Here are the daily temperature departures for the summer of 2017 from the long-term daily normals for three sites in Illinois – Chicago, Champaign, and Carbondale.
Chicago experienced an early summer heat wave in the first two weeks of June with temperatures reaching up into the 90s, which was well above normal for that time of year. This was followed by cooler conditions in the second half of June. Periods of cooler and warmer than normal weather alternating throughout July, which is pretty common. August has been running cooler than normal so far. The lesson here is that early season heat waves do not necessarily set the tone for the rest of the summer (thank goodness). For Chicago, June was 3.5 degrees above normal, July was 0.6 degrees above normal, and August so far is 2.5 degrees below normal.
Champaign also experienced heat in the first half of June and a little more pronounced cooling than Chicago in the second half of June. July alternated between relatively warm and cool periods. August has been quite cool so far. For Champaign, June was 0.7 degrees above normal, July was 1.5 degrees above normal, and August so far is 2.9 degrees below normal.
Carbondale was warm in the first half of June, and cool in the second half. However, these warm and cool periods were not as pronounced as in Chicago and Champaign. Days in July were more often warmer than normal, while late July and all of August have been much cooler than normal so far. For Carbondale, June was 0.5 degrees below normal, July was 0.6 degrees above normal, and August so far is 6.2 degrees below normal.
Why are there different responses across the state from the periods of warmer and cooler than normal periods? It is, in part, due to climatology. In the early June heat, temperatures reached into the 90s across the state. However, those temperatures are much more unusual in northern Illinois than southern Illinois. Therefore, the daily departures from normal were much larger in Chicago than Carbondale.
Also, it was wetter in southern Illinois during the first half of June, and likely more cloudiness, which would lead to slightly lower temperatures during that period.
The reverse was true in August. High temperatures were in the 70s and low 80s across the state, which was much more unusual in southern Illinois than northern Illinois.
Temperature departures from normal for the first half of June. The heat in northern Illinois was an extension of the heatwave in the upper Midwest and High Plains. The deeper the orange/red, the warmer the temperature departure.
Temperature departures from normal for the first half of August. The deeper the green, the cooler the departure from normal.