TROPICAL Storm Isaac gained fresh muscle yesterday 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 on 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. The US National Hurricane Centre said yesterday that Isaac had started lashing the Keys with rain and wind by late morning.

A steady line of cars moved north Saturday along the Overseas Highway, the only road linking the Florida Keys. Residents boarded up windows, laid down sandbags and shuttered businesses ahead of the approaching storm. Even Duval Street, Key West’s storied main drag, was subdued for a weekend, though not enough to stop music from playing or drinks from being poured.

“We’ll just catch every place that’s open,” said Ted Lamarche, a 48-year-old pizzeria owner visiting Key West to celebrate his anniversary with his wife, Deanna. They walked along on Duval Street, where a smattering of people still wandered even as many storefronts were boarded up and tourists sported ponchos and yellow slickers.

“Category None!” one man shouted in a show of optimism.

The Keys were bracing storm surge of up to four feet, strong winds and the possibility of tornadoes. The island chain’s two airports closed Saturday night, and volunteers and some residents began filing into shelters.

“This is a huge inconvenience,” said Dale Shelton, a 57-year-old retiree in Key West who was staying in a shelter.

Isaac has already left a trail of suffering across the Caribbean.

The storm’s centre made landfall on Saturday near the far-eastern tip of Cuba, downing trees and power lines. In the picturesque city of Baracoa, the storm surge flooded the seaside Malecon and a block inland, destroying two homes.

At least four people were reported dead in Haiti, including a 10-year-old girl who had a wall fall on her, according to the country’s Civil Protection Office. The government also reported “considerable damage” to agriculture and homes. Nearly 8000 people were evacuated from their houses or quake shelters and more than 4000 were taken to temporary shelters.

The Grise River in Haiti overflowed north of Port-au-Prince, sending chocolate-brown water spilling through the sprawling shantytown of Cite Soleil, where many people grabbed what possessions they could and carried them on their heads, wading through waist-deep water.

Scores of tents in quake settlements collapsed. In a roadside lot in Cite Soleil, the dozens of tents and shelters provided by international groups after the earthquake were tossed to the ground like pieces of crumpled paper, and the occupants tried to save their belongings.

After Isaac passes the Keys, it will move over the warm, open waters of the Gulf of Mexico and is expected to gain significant strength. It could ultimately make landfall on the northern Gulf Coast late on Tuesday or early Wednesday. However, forecasters have stressed that the storm’s exact path remains highly uncertain.

“Definitely the northern Gulf Coast should be preparing for a hurricane right now,” Jessica Schauer, a meteorologist with the US National Hurricane Centre in Miami, told The Associated Press in a phone interview.

A Category 2 hurricane is capable of top sustained winds of 154 to 177 kph.

┬áIsaac isn’t likely to hit Tampa head-on, but it could still lash the city with rain and strong winds just as the convention ramps up. A tropical storm warning is in effect for parts of Florida’s west coast, including Tampa Bay.

Convention officials said they would convene briefly today, then immediately recess until tomorrow afternoon, when the storm is expected to have passed. Florida Governor Rick Scott, a Republican, declared a state of emergency and cancelled his plans to attend convention events on Sunday and Monday.

As of 8 am EDT, the storm was centred about 129 kilometres southeast of Key West, according to the US National Hurricane Centre in Miami. Isaac had top sustained winds of 100 kph.

It was moving to the west-northwest toward the Keys at 29 kph. Tropical storm-force winds extended outward up to 335 km from the center, meaning storm conditions were possible in many places even if Isaac does not pass directly overhead.

A hurricane warning was in effect for the Keys, including the Dry Tortugas and for the west coast of Florida from Bonita Beach south to Ocean Reef, among some other areas, authorities said.

Meanwhile, a hurricane watch was in effect from east of Morgan City, Louisiana – including the New Orleans metro area – eastward to Indian Pass, Florida.