A killer polar vortex gripping parts of the US is set to ease up and give way to a dramatic, never-before-seen warm-up.
Brutal winds sent the mercury plummeting to -34C in the Midwest, which has been struck by a historic deep freeze, but temperatures are expected to soar by up to 80 degrees within days.
Meteorologists said past record-cold waves have never dissipated so quickly as they predicted that the region would be seeing “spring-like” temperatures by the week’s end.
Temperatures in Chicago have been at near-record lows. Amid the freezing conditions, the city reportedly experienced “frost quakes” with residents reporting loud booms.
At least 21 people have been killed and millions were left stranded since the deep freeze took hold at the weekend.
More than 4,000 flights were cancelled at Chicago's two international airports, with transport officials saying operations are not expected to return to normal until Saturday.
But Jeff Masters, Weather Underground meteorology director, said Michigan and Illinois were headed dramatically warmer temperatures by Monday.
He said the swing in temperature would be “unprecedented”, adding that the polar vortex may return in late February but such intense cold will not be felt.
Forecasters had said the some of the coldest wind chills were recorded in International Falls, Minnesota, at -48C.
The weather has spread east with states including Massachusetts, New York and Pennsylvania beginning to experience bitterly cold temperatures.
The fatalities from the recent spell of freezing weather include a 75-year-old man hit by a snow plow in the Chicago area as well as a University of Iowa student who was found dead on campus.
Television station KCRG reported the body of 18-year-old Gerald Belz was found behind an academic hall just before 3am on Wednesday.
He was taken to a hospital where he died.
Deaths related to extreme cold weather have been reported in Michigan, Iowa, Indiana, Illinois, Wisconsin and Minnesota. Some died in weather-related traffic accidents while other fatalities were from apparent exposure to the elements.
The extreme conditions are being caused by a spinning pool of cold air which is known as a polar vortex.
Weather officials in Iowa urged people to "avoid taking deep breaths" and to "minimise talking" if they go outside.
The National Weather Service said frostbite was possible within just 10 minutes of being outside.
A state of emergency has previously been declared in the Midwestern states of Wisconsin, Michigan and Illinois as well as in the southern states of Alabama and Mississippi.
As the cold gripped more than 30 record lows were shattered across the Midwest.
Illinois State Police officers also rescued 21 people who were stranded in a charter bus that broke down in sub-zero temperatures after the vehicle's fuel reportedly turned to gel in its engine.
Streets in Chicago were nearly empty on Wednesday, with few people walking outside in the painfully cold air, while the US Postal Service has halted deliveries from parts of the Dakotas through to Ohio.
Measures are in place to protect the homeless and other vulnerable people from the bitter cold.
These included turning some city buses into mobile warming shelters in Chicago.
