All of it was the result not just of the cold, but of how quickly temperatures dropped. One culprit: the polar vortex, a giant mass of cold air normally contained above the North Pole by strong bands of circulating winds. In recent weeks, the vortex has broken apart, with one block of cold air escaping into Canada and the United States, and another into Europe. “It’s sort of like a dam bursting into two or three pieces, and those pieces took the cold air with them,” said Judah Cohen, director of seasonal forecasting at Atmospheric and Environmental Research, a weather risk-assessment firm. From there, the systems worked in tandem to suck cold air from the Northwest into regions that had just been pummeled with up to two feet of snow.

Source: New York Times January 22, 2019 00:11 UTC