A polar vortex collapse. Now, meteorologists are calling for another collapse in the middle of March, one that could plunge the U.S., Canada, and parts of Europe into a bitterly cold spell just days before the official arrival of spring.

Winter isn't over just yet. A major shift in the weather pattern could have millions across the United States once again reaching for their winter coats later this month before milder springlike air arrives for good.

A polar vortex is a ‘large area of low pressure and cold air surrounding both of the Earth’s poles’ that ‘ALWAYS exists near the poles, but weakens in summer and strengthens in winter’, according to the National Oceanic and Atmospheric Administration.