The coldest, most dangerous blast of polar air in decades has gripped the US midwest and pushed toward the east and south and eastern Canada.
It closed schools and day care centres, grounded flights and forced people to pull their hoods and scarves tight to protect exposed skin from nearly instant frostbite.
Many across the nation went into virtual hibernation, while others dared to venture out in temperatures that plunged well below -18C (0F).
"I'm going to try to make it two blocks without turning into crying man," said Brooks Grace, who was wrapping up to do some banking and shopping in Minneapolis, where temperatures reached -31C (-24F) with wind chills of -45C (-48F). "It's not cold - it's painful."
Forecasters said some 187 million people in all could feel the effects of the "polar vortex" by the time it spread across the country last night and today.
Record lows were possible in the east and south, with wind chills expected to reach -23C (-10F) in Atlanta and -24C (-12F) in Baltimore.
From the Dakotas to Maryland, schools and day care centres shut down.
For a big swath of the midwest, the bone-chilling cold moved in behind another winter wallop - more than a foot of snow and high winds that made travelling treacherous.
Several deaths were blamed on the snow, ice and cold since Saturday, including that of a one-year-old boy who was in a car that went out of control and collided with a snowplough yesterday in Missouri.
It took authorities using 10-ton military vehicles known as "wreckers" until early yesterday to clear all the chain-reaction accidents caused when several lorries jackknifed along snowy highways in southern Illinois.
The crash stranded about 375 vehicles, but there were no fatalities or injuries, largely because motorists either stayed with their cars or were rescued and taken to nearby warming centres if they were low on petrol or did not have enough coats or blankets, said Jonathon Monken, director of the Illinois emergency management agency.
Others got stuck in the snowdrifts, including the Southern Illinois men's basketball team, which had to spend the night sleeping in a church.
In the eastern US, higher temperatures yesterday helped melt piles of snow from a storm last week, raising the risk that roads would freeze over as the cold air moved in later, said Bob Oravec from the weather prediction centre in College Park, Maryland.
The snap was set to be dramatic: Springfield, Massachusetts, enjoyed 13C (56F) yesterday morning but faced an overnight low of -14C (6F).
More than 3,700 flights were cancelled by late yesterday afternoon, following a weekend of travel disruption across the US.
Airline officials said de-icing fluid was freezing, fuel was pumping sluggishly, and workers were having difficulty loading and unloading luggage.
JetBlue Airways stopped all scheduled flights to and from New York and Boston yesterday. Southwest ground to a halt in Chicago earlier in the day, but by the evening, flights resumed in "a trickle," a spokesman said.
The authorities in Indiana and Kentucky warned people not to leave their homes at all unless they needed to go somewhere safer.
Utility crews worked to restore power to more than 40,000 Indiana customers affected by the weekend storm and warned that some people could be in the cold and dark for days.
Officials in Chicago and other cities checked on the homeless for fear they might freeze to death on the street.
In Canada, much of Saskatchewan and Manitoba were in a deep freeze yesterday with temperatures near or below minus -30C (-22F) and wind chills colder than -40C (-40F).
In Newfoundland, about 5,000 customers remained without power because of rolling blackouts in recent days, but Premier Kathy Dunderdale said it was not a crisis and government services were still operating.