Millions of Americans have hunkered down in anticipation of a record-setting "polar vortex" expected to bring dangerously low temperatures to much of the United States.
The rotating pool of cold, dense air was forecast to affect more than half of the continental US throughout Sunday and into Monday and Tuesday, with wind-chill warnings stretching from Montana to Alabama.
The Arctic blast could send temperatures in the US to their lowest in two decades.
"Combined with gusty winds, these temperatures will result in life-threatening wind-chill values," the National Weather service said, noting heavy snow from the eastern Plains to the Great Lakes could bring up to a foot of accumulation.
Weather forecasters said with the wind chill, temperatures could plunge as low as -51C in some areas, prompting authorities in several towns and cities to warn residents to stay indoors and stock up on food.
In such extreme cold, exposed skin would suffer frostbite in as little as five minutes, experts say.
Several states in the Midwest had already received up to a foot of new snow, creating dangerous travel conditions, and prompting church and school closures. Thousands of flights had been cancelled or delayed.
The National Weather Service said the icy temperatures were caused by a relatively infrequent alignment of weather conditions, allowing the Arctic polar vortex to be displaced unusually far south.
"The weather pattern across North America right now is set up to be very favourable for the southward transport of Arctic air," said meteorologist Bob Oravec.
"It's not going to be long-lived," he added, noting temperatures would start to moderate countrywide by the end of the week.
In New York, which declared a state of emergency when storm Hercules swept in on Thursday, John F Kennedy Airport halted operations for more than two hours after a Delta Airlines jet from Toronto slid into a snowbank.