Hurricane Isaac intensified on Tuesday, lashing New Orleans with rains and strong winds seven years after Hurricane Katrina devastated the "Big Easy" and killed 1,800 people on the US Gulf Coast.
The National Hurricane Center in Miami warned the category one hurricane's maximum sustained winds had increased to 80 miles (130 kilometers) per hour, and could yet gain more pace before making landfall later Tuesday.
Isaac was "getting better organized," the NHC said in its 2100 GMT bulletin, warning that flooding from the storm surge was "expected" in coastal areas.
States of emergency were declared in Louisiana and Mississippi, allowing authorities to coordinate disaster relief and seek emergency federal funds.
New Orleans Mayor Mitch Landrieu said the city could expect up to 16 inches (40 cm) of rain or more because the hurricane was moving slowly over the area.
"We have dodged a bullet in the sense that this is not a category three storm," he said, "But a category one at this strength... is plenty big enough to put a big hurt on you if you fall into complacency. Let's not do that."
US President Barack Obama urged people to take the threat seriously, warning of the possibility of major flooding and damage.
"I want to encourage all residents of the Gulf Coast to listen to your local officials and follow their directions, including if they tell you to evacuate," Obama said.
"Now is not the time to tempt fate. Now is not the time to dismiss official warnings. You need to take this seriously."
Obama said he had managed a wide-ranging effort by federal and local governments to make preparations.
His televised appearance showed the power of an incumbent to intervene at politically advantageous moments, just as Republicans met to nominate Mitt Romney as their candidate for the November presidential election.
The National Hurricane Center said that in some Gulf Coast areas -- such as the Mississippi-Alabama border east to Florida -- a hurricane warning has been replaced with a tropical storm warning.
But a full-blown hurricane warning remained in effect for metropolitan New Orleans, a city known as the Big Easy for its jazz and easy-going life-style.
As of 2100 GMT, the eye of the hurricane was heading northwest and its eye was about 30 miles (45 kilometers) southwest of the mouth of the Mississippi River, and about 100 miles southeast of New Orleans, the center said.
Louisiana Governor Bobby Jindal said the state contacted Washington about getting reimbursed for hurricane-preparation spending -- an allusion to agonizing delays in getting federal help after Katrina blasted the city.
"We sent a letter yesterday to the president. We have learned from past experiences that you can't wait. You have to push the federal bureaucracy," Jindal said.
The timing of the storm -- set to arrive on the seventh anniversary of Katrina -- had many here on edge.
"It brings back a whole lot of memories," said Melody Barkum, 56, who spent days stranded on a roof without food or water after Katrina struck. "I'm not afraid. If I can survive Katrina, I can survive this."
Katrina left behind a devastating sprawl of destruction and death when it hit New Orleans on August 29, 2005, and a bungled response by the Bush administration tarnished the president's second term in office.
Thousands of people were left stranded on the roofs of their houses for days after Katrina's storm surge smashed levees long-warned to be inadequate, flooding 80 percent of the low-lying city.
Those who made it to dry land faced deadly violence and looting as the city descended into chaos and officials failed to even provide water and food -- let alone security and medical aid -- in the sweltering heat.
Officials insisted billions of dollars spent to reinforce the city's storm levees and pumps will protect the Big Easy from inundation this time, and Isaac is nowhere near Katrina's strength.
But Isaac will still pack powerful winds expected to knock out power lines and churn up a massive surge of sea water as much as 12 feet (3.7 meters) deep that will roll up across the Gulf Coast.
Mandatory evacuations were ordered in a number of coastal counties in Louisiana and Alabama, where people typically build their homes on stilts.
The slow-moving and massive storm could dump as much as 20 inches (50 centimeters) of rain on isolated areas and spawn tornadoes, the NHC warned.
Jindal 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 Monday.
"If you plan on hunkering down at home, today is the day to get supplies. I strongly encourage people not to wait."
Those heeding the call included Tammy Edmondson, who looked anxious as she picked through the grocery shelves at a Target store with her daughter.
Edmondson said she left town ahead of Katrina and that it was a month before she could go home.
"We had a lot of damage -- we're still fixing some of it," she told AFP. "That's why I'm starting to panic."