Overview
September was warm and dry for Illinois.
Temperatures averaged 69.1 degrees, 2.3 degrees above normal and tied for the 25th warmest September on record statewide (Figure 1).
Precipitation statewide was 1.06 inches, 2.29 inches below the 30-year normal and 7th driest September on record (Figure 1).
Mean streamflow statewide was below the median for the month.
Mean provisional streamflow aggregated statewide was below the long-term median flow for September, about 85% of median (Figure 1). Monthly mean discharge values ranged mainly from normal to much below normal for September. The September 2025 monthly mean flow of the Illinois River at Valley City was the lowest reported for the calendar month of September in the period of record of the gage (1939 to present) (Figure 1).
Water surface levels at the end of September were below the full pool or target level at 16 of 19 reporting reservoirs. At the end of September, Lake Shelbyville was 1.8 feet below the seasonal target level, Carlyle Lake level was 1.3 feet below the seasonal target level, and Rend Lake was 0.2 feet above the spillway level.
Lake Michigan’s mean level was below its long-term mean for the month.
Soil moisture at 2-inch depths averaged 0.17 water fraction by volume (wfv) across the state at the end of September with no overall change in September. However, there were large regional differences with soil moisture levels improving in the south, declining in the north, and remaining low in the central region.
Shallow groundwater levels were 1.73 feet below the long-term average at the end of September. Levels averaged 1.05 feet below August and 1.40 feet below last year (Figure 1).
Weather/Climate Information (Trent Ford)
The following description of temperatures, precipitation, severe weather, and drought comes from data compiled by networks that report to the National Oceanic and Atmospheric Administration (NOAA). These data are provisional and may change slightly over time.
September in Illinois was warmer and drier than normal statewide.
Temperatures averaged 69.1 degrees in September, 2.3 degrees above normal and tied for the 25th warmest on record 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, Table 1). 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.
Precipitation statewide in September was 1.06 inches, 2.29 inches below the 30-year normal and 7th driest on record statewide.
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, Table 1).
Drought: 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 have 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.
Most of the state north of Interstate 64 was 5 to 10 inches drier than normal in this past water year (October 1 – September 30), with most of the deficit coming in the calendar year 2025. Parts of east-central Illinois, including much of Champaign County, have had 1 foot less precipitation than normal since the start of 2025. This year through October 1st 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.
Illinois Climate Network (ICN) (Jennie Atkins)
The Illinois Climate Network (ICN) consists of 19 stations across the state which collects hourly weather and soil information.
Winds averaged 3.3 mph in September, down 0.4 mph from August and 1.7 mph lower than the long-term network average. ICN Monmouth (Warren County) was the month’s windiest station, averaging 5.6 mph for the month. The highest recorded wind gust was 33.8 mph, measured at ICN Snicarte (Mason County) on September 21.
September was warm across the network as 16 out of the 19 stations had higher than normal monthly averages. Overall, ICN averaged 68.9° for the month, 3.9° lower than August but 2.1° higher than the long-term average. Station highs ranged from the high 80s to low 100s with lows in the 30s and 40s. ICN Brownstown (Fayette County) recorded the month’s highest temperature of 100.7° on September 14. The coolest was 36.0°, measured at ICN DeKalb (Dekalb County) on September 8.
Soil temperatures fell 6° to averages in the low to mid 70s. Under bare soils, temperatures ranged from 35.8 to 105.7° at 2 inches and 49.6 to 105.9° at 4 inches. Temperatures under sod ranged from 57.2 to 83.6° at 4 inches and 56.9 to 90.2° at 8 inches.
ICN was dry again in September with a network average of 1.31 inches for the month, 0.20 inches less than August and 1.84 inches lower than the long-term average. All but two stations recorded lower than normal precipitation as over half had monthly totals less than an inch.
The highest totals were in the south where ICN Carbondale (Jackson County) and ICN Dixon Springs (Pope County) recorded 3.55 and 3.21 inches, respectively, the month’s highest totals. The northern stations has a much drier month. ICN Big Bend (Whiteside County) received only 0.15 inches, the month’s lowest total.
Soil moistures varied greatly across the network. Improvements were seen at the southern stations through the first 8 inches as levels at 2 inches increased 44% in September to an end of the month average of 0.22 water fraction by volume (wfv).
However, soil moisture declined in the north. Levels at 2 inches were 40% lower at the end of the month. The declines were observed through the top 20 inches.
The central stations showed little change over the month. Soil moisture was near the wilting point for most stations at the beginning of September with no improvement over the month.
