Tropical Storm Isaac swirled into the Gulf of Mexico early Monday, with U.S. forecasters saying it is expected to become a hurricane on Tuesday.
The storm had sustained winds of 100 kilometers an hour. It battered Haiti and Cuba and then lashed southern Florida as it headed toward the northern Gulf Coast.
The National Hurricane Center says Isaac could hit the Gulf Coast somewhere between Florida and Louisiana.
Alabama, Florida, Louisiana and Mississippi have declared states of emergency as they brace for as much as 46 centimeters of rain from the storm.
U.S. energy companies shut offshore oil rigs and started closing coastal refineries.
New Orleans Mayor Mitch Landrieu told residents to be prepared to live without regular water or power supplies for several days. He also urged them to leave if an evacuation order is issued.
Many did not leave in 2005 when Hurricane Katrina struck New Orleans. Katrina ravaged New Orleans and much of the Gulf Coast, killing more than 1,800 people and leaving thousands more homeless. Wednesday marks Katrina's seventh anniversary.
Isaac battered eastern Cuba on Saturday, after killing at least 19 people in Haiti, which is still recovering from a devastating 2010 earthquake. Two people were killed in the Dominican Republic. There were no reports of fatalities in Cuba.
The threat of the storm has caused the U.S. Republican Party to postpone the start of most events at its national convention in Tampa, Florida, from Monday to Tuesday afternoon.