Tens of millions of Americans are enduring bone-chilling temperatures, blizzard-like conditions, power outages and cancelled holiday gatherings due to a winter storm that forecasters say is nearly unprecedented in its scope.

More than 200 million people, or about 60 percent of the United States population, were under some form of winter weather advisory or warning on Friday, the National Weather Service said. The agency’s weather map “depicts one of the greatest extents of winter weather warnings and advisories ever”, forecasters said in a statement.

More than 4,200 flights within, into or out of the US were cancelled by midday Friday, according to the tracking site FlightAware, causing more mayhem as travellers try to make it home for the year-end holidays. Hundreds of thousands of homes and businesses were without power on Friday morning.

In Canada, WestJet cancelled all its flights on Friday at Toronto Pearson International Airport beginning at 9am (14:00 GMT).

“This is not like a snow day when you were a kid,” US President Joe Biden warned on Thursday after a briefing from federal officials. “This is serious stuff.”

Forecasters are expecting a bomb cyclone when atmospheric pressure drops very quickly in a strong storm. The phenomenon is forecast to develop near the Great Lakes. That will stir up blizzard-like conditions, including high winds and heavy snow.

Among those passengers with cancelled flights was Ashley Sherrod, who planned to fly from Nashville, Tennessee, to Flint, Michigan, on Thursday afternoon. Sherrod was debating whether to drive or risk booking a Saturday flight that she worries will also be cancelled: “My family is calling. They want me home for Christmas, but they want me to be safe too.”

The cold has led to high demand at homeless shelters, including in Detroit, where some shelters were at capacity on Thursday.

“We are not sending anyone back into this cold,” Aisha Morrell-Ferguson, a spokeswoman for COTS, a family-only shelter, told the Detroit News.

In Portland, Oregon, officials opened four emergency shelters. In the city’s downtown, Steven Venus tried to get on a light-rail train to escape the cold after huddling on the sidewalk overnight in below-zero temperatures.

“My toes were freezing off,” he said with a sleeping bag wrapped around his head near a flimsy tent where another homeless person was taking shelter.

In Buffalo, New York, forecasters predicted a “once-in-a-generation storm” because of heavy lake-effect snow, wind gusts as high as 105 kilometres per hour (65 miles per hour), whiteouts and the potential for extensive power outages.