Surface Water Information (Bill Saylor)
Provisional monthly mean flows for this month for 26 streamgaging stations located throughout Illinois are shown in Table 2, compared to statistics of past record of monthly mean flows at those stations for the same month. Both recent and long-term data are retrieved from USGS online data services following the end of the month. Years of record values in Table 2 represent the number of past monthly values included in the Table 2 statistics; at some stations, the available record may not be continuous. Additional source data may be available from USGS.
The statewide percent of historical mean flow and percent of historical median flow are calculated by dividing the sum of the average flows this month at stations in Table 2 by the sum of the historical mean and median flows calculated for the month, respectively, at the same stations. This method is intended to weight individual observations proportionately in the aggregate comparison. (The Illinois River and Rock River stations are excluded from the statewide calculation because other rivers listed in Table 2 contribute to their flow.)
Mean provisional flow aggregated statewide, using the available monthly mean data shown this month in Table 2, was below the median value for September (approximately 85 percent of the median) and below the mean for September (approximately 40 percent of the mean). In September, monthly mean discharge values ranged mainly from normal to much below normal. The September 2025 monthly mean flow of the Illinois River at Valley City was the lowest reported for the calendar month of September in the period of record of the gage (1939 to present). The September 2025 monthly mean flow of the Sangamon River at Monticello was the second lowest reported for the calendar month of September in the record of the streamgage. (The lowest monthly mean flow value reported at this gage for a calendar month was in September 1988.)
Water-Supply Lakes and Major Reservoirs. Table 3 lists selected reservoirs in Illinois, their normal pool or target water surface elevation, and data related to observed variations in water surface elevations. Reservoir levels are obtained from a network of cooperating reservoir operators who report water levels each month. Current reservoir levels reported in Table 3 are representative of the end of the reported month and are presented as the difference in feet from seasonal target level or from full pool, as applies. Years of record represent the number of past reports for the same month used to calculate the average of the month-end values presented in Table 3. For some reservoirs, this average does not include additional period of record prior to a substantial change in reservoir operation. Most reservoirs in Table 3 serve as public water supplies, with the exceptions noted in the last column.
Compared to end-of-August water levels at 16 reservoirs for which levels were reported last month and this month, reported end-of-September water levels were lower at 13 reservoirs, higher at 2 reservoirs, and about the same as at the end of August at 1 reservoir (where supplemental supply was in use). For the 19 reservoirs with measurements reported for the end of September, water levels were below normal target pool or spillway level at 16 reservoirs, above normal target pool or spillway level at 1 reservoir (Rend Lake), and at about the target level or spillway level at 2 reservoirs (in systems where supplemental supply was in use). Salem Lake inflow has been supplemented by pumping from Carlyle Lake. Sparta Reservoir inflow has been supplemented by pumping from the Kaskaskia River.
Major Reservoirs. Compared to water levels at the end of August, at the end of September the water level at Lake Shelbyville was 0.4 feet lower, Carlyle Lake was 0.3 feet lower, and Rend Lake was 0.7 feet lower. At the end of September, Lake Shelbyville was 1.8 feet below the seasonal target level, Carlyle Lake level was 1.3 feet below the seasonal target level, and Rend Lake was 0.2 feet above the spillway level.
Great Lakes. Current month mean and end-of-month values are provisional and are relative to International Great Lakes Datum 1985. The September 2025 mean level for Lake Michigan was 578.6 feet. The monthly mean level one year ago (September 2024) was 579.3 feet. The long-term average lake level for September is 579.2 feet, based on 1918-2023 data. In this period of record, the lowest mean level for Lake Michigan for September occurred in 1964 at 576.6 feet, and the highest mean level for September occurred in 1986 at 582.0 feet. The month-end level of Lake Michigan was 578.5 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Water tables continued to fall in September. Wells ended the month 1.05 feet lower.
All but one well declined over the month with most 0.5 to 2.0 feet lower at the end of September. The Olney (Richland County) well rose over the month with a level 1.40 feet higher than in August.
Most declines were between 0.5 and 2 feet. The Good Hope (McDonough County) had the largest decline ending September 2.09 feet lower than August.
Eighty percent of wells were lower than last year. The Olney well, despite September’s increase, was 5.90 feet lower than 2024. Significant decreases were observed from the 15-year averages and the period of record for most wells.
Wells averaged 1.40 feet below September 2024, 1.87 feet below the 15-year average, and 1.73 feet below the long-term record (Table 4). September was the ninth consecutive month with water tables averaging lower than the long-term record.
Data sources for the IWCS include the following:
CPC - Climate Prediction Center
ISWS - Illinois State Water Survey
MRCC - Midwestern Regional Climate Center
NCEI - National Centers for Environmental Information
NWS – National Weather Service
SPC – Storm Prediction Center
USACE – U.S. Army Corps of Engineers
USDM – US Drought Monitor
USGS – US Geological Survey
WARM – Water and Atmospheric Resources Monitoring Program