KEY WEST, Fla. (AP) — Tropical Storm Isaac gained fresh muscle Sunday as it bore down on the Florida Keys, with forecasters warning it could grow into a dangerous Category 2 hurricane as it nears the northern Gulf Coast.

Isaac drew new strength early Sunday during a warm-water crossing of the Florida Straits after causing weekend havoc in Cuba, where it downed trees and power lines. Before that, Isaac was blamed for four deaths in Haiti.

On Key West, locals followed time-worn storm preparedness rituals while awaiting the storm after Isaac swamped the Caribbean and shuffled plans for the Republican National Convention. Forecasters said the storm was expected to reach the archipelago sometime Sunday at or near hurricane strength.