It appears likely that this December will go down as one of the top 15 warmest ever in Chicago and likely one of the top 10 warmest ever in Rockford. So naturally that leads one to ask how other winters with unseasonably mild Decemebers turned out? The charts below show how January and February temperatures fell out following the top 15 warmest Decembers at both Rockford and Chicago.
Probably one of the first things that becomes apparent based on the charts above is that having an unseasonably warm Decemeber doesn't serve as a particularly good predictor of what January and February may have in store temperature wise. Interestingly, probably one of the things that does stand out the most is that very few Januarys or Februarys went on to produce near normal temperatures, with the majority of the months (77% in Chicago and 80% in Rockford) going on to produce notably below or above average temperatures.
At first glance, in Chicago, there certainly would seem to be a tendency for warmer than average temperatures, especially in January. However, this chart should be used with caution for Chicago as 11 out of the 15 warmest Decembers (and subsequent Jan and Feb) were back when the official temperature reading was taken downtown. Temperatures downtown tend to be warmer in the winter than at O'Hare or Midway, where the official station has been located since July 1942, and because the official readings were taken downtown so many of the years above it results in a warm bias.
For that reason, looking at Rockford's temperatures may offer more insight in the predictive powers a warm December may hold for the remainder of winter. Unfortunately, looking at Rockford temperatures, there is a wide spread with many years seeing an abrupt switch to unseasonably cold weather and other remaining unseasonably warm. As it turns out, there is a slight tendency for warm Januarys with 8 out of 15 ending up above or much above average, though it's hardly a slam dunk with 5 Januarys ending up below or much below average. Februarys actually have a slight tendency toward cold, with 7 out 15 ending up 2F or more degrees below average and only 4 ending up above average.
It's important to remember that our skill in predicting seasonal temperatures is very limited and many factors that cannot be anticipated more than a week or two out often play major roles in the predominant weather patterns. The charts above simply show what has happened in the past, and should not be used as a forecast for this winter. For a forecast for the remainder of this winter refere to the latest 30 and 90 day outlooks from the Climate Prediction Center. The National Weather Service Chicago also recently put out a detailed explanation of the warm December which can be found here.