The hurricane's landfall could still come in Mississippi and affect Alabama and Florida, but it looked likely to come ashore Monday morning on the southeastern Louisiana coast, said Ed Rappaport, deputy director of the National Hurricane Center in Miami. That put New Orleans squarely in the crosshairs.

"If it came ashore with the intensity it has now and went to the New Orleans area, it would be the strongest we've had in recorded history there," Rappaport said in a telephone interview Sunday morning. "We're hoping of course there'll be a slight tapering off at least of the winds, but we can't plan on that. So whichever area gets hit, this is going to be a once in a lifetime event for them."

At 8 a.m., Katrina's center was about 250 miles south-southeast of the mouth of the Mississippi River, the hurricane center said. It was moving west-northwest at about 12 mph and a gradual turn toward the north-northwest was expected. Hurricane force-wind of at least 74 mph extended up to 85 miles from the center.

The storm had the potential for storm surge flooding of up to 25 feet, topped with even higher waves, as much as 15 inches of rain, and tornadoes.

"All indications are that this is absolutely worst-case scenario," Ivor van Heerden, deputy director of the Louisiana State University Hurricane Center, said Sunday afternoon.

The center's latest computer simulations indicate that by Tuesday, vast swaths of New Orleans could be under water up to 30 feet deep. In the French Quarter, the water could reach 20 feet, easily submerging the district's iconic cast-iron balconies and bars.

Estimates predict that 60 percent to 80 percent of the city's houses will be destroyed by wind. With the flood damage, most of the people who live in and around New Orleans could be homeless.

"We're talking about in essence having — in the continental United States — having a refugee camp of a million people," van Heerden said.

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Experts have warned about New Orleans' vulnerability for years, chiefly because Louisiana has lost more than a million acres of coastal wetlands in the past seven decades. The vast patchwork of swamps and bayous south of the city serves as a buffer, partially absorbing the surge of water that a hurricane pushes ashore.

Experts have also warned that the ring of high levees around New Orleans, designed to protect the city from floodwaters coming down the Mississippi, will only make things worse in a powerful hurricane. Katrina is expected to push a 28-foot storm surge against the levees. Even if they hold, water will pour over their tops and begin filling the city as if it were a sinking canoe.

After the storm passes, the water will have nowhere to go.

In a few days, van Heerden predicts, emergency management officials are going to be wondering how to handle a giant stagnant pond contaminated with building debris, coffins, sewage and other hazardous materials.

"We're talking about an incredible environmental disaster," van Heerden said.

He puts much of the blame for New Orleans' dire situation on the very levee system that is designed to protect southern Louisiana from Mississippi River floods. Before the levees were built, the river would top its banks during floods and wash through a maze of bayous and swamps, dropping fine-grained silt that nourished plants and kept the land just above sea level.

The levees "have literally starved our wetlands to death" by directing all of that precious silt out into the Gulf of Mexico, van Heerden said.