Overview
October was another warm month in Illinois.
Air temperatures averaged 61.9 degrees for the month, 7.1 degrees warmer than the 1991-2020 normal (Figure 1).
Precipitation averaged 1.80 inches, 1.46 inches below the 1991-2020 normal (Figure 1).
Soil moisture declined 23%, on average, at 2-inch depths and 13% at 4 inches. Moisture levels declined in the first 39 inches but remained steady at the 59-inch depths.
Mean streamflow statewide was above the median for the month.
Mean provisional streamflow aggregated statewide was above the long-term median flow for October, about 265% of median (Figure 1). Monthly mean discharge values ranged mainly from below normal in parts of southern Illinois to much above normal for October in parts of northern Illinois.
Water surface levels at the end of October were below the full pool or target level at 17 of 19 reporting reservoirs. At the end of October, Lake Shelbyville was 0.4 feet below the seasonal target level, Carlyle Lake level was 0.1 foot below the seasonal target level, and Rend Lake was 1.4 feet below the spillway level.
Lake Michigan’s mean level was slightly below its long-term mean for the month.
Shallow groundwater levels were 0.81 feet below the long-term average at the end of October (Figure 1). Levels averaged 0.66 feet below September and 1.18 feet above last year.
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.
October was the third warmest on record in Illinois, and much drier than normal.
Temperatures averaged 61.9 degrees in October, 7.1 degrees above the 1991-2020 normal, and the 3rd warmest October on record since 1895. It was the warmest October in Illinois since 1963.
October average temperatures ranged from the high 50s in northern Illinois to the mid-60s in southern Illinois, between 5 and 9 degrees above average (Figure 2, Table 1). High temperatures regularly reached into the high 80s and low 90s in early October, including 95 degrees in Casey and 93 in Mt. Vernon. Low humidity later in the month helped nighttime temperatures dip in the 30s on a few nights, including lows of 30 in Elgin and 31 in Rockford.
The summer-like heat in early October broke 98 daily high maximum temperature records and 75 daily high minimum temperature records. Last month was a top 5 warmest October on record in several places including Peoria, Champaign-Urbana, and Belleville.
Precipitation statewide in October was 1.80 inches, 1.46 inches below the 1991-2020 normal and the 33rd driest October on record.
The largest precipitation deficits in October were in the central and southern thirds of the state. Overall, October total precipitation ranged from around a half an inch in east-central Illinois to just under 5 inches in northeast Illinois. Northern Illinois was within 1 inch of normal October rainfall, while southern and central Illinois were 1 to 4 inches drier than normal (Figure 3, Table 1).
Illinois Climate Network (ICN) (Jennie Atkins)
The Illinois Climate Network (ICN) consists of 20 stations across the state which collects hourly weather and soil information.
October winds were calmer than normal. Averaging 4.5 mph or 2.0 mph below the network’s long-term average. ICN Monmouth (Warren County) averaged a wind speed of 7.9 mph, the network’s highest. The highest recorded wind gust was 44.8 mph, measured at ICN Snicarte (Mason County) on October 13.
It was warmer across the network in October. Air temperatures averaged 61.2 degrees, 11.2 degrees cooler than September but 6.4 degrees above the long-term average. Station highs were in the high 80s to low 90s with lows in the 30s. The highest recorded temperature was 92.9 degrees, recorded at ICN Perry (Pike County) on October 13. The lowest temperature was 30.6 degrees at ICN Stelle (Ford County) on October 18.
Soil temperatures were 10 to 13 degrees lower than September and 3 to 5 degrees higher than the long-term averages with averages in the low to mid 60s. Under bare soils, temperatures ranged from 34.3 to 92.8 degrees at 2 inches and 39.6 to 90.5 degrees at 4 inches. Temperatures under sod ranged from 48.9 to 78.8 degrees at 4 inches and 54.4 to 82.6 degrees at 8 inches.
October was dry at most stations. The network averaged 1.70 inches of precipitation for the month, 1.60 inches less than September and 1.58 inches below the long-term average. The driest stations were in southern and central Illinois where ICN Olney (Richland County) recorded 1.06 inches and ICN Charleston (Coles County) measured 0.25 inches, the month’s lowest.
Only one station – ICN Monmouth (Warren County) – had a monthly total above its long-term average. Monmouth recorded 3.40 inches in October, 0.66 inches above normal.
The drier weather caused soil moisture to decline through the top 39 inches. At 2-inch depths, moisture levels declined 23%, on average, across the network. The northern stations saw larger drops with levels 46% lower at the end of October. Only at the southern stations did soil moisture remained steady because levels in that region were already near the wilting points at the beginning of October.
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. Streamflow data is retrieved from USGS online data services. 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 above the median value for October (approximately 265 percent of the median) and above the mean for October (approximately 120 percent of the mean). In October, monthly mean discharge values ranged from below normal to normal in much of southern Illinois, to above normal to much above normal in northern Illinois. The October 2026 monthly mean flow posted for Skillet Fork at Wayