Tropical Storm Isaac rolled over the open Gulf of Mexico on Monday, where it was expected to grow into a hurricane before hitting land somewhere between Louisiana and Florida close to the seventh anniversary of the devastating Hurricane Katrina.

The storm that left 24 dead in Haiti and the Dominican Republic over the weekend promised a drenching but little more for Tampa, where the Republican National Convention had pushed back its start to Tuesday in case Isaac passed closer to the gulfside city.

The National Hurricane Center predicted Isaac would grow to a Category 1 hurricane over the warm Gulf and possibly hit late Tuesday somewhere along a roughly 300-mile (500-kilometre) stretch from the bayous southwest of New Orleans to the Florida Panhandle.

That would be one day shy of seven years after Hurricane Katrina struck in 2005, although Katrina was a much stronger Category 5 storm with winds over 252 kilometres an hour. Isaac was expected to have top winds of around 145 kilometres an hour when it hits land.

Earlier predictions said Isaac would be a Category 2 hurricane, but National Hurricane Center director Rick Knabb told ABC’s “Good Morning America” that Isaac wouldn’t be as strong as they initially thought. But Knabb warned that such storms often do not stick to forecasters’ predictions.

The size of the warning area and the storm’s wide bands of rain and wind prompted emergency declarations in four states, and hurricane-tested residents were boarding up homes, stocking up on food and water or getting ready to evacuate.

Federal Emergency Management Agency officials said the updated levees around New Orleans are equipped to handle storms stronger than Isaac. Levee failures led to the catastrophic flooding in the area after Katrina, which killed 1,800.

FEMA Administrator Craig Fugate, in a conference call with reporters, said people shouldn’t focus just on New Orleans.

“This is not a New Orleans storm. This is a Gulf Coast storm. Some of the heaviest impact may be in Alabama and Mississippi,” he said.

The storm stopped work on rigs that account for 24 per cent of daily oil production in the U.S. portion of the Gulf of Mexico and eight per cent of daily natural gas production there, the federal Bureau of Safety and Environmental Enforcement said in its latest update Sunday.

At 11 a.m. ET, Isaac remained a tropical storm with top sustained winds of 100 kilometres an hour. Its centre was about 500 kilometres southeast of the mouth of the Mississippi River, and it was moving northwest at 22 kilometres an hour.

If the storm hits during high tide, it could push floodwaters as deep as four meters on shore in Louisiana, Mississippi and Alabama and up to 1.8 metres in the Florida Panhandle.

Louisiana Gov. Bobby Jindal called a state of emergency, and 53,000 residents of St. Charles Parish near New Orleans were told to leave ahead of the storm. The governors of Mississippi, Florida and Alabama also declared states of emergency.

Even though the storm was moving well west of Tampa, tropical storm-force winds and heavy rains were possible in the area because of Isaac’s large size, forecasters said.

Only about 200 protesters were marching outside the Republican convention Monday, a fraction of the 5,000 who had been expected. Republicans planned to briefly gavel the gathering to order Monday afternoon and then recess until Tuesday.

Isaac caused 555 flights to be cancelled at Miami International Airport. But officials said damage in Florida appeared to be minimal.

The Gulf Coast hasn’t been hit by a hurricane since 2008, when Dolly, Ike and Gustav all struck the region.

Before reaching Florida, Isaac was blamed for 19 deaths in Haiti and five more in the Dominican Republic, and it downed trees and power lines in Cuba.