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 Florida Keys are already experiencing rain and strong winds.
The Republican Party has also been forced to delay by a day the start of its convention, scheduled to take place in Tampa, Florida.
The US National Hurricane Center said that Isaac currently has wind speeds of 105km/h (65mph). The storm could reach hurricane force in the next 24 hours, it added.