Dangerously cold temperatures will hit much of the Midwest and bring a deep freeze to parts of the U.S. — and the weather term we can't stop talking about is to blame. Raise your snow shovel to the polar vortex.

The National Weather Service warned that wind chills will drop to their lowest readings since the mid-1990s across the Upper Midwest.

The National Weather Service in Des Moines, Iowa, said "this is the coldest air many of us will have ever experienced" and, warned that if you're outside, "avoid taking deep breaths, and try to talk as little as possible, according to USA Today.

"Some locations in the Midwest will be below zero continuously for 48-72 hours," according to AccuWeather Senior Meteorologist Mike Doll.

And the warnings keep on coming.

So why now?

The vortex, in case you need a reminder, is a large area of low pressure located about 60,000 feet up on the atmosphere over both the poles. That's the polar part. The vortex part describes the counter-clockwise flow of air that keeps the cold polar air up at the poles. Sometimes, however, that flow of air is disrupted, either by the winds changing direction or stopping entirely. Either of these events allows the vortex area to warm, and the cold polar air goes south, causing frigid conditions in much of North America, Europe and Asia.

Sometimes this cold air is trapped by the jet stream and hangs around. Think back to March 2018 when the U.S. experienced a four-punch combo of nor'easters, or Europe getting pummeled in March, and you'll have an idea of how long that cold air can linger.

The promise of this extraordinarily brisk weather was initially forecasted by Judah Cohen, a climate researcher at Atmospheric and Environmental Research, a private meteorological research and risk analysis firm that provides data to government agencies like NASA and the Department of Defense. Cohen studies polar vortex conditions and prediction models every day, looking for potential disturbances that could turn a normal winter into a harsh one.

A 3-way split

Boston covered in snow early in March 2018 This wasn't Boston in December. It was Boston in March 2018. And if anyone living there has forgotten that weather pattern, we are diving in again this year. (Photo: Kaleb Kloppe/Flickr)

Earlier this month, the polar vortex split into two separate "sister" vortices, and now those storms are barreling down on the eastern and central regions of the U.S.

The first storm swept across the Midwest into the Northeast on Jan. 16-18. The second storm — what's happening now — is expected to pack more of a punch with heavy snow hitting the upper Midwest to northern New England.

But those storms aren't the only impact from the fractured polar vortex. An arctic blast is expected to follow after the storms and is on track to be the coldest of the season. The blast will likely hit the central and eastern regions later this week.

Cohen told The Washington Post that the impact of these storms could last four to six weeks, even possibly up to eight weeks. Cohen said people living in those regions should expect "intense periods of winter weather becoming more frequent including more frequent episodes of arctic outbreaks."

Axios points out that in the past, polar vortex splits have been linked with major snowstorms, including one in 2010 when the Mid-Atlantic was engulfed in blizzards.

Of course, weather forecasting, while a science, isn't always an exact science. The variables meteorologists use in their models differ, and that can influence the results.

Editor's note: This story was originally published in December 2018 and has been updated with new information.

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