The Polar Vortex is preparing to unleash some bitter cold.
As a result, a surge of Arctic Air is blasting into Chicago, Midwest and the eastern part of the United States starting this week and into February. It brings below-zero temperatures and wind chills, along with much sadness for people who are already weary of winter.
The Polar Vortex gained notoriety in 2014 when it brought cold temps to Chicago and the Midwest, and became a favorite catch phrase to describe frigid temps.
But that's not the full story. The Polar Vortex is actually a circulation of cold air in the Arctic Circle - about 7 to 10 miles above the ground in the stratosphere.
Usually that circulation is really strong and keeps most of the cold temps bottled up above the North Pole.
However, sometimes, that circulation weakens. Then, little pieces of energy breaks off and splits and sends cold Arctic Air southward.
And, the Polar Vortex is nothing new.
Every winter, it brings cold temps - some years worse than others - and then goes back to circling about the Arctic Circle during the off-season.
Polar Vortex Explainer: Get ready for bitter cold the next few weeks