The preliminary statewide average September temperature was 68.5 degrees, 1.8 degrees above the 1991-2020 average and 22nd warmest on record going back to 1895. The preliminary statewide total September precipitation was 2.73 inches, 0.62 inches below the 1991-2020 average and the 49th wettest on record.
Data are provisional and may change slightly over time.
Fall Start and Summer Finish
September is a transition month as we move from summer into fall. In true September form, the month began with a brief taste of fall-like weather. Temperatures in Decatur were 5 to 12 degrees below normal in the first 10 days of the month (Figure 1). Milder temperatures moved in and persisted through the last three weeks of the month.
September average temperatures ranged from the mid-60s in northern Illinois to the low 70s in southern Illinois. Most of the state was between 1 and 5 degrees above normal, with the highest departures in the northern third of the state (Figure 2). High temperatures regularly reached the 90s in mid-September, including 96 degrees in Mt. Vernon and 95 in Peoria. Meanwhile, cooler air in early and later parts of the month brought nighttime low temperatures in the 30s across much of the state, including 36 in Elgin and 39 in Normal. The coolest point in the state was Stockton with an average temperature of 64.3 degrees and the warmest part of the state was Smithland Lock & Dam in Massac County with an average temperature of 72.9 degrees. Last month was the second warmest September on record in Chicago.
Overall, the preliminary statewide average September temperature was 68.5 degrees, 1.8 degrees above the 1991-2020 average and 22nd warmest on record going back to 1895.
Tropical Weather Brings September Rain
September is not usually a very wet month across Illinois, and the first two-thirds of last month played to type with very dry conditions in the Midwest. However, September is also the heart of Atlantic hurricane season, and tropical weather broke up our dry streak, at least in southern Illinois, toward the end of the month. Total September precipitation ranged from less than 1 inch in far northwest Illinois to over 8 inches in parts of southern Illinois. Most areas of the state south of Interstate 64 were 1 to 5 inches wetter than normal for the month while much of central and northern Illinois were 1 to 3 inches drier than normal (Figure 3).
September was extremely dry in northwest Illinois. In fact, it was the second driest September on record just across the river in Dubuque Iowa, with only 0.08 inches total. It was also the second driest September on record on Illinois’ side of the river in Mount Carroll, with 0.23 inches total. Meanwhile, it was the fourth wettest September on record at the Carbondale Water Treatment Plant, with over 8 inches for the month.
The preliminary statewide total September precipitation was 2.73 inches, 0.62 inches below the 1991-2020 average and the 49th wettest on record.
A Mixed Bag of Drought in September
The first three weeks of September were extremely dry across much of the Midwest. Figure 4 shows precipitation rankings from Sept. 1 to Sept. 20 across the region, with 1 indicating that period was the driest on record for that station. Much of the region had a top-five driest start to September on record, including the driest first 20 days of the month in Quincy, Danville, Moline, and Rockford. The dryness culminated in large expansion of moderate drought across Illinois in mid-September, including over 45% of the state in moderate to severe drought as of the Sept. 17 U.S. Drought Monitor.
The remnants of Hurricane Helene were pulled to the northwest and moved into the lower Ohio River Basin in the final week of the month. The tropical system brought widespread and, at times, heavy rain across southern Illinois. Three-day rainfall totals exceeded 6 inches in parts of southern Illinois; however, the previously dry soils helped absorb much of the rain and mitigate significant flooding in the region. The hurricane remnants provided widespread drought relief across southern Illinois, dramatically reducing drought extent and severity in the latest U.S. Drought Monitor. The heavy rain across the Ohio River Basin also helped improve streamflow along the Ohio and Mississippi Rivers. Figure 4 shows the level of the Ohio River at Cairo, which jumped more than 15 feet in just a week in response to Helene’s rainfall. The rain temporarily assuaged low river level concerns and impacts to barge traffic.
Outlooks
October — in my humble opinion — is the best weather month of the year. Days become noticeably shorter, nights become cooler, and hoodies and shorts become the ultimate nightwear. The Climate Prediction Center shows higher odds of a warmer- and drier-than-normal October (Figure 5) but no doubt there will be some beautiful fall weather.
Meanwhile, October also brings in the first look at NOAA’s official winter prediction. This year, NOAA is leaning hard into La Niña with elevated chances a wetter than normal winter across Illinois, and equal chances of cooler and warmer conditions (Figure 7).
