Why it's going to be so cold next week
What exactly is the polar vortex and why is it here, bringing temperatures far below zero for the next few days?
The polar vortex is a circulation pattern near the poles in a layer of upper atmosphere called the stratosphere. It's always present but is strongest during winter.
The structure will intermittently weaken as a result of warming toward the poles. When that happens, the vortex can split into two or more smaller circulations that can travel far beyond their typical range.
The result is cold air will be forced further south - into Europe, or in our case, the Eastern half of the United States.
Our weather for the middle of the week is strongly influenced by a piece of this upper level polar circulation that has been dislodged and is centered over the Hudson Bay in Canada.
At the surface, this translates into a deep trough over the Great Lakes that sends arctic Canadian air over the Midwest. It is associated with an active storm track and prolonged periods of well below average and subzero temperatures for Illinois and the Midwest.
According to the National Weather Service, after a storm system passes through the region, temperatures will drop sharply Monday evening, with the coldest air forecast overnight Tuesday.
Widespread subzero lows will continue through Thursday morning. Nighttime wind chills approaching -30 degrees or lower are forecast.
Daytime highs on Wednesday could remain below zero in Chicago. That hasn't happened since January 2014 but is more frequent in the suburbs and northern counties.
Historical notes:
• Average annual normal number of days with below zero lows at Chicago is 7, and Rockford is 12.
• Chicago's record coldest official temperature is -27 degrees on Jan. 20, 1985.
• Rockford's record coldest official temperature is -27 degrees on Jan. 10, 1982.
Recent cold outbreaks:
• Dec. 26, 2017 - Jan. 6, 2018. Chicago remained below 20 degrees with wind chill warnings across northern Illinois from Dec. 31 through Jan. 2.
It was the coldest New Year's Day on record in Chicago and Rockford, with wind chill readings of -20 to -40 degrees.
• Winter of 2013-14. The third coldest on record for Chicago - 23 days with subzero lows in Chicago and 36 in Rockford. Chicago hit -16 degrees Jan. 6 with a high that day of -2 degrees and a wind chill of -42 degrees.