LAST CHRISTMAS DAY, in New Orleans, it started to snow. We were eating Creole spiced turkey with all the trimmings at Tujagues restaurant in the French Quarter, when the waiter, his eyes wide with wonder, announced: “It’s snowing in New Orleans on Christmas Day. Ain’t never seen that before. This sure is the darndest weather.” A single saxophone was playing a jazzed-up Silent Night outside, and we all piled out to stare at the sky as the children danced on the pavement and fat flakes drifted down over the wrought-iron balustrades.

My sister, who has lived in New Orleans for a decade, got out of town on Sunday before the hurricane struck. She loaded her young son and some belongings into the station wagon and didn’t stop until they reached Huntsville, Alabama. Behind her, the darndest weather was laying waste to the city: a great tide of filthy water washing down Bourbon Street, on the day the music died. Wars and Prohibition and the Great Depression and moral disapproval couldn’t stop the party in New Orleans, but Hurricane Katrina could.

New Orleans holds a unique place in the American imagination because, in truth, it is a place apart from America. In a country inclined to puritanism, the Big Easy is the almost Mediterranean enclave where America came to drink and eat too much, to stay up late and behave badly. No American city, with the exception of New York, has such a vivid mythology; no city, including New York, is more conscious of its own romance. The normal rules do not apply here. New Orleans is famed for its jazz and blues, but it also has what so many American cities lack: soul. Corrupt, voluptuous and profoundly literate, New Orleans marches to the beat of a different drummer, and if it wants to defy convention by snowing on Christmas Day for the first time in living memory, then that is what it does.

I first fell in love with New Orleans as a student when I travelled down from New York for Mardi Gras: a lost weekend that still tastes, 15 years later, of a cocktail called (I shudder) a Hurricane. Even the street names beckoned huskily: Amour, Abundance, Treasure.

The city is a spicy human gumbo: a mixture of Creole and Cajun, white and black, French, English, Spanish Irish, German and African, the kitsch and the cool. Congo Square in New Orleans was the first place where slaves could freely sing and dance and practise voodoo while African culture was outlawed across the South. The city became a haven of African spirituality, from which jazz, the blues and Louis Armstrong would emerge.

People who don’t fit with the rest of America gravitate here. “This city is the flagrant vice capital of the civilised world,” declares Ignatius J. Reilly in John Kennedy Toole’s Confederacy of Dunces. “This city is famous for its gamblers, prostitutes, exhibitionists, anti-Christs, alcoholics, sodomites, drug addicts, fetishists, onanists, pornographers, frauds, jades, litterbugs and lesbians, all of whom are only too well protected by graft.” No doubt, even now, some Bible-thumpers will be hailing divine retribution for decades of decadence, as Sodom-by-Sea is swamped.

But in the city the sacred and the profane happily cohabited, as Walker Percy pointed out: “Out and over a watery waste and there it is, a proper enough American city, and yet within the next few hours the tourist is apt to see more nuns and naked women than he ever saw before.”

Even buttoned-up Midwesterners find it hard to resist the city’s steamy Southern embrace. This is the world of William Faulkner and Tennessee Williams, dripping with Spanish moss and genteel decay and languid melancholy. “The air was so sweet in New Orleans,” wrote Jack Kerouac (once the Benzedrine had worn off). “You could smell the river, and every kind of tropical exhalation with your nose suddenly removed from the dry ices of a Northern winter.”

The river, and the lake: held back by a combination of earthworks and hope, both manufactured by Man. Here is a city reclaimed from water, reflected in the lichen-greening walls, the running to seed at speed, the good times rolling on beside the rolling Mississippi: there is a sensation of dancing on borrowed time. Even the dead are restless, buried in raised graves above the sodden earth, for this is a city suspended above the swampy estuaries of the Mississippi.

I grieve for New Orleans. I wonder what has happened to that hopeless caricaturist in Jackson Square, the carriage driver who christened his intractable mule Mother-in-Law, and the battered jazz trumpeter whose swooping rendition of When the Saints Go Marching In inspired my son to take up the instrument. I wonder if my sister can bear to return.

It is one thing to see your city attacked, but another to see it drowned, wiped away by flood and ravaged by looters. The “city that care forgot” will never be as carefree again; the Big Easy has never faced a harder moment. Yet for a place that celebrated vibrant life, death was always a part of New Orleans culture: in the cemeteries crammed with victims of yellow fever and malaria, in voodoo morbidity and in the obsession with ghosts and vampires.

The New Orleans street funeral parade is the city’s way of grieving, a wake set to jazz: the march to the churchyard to the sombre strains of Just a Closer Walk with Thee, then breaking into the joyful chorus, the living dancing with the dead. That, I suspect, is how New Orleans will cope with this cataclysm, when the music starts again: a mournful dirge, and then a soul-raising chorus of hope.

On Christmas Day, when it had stopped snowing, we walked along the levee beside Lake Pontchartrain. “How mad is this?” my sister said. “Who builds a city on a swamp six feet below sea level?” Then, in an authentic N’Awlins drawl, she added: “It really didn’t ought to be here.” Then we forgot about the rising water, and went to Tipitina’s on Napoleon Avenue, and danced through the night.

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