Arctic-like temperatures that dropped as low -49°C paralysed America's Midwest on Thursday and were blamed for at least 21 deaths.
A forecast of warmer-than-normal weather by the weekend offered little comfort to vulnerable populations such as the homeless and elderly enduring cold that caused frostbite in minutes and made being outdoors potentially deadly.
The death toll from the cold rose from 12 after at least nine more people in Chicago were reported to have died from weather-related injuries, according to Stathis Poulakidas, a doctor at the city's John H Stroger Jr Hospital.
Dr Poulakidas, a trauma specialist, said the hospital had seen about 25 frostbite victims this week. He said the most severe cases risked having fingers and toes amputated.
Among those believed to have died from the cold was University of Iowa student Gerard Belz. The 18-year-old was found unresponsive on campus on Wednesday morning just a short walk from his dormitory, according to university officials. Police told a local television station they believed the cold played a factor in his death. The wind chill at the time officers found Belz was -46°C, according to the National Weather Service.
Homeless and displaced people were particularly at risk, with Chicago and other cities setting up warming shelters. But many braved the cold in camps or vacant buildings. A 60-year-old woman found dead in an abandoned house in Lorain, Ohio, was believed to have died of hypothermia, Lorain County Coroner Stephen Evans said.
"There's just no way if you're not near a heat source that you can survive for very long out in weather like this," Dr Evans told the Chronicle-Telegram newspaper.
It has been more than 20 years since a similar blast of frigid air covered a swath of the US Midwest and North-east, according to the National Weather Service.
The bitter cold was caused by the mass of air known as the polar vortex drifting south from its usual position over the North Pole.
Homes and businesses used record amounts of natural gas to fight the cold, according to financial data provider Refinitiv. Utilities appealed to consumers to conserve energy to avoid power outages.
In Detroit, General Motors suspended operations at 11 Michigan plants to cut natural gas consumption. Fiat Chrysler canceled a shift on Thursday at two of its automobile plants.
Snow and ice created treacherous travel conditions, with 26 road collisions reported within two hours on Thursday in eastern Iowa's Johnson County, emergency communications centre chief Tom Jones told the Iowa City Press-Citizen.
For the second day in a row, the intense cold and windy conditions forced US airlines to cancel more than 2,000 flights. Chicago was hardest hit, with O’Hare International Airport experiencing more than 700 cancellations, according to the FlightAware tracking site.
Heavy snow hitting Chicago off the Great Lakes was set to start reducing on Thursday night, the weather service said.
More than 30 record lows were shattered across the Midwest. Cotton, Minnesota, had the lowest national temperature recorded early on Thursday at -48°C, before the weather warmed up, the weather service reported.
Temperatures in the Upper Midwest were forecast to rise above -18°C on Friday, and reach as high 4°C by Saturday, while the central Plains would experience temperatures of about 15°C, the weather service said.
