Forecasters say Isaac has strengthened into a Category 1 hurricane with 120 km/h winds as it makes its way toward Louisiana.
The U.S. National Hurricane Center said Tuesday that Isaac gained strength as it moved over the warm, open waters of the Gulf of Mexico.
Isaac is expected to make landfall over southeastern Louisiana, possibly the New Orleans area, either late Tuesday or early Wednesday
Shelters were open for those who chose to stay or missed the chance to get away before the outer bands of the large storm blow ashore.
In Houma, a city southwest of New Orleans, people filled a municipal auditorium-turned-shelter. Simon and Crystal Naquin were there with their teenage sons because the camper they call home sits in a flood-prone spot between a navigation canal and lower Bayou Caillou.
Simon Naquin said he rode out hurricanes when he was younger, but doesn't do that anymore since seeing the damage wrought by hurricanes Andrew, Katrina and Rita.
"Now that I got kids, I've seen too much to say, `Stay,"' said Naquin, who shared a twin air mattress with his wife while their sons read and snacked on jambalaya amid a pile of blankets and next to a stash of water bottles and food.
U.S. President Barack Obama warned of the possibility of flooding caused by Isaac, and he encouraged residents along the Gulf coast to listen to local officials if evacuation orders are issued.
"Now is not the time to tempt fate," Obama said in a brief appearance at the White House. "Now is not the time to dismiss official warnings. You need to take this seriously."
Forecasters warned that Isaac was a large storm whose effects could reach out 322 kilometres from its centre. Water may be worse than wind because the storm could push walls of water while dumping rain to flood the low-lying coast in Louisiana, Mississippi, Alabama and the Florida Panhandle.
Isaac's track is forecast to bring it to New Orleans seven years after Katrina hit as a much stronger storm on Aug. 29, 2005. By Tuesday afternoon it was about 220 km southeast of New Orleans, with sustained winds of 120 km/h.
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This time, federal officials say the updated levees around the city are equipped to handle storms stronger than Isaac. The Army Corps of Engineers was given about $14 billion US to improve flood defences, and most of the work has been completed.
New Orleans Mayor Mitch Landrieu did not activate a mandatory evacuation Monday. Instead, officials urged residents to hunker down and make do with the supplies they had.
But with landfall expected near the Katrina anniversary, anxiety was high, especially in the Lower 9th Ward, wiped out by Katrina after floodwalls burst and let the waters rush in.
"I don't really trust the levees," said Robert Washington, who planned to evacuate along with his wife and five children. "I don't want to take that chance. I saw how it looked after Katrina back here."
He leaned over the banister of his porch railing and looked out onto empty lots where houses stood before Katrina. His neighborhood, just a few blocks away from where the floodwall protecting the Lower 9th Ward broke open, remains largely empty.
A few streets over, Arthur Smith was unpacking supplies from his car and taking them into his renovated house.
"We have the lamp oil, the water, non-perishable food items, the radio that works without a battery," he said, listing some items on his checklist.
He was planning to either evacuate or hunker down with his 76-year-old mother and sister. He said that decision would be made Tuesday morning once Isaac's forecast became better defined.
As of 12:20 p.m. Tuesday, the storm system was centered about 115 km south-southeast of the mouth of the Mississippi River and moving northwest at 17 km/h.
Although Isaac's approach on the eve of the Katrina anniversary invited comparisons, the storm is nowhere near as powerful as Katrina was when it struck. Katrina at one point reached Category 5 status with winds of more than 252 km/h, and made landfall as a Category 3 storm.
Still, forecasters at the National Hurricane Center warned that Isaac, especially if it strikes at high tide, could cause storm surges of up to 3.7 metres along the coasts of southeast Louisiana and Mississippi and up to 1.8 metres as far away as the Florida Panhandle..
Rain from the storm could total up to 355 millimetres, with some isolated areas getting as much as 508 millimetres, along the coast from southeast Louisiana to the extreme western end of the Florida Panhandle.