New Orleans braced for another major storm Monday, seven years after Hurricane Katrina swamped the fabled US city of jazz, leaving behind a devastating sprawl of destruction and death.

A hurricane warning was issued for New Orleans and nearby areas as Tropical Storm Isaac churned toward the northern Gulf of Mexico coast, with sustained winds of 65 miles (100 kilometers) per hour.

Alabama governor Robert Bentley has ordered mandatory evacuations in the coastal counties of Mobile and Baldwin. The storm is likely to reach hurricane force and make landfall on Tuesday.

Louisiana Governor Bobby Jindal, who has recommended voluntary evacuations within the hurricane watch area, urged people to prepare for the worst.

"If you are in low lying areas and are thinking about evacuating, today is the day to do that," he said in a statement issued on Monday, predicting that Isaac would hit New Orleans as a category one hurricane.

"If you plan on hunkering down at home, today is the day to get supplies. I strongly encourage people not to wait," added Jindal, who is not attending the weather-affected Republican National Convention in Tampa, Florida.

Isaac was swirling in the Gulf of Mexico about 280 miles (450 kilometers) southeast of the mouth of the Mississippi River, and is expected to gather pace in the next 48 hours, the National Hurricane Center said.

It was heading northwest at 14 miles per hour.

"Isaac was expected to become a hurricane before reaching the northern Gulf coast," the agency added.

Forecasters fretted about dangerous surges in water levels and a slow-moving storm dumping up to 18 inches (46 centimeters) of rain on southeastern Louisiana, southern Alabama, Mississippi and the western Florida panhandle.

Mississippi activated 1,500 National Guard troops, while Florida called up only a few dozen service personnel and Louisiana issued orders to approximately 4,100 soldiers and airmen in preparation of the storm making landfall.

Louisiana, Mississippi and Alabama issued emergency warnings on Sunday, 24 hours after Isaac forced the main program of the Republican convention to be postponed by one day.

"Under current forecasts, New Orleans may feel winds as early as Monday night, with heavy weather Tuesday and Wednesday," said city mayor Mitch Landrieu, declaring an emergency and urging officials to prepare.

The NHC said "preparations to protect life and property should be rushed to completion," in all areas subject to the hurricane warning.

The agency also warned that Isaac could spawn "isolated tornadoes" over central and southern Florida on Monday.

Hurricane Katrina made landfall along the Mississippi-Louisiana border on August 29, 2005 -- eventually killing around 1,800 people in New Orleans, a city famed for its music, easygoing atmosphere and Creole cuisine.

It was the third deadliest US hurricane on record.

Though 1.4 million residents and visitors were ordered to evacuate as the monster storm approached, many could not or would not and were left stranded.

A lack of preparation and bungled coordination forced residents to take shelter in attics, and then break through their roofs to escape rising water.

Sunday's emergency declarations indicated the importance of official efforts to safeguard the city, as Isaac brought rain and choppy seas to the Florida Keys after battering Haiti and Cuba over the weekend.

In Haiti, the death toll from Isaac surged to 19 as reports of damage and destruction trickled in from remote areas, authorities said on Monday.

In Tampa, the Republican convention began with a whimper as the bad weather reduced the gala opening of Mitt Romney's coronation as the party's presidential candidate to a symbolic session of less than two minutes.

It was supposed to be a raucous launch-pad for four days of carefully choreographed political theater, but Isaac has hogged the spotlight.

Party officials have said primetime speaking slots on Tuesday, Wednesday and Thursday, including speeches by Romney, his wife Ann and running-mate Paul Ryan, remain unchanged.