Weather upending routines and causing havoc is nothing new for New Orleans, which has contended with hurricanes, heat waves and the occasional ice storm.

But snow? Not so much. The city went 15 years without measurable snow until Tuesday morning.

“I would call it a poet’s snow,” waxed Kevin Bennett, a writer who pulled a folding chair outside in the French Quarter to smoke a cigarette in the blur of whirling white flakes. “I love it.”

For the vulnerable, though, the bitter cold was a real danger. “It’s horrible, in its own beautiful way,” said Chad Rigsby, 52, after he pulled off the blanket he had slept under on Monday night.

Major highways and bridges were closed on Tuesday. Many roadways had become impassable. Classes were canceled in local schools, and many businesses could not open as the snow essentially brought New Orleans to a standstill. Officials warned residents to hunker down, since the snow was expected to continue into Tuesday evening.

Gov. Jeff Landry said the same entreaty applied to a vast swath of the state, as forecasters predicted levels of snowfall that could break records. “This creates a very dangerous situation, not only for travel but for people, pipes, pets and plants,” Mr. Landry, a Republican, said in a news conference on Monday.

Jay Grymes, a longtime television meteorologist in Baton Rouge who became the state climatologist last year, said the state had not experienced wintry conditions like this since the 1960s.