Hurricane Issac has been downgraded to a tropical storm, after dumping huge amounts of rain on the US Gulf Coast as its moves inland with the threat of more heavy rain.
During the next few days, heavy rain is expected to spread inland and water levels will remain high along the northern Gulf Coast, the US National Hurricane Center said.
"Even though Isaac is no longer a hurricane, life-threatening hazards from storm surge and inland flooding are still occurring," the NHC said on Thursday.
Isaac, which is packing winds of 95kmh, is moving towards the northwest near 9 kmh and is expected to continue in this direction through tonight, followed by a turn toward the north-northwest by Thursday night or early Friday, the NHC said.
It has already caused widespread flooding in the southeast, including New Orleans, where seven years ago Hurricane Katrina caused massive destruction.
New Orleans flooding
Hundreds of thousands of people were without power across Louisiana's southern parishes, including more than 250,000 in New Orleans and its suburbs, power provider Entergy reported.
Isaac's centre slowed over the Gulf, whipping winds of 130km per hour (kph) across parts of Florida, Alabama, and Mississippi.
A storm surge also overflowed parts of a 29km levee in rural Plaquemines Parish south of New Orleans, flooding areas and possibly trapping some people in their homes, according to parish officials.
"The biggest test for New Orleans is going to be the city's brand new pumps and levees and also the big barrier which is about 20km outside the city designed to stop the water getting anywhere near it," Al Jazeera's John Terrett reported.
"They're very confident that all that is going to work this time."
Oil production in the area has nearly ground to a halt, and ports and coastal refineries curtailed operations as Isaac approached.
The Alliance refinery in Belle Chasse, which is owned by Phillips 66 and produces 247,000 barrels a day, lost power on Tuesday night, the Times Picayune newspaper reported.
But the company said it had already shut the facility down.
The storm has brought high winds, soaking rains and storm surges that will pose the first major test for multibillion-dollar flood protections put in place in New Orleans after Hurricane Katrina devastated the city and coast seven years ago.
President Barack Obama declared federal emergencies in the areas hit hard by Isaac, moving to speed federal aid and recovery efforts to the hurricane-battered Gulf Coast.
Obama signed the orders late on Wednesday covering 35 parishes in Louisiana and 34 counties in Mississippi.
The White House said in a statement that the disaster declarations ordered authorities to expedite federal aid in areas affected by Isaac.
The move makes the federal funding available to state and eligible local governments and certain nonprofit organisations for emergency work in the stricken areas.