Tropical Storm Isaac has gained fresh muscle as it nears the Florida Keys, threatening to be at or near hurricane strength approaching the island chain.
Forecasters warned Isaac could grow into an extremely dangerous Category 2 hurricane on an expected track towards the northern Gulf Coast.
Isaac drew new strength during a warm-water crossing of the Florida Straits after causing weekend havoc in Cuba, where it downed trees and power lines, and after leaving seven dead earlier in Haiti.
On Key West, locals followed time-worn rituals as they prepared for a lashing from Isaac, which swamped the Caribbean and changed plans for the Republican National Convention.
"Currently Isaac is a tropical storm that's expected to become a hurricane as it reaches Key West... then it will move into the Gulf of Mexico and is expected to strengthen further," said meteorologist Jessica Schauer at the US National Hurricane Centre in Miami.
"Our forecast is that as the system moves northward it is forecast to strengthen to a Category 2," she said, adding an eventual landfall is expected on the northern Gulf Coast. "Definitely the northern Gulf Coast should be preparing for a hurricane right now."
She said Isaac could make landfall on the northern Gulf Coast late Tuesday night or early Wednesday morning.
A Category 2 hurricane is capable of top sustained winds of 96-100 mph. But Ms Schauer cautioned that forecasts that far out in time are subject to greater uncertainty. Nonetheless she said a wide area of the northern Gulf Coast should be bracing for the threat.