NEW ORLEANS – A monstrous Hurricane Katrina barreled toward New Orleans yesterday with 160-mph wind and a threat of a 28-foot storm surge, forcing a mandatory evacuation of the city, which sits below sea level, and prayers for those who remained to face a doomsday scenario.

“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. “It’s very frightening.”

Katrina intensified into a Category 5 giant over the warm water of the Gulf of Mexico, reaching top winds of 175 mph before weakening slightly on a path to hit New Orleans around sunrise today. That would make it the city’s first direct hit in 40 years and the most powerful storm ever to slam the city.

Forecasters warned that Mississippi and Alabama were also in danger because Katrina was such a big storm, with hurricane-force winds extending up to 105 miles from the center. In addition to the winds, the storm packed the potential for a surge of 18 to 28 feet, 30-foot waves, and as much as 15 inches of rain.

“The conditions have to be absolutely perfect to have a hurricane become this strong,” said the National Hurricane Center’s director, Max Mayfield, noting that Katrina may yet be more powerful than the last Category 5 storm, 1992’s Hurricane Andrew, which at 165 mph leveled parts of South Florida, killed 43 people and caused $31 billion in damage.

“It’s capable of causing catastrophic damage,” he said. “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.”

By evening, the first squalls, driving rains, and lightning began hitting New Orleans. A grim Mayor C. Ray Nagin earlier ordered the mandatory evacuation for his city of 485,000, conceding Katrina’s storm surge pushing up the Mississippi River would swamp the city’s system of levees, flooding the bowl-shaped city, and causing potentially months of misery.

“We are facing a storm that most of us have long feared,” he said. “This is a once-in-a-lifetime event.”

[President Bush yesterday encouraged people in the hurricane’s path to make safety their foremost concern and to move to higher ground as instructed, the Associated Press reported.

“We cannot stress enough the danger this hurricane poses to Gulf Coast communities, Mr. Bush said. “I argue all citizens to put their own safety and the safety of their families first by moving to safe ground.”]

As many as 100,000 inner-city residents didn’t have the means to leave and an untold number of tourists were stranded by the closing of the airport, so the city arranged buses to take people to 10 last-resort shelters.

For years, forecasters have warned of the nightmare flooding a big storm could bring to New Orleans, a bowl shaped city bounded by the half-mile wide Mississippi River and massive Lake Pontchartrain.

As much as 10 feet below sea level in spots, the city is as the mercy of a network of levees, canals, and pumps to keep dry.

Scientists predicted Katrina could easily overtake that levee system, swamping the city under a 30-feet cesspool of toxic chemicals, human waste, and even coffins that could leave more than 1 million people homeless.

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

Jefferson Parish’s president, Aaron Broussard, said some who have ridden out previous storms in the New Orleans area may not be so lucky this time.

“I’m expecting that some people who are die-hards will die hard,” he said.

By 11 p.m. EDT, Katrina’s eye was about 105 miles south of the mouth of the Mississippi River and 170 miles south-southeast of New Orleans. The storm was moving toward the north-northwest at about 10 mph and was expected to turn toward the north. A hurricane warning was in effect for the north-central Gulf Coast from Morgan City, La., to the Alabama-Florida line.

Despite the dire predictions, a group of residents in a poor neighborhood of central New Orleans sat on a porch with no car, no way out, and, surprisingly, no fear.

“We’re not evacuating,” said Julie Paul, 57. “None of us have any place to go. We’re counting on the Superdome. That’s our lifesaver.”

The 70,000-seat Superdome, the home of football’s Saints, opened at daybreak yesterday, giving first priority to frail, elderly people on walkers, some with oxygen tanks. They were told to bring enough food, water, and medicine to last up to five days.

“They told us not to stay in our houses because it wasn’t safe,” said Victoria Young, 76, who sat amid plastic bags and a metal walker. “It’s not safe anywhere when you’re in the shape we’re in.”

The evacuation was slow going. Highways in Louisiana and Mississippi were jammed all day as people headed away from Katrina’s expected landfall. All lanes were limited to northbound traffic on Interstates 55 and 59, and westbound on I-10. At the peak, 18,000 vehicles an hour were streaming out of southeastern Louisiana.

By last night, most major highways were cleared out and state police warned that late escapes would be impossible after high winds hit elevated expressways over the surrounding swamps.

Evacuation orders were also posted along the Mississippi and Alabama coast, and in barrier islands of the Florida Panhandle, where crashing waves swamped some coastal roads.

New Orleans has not taken a major direct hit from a hurricane since Betsy blasted the Gulf Coast in 1965. Flood waters approached 20 feet in some areas, fishing villages were flattened, and the storm surge left almost half of New Orleans under water and 60,000 residents homeless. Seventy-four people died in Louisiana, Mississippi, and Florida.

Tourists stranded by the shutdown of New Orleans’s Louis Armstrong Airport and the lack of rental cars packed the lobbies of high-rise hotels, which were exempt from the evacuation order to give people a place for “vertical evacuation.”

Tina and Bryan Steven, of Forest Lake, Minn., sat glumly on the sidewalk outside their hotel in the French Quarter.

“We’re choosing the best of two evils,” said Mr. Steven. “It’s either be stuck in the hotel or stuck on the road. … We’ll make it through it.”

His wife, wearing a Bourbon Street T-shirt with a lewd message, interjected: “I just don’t want to die in this shirt.”