Americans were urged to stay indoors and stock up on food after a fierce winter storm killed 11 people - with forecasters predicting that the Arctic blast could bring record low temperatures.
There was no sign of respite from the brutal chill that struck last week in parts of the northeastern US and Canada, prompting New York and New Jersey to declare a state of emergency.
After a heavy snowfall last Thursday, one of the coldest Arctic blasts in the past two decades is this week set to plunge America's Midwest into near-record freezing conditions.
Temperatures in Chicago today were expected to hover around minus 23 degrees Celsius, weather analysts said.
And strong gusts could bring what forecasters at the National Weather Service called "very dangerous levels" of wind chill.
"Incredibly, it may feel as cold as minus 45 Celsius to minus 50 Celsius on Sunday night over sections of the north-central states with the frigid air remaining in place into next week," it said.
In such conditions, exposed skin would suffer frostbite in as little as five minutes, forecasters cautioned.
Meteorologists blamed a "polar vortex" - a pool of cold, dense air rotating anticlockwise.
National Weather Service meteorologist Phillip Schumacher said: "It's just a large area of very cold air that comes down, forms over the North Pole or polar regions and usually stays in Canada. But this time it's going to come all the way into the eastern United States."
The authorities have urged people in the worst-hit areas to stay at home for their own safety and to allow rescue and clean-up teams to get to work as quickly as possible.
"Chicagoans are a hearty bunch," said Matt Smith, spokesman for the Chicago Department of Family and Support Services.
"But when temperatures get as cold as they are predicted, you want to start thinking out things in advance," he said.
He added that the department had advised people to make sure they have medical supplies and food.
In Minnesota, Governor Mark Dayton has already announced schools will be closed today "to protect all our children".
The storm - named Hercules by the US Weather Channel - has been the first big test for New York City's new Mayor Bill de Blasio, who took up his job last Wednesday. "If you want safe, clear streets, stay home," he said.
Thousands of domestic and international flights have been cancelled or delayed in several US cities including at New York's John F. Kennedy Airport and in Chicago, Boston and Philadelphia, while thousands of miles of roads were also blocked.
Added to the mix was a flood alert for much of the Atlantic coastline of Massachusetts.
On Friday, a worker was crushed to death by a 30-metre pile of salt being prepared to treat roads in the Philadelphia area.
A 71-year-old woman suffering from Alzheimer's disease froze to death after walking out into the cold and getting lost in northern New York state, according to authorities.
At least nine other deaths were blamed on the Hercules snowstorm.
Eastern Canada has also been badly hit, and residents in many parts of Newfoundland lost power. Temperatures there plunged to minus 15 Celsius, but the wind chill made it feel as cold as minus 35 Celsius.
The chill was also set to hit yesterday's crucial National Football League play-off between the Green Bay Packers and the San Francisco 49ers.
Some media outlets speculated that the eagerly anticipated clash at the open-air Lambeau Field in Green Bay, Wisconsin, might be the coldest NFL game in history as freezing temperatures and blustery winds threatened a wind chill factor as low as minus 37 Celsius.
Additional reporting by Associated Press