As New Orleans battened down its hatches Sunday, evacuations were ordered, the Superdome was turned into a shelter, and emergency plans swung into effect against a flood threat the below-sea-level city has long dreaded.
The danger is also real in Alabama and Mississippi, where many in coastal areas rushed Sunday to get out of harm's way and onto higher ground.
The storm might spare New Orleans a direct hit, while posing a greater danger to the coastal Mississippi cities of Gulfport and Biloxi.
Three residents of a Louisiana nursing home died late Sunday while trying to get out of the path of the storm, according to CBS News Affiliate WWL-TV, which reports the three people were killed in an incident involving a school bus in the Baton Rouge area.
Click here for live Webcast coverage of Hurricane Katrina, from CBS News Affiliate WWL-TV in New Orleans.
Because of its size, with hurricane-force winds extend up to 105 miles from the center, and its potential to spawn tornadoes - even areas far from the landfall could be devastated.
"It's capable of causing catastrophic damage," said Max Mayfield, director of the National Hurricane Center, referring to the danger for New Orleans. "Even well-built structures will have tremendous damage. Of course, what we're really worried about is the loss of lives... New Orleans may never be the same."
Much of coastal Alabama was evacuated Sunday as emergency officials warned that Hurricane Katrina could bring historic flood levels to Mobile's downtown riverfront and leave bayou and beachfront roads under a sea of water. Mobile Mayor Michael Dow said the possible flooding could be worse than the 9-foot surge that soaked downtown and turned a key interstate connector into a lake during Hurricane Georges in 1998.
Evacuations were also ordered in coastal Mississippi, as the many floating casinos in the area packed up their chips and closed.
Hundreds of thousands in the three states heeded official advice Sunday to evacuate, some heading to shelters and others clogging the roads as they tried to reach friends, relatives and motels on higher ground.
"Have God on your side, definitely have God on your side," Nancy Noble said as she sat with her puppy and three friends in six lanes of one-way traffic on gridlocked Interstate 10 in Louisiana. "It's very frightening."
"I'm really scared," said Linda Young as she filled her gas tank near New Orleans. "I've been through hurricanes, but this one scares me. I think everybody needs to get out."
Because much of New Orleans is below sea level in a basin, a complex levee system is New Orleans' only protection from major flooding. Chief Joseph Matthews of the Office of Emergency Preparedness tells CBS News that if the levees wind up underwater, the resulting floods could take as long as two weeks to drain.
Dr. Walter Monsour, director of emergency management in the New Orleans area, says the city is "going to experience a significant tidal surge" and is asking evacuees to stay out of town until well after the storm - to give authorities time to assess the expected damage.
Rain began falling on southeastern Louisiana at midday Sunday, the first hints of a storm with a potential surge of 18 to 28 feet, topped with even higher waves, tornadoes and as much as 15 inches of rain.
"We are facing a storm that most of us have long feared," Mayor Ray Nagin said in ordering the mandatory evacuation for his city of 485,000 people, surrounded by suburbs of a million more. "The storm surge will most likely topple our levee system."
"This is very serious, of the highest nature," said Nagin. "This is a once-in-a-lifetime event."