Tropical Storm Isaac has gained strength as it approaches south Florida, with forecasters warning it could develop into a more dangerous Category 2 hurricane.
The governors of Louisiana, Mississippi and Alabama declared states of emergency on Monday after Isaac hit the northern Gulf Coast on Sunday. On its way to the US, the storm left seven people dead in Haiti and caused havoc in Cuba, bringing down trees and power lines.
Florida declared a state of emergency on Saturday and funding has been put into place to implement emergency plans to respond to the storm.
Louisiana Governor Bobby Jindal said that 15 low-lying parishes outside New Orleans's newly built $14.5bn flood-defence system would probably come under mandatory evacuation orders by Monday.
Robert Bentley, the governor of Alabama, ordered mandatory evacuations in the southern counties of Mobile and Baldwin.
Rick Scott, Florida governor, cancelled his plans to attend the Republican convention in Tampa, which was to run on Sunday and Monday.
Isaac has forced the party to make changes to their schedule, with party officials scrambling to make sure candidate Mitt Romney's message to voters does not get blown off course.
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 of 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."
Expecting earlier that Tampa would be in Isaac's sights, Republican 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, reporting from Tampa, said that there are several reasons Republicans are considering delaying the convention.
|Isaac, which caused havoc across eastern Cuba, was expected to strengthen into a hurricane before hitting the Florida Keys [Reuters]
"First of all, there is the safety issue. 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 programme 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," Dennis Feltgen, National Hurricane Centre spokesman, said.
The Keys were bracing for storm surges of up to 4ft, 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.
On Mississippi's Gulf Coast, residents started stocking up on supplies and securing their homes.
"It is packed. They are clearing the shelves," said Lindy Stewart after shopping at a Sam's Club in Gulfport. Stewart said she bought bread, lunch meat and other "stuff you need to survive a couple of days without power".
The Mississippi State Port Authority ordered the port of Gulfport cleared of all cargo vessels and cargo containers.
Isaac is also expected to pass directly over the US' offshore oil patch, which produces about 23 per cent of US oil output and seven per cent of its natural gas output.
The storm has already left a trail of suffering across the Caribbean.