New Orleans ? More than 1.2 million people in metropolitan New Orleans were warned to get out Tuesday as 140-mph Hurricane Ivan churned toward the Gulf Coast, threatening to submerge this below-sea-level city in what could be the most disastrous storm to hit in nearly 40 years.

Residents streamed inland in bumper-to-bumper traffic in an agonizingly slow exodus amid dire warnings that Ivan could overwhelm New Orleans with up to 20 feet of filthy, chemical-polluted water. About three-quarters of a million more people along the coast in Florida, Mississippi and Alabama also were told to evacuate.

Forecasters said Ivan, blamed for at least 68 deaths in the Caribbean, could reach 160 mph and strengthen to Category 5, the highest level, by the time it blows ashore as early as Thursday somewhere along the Gulf Coast.

With hurricane-force wind extending 105 miles from its center — and forecast to continue as much as 150 miles inland — Ivan could cause significant damage no matter where it strikes. Officials ordered or strongly urged an estimated 1.9 million people in four states to flee to higher ground.

“I beg people on the coast: Do not ride this storm out,” Mississippi Gov. Haley Barbour said, urging people in other parts of the state to open their homes to relatives, friends and co-workers.

At 10 p.m. CDT Tuesday, Ivan was centered about 295 miles south-southeast of the mouth of the Mississippi River and moving north-northwest at 12 mph.

The National Hurricane Center in Miami posted a hurricane warning for about a 300-mile swath from Apalachicola in the Florida Panhandle to New Orleans and Grand Isle in Louisiana. Forecasters said Ivan could bring a coastal storm surge of 10 to 16 feet, topped by large, battering waves.

New Orleans, the nation’s largest city below sea level, is particularly vulnerable to flooding, and Mayor Ray Nagin urged residents to get out while they can. The city’s Louis Armstrong Airport was ordered closed Tuesday.

Up to 10 feet below sea level in spots, New Orleans is a bowl-shaped depression that sits between the half-mile-wide Mississippi River and Rhode Island-size Lake Pontchartrain. It relies on levees, canals and huge pumps to keep dry.

The city has not taken a major direct hit from a hurricane since Betsy in 1965, when an 8- to 10-foot storm surge submerged parts of the city in seven feet of water. Betsy, a Category 3 storm, was blamed for 74 deaths in Louisiana, Mississippi and Florida.

Experts said Ivan could be worse.

Walter Maestri, an emergency manager in New Orleans, warned that if Ivan made a direct hit, 50,000 people could drown, and this city of Mardi Gras and jazz could cease to exist.

“This could be The One,” Maestri said. “You’re talking about the potential loss of a major metropolitan area.”