August: the statewide average temperature for Illinois in August was 74.9 degrees, 1.3 degrees above normal. The statewide average rainfall was 5.25 inches, 1.66 inches above normal and the 12th wettest August on record.
The warmest daily high temperature was 97 degrees reported at Chicago Botanical Gardens (8/5), Chicago O’Hare (8/4), Jerseyville (8/7), and Pittsfield (8/29). The coolest reading was 42 degrees reported at Rochelle (8.22).
Rainfall for August was heaviest in the west and southern portions of the state with some areas up to 5 to 8 inches above normal (see maps). Meanwhile, parts of east-central and northeast Illinois had below-normal rainfall. The largest monthly total was in Arlington Heights (IL-CK-163) with 10.22 inches.
Summer: the statewide average temperature for the summer (June, July, and August) was 75.0 degrees, 1.4 degrees above normal. The statewide average precipitation for the summer was 14.86 inches, 2.99 inches above normal and the 13th wettest summer on record.
Rainfall was heaviest in the northwest and south with some areas reporting totals that were 6 inches or more above normal. Meanwhile, there were a few spots around the state that were persistently dry, like the area around Kankakee. The largest summer rainfall total was 24.50 inches at Rockford (IL-WN-26). One of the lowest summer rainfall totals was 5.92 inches reported at Ashkum (IL-IR-2) in Iroquois County (south of Kankakee). That is about half the normal summer rainfall for that area.
Here are the maps for the last 90 days, which shows nearly all of this summer’s rainfall.