La Niña.
If it seems like the current cold snap is unusual, you are right.
As of 5 a.m. EST this morning, Sunday, Dec. 18, the average temperature across the Lower 48 states of the U.S. is colder than any time all last winter.
As this plot of hourly temperatures shows, the average temperature is 18 deg. F, which is 2 deg. colder than any time last winter (graphic courtesy of Weatherbell.com):
Even the unusual warmth remaining in the Southeast U.S. is not enough to offset the frigid airmass much of the country is now experiencing.
Via Roy Spencer