CHAMPAIGN — The statewide average temperature of 69.4 degrees in September was 2.6 degrees above average, but the humidity was lower than in the previous months, according to Illinois State Climatologist Trent Ford at the University of Illinois’ Illinois State Water Survey.
Most of September passed with temperatures above average across the state. The highest maximum temperature recorded last month was 94 degrees in Alton, while the lowest minimum temperature recorded last month was 36 degrees in Galena.
Overall, the September average temperature ranged from the mid- to high 60s in northern and north-central Illinois to the low 70s in southern Illinois. Last month was 3 to 5 degrees warmer than normal in northern Illinois and within 1 degree of normal in southern Illinois
Despite the higher temperatures, the unusually high humidity across the state in July and August disappeared in September. The conditions were less humid than the Septembers in 2018, 2019, and 2020.
The preliminary statewide average total September precipitation was 2.38 inches, 0.97 inches below the 1991–2020 average. Last month’s precipitation totals ranged from less than an inch in northwest Illinois to over 5 inches in east-central and southern Illinois. September was 2 to 3 inches drier than normal throughout northern Illinois and near normal to about 1 inch wetter than normal in central and southern Illinois.
Very dry conditions last month added to existing precipitation deficits in northern Illinois. In response, the latest U.S. Drought Monitor expanded severe and moderate drought across the northern third of the state and expanded extreme drought in McHenry County. The 8.78 percent of the state in severe drought is the highest percentage since fall of 2013.
Dry conditions have left substantial soil moisture deficits across northern Illinois, and a near to wetter than normal winter will be necessary to recharge dry soils. A healthy, persistent snowpack across northern Illinois would also go a long way to help improve soil moisture conditions next spring.
The most recent Climate Prediction Center outlooks for the second week of October show strongly elevated odds of above normal temperatures sticking around, with the highest odds of near normal precipitation.
Meanwhile, outlooks for the entire month of October also show strongly elevated odds of above normal temperatures, though with slightly higher odds of wetter than normal weather throughout the month.