Tropical Storm Isaac has gained strength as it bore down on south Florida, with forecasters warning that the storm could grow into a more dangerous Category 2 hurricane.
Isaac hit the northern Gulf Coast on Sunday after leaving seven dead in Haiti and causing havoc in Cuba, where it downed trees and power lines.
Forecasters said the storm was expected to reach the Florida Keys later on Sunday at 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,” Jessica Schauer from the US National Hurricane Centre said.
“Our forecast is that as the system moves northward it is forecast to strengthen to a Category 2.
“Definitely the northern Gulf Coast should be preparing for a hurricane right now.”
A Category 2 hurricane is capable of top sustained winds of 154-177 kph.
Florida Governor Rick Scott declared a state of emergency and cancelled his plans to attend the Republican convention in Tampa, which was to run on Sunday and Monday.
Isaac has forced Republicans to make changes to their national convention, where party officials scrambled to make sure candidate Mitt Romney’s message to voters does not get blown off course.
Expecting earlier that Tampa would be in Isaac’s sights, convention organisers had cancelled Monday’s opening events and tried to squeeze as many speakers and activities into three days as had been planned for four.
Al Jazeera’s Alan Fisher reported from Tampa that there are several reasons Republicans are considering delaying the convention.
“First of all, there is the safety issue … and also, they don’t want to be seen to be partying while parts of this coast, perhaps even as far along as Louisiana, are struggling because of the weather,” he said.
“The convention here has decided that it will start as scheduled on Monday, but they will immediately adjourn and so essentially all of Monday’s program has been postponed.”
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, officials said.
Meanwhile, authorities said a new hurricane watch has been issued from the mouth of the Mississippi River, not including the New Orleans metro area.
From Key West, a steady line of cars moved north along the Overseas Highway, the only road linking the Florida Keys, while residents boarded up windows, laid down sandbags and shuttered businesses ahead of the approaching storm.
When it hits, winds will be “enough to knock you over,” National Hurricane Centre spokesman Dennis Feltgen said.
The Keys were bracing for storm surges of up to four feet, strong winds and the possibility of tornadoes.
The island chain’s two airports closed on 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.
At least four people were reported dead in Haiti, including a 10-year-old girl who died after a wall fell 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 earthquake shelters and more than 4,000 were taken to temporary shelters.