The most severe outbreak of frigid arctic air in years is due to surge over the Upper Midwest and Great Lakes by the middle of this week.

A large lobe of the polar vortex, the zone of frigid air winding around the North Pole, is breaking off, and temperatures will plunge as much as 40 degrees below normal close to its core. Some of this teeth-chattering cold will spill into the Mid-Atlantic and Northeast as well, but it won’t be quite as extreme.

The hardest-hit zone will span from Minnesota and Iowa through Michigan, including Minneapolis, Des Moines, Green Bay, Milwaukee, Madison, Chicago, Indianapolis and Detroit.

“There’s no mild way of saying it. Brutal cold is coming,” the National Weather Service office serving Chicago tweeted. It is predicting air temperatures close to minus-20 and wind chills of minus-40 Wednesday morning.

There is an outside chance low temperatures in the Windy City will touch minus-20 on consecutive days Wednesday and Thursday, which has happened only four other times in recorded history.

In Green Bay, the weather service is warning of “dangerous” cold with wind chills dropping to minus-35 to minus-50 Wednesday and Thursday.

Some of the cold will reach the Mid-Atlantic and Northeast, but its intensity is likely to ease as it nears the coast.