A “polar vortex” dropped over a huge section of the United States on Monday, sending temperatures plunging across the Midwest as the East Coast braced for the bitter Arctic blast.

Meteorologists predicted the lowest temperatures in two decades in some areas of the country, with the mercury at -35 degrees Fahrenheit (minus 37 Celsius) in Milwaukee.

Icy roads and snow storms created travel chaos, with thousands of flights canceled or delayed.

The northeastern United States has endured heavy snow and deadly freezing conditions since the start of the year, and the extreme cold has claimed about a dozen lives nationwide.

“The coldest weather in years will be making its presence known from the Upper Midwest to the Mid-Atlantic region for the beginning of the work week,” the National Weather Service said.

Wind chill warnings were in effect from Montana as far south as Tennessee and Alabama, with temperatures feeling as cold as -50 degrees Fahrenheit, it said, describing the conditions as “life-threatening.”

On the East Coast, a brief lull in the deep freeze greeted commuters today, but temperatures were expected to drop by 40 degrees overnight.

In Milwaukee, on the shores of Lake Michigan, officials warned residents to stay indoors and urged schools to shut down.

“Police are reaching out to homeless citizens in order to help them find the nearest shelter,” city spokeswoman Sarah DeRoo told.

All schools were closed in Minnesota. In Cook County in Illinois, which includes Chicago, warming centers were opened for anyone suffering, including those whose home heating did not work.

“This winter storm will be one for the record books, and we want to make sure everyone stays safe and warm until it passes,” Illinois state governor Pat Quinn said.

Exposed skin could suffer frostbite in as little as five minutes in such conditions, experts warn.

More than 3,000 flights within, into or out of the United States were canceled today, according to industry tracker FlightAware, including nearly 800 at Chicago O’Hare International, one of the country’s busiest airports.

Hundreds more flights were canceled or delayed from other major hubs like New York’s John F Kennedy or Newark International.

A Delta passenger airplane skidded into a snow bank at JFK yesterday.

Nobody was hurt, but the plane had to be towed to its gate.