We've known for about a week that a mass of cold air was bearing down on us. It formed over Siberia, passed over the North Pole and Canada, and has now reached Chicago.
Cassie and I went out just now for 22 minutes, and in that time the temperature dropped 0.4°C (0.7°F), which may not sound like a lot until you do the math (1.2°C/2.2°F per hour). And it will continue doing so until early Monday morning, plateauing but not rising during daylight hours:
Fortunately for us (but unfortunately for a lot of insects and plants), we have almost no snow on the ground after two days of above-freezing weather. Without the snow to lower its albedo, the ground will absorb the abundant sunlight to keep us warmer than we otherwise would be:
The dump of arctic air will be in full swing Sunday night with a lobe of roughly -27° to -30°C 850 mb air waiting in the wings just to our north and west, and this will traverse northern Illinois fully on Monday night. From a climatology perspective, this is just about as cold as we`ve seen (at 850 mb) in the nearby upper air database (Quad Cities and Lincoln, IL). Out of curiosity, went back and took a look at the arctic outbreak from this time last year (January 14 - 17), which featured low temperatures in the -5° to -15°F range and wind chills solidly down towards -30°F (and even lower than that on short time periods). 850 mb temperatures in this case were actually notably "warmer", generally around -20°C. They key difference was a widespread and dense snowpack which we obviously don`t have this time around, which just goes to show the power of snowpacks in altering these arctic airmasses. In this case, little/snow in place, even amidst near-record cold just off the deck, the peak of this arctic episode looks to wind up a bit under where we found ourselves one year ago.
Also, yesterday we had a late-March-like 8.3°C high temperature at O'Hare, and the temperatures should go above zero again on Thursday. The coldest normal temperatures in January occur on the 23rd to the 25th, so this is pretty much right on time.
The worst part of this will fall on Cassie. She only has the two fur coats her parents gave her, and she has no sense of cold. I've explained to her that she's only going to get 5-minute walks from Sunday to Tuesday. She responded by rolling on her back and demanding a belly-rub. I'm envious.