The states of Louisiana, Mississippi and Alabama and Florida have all declared states of emergency as Tropical Storm Isaac makes landfall.
The storm has already caused havoc across the Caribbean, with at least six people killed in Haiti.
Hundreds of flights in and out of Miami and the surrounding the region have been cancelled, disrupting tourism.
The Republican Party has also been forced to delayed by a day the start of its convention, scheduled to take place in Tampa, Florida.
The National Hurricane Center (NHC) in the United States said Isaac presently has wind speeds of 65 mph (105km/h).
The storm could reach hurricane force in the next 24 hours, added NHC.
The Florida Keys are already experiencing rain and strong winds.