With yesterday's temperatures more like April than January, Chicago magazine's explanation of it is timely:
So what is going on? It's the warmest La Niña on record. That brings the global temperature down, but causes different effects in different places. Chicago is going through a near-record warm spell—strong La Niñas correlate with above average temperatures, like the 18°C we hit in 1989 when the mean January max was 11°C, 2°C higher than this month's mean. Meanwhile, Alaska and northern Europe are suffering through deadly cold snaps.
This came to me through the WGN weather blog, which in the same story points out that groundhogs are less accurate than random chance at predicting the weather. Just a heads-up for tomorrow.