Forecasters in Miami say Isaac has gained added strength and should be at or near hurricane force by the time it reaches the Keys later in the day.
- Republican convention, Florida brace for Isaac
The U.S. National Hurricane Center said Isaac's maximum sustained winds had strengthened some to 100 km/h. It said more strengthening is expected in the next 48 hours.
At 5 a.m. ET, the storm was centered about 330 kilometres east-southeast of Key West and moving to the northwest at 30 km/h, the center reported.
Forecasters say a hurricane watch has been issued from the mouth of the Mississippi River eastward to Indian Pass, Fla., not including the New Orleans metro area. A hurricane warning remains in effect for areas including the Florida Keys.
Residents brace for possible hurricane
A steady line of cars moved north along the Overseas Highway, the only road linking the Keys, while 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.
Authorities have warned tourists to head for the mainland.
When it hits, winds will be "enough to knock you over," National Hurricane Center spokesman Dennis Feltgen said.
The Keys were bracing for a 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.
The hurricane center said the storm, which was swirling off the northern coast of Cuba overnight, was expected to move near or over the Florida Keys later in the day or Sunday night. Isaac was then forecast to move over the southeast Gulf of Mexico on Monday. It was moving to the northwest toward the Keys at 30 km/h.
Isaac leaves trail of destruction across Caribbean
The storm's center made landfall 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 8,000 people were evacuated from their houses or quake shelters and more than 4,000 were taken to temporary shelters.
The Grise River in Haiti overflowed north of Port-au-Prince, sending 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.
"From last night, we're in misery," said Cite Soleil resident Jean-Gymar Joseph. "All our children are sleeping in the mud, in the rain."
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.
"They promised they were going to build us a sturdy home and it never came," Jean-Robert Sauviren, an unemployed 63-year-old father of six said as he stood barefoot in the water and held aloft his arms. "Maybe we don't deserve anything."