 85 kilometres per hour.
The centre said a storm warning was in effect for a stretch of coastline from south-central Louisiana to the panhandle of Florida. The watch area included New Orleans.
Meteorologists issued a hurricane watch on Thursday, but that was downgraded to a tropical storm watch by mid-day Friday.
White House spokesman Jay Carney said there were "extensive activities" underway by the Federal Emergency Management Agency (FEMA) to support state and local partners along the Gulf Coast.
FEMA has activated its National Response Coordination Centre in Washington and deployed people to work at emergency operation centres in Alabama, Florida, Louisiana and Mississippi, he said.
The storm prompted the US government to recall FEMA emergency staff who had been furloughed earlier this week due to the government shutdown over a budget impasse.
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