At least eight people have died in one of the worst cold snaps to hit the US Midwest in decades.
An Iowa student found dead outside a college building is among the victims.
Hospitals have been treating patients reporting frostbite as life across a swathe of the country grinds to a halt.
Ninety million people - a third of the US - have seen temperatures of -17C (0F) or below. Some 250 million Americans overall have experienced the "polar vortex" conditions.
However southern states such as Florida have escaped the brutal chill.
How did the fatalities occur?
- University of Iowa student Gerald Belz, 18, was found unresponsive behind a campus building before dawn on Wednesday and later died in hospital. Officials said weather was a factor. His father told local news channel KCRG that Gerald was a "mama's boy with a tough exterior".
- A 70-year-old man in Detroit, Michigan, was found dead in front of a neighbour's home on Wednesday
- Another Michigan man in his 70s was found frozen to death in his neighbourhood. Officials said he was "inadequately dressed for the weather" and was probably disoriented
- On Tuesday, 55-year-old Charley Lampley froze to death in a garage in Milwaukee, Wisconsin, having "apparently collapsed after shovelling snow", according to a medical examiner
- An 82-year-old man in Pekin, Illinois, died from hypothermia after apparently falling outside his home on Tuesday
- A 75-year-old man was fatally struck by a snow plough near Chicago on Monday. The driver has since been placed on paid leave pending an investigation, according to WGN9 News
- In northern Indiana, a young couple died after a collision on icy roads
What's the forecast?
Thursday could see America's third largest city of Chicago breaking its 1985 record low of -32C (-25F), according to meteorologists. The city has already passed the record low for 31 January.
The official low temperature at Chicago this morning was -21F. This shatters the previous record for Jan 31st, which was -12F set back 1985. In records that date back to the 1870s, this is only the 15th time Chicago has seen a daily low this cold or colder, yesterday was 14th.— NWS Chicago (@NWSChicago) January 31, 2019
The National Weather Service (NWS) announced Rockford, Illinois, west of Chicago, broke its all-time low record of -32C (-27F) when temperatures dipped to -34C (-30F) on Thursday morning.
Cities across Iowa have also broken temperature records.
In news that pretty much no one wants to hear ... we broke numerous low temperature records this morning across the state! #iawx— NWS Des Moines (@NWSDesMoines) January 30, 2019
(Yes, those are air temps, NOT wind chill.)
Cotton, Minnesota, was the coldest place in the US on Thursday, however, with a low of -48C (-56F) based on preliminary data.
The chill is drifting eastward on Thursday, bringing sub-zero temperatures to north-eastern cities such as Boston.
Areas downwind of the Great Lakes are expected to be buried by intense "snow lakes" into Thursday night.
The region near Buffalo, New York, should see the heaviest snowfall. Snow could fall at rates of 3in-5in (7cm-12cm) per hour.
The icy cold is expected to loosen its grip on Friday.
With wind chill factored in, the Midwest and Great Lakes have felt temperatures closer to -40C (-40F) and -53C (-63F), which is enough to cause frostbite in under five minutes.
But by the end of the weekend, Chicago could see temperatures as high as 10C (50F).
"It's going to be at least a 60-degree swing for Chicago," David Hamrick, a National Weather Service forecaster, told Reuters news agency.
The Wind Chill City
Analysis by Chris Buckler, BBC News, Chicago
On the icy streets of Chicago they are used to bitter winters but this was too cold even for some people who live here.
Rush hour hasn't existed in this normally bustling city for the last couple of days as many have chosen to stay at home rather than brave such extreme elements.
Those who did go to work arrived bundled up in layer after layer of clothing. Anything exposed - like eyebrows and lashes - were covered in frost.
"They've frozen shut a couple of times", one man told me about his watering eyes.
The Windy City could be renamed the Wind Chill City given the number of warnings there have been this week.
Below several skyscrapers there are still signs pointing out the dangers of falling ice.
You could argue that a slight thaw has begun given that cracks have started to appear on what was a solid sheet of ice covering the Chicago river.
But temperatures are expected to remain below freezing until Saturday at the earliest.
How is the cold snap affecting daily life?
The Arctic weather could cost the US billions of dollars. In 2014, a similar polar freeze cost the country an estimated $5bn (£3.8bn), CBS News reported.
In Minnesota, residents have been asked by natural gas company Xcel Energy to reduce their home thermostats to 17C (63F) in order to help the company handle heating demands.
Michigan residents have had similar requests from their utility companies as providers struggle to keep the states warm.
We understand the timing of our request is not ideal given today’s cold temperatures, however, without additional reductions, we run the risk of not being able to deliver natural gas to families and critical facilities across Michigan – a scenario none of us want to encounter.— Consumers Energy (@ConsumersEnergy) January 31, 2019
Native American tribes in the northern Midwest states have been helping their members obtain heating supplies during the chill as many live in poor-quality housing, the Associated Press reported.
Detroit has had more than two dozen water mains freeze over this week. A city spokesman told the Associated Press the pipes were installed up to 1.8m below the frost line, but with such drastically low temperatures, the ground has still frozen through.
The US Postal Service has suspended all mail deliveries for the second day to parts of six states.
More than 2,300 flights have been cancelled and another 3,500 delayed due to the polar vortex.
As ice and snow continue to build up, roads have become increasingly dangerous across the northern US. In Illinois, police said they had assisted more than 1,300 motorists and received 460 calls in eight hours - 10 times the norm.
One woman in the state was caught speeding at 115mph (185km/h) on a snowy road with a 35mph (56km/h) speed limit.
At least two people were critically injured in a 27-car pile-up on icy roads in Wyomissing, Pennsylvania.
Twenty-one vehicles, including a lorry, were involved in a pile-up in Buffalo, New York, during a snowstorm on Wednesday. Officials have not released details on any injuries, WKBW News reported.
It is reported that the lorry should not have been on the road at the time, due to a weather-related ban.
In Minnesota, prison visits were cancelled over the cold, according to the New York Times.
How cold is Chicago?
The Illinois city became a frozen ghost town after temperatures fell to -30C (-22F), colder than parts of Antarctica.
A Good Samaritan paid for 70 homeless Chicagoans to stay in a hotel as temperatures dropped.
Salvation Army spokeswoman Jacqueline Rachev told the Washington Post: "It's a deadly situation for anyone. We're thrilled that someone was in a position to be able to do this."
Most of the thousands of cancelled flights this week were coming out of Chicago's airports - O'Hare International is ranked as one of the top 10 busiest airports in the world.
Amtrak also cancelled all trains into Chicago on Wednesday, affecting 55 trains, and said most would be cancelled on Thursday as well.
As the Midwestern city is one of the company's hubs, train services nationwide could be impacted.
The chill was even too much for Chicago's Disney on Ice show, which cancelled its Wednesday performance.
More than 600 local schools have shut, keeping 360,000 students at home.
What about Canada?
Most of Canada was under some sort of weather warning - from extreme cold in the Prairies, Quebec and Ontario to heavy snows in Alberta and Nova Scotia.
In Toronto, where winters tend to be milder compared to cities such as Montreal and Ottawa, temperatures had plummeted to -18C (0F). Icy roads and several transit delays made for a hellish commute for the city's residents.
Environment Canada issued extreme cold warnings for most parts of Ontario, Quebec and Manitoba, urging residents to limit their exposure to cold and keep pets indoors.
Advocates expressed concern for homeless people living in cities hit by the extreme temperatures.
In Winnipeg, the capital of Manitoba, temperatures were -40C (-40F), with wind chill factors making it feel like -50C (-58F).
"If you don't get it in December, you will get it in January or February or March. What do you expect? It's 'Winterpeg'," Caroline Rodriguez told CBC.
In Calgary, Alberta, temperatures were a relatively balmy -3C (27F), but were expected to plummet overnight and through the weekend.
In parts of rural northern Alberta, 15in of snow were expected to fall.