Seventy-five percent of the continental US population is expected to face below-freezing temperatures this week as a polar vortex hits the region.

The US Northeast and Midwest are forecast to be worst hit, with Chicago on Wednesday expected to see a high of just 14 degrees below zero and a low of minus 29 (minus 26 to minus 34 in Celsius).

Medical and weather authorities warn that people could get frostbite and hypothermia from as little as five minutes outside.

Thousands of flights have also been canceled, with airlines waiving flight-change fees to accommodate for the weather.

A dangerous polar-vortex weather system is bringing extreme low temperatures to large parts of the US this week, with authorities warning residents about hypothermia and frostbite, thousands of flights canceled, and at least two people already dead in connection with the cold.

This week, regions from North Dakota into Missouri, and as far east as upstate New York, are expected to drop to about 50 degrees below average for late January, The Washington Post reported Monday.

Some 220 million people, or 75% of the continental US population, are expected to face below-freezing temperatures this week, CNN said.

Weather forecasters and doctors have warned of cold and wind so intense that it could give people frostbite within five minutes of exposure.

This map shows the lowest that windchills are expected to fall this week. Windchill figures are adjusted down from actual temperatures to reflect that strong wind removes heat much faster than still air.

A map showing the coldest windchill temperatures expected across the US Midwest and Northeast from Monday to Friday. NWS Eastern Region/Twitter

The polar vortex refers to an air pattern that keeps cold air in the polar regions.

Sometimes, that mass of air expands and get pushed away from its normal pattern, in this case allowing cold air from the North Pole to pour over the US.

President Donald Trump, who has long questioned the science behind climate change, spoke sarcastically of the cold on Twitter on Monday.

"What the hell is going on with Global Waming? Please come back fast, we need you!" he tweeted. Weather and climate are two different things: Weather on one day or even over a longer period in one large location doesn't define the global climate.

"A single cold snap in one part of the world does nothing to invalidate the long-term trends," Andrew Freedman of the news website Axios tweeted.

Read more: Trump tweets falsehoods about climate change and cold weather — again



Donald Trump reading The Wall Street Journal on the campaign trail in Jacksonville, Florida, in October 2015. In a tweet Monday, he sarcastically doubted the science behind climate change while mentioning the cold weather. REUTERS/Daron Dean

Under-25s 'never felt this cold before'

Forecasters predict a high of minus 14 degrees Fahrenheit (minus 26 degrees Celsius) in the northern cities of Chicago and Minneapolis on Wednesday, The New York Times reported Monday.

Chicago could see a low of minus 29 (minus 34 Celsius), The Washington Post added. This would surpass the city's coldest recorded temperature of minus 27 on January 20, 1985, according to the National Weather Service.

Windchills could hit minus 50 (minus 45.6 Celsius) in Chicago and minus 60 (minus 51 Celsius) in Minneapolis that same day, The Times added.

Americans who are 25 or younger will have "never felt this cold before," the CNN meteorologist Tom Sater said.

A commuter in Chicago on Monday. The city may see a historic low of minus 29 degrees Fahrenheit (minus 34 degrees Celsius) on Wednesday. Kiichiro Sato/AP

Frostbite in less than 5 minutes

Experts say residents could contract frostbite and hypothermia from even very brief exposure to the elements.

"You're talking about frostbite and hypothermia issues very quickly, like in a matter of minutes, maybe seconds," Brian Hurley, a meteorologist at the Weather Prediction Center, told the Associated Press.

The lack of blood circulation to hands and feet can cause frostbite. Hypothermia occurs when the body is exposed to extremely cold conditions over a prolonged period, and as a result the body temperature becomes dangerously low.

The National Weather Service's Chicago office warned that "dangerously cold wind chills could cause frostbite on exposed skin in as little as 5 minutes."

Amanda Jackson, an emergency-room doctor at SSM Health St. Mary's Hospital in Madison, Wisconsin, also told the local Channel 3000 news that people with exposed skin could see signs of frostbite — "a tingling sensation and then a throbbing pain" — in less than five minutes.

Eric Ernest, a doctor at the Nebraska Medicine Emergency Department, told Omaha's local WOWT 6 News that people could incur frostbite "anywhere from 10 to 20 minutes of exposed skin."

A woman in a bus shelter in Vernon Hills, Illinois, on Monday. Medical and weather authorities are warning people that they could contract frostbite and hypothermia within minutes, even seconds. Nam Y. Huh/AP

A 22-year-old man in Rochester, Minnesota, was found dead amid cold temperatures outside his relatives' house, to which he didn't have a key, the local KIMT 3 News reported. Authorities believe he suffered from frostbite and hypothermia, though an autopsy has not yet been carried out.

The police found blood smeared on several doors and footprints that suggested that he fell multiple times. Officers also found clothing near the house, seeming to confirm the hypothermia theory — people in the final stages of hypothermia can feel irrationally hot and take their clothes off.

Two deaths have already been reported in connection with the cold weather this week, CNN said.

Thousands of flights have been canceled as a result of the snow. Reuters/Christian Hartmann

Classes and flights canceled

Nearly 2,300 flights have already been canceled, with another 11,800 delayed, Reuters reported.

American Airlines, United Airlines, JetBlue Airways, Delta Air Lines, and Spirit Airlines have waived flight change fees from the Midwest and the Northeastern to accommodate for the weather, CNBC reported. Southwest Airlines said people could delay their flights from up to two weeks later without having to pay the fare difference.

The city of Peoria, Illinois, closed all public schools and canceled trash collections on Wednesday because of the cold weather and snow.

Public schools in Milwaukee also closed on Tuesday and in Chicago on Wednesday.

Cars seen Monday in Wheeling, Illinois. Many cities have canceled classes for public schools this week to accommodate for the cold weather. Nam Y. Huh/AP

'All hands are on deck'

Chicago has opened community centers, libraries, and police stations as warming centers. Mayor Rahm Emanuel told The New York Times he was working with local officials to help homeless people living alongside highways, adding: "All hands are on deck."

—Mayor Rahm Emanuel (@ChicagosMayor) January 28, 2019

Officials in Chicago and Peoria instructed landlords to provide heat to their tenants.

Officials in South Bend, Indiana, also pledged to open a warming center for vulnerable people.