Desperately cold weather is now gripping the Midwest and Northern Plains of the United States, as well as interior Canada. The culprit is a familiar one: the polar vortex. A large area of low pressure and extremely cold air usually swirls over the Arctic, with strong counter-clockwise winds that trap the cold around the Pole. But disturbances in the jet stream and the intrusion of warmer mid-latitude air masses can disturb this polar vortex and make it unstable, sending Arctic air south into middle latitudes. That has been the case in late January 2019. Forecasters are predicting that air temperatures in parts of the continental United States will drop to their lowest levels since at least 1994, with the potential to break all-time record lows for January 30 and 31. With clear skies, steady winds, and snow cover on the ground, as many as 90 million Americans could experience temperatures at or below 0 degrees Fahrenheit (-18° Celsius), according to the National Weather Service (NWS).