What is causing the extreme cold warnings affecting millions of people across the United States?

The extreme cold warnings affecting over 87 million people across 22 states are being caused by a polar vortex. This weather phenomenon refers to super cold air that normally swirls over Earth's poles but can escape and make areas extremely frigid when it moves southward. According to the National Weather Service, this polar vortex is triggering massive temperature drops, with wind chills potentially reaching 60 degrees below zero in northern states and even southern states experiencing wind chills in the teens. Additionally, a snowstorm is expected to develop near the Rockies and move eastward throughout the week.