10:39 AM CDT on Sunday, August 28, 2005

Mary Foster / Associated Press

Mayor Ray Nagin ordered an immediate mandatory evacuation Sunday for all of New Orleans, a city sitting below sea level with 485,000 inhabitants, as Hurricane Katrina bore down with wind revved up to nearly 175 mph and a threat of a massive storm surge.

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, the National Hurricane Center said.

Only three Category 5 hurricanes - the highest on the Saffir-Simpson scale - have hit the United States since record-keeping began. The last was 1992's Hurricane Andrew, which leveled parts of South Florida, killed 43 people and caused $31 billion in damage. The other two were the 1935 Labor Day hurricane that hit the Florida Keys and killed 600 people and Hurricane Camille, which devastated the Mississippi coast in 1969, killing 256.

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."

He said loss of life was "what inevitably occurs" with a storm this strong.

"We're in for some trouble here no matter what," he said.