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The polar vortex dives into the Lower 48 this week. Here’s how cold it will get.

Wind chill temperature forecast on Thursday and Friday. (Capital Weather Gang)

The coldest December air in years is dropping south into the Midwest and Northeast this week. With temperatures forecast to be 20 to 35 degrees below average, record lows are likely Thursday and Friday.

The most frigid air is going to stream in across the Upper Midwest on Wednesday night into Thursday. All of Minnesota and Wisconsin, parts of the Dakotas, Iowa and the Chicagoland area will wake up to subzero temperatures Thursday morning and wind chills colder than minus-20.

Daytime temperatures aren’t expected to get out of the single digits. Records for cold temperatures could be broken in northern Illinois, Michigan, Ohio, Kentucky and West Virginia on Thursday.

By Friday, the most intense lobe of the polar vortex will be over New England.

New England meteorologists are noting this could be the most intense blast of Arctic air the region has seen in over 30 years. Eric Fisher at WBZ scoured through old weather-balloon data and found that this week’s forecast is similar to that of the coldest December reading on record in Chatham, Mass. — Christmas Day in 1983.

“I’ve been consistently amazed at how well we have mimicked the strong 1982-83 El Niño and subsequent exit to weak La Niña,” Fisher wrote Tuesday morning, noting that this year we are transitioning from a very strong El Niño to a La Niña. “That December was BRUTAL across the Lower 48, much like what we’re seeing now.”

Remarkably, the GFS weather model is predicting early-morning temperatures more than 40 degrees below average in Upstate New York on Friday. Our color charts don’t even go colder than that.

The National Weather Service in Boston is warning of “dangerously cold” weather on Thursday and Friday. Overnight lows will drop into the single digits and below zero in southern New England. When combined with wind gusts up to 45 mph, the wind chill will be a bone-chilling minus-15 to minus-25 degrees.

Record-cold highs are possible — if not likely — in Maine, the Boston metro area, New York City and the D.C. metro area on Friday.

In D.C., we have to look back to 2008 to find similarly-cold air. Thursday’s and Friday’s high temperatures in the capital are forecast to be in the 20s — the last December to have consecutive days in the 20s was in 2000.

Coldest December weather in more than a decade possible in D.C.

Frigid air 15-30 degrees below normal is spilling into the Lower 48. Capital Weather Gang's Jason Samenow has your forecast and possible snow predictions. (Video: Ashleigh Joplin/The Washington Post)

The polar vortex is a weather term that was popularized in 2014, though it’s a term that meteorologists have always used to describe a common phenomenon: the circulation of air around the Arctic circle. As long as the jet stream is strong, it keeps cold, polar air confined to the north. At times, the jet weakens, and cold air is allowed to spill south into the mid-latitudes, where the Lower 48 is located.

The whole vortex does not shift south during these cold outbreaks. Instead, little pieces — some stronger than others — break away from the main circulation. It just so happens that one of these breakaway lobes will traverse south through Canada this week and wind up over New England on Friday.