A student reacts to subzero temperatures while walking to class at the University of Minnesota in Minneapolis, Minnesota, U.S., January 29, 2019.

Forecasters warned late Tuesday that the harsh winter blast that has already killed at least five people would worsen dramatically on Wednesday, saying the expected conditions could be the coldest in a generation.

The National Weather Service described the polar vortex system surging from the north as "one of the coldest arctic air mass intrusions in recent memory." It said bitterly cold, very dangerous wind chills would spread across much of the eastern two-thirds of the country, "likely leading to widespread record lows and low maximum temperatures" plunging more than 20 degrees below zero in the Midwest.

Frigid, icy conditions have been blamed for at least five deaths so far:

An 82-year-old man died Tuesday afternoon after he was found suffering from hypothermia outside his home in Pekin, Illinois, the Peoria County Coroner's Office told NBC affiliate WEEK of Peoria.

A man was found frozen in a detached garage near his home in Milwaukee County, Wisconsin, apparently after having been shoveling snow, the county medical examiner's office said Tuesday.

A man was killed when he was struck by a city snow plow at the end of his driveway Monday morning in Libertyville, Illinois, southwest of Waukegan, city officials told NBC Chicago. The snow plow driver was placed on paid administrative leave pending an investigation.

A 31-year-old man was killed Monday when he lost control of his vehicle, struck a light pole and was ejected on icy I-80 north of Des Moines. The state patrol said the man was driving too fast for the weather conditions.

A 9-year-old Nebraska boy died Sunday when the vehicle he was traveling in lost traction on icy Interstate 80 in Cass County and rolled into a ditch. Five other people were injured, the Iowa State Patrol said.

Almost 1,000 arrivals and departures scheduled for Wednesday were pre-emptively canceled at O'Hare International Airport in Chicago. More than 320 were scrubbed in advance at the city's other major airport, Midway International.

Amtrak said all trains scheduled to and from Chicago on Wednesday were also canceled.

Forecasters said subzero temperatures would cover large parts of the East and the Midwest where as many as 75 million Americans live on Wednesday. By Wednesday night, 85 percent of the U.S. land area and 230 million Americans will have experienced freezing temperatures, they said.

Record lows were forecast over a wide stretch of the Midwest: Rochester, Minnesota, was forecast to reach 31 degrees below zero overnight. Milwaukee was expected to drop to minus-28 degrees. Des Moines, Iowa, could hit minus-21.

"Historic cold, unprecedented cold, these are all adjectives you could use to describe this," said Kathryn Prociv, a meteorologist for NBC News.

"These are some of the coldest temperatures an entire generation has ever felt, talking about the millennials," Prociv said. "A lot of these temperatures will be the coldest since about 1994, when a lot of them were just being born."

The U.S. Postal Service said it would suspend delivery services on Wednesday in Minnesota, western Wisconsin, Iowa and western Illinois. There will also be no mail or package pickups, it said.

"You're talking about frostbite and hypothermia issues very quickly, like in a matter of minutes, maybe seconds," said Brian Hurley, a meteorologist with the National Weather Service's Weather Prediction Center.For emergency responders, such conditions are life-threatening but part of the job.

When any equipment gets wet, it "freezes to the ground," Rochester fire Capt. Greg Neumann said.

"Even the hose lines get froze up," Neumann told NBC affiliate KTTC. "They grab the snow, so everything is three times heavier than they actually are."

But in Kentucky, Gov. Matt Bevin complained that people just aren't tough enough. In an interview with WHAS radio of Louisville, Bevin said he wasn't happy that dozens of districts were canceling classes Wednesday.

"Now we cancel school for cold," Bevin told Terry Meiners, the host. "I mean, there's no ice going with it or any snow. What happens to America? We're getting soft, Terry. We're getting soft."

Bevin said that he was being "only slightly facetious" and that he had a real concern — "that in America, on this and any number of other fronts, we're sending messages to our young people that if life is hard, you can curl up in the fetal position somewhere in a warm place and just wait 'til it stops being hard. And that just isn't reality."