The eastern half of the US has been hit by a dangerously cold whirlpool of dense air known as a "polar vortex" which threatens to break decades-old weather records.
The bitter weather comes after a heavy snowstorm hit much of the region last week. Schools in Chicago and other Midwest cities were closed and people warned to stay indoors.
More than 1,000 flights were canceled on Sunday at airports throughout the Midwest.
"It's just a dangerous cold," said a National Weather Service spokesman.
The forecast is extreme: -35 C in North Dakota, and -26 C in Minneapolis, Indianapolis and Chicago. Wind chills - what it feels like when high winds are factored into the temperature - could drop into the -45 to -50 C.
In New York the temperature was expected to drop sharply from about 11 C to about -12 C overnight as arctic air moves in.
It has not been this cold for almost two decades in many parts of the country. Frostbite and hypothermia can set in within minutes.
"I have seen frostbite occur through clothing," said Douglas Brunette, an emergency room doctor in Minneapolis. "It's not enough just to be covered. You need clothes made for the elements. You need to repel the wind."
The Indianapolis mayor upgraded the city's travel emergency level to "red," making it illegal for anyone to drive except for emergencies or seeking shelter. The last time the city issued such a travel warning was 1978.
Elnur Toktombetov, a Chicago taxi driver, said that an hour into his shift, his cab's windows were still coated with ice on the inside
Many cities came to a virtual standstill. School was called off for the state of Minnesota. Government offices and courts in several states closed.
Southern states were bracing for possible record cold temperatures, too. With two freezing nights ahead, Louisiana citrus farmers could lose any fruit they cannot pick in time.