A critical blockade against some of nature’s coldest air has temporarily broken down—and much earlier in the year than normal. Disruptions in this circulation, known as the polar vortex, often push intensely cold air out of the Arctic toward lower latitudes.

Experts still aren’t entirely sure how the current collapse will play out in our everyday weather. But it’s another sign that we may be in for a wild ride over the weeks and months to come.

The polar vortex is a band of upper-level winds wrapping around the Arctic Circle like a dam. This circulation is usually strong and stable, helping to keep the season’s coldest air locked near the North Pole. But the winds occasionally weaken or even reverse course, which can set off an atmospheric chain reaction, allowing bitterly cold air to spill southward.

The manner in which the polar vortex breaks down determines what kind of weather conditions we might expect. Upper-level winds dipping southward in troughs are responsible for brief spells of cold temperatures. These “lobes” of the polar vortex, as they’re known, are relatively common in a typical winter.

Sometimes, this circulation can splinter into smaller pieces. If the vortex were a flowing river, these fragments are like leaves stuck on the riverbank, forcing extremely cold air to linger over the same areas for days or weeks at a time.

One such case made this phenomenon a household name. A highly disrupted polar vortex in early 2015 sent a weeks-long burst of cold air sweeping over the eastern US.

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