Here we are now several days into January, which is climatologically the coldest month during the year, and instead of dealing with a lot of cold and snow, we have actually been experiencing more spring-like conditions of late. The conditions that we have experienced have not been record breaking for us in northern Illinois and northwest Indiana. So the question that comes to mind is how warm have things really been lately in terms of what is typical for this time of year? To answer this question, consider the graphics below.
These graphics display the daily high temperatures for the period of December 1st to January 6th for both Chicago and Rockford. The red line indicates the daily high temperatures that we have experienced over the past 37 days. For comparison the same 37 day period from last year is shown in blue, while the 1981 to 2010 daily averages are plotted represented by the light green line. The colored filled areas in the figure represent temperatures that are either within the typical range (green area), abnormal warm (red area) or abnormal cold (blue area). Finally, the daily record high and low temperatures are represented by the truncation in the red and blue color filled areas.
It is apparent that daily high temperatures since December 1st have been consistently above average at both Chicago and Rockford. But is this abnormal? It is not necessarily abnormal to experience temperatures above average. There actually is a large amount of day to day temperature variability during the winter season. Because of this, it is good practice to consider a range of temperatures as being “typical” for the time of year. In the figures below the “typical” range of temperatures is within the green shaded area. This is a range that represents a vast majority of daily high temperatures in the past. However, on some occasions daily temperatures will fall outside of this typical range; these are considered days of either abnormally warm or cold temperatures.
As noted above, none of this warmth over the past 37 days has been record breaking. However, it definitely has been abnormally warm on several occasions. There was a two day period on the 3rd and 4th of December in which temperatures topped out in the low 50s before a cool down produced more typical temperatures through the 11th. Then from the 12th through the 15th of December, temperatures once again warmed considerably above average. Several more of these up and down swings in temperatures occurred through the rest of December. However, notice that overall the daily high temperatures remained above the daily averages. These unseasonably warm high temperatures in December helped lead to the 15th warmest December on record at Chicago and the 8th warmest at Rockford. These warm conditions have continued into the first week of January as well. In fact, at the end of the period, January 5th and 6th you can see that near record warmth occurred.
If we compare this year’s temperatures to those observed last year at this time you can see that there is a remarkable difference. Overall, last year’s daily high temperatures during this same 37 day period were either at or consistently below average, with the exception of a brief warm spell around the 1st of 2011. There were even a few periods of abnormal cold daily high temperatures. Overall, December of 2010 ended up being the 17th coldest on record at Chicago and the 19th coldest at Rockford.