Updated Jan. 28, 2019

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Find your long johns, break out the thick socks and raid the supermarket. After a month of relatively mild winter weather, the Midwest and the East Coast are experiencing what has become a seasonal rite of passage: the polar vortex.

The phrase has become synonymous with frigid temperatures that make snowstorms more likely. And if it seems as if these polar freezes are happening more often, you’re right. “They are definitely becoming more common,” said Jennifer Francis, a senior scientist at the Woods Hole Research Center. “There have been a couple of studies that have documented that.”

Colder temperatures have been arriving later in winter over the past few years, according to Judah Cohen, a climatologist at Atmospheric and Environmental Research, a weather risk assessment firm. But because of changes to the polar vortex, when wintry weather does arrive, it’s often more intense — witness the four back-to-back nor’easters last year.

“I’ve been making that argument that winter is shortening, but you’re getting these more intensive periods in that shorter winter,” Dr. Cohen said.