The polar vortex, which wreaked havoc for much of last year's winter.
From CBS News:
Prepare yourself for an invasion from the north. A blast of polar air is about to send temperatures plunging in the heart of America.
It's the return of the polar vortex that brought misery a year ago. A mass of whirling cold air will dip southward this weekend, sending the mercury plunging.
As the cold air moves south and east, it has the potential to affect as many as 243 million people with wind chills in the single digits in some places and snow.