Parts of the US are braced for potentially record-breaking low temperatures as a “polar vortex” brings more freezing weather.
A winter storm has already blanketed areas of Canada and the north-eastern US with up to 2ft (60 cm) of snow.
It has been blamed for 16 deaths in recent days and the cancellation of some 3,700 flights over the weekend.
Schools have been closed in several US states and residents urged to stay indoors for their own safety.
Temperatures in the north and central US could feel as low as -51C (-60F) with the effect of wind chill, forecasters say.The plunging temperatures result from the polar vortex, an anti-clockwise pool of cold, dense air.
The vortex has been very strong, with the cold air locked in to Arctic Canada for a long time. Stuck in one place, the air has got colder and colder.
Cold air is dense so once it is released it travels a long way and that is why it is penetrating so far southwards, John Hammond from the BBC Weather Centre says.
“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.
The weather service said “an incredibly strong surge of bitterly cold Arctic air” was sweeping across the country until Tuesday.
It is set to continue to the north-east, where residents are still digging out from the week’s deadly snowstorm.
It could bring some of the coldest temperatures in two decades.
“The last really big Arctic outbreak was 1994,” Reuters news agency quoted Bob Oravec, a forecaster with the National Weather Service, as saying.
“Outbreaks like this don’t occur every day.”
In Canada, the temperature dropped as low as -29C (-20F) in Toronto and -38C (-36F) in Quebec City with wind chill on Thursday – the lowest seen in at least two decades there.
Heavy snow has also affected US states including New York, Connecticut and Massachusetts. Boston was said to be the worst hit with nearly 18in (45cm) of snow.
Schools have also been closed across the state of Minnesota for Monday, as well as for parts of Wisconsin, Illinois and Indiana, among others.
In other developments across the region:
- There was another power failure in Newfoundland, in Canada, late on Sunday leaving 90,000 homes without power. It is a blow to residents already trying to deal with rolling blackouts and the aftermath of a blizzard,CTV News reports
- The National Weather Service in Kansas City predicted a low of -8C (18F) for Monday morning, shy of the record low for this date of -13C (9F) 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, as the cold weather can have a devastating impact on herds
- In the city of Cleveland, 20 neighbourhood recreation centres will open as warming centres on Monday and Tuesday, Cleveland.com reports
- State offices were to stay closed in Indiana where a state of emergency was declared in Lake County, allowing only emergency or law enforcement vehicles on the roads.