A polar vortex is an area of circulating cold air that ring the planet’s North and South poles. Sometimes that vortex destabilizes, sending surges of Arctic air south.
Windchill temperatures could hit minus 50 degrees Fahrenheit in Illinois, Indiana, North Dakota, South Dakota, and parts of Minnesota, far below average for this time of year.
In these temperatures, frostbite can strike within minutes.
If Arctic ice continues to melt, polar-vortex events could get more frequent.