KEY WEST, Fla. (AP) — Tropical Storm Isaac gained fresh muscle Sunday as it bore down on the Florida Keys and forecasters warned Isaac could grow into an extremely dangerous Category 2 hurricane as it begins tracking toward the northern Gulf Coast.

Isaac's 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, and after leaving four dead earlier 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 later Sunday or Sunday night at or near hurricane strength.

"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 with the U.S. National Hurricane Center in Miami.

"Our forecast is that as the system moves northward it is forecast to strengthen to a Category 2," she said, adding an ultimate landfall is possible on the northern Gulf Coast late Tuesday night or early Wednesday morning.

"Definitely the northern Gulf Coast should be preparing for a hurricane right now," she added, speaking with The Associated Press by telephone.

A Category 2 hurricane is capable of top sustained winds of 96-100 mph (154-177 kph). Schauer cautioned that Isaac also poses a threat of destructive storm surges though she noted forecasts extending out as far as Tuesday or Wednesday are subject to greater uncertainty.

Thus, she said, a wide swath of the Gulf Coast, including the thousands of people gathering for the Republican National Convention in Tampa set to begin Monday, should remain alert to the storm's progress.

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