A "once-in-a-generation" winter storm with temperatures as low as -40 degrees Fahrenheit (Celsius) caused Christmas travel chaos in the United States on Thursday, with thousands of flights cancelled and major highways closed.

Heavy snow and howling winds upended holiday plans at one of the busiest times of the year, as a huge cold front swept down from the Arctic and took freezing hold of the middle of the country.

Tens of millions of people were under winter storm advisories or warnings, with meteorologists saying it was so cold in places that anyone venturing outside risked frostbite within minutes.

"This is not like a snow day when you were a kid," President Joe Biden told reporters. "This is serious stuff."

Blinding whiteouts and hazardous road conditions were already spreading, even as 100 million people were expected to take to the roads, according to the American Automobile Association.

'NO TRAVEL'

The I-90, a major highway running across the north was shuttered in South Dakota, with officials saying it would not reopen until Friday.

"Crews are using all available resources from across the state to clean-up and restore travel," South Dakota Department of Transport said.

"Multiple highways are currently listed as 'Road Impassable'… travel on the road segment is physically impossible due to widespread deep snow and drifts."

Around 100 motorists were stranded near Rapid City in the state, Pennington County Sheriff's Office tweeted.

"NO TRAVEL advised," it added.

Plane tracking website Flightaware.com showed more than 22,000 flights had been delayed on Thursday, with 5,500 cancelled outright, many at Chicago O'Hare or Denver, both international hubs.

Madison Painter told CNN she and her fiance had decided to drive 700 miles (1,100 kilometers) after their flight from Chicago to Atlanta had been cancelled.

"I wanted to get home to our families," she said.

Holiday travel volumes are expected to be close to pre-pandemic levels, with the busiest day on Thursday, three days before Christmas.

'Bomb cyclone'

AccuWeather forecasters have said the storm could rapidly strengthen into what is known as a "bomb cyclone" through a process known as "bombogenesis," when the barometric pressure drops and a cold air mass collides with a warm air mass.

The National Weather Service warned snow squalls — bursts of snow lasting an hour or two — had already happened or were expected from the Central Plains to the mid-Atlantic and Northeast.

Forecasters in Montana said they were expecting their coldest night of the Arctic snap Thursday, with temperatures down as low as -40 Fahrenheit (-40 Celsius), and windchill taking the temperature to a bone-crushing -60 Fahrenheit — only a little warmer than Mars, according to NASA data.

Rich Maliawco, lead forecaster for the National Weather Service in Glasgow, Montana, said an encroaching high pressure system was driving clouds away.

"When you've got skies that clear out… the temperatures are just going to fall considerably," he told AFP.

While long-time residents have experienced temperatures in this range before, extreme weather like this can be dangerous, Maliawco said.

"When it's this cold, anybody can run into trouble," he said.

"With these kinds of wind chills, if you're not wearing those warm layers… unprotected skin can get frostbite in less than five minutes."

Conditions were cold enough for people to post videos of themselves carrying out the "boiling water challenge," where boiling water is thrown into the air and instantly freezes.

"We created our own cloud @ -17° F (-27° C) at the #Missoula International Airport," tweeted NWS Missoula in Montana.

State of emergency

In Minneapolis and Saint Paul, more than eight inches (20 centimeters) of snow accumulated over a 24-hour period, the NWS said in a Thursday morning update.

Farther east in Buffalo, New York, forecasters called it a "once-in-a-generation storm" with wind gusts of more than 65 miles (105 kilometers) per hour, wind chills as low as 10 to 20 degrees F below zero, and power outages.

New York Governor Kathy Hochul joined the governors of several other states in declaring a state of emergency, warning of a long list of possible calamities.

"Heavy rain and snow, strong winds, coastal and lakeshore flooding, flash freezing, extremely low wind chills and power outages all possible," an announcement said.