CHAMPAIGN — Rainfall was above average in July in Illinois with slightly cooler temperatures and very high humidity, even by recent climate standards, according to Illinois State Climatologist Trent Ford at the University of Illinois’ Illinois State Water Survey.
The preliminary statewide average total July precipitation was 5.37 inches, 1.31 inches above the 1991–2020 average. The statewide average July temperature was 74.6 degrees, 0.8 degrees below average.
High humidity and southerly flow off the Gulf of Mexico helped continue to fuel scattered thunderstorms across the state in July.
Total July precipitation ranged from less than 2 inches in the northeast part of the state to more than 10 inches in south-central Illinois. Long-running precipitation deficits in northern Illinois grew as July rainfall was 2 to 3 inches below normal there, within 1 inch of normal in most of central Illinois, and 2 to 5 inches above normal in southern Illinois.
Most of northeast Illinois was unable to make progress reducing long-running precipitation deficits in July. The latest version of the U.S. Drought Monitor released on July 29 showed most of northern Illinois from Jo Daviess County to Cook County remained in moderate drought, and parts or most of Boone, McHenry, and Lake Counties were in severe drought.
Apart from a late-month heat wave, temperatures in July were mostly below the 1991–2020 normal.
Most places observed a single-digit number of days with a high temperature above 90 degrees. For example, Carbondale observed only 9 days in July with a high temperature above 90 degrees. Comparatively, Carbondale recorded 29 days with a high above 90 degrees in 2012.
Most of central Illinois was between 1 and 2 degrees below normal for the month of July, while northern and southern Illinois were less than 1 degree below normal for the month. Last month, 45 daily low maximum temperature records were broken across the state, including a 67-degree high temperature on July 11 in Kewanee. Additionally, three daily low minimum temperature records were broken in July.
Growing degree days
The cooler July temperatures maintained growing degree day accumulation departures from the end of June. As of the start of August, growing degree day accumulation is ahead of normal in northern Illinois and slightly to much below normal in southern Illinois.
Climate Prediction Center outlooks show strongly elevated odds of warmer than normal conditions and elevated odds of wetter than normal conditions in the second week of August.