Parts of the US and Canada are braced for potentially record-breaking low temperatures as a blast of arctic air blows across North America.
A winter storm has already blanketed areas of Canada and the north-eastern US with up to 2ft (60cm) of snow.
Some 3,700 flights were cancelled on Monday, on top of thousands grounded over the weekend.
Schools have been closed in several US states and residents urged to stay indoors for their own safety.
The storm and the deep cold have been blamed for 16 deaths in recent days.
In developments across the region:
- A power failure in Newfoundland, Canada, late on Sunday left 5,000 homes without electricity as of Monday afternoon. It was a blow to residents already trying to deal with rolling blackouts and the aftermath of a blizzard, CTV News reports
- New York Governor Andrew Cuomo announced on Monday that parts of major highways would be closed due to the extreme weather
- The National Weather Service in Kansas City predicted a low of -22C (-8F) for Monday, shy of the record low for this date of -25C (-13F) set in 1912. But life-threatening wind chills will make it feel much colder, the Kansas City Star reports
- South Dakota's Keloland.com carries a report about farmers trying to keep their cattle warm - cold weather can have a devastating impact on herds
- In Indiana, a state of emergency has been declared in several north-west counties, allowing only emergency or law enforcement vehicles on the roads
- In Thunder Bay, Ontario, where the temperature on Monday morning was -33C (-27F), skate rental business owner Roy Laham told the Chronicle-Journal newspaper he had only 20 customers on Sunday, down from the usual 200-300
- Authorities in Minnesota ordered school closures due to weather, the first time in 17 years
Temperatures in the north and central US could feel as low as -51C (-60F) with the effect of wind chill, forecasters say.
"The coldest weather in years will be making its presence known from the Upper Midwest to the Mid-Atlantic region for the beginning of the work week," the US National Weather Service said in a statement.
The states of Ohio, South Dakota and Illinois are among those set to be hit. Tennessee and Kentucky are forecast to see several inches of snow.
Illinois Governor Pat Quinn called the storm "one for the record books" on Monday.
An emergency room doctor in Minneapolis, Minnesota, told the Associated Press news agency he was seeing frostbite injuries occur even through protective clothing.
On Monday evening the frigid weather is set to fall on the north-east US, where residents are still digging out from the week's deadly snowstorm.
In eastern Canada, the temperatures are expected to drop quickly throughout Monday. Freezing rain caused long airport delays in Toronto and Ottawa.
In New York City, temperatures are expected to drop to roughly -12C (10F) overnight.
Environment Canada's David Phillips told the BBC the rapid temperature drop expected - about 24 degrees centigrade in 24 hours in cities such as Toronto - means residents could be driving or walking on roads that are "like a rink of ice".
Cold temperatures could bring lows not seen in 20 years.
Some parts of Canada could see as much as 23in of snow over just a few days, Mr Phillips said.