The preliminary statewide average September temperature was 68.8 degrees, 2.0 degrees above the 1991–2020 average and tied for the 27th warmest on record going back to 1895. The preliminary statewide total September precipitation was 2.60 inches, 0.75 inches below the 1991–2020 average and the 41st driest 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 very September form, the month began with a brief taste of fall-like weather. Temperatures in Monmouth were 5 to 12 degrees below normal in the first 10 days of the month (Figure 1). Summer heat returned in the latter two-thirds of September, with several days that were 5 to 15 degrees warmer than normal statewide.
September average temperatures ranged from the high 60s in northern Illinois to the mid-70s in southern Illinois, between 2 and 5 degrees above normal (Figure 2). High temperatures regularly reached the 90s in mid- and late September, including 99 degrees in Mt Vernon and Charleston. Meanwhile, cooler air in the first half of the month brought nighttime low temperatures in the 30s across much of the state, including 37 in Lincoln and 38 in McHenry. 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 72.6 degrees.
The summer-like heat last month broke 51 daily high maximum temperature records and 3 daily high minimum temperature records across the state. The brief cold spell in early September broke 6 daily low maximum temperature records and 17 daily low minimum temperature records. Overall, the preliminary statewide average September temperature was 68.8 degrees, 2.0 degrees above the 1991–2020 average and tied for the 27th warmest on record going back to 1895.
Another Dry Month in Illinois
September is not usually a very wet month across Illinois, but September precipitation is still important to finish the soybean crop, help along late harvested specialty crops, give our trees and other plants one last drink before winter, and keep our streams above low flow. Unfortunately, very dry conditions last month, following an extremely dry August, did none of those things. Total September precipitation ranged from less than a quarter of an inch in northwest Illinois to over 5 inches in parts of southern Illinois. Only the south seven counties were wetter than normal last month, while a large swath of central and northern Illinois were 2 to 4 inches drier than normal (Figure 3).
The preliminary statewide total September precipitation was 2.60 inches, 0.75 inches below the 1991–2020 average and the 41st driest on record.
Drought Returns in Illinois
Very dry weather in August and September caused a significant expansion and intensification of drought across the state. The September 23rd edition of the U.S. Drought Monitor had moderate drought in 67% of the state and severe drought in nearly 20% of the state, when just 3 months ago only 10% of the state was in any drought level. The dry transition from summer to fall was a substantial pattern change from the extremely wet conditions in spring in southern Illinois. In fact, some areas of the south-central part of the state were having a top-5 wettest year as recently as mid-July, but has only had 10-25% of normal rainfall since then. Meanwhile, the recent dryness has only added to what has been a very dry calendar year and water year (October 1 – September 30) in parts of central and northern Illinois.
Figure 4 shows precipitation departures from normal for the 2024-25 water year (October 1, 2024 – September 30, 2025) and the 2025 calendar year so far. Most of the state north of Interstate 64 was 5 to 10 inches drier than normal in this past water year, with most of the deficit coming in the calendar year 2025. Parts of east-central Illinois, including much of Champaign County, has had over 1 foot less precipitation than normal since the start of 2025. This year to date is the 6th driest on record in Champaign and the driest since 1988. Amazingly, we’ve had 5 fewer inches of precipitation this year in Champaign than in 2012. This year to date is also the 10th driest year on record in Peoria.
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 fashion. However, we may need to wait a bit longer for real fall, because the Climate Prediction Center shows higher odds of a warmer than normal October (Figure 6). Unfortunately, the outlooks also show best chances of another drier than normal month in October.
The most recent outlooks for climatological winter are continuing to lean toward La Niña effects of near normal temperatures but above normal precipitation across much of the state. A wetter late fall and winter would be welcome to improve drought conditions and recharge our soils and streams. For now, let’s enjoy the best of the four seasons.
