Illinois State Climatologist Jim Angel has a report:
Based on preliminary data, the statewide average temperature for May in Illinois was 21.4°C, 4.4°C above normal and the warmest May on record. The old record was 20.8°C set back in 1962. A brief examination of daily records indicates that Springfield, Champaign, Quincy, and Carbondale all had daily mean temperatures at or above normal for each day of the month. On the other hand, Chicago, Rockford, and Peoria had a few dips into the below-normal territory but overall finished above-normal for the month.
In Chicago, we also had the warmest last week of May ever. Fun times. And today, it's 14°C by the Lake.