Tropical Storm Isaac lashed the Florida Keys just off the U.S. mainland, at or near hurricane strength.
The U.S. National Hurricane Center
says Isaac has maximum sustained winds of 100 kilometers per hour with higher gusts and is expected to drop more than 30 centimeters of rain.
Isolated tornadoes are also possible over portions of the Keys and central and southern Florida.
A hurricane watch has been issued from the mouth of the Mississippi River eastward to Indian Pass, Florida. The watch area does not include metropolitan New Orleans, where Hurricane Katrina left thousands homeless in 2005.
Isaac battered eastern Cuba Saturday, after killing at least four people in Haiti, which is still recovering from a devastating 2010 earthquake. 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.
Protesters who have set up camp in Tampa, dubbed "Romneyville," near the site of the Republican convention will likely need to temporarily abandon their outdoor accommodations.