(CNN) — Even for the hardiest, cold-tested Americans, the deep freeze sweeping over the Midwest will be brutal.

Officials warned of almost instant frostbite as temperatures in the region plunged below zero Wednesday. Some state offices are closed and postal workers won’t deliver mail in 10 states. Thousands of flights have been canceled along with dozens of train services — most of them in and out of Chicago.

About 212 million people — or 72% of the continental US population — will see temperatures drop below freezing over the next few days. And more than 83 million Americans — about 25% of the US population — will suffer subzero temperatures at some point between Wednesday and Monday.

With at least five deaths linked to the extreme conditions this week, authorities are urging people to bundle up, stay inside and check up on the elderly and vulnerable in what experts are describing as “the coldest air in a generation.”

Chicago will be below zero for days

While most of the Midwest will see frigid temperatures, Chicago will be “the epicenter of the extreme cold,” CNN meteorologist Dave Hennen said.

Chicago could reach a record low temperature of 27 below zero by Thursday morning. Its daytime high Wednesday is forecast to be 15 below zero.

The National Weather Service in Chicago tweeted Tuesday night that the temperatures had already started dipping.

“Chicago officially fell below zero prior to 6 p.m. at O’Hare and it may not get back to zero until Thursday evening,” it said.

It’ll be so cold, Chicago-area residents would be better off warming up in parts of Antarctica. Priestley Glacier, Antarctica, will have a Wednesday high temperature of 6 degrees Fahrenheit and a low of 7 below zero.

More than 2,700 flights involving US airports are canceled for Tuesday and Wednesday, including about 1,550 in and out of Chicago airports, according to FlightAware.com.

Amtrak also canceled all service to and from Chicago on Wednesday due to weather, including short-distance trains and long-distance overnight trains. It said it typically operates 55 trains daily to and from the Chicago hub.

Mail delivery will also be canceled in Illinois, Iowa, Minnesota, North Dakota, South Dakota, Wisconsin, and parts of Kansas, Missouri, Montana and Nebraska.

In Minnesota, frostbite can hit in minutes

Frigid temperatures are not the only concern. In Minnesota, blustery weather could mean wind chills approaching negative 70. In Ponsford, the wind chill was negative 66, CNN meteorologist Michael Guy said.

“These are VERY DANGEROUS conditions and can lead to frostbite on exposed skin in as little as five minutes where wind chill values are below -50,” the National Weather Service tweeted. “Best thing you can do is limit your time outside.”

Hennen described it as the “coldest air in a generation.” Temperatures will plunge to 20-40 degrees below zero between Tuesday and Thursday in the Upper Midwest, Hennen said. In northern Minnesota, wind chills were forecast to drop to 65-70 degrees below zero, which would rival the coldest wind chill ever recorded in the state (71 below) in 1982.

Frostbite is an issue in central Iowa, too

In central Iowa, wind chills are also a major concern.

The National Weather Service forecast dangerous wind chills of negative 45 degrees for Des Moines, negative 57 for Waterloo and negative 60 for Mason City into Wednesday night.

“This is the coldest air many of us will have ever experienced,” it tweeted.

Wind chill refers to how cold people and animals feel when they’re outside, according to the weather service. It’s how much heat is lost from exposed skin while it’s windy and cold. The faster the wind, the more heat is drawn from the body, which lowers the skin temperature and, ultimately, the internal body temperature.

Frostbite is caused by freezing of the skin and underlying tissues. It’s most common on the fingers, toes, nose, ears, cheeks and chin. Severe cases can kill body tissue.

North Dakota residents told to avoid roads

In North Dakota, authorities issued a “no travel” advisory for the state’s northeast region, warning motorists to stay off the road in those areas due to zero visibility from blowing snow. The region includes Grand Forks and surrounding areas.

The North Dakota Highway Patrol said it also issued a travel alert for southeast North Dakota due to blowing snow. Cities included in the travel alert are Fargo, Casselton and surrounding areas.

“A travel alert means conditions are such that motorists can still travel in these areas, but should be advised of changing conditions. Motorists are encouraged to wear seat belts, reduce speeds and drive according to the conditions,” it said.

The wind chill at Grand Forks International Airport was 61 degrees below zero, the National Weather Service said. Extreme cold will continue through Thursday, with wind chills down into the negative 60s, according to the National Weather Service.

State offices closed in Michigan

In west Michigan, with wind chills between negative 20 and negative 40 expected Wednesday through Thursday morning, the National Weather Service warned residents that “these temperatures the next few days are nothing to mess with.”

“We are not used to this. Take steps to prevent frostbite and hypothermia,” it said.

All state offices will be closed Wednesday and Michigan Gov. Gretchen Whitmer has declared a state of emergency.

“Such widespread, extreme conditions have not occurred in Michigan for many years. It’s imperative that we are proactive with record-low temperatures being predicted by the National Weather Service,” she said.

“Wind chills are predicted as low as 50 degrees below zero in many places, such as metro Detroit which is especially unaccustomed to these temps.”

Deaths linked to brutal weather

As millions grapple with the frigid temperatures, at least five deaths have been linked to the extreme weather this week.

In Rochester, Minnesota, a man died Sunday outside the home he was staying in with a relative. He didn’t have keys to the home and was unable to enter after being dropped off that morning. The single-digit temperatures that dipped below zero may have played a role in his death, police said.

And in Illinois, a man died Monday in a crash involving a village plow truck and a pedestrian, Libertyville police said. The plow truck driver has been placed on paid administrative leave pending the results of an investigation. The same day, Ethan and Shawna Kiser were killed when their Saturn Vue spun sideways and into the path of a GMC Yukon in northern Indiana, authorities said. The couple was 22 and 21, respectively.

A 55-year-old man was found dead Tuesday in the detached garage of his Milwaukee home after he apparently collapsed while shoveling snow, the medical examiner’s office said.

CNN’s Joe Sutton and Dave Alsup contributed to this report.

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