A weather phenomenon known as a polar vortex is likely to hit a large swath of North America this week and into next, potentially disrupting travel for millions of people during the Thanksgiving holiday.

A polar vortex happens when air in the Arctic is destabilized, often by intruding warming air currents, and moves extreme cold air southward. The type of polar vortex this current system could become is known as a “sudden stratospheric warming” event — if it indeed comes to pass, this would be the earliest instance of it happening during a winter season on record .

The polar vortex will likely hit parts of western and central Canada, and could later move on to eastern parts of the country as well. It’s also expected to stretch over much of the U.S., “including northern parts of the Pac

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