States of emergency have been declared in Florida, Louisiana, Mississippi and Alabama due to Tropical Storm Isaac.
It hit the Florida Keys and is heading towards the northern Gulf Coast of the state.
Hundreds of flights have been cancelled from Miami, Fort Lauderdale and other southern Florida airports.
Cruise ships have been forced to alter itineraries to avoid the storm that has already killed at least nine people, including eight in Haiti.
Isaac is expected to strengthen to a Category 2 hurricane with winds of 105 mph and make landfall somewhere between Florida and Louisiana by the middle of the week.
There is a chance it may hit New Orleans on the seventh anniversary of the devastating Hurricane Katrina.
Isaac is already a large storm and could bring significant damage to areas within 200 miles of its centre.
Power cuts have affected areas from Key West to Fort Lauderdale, hitting more than 6,000 customers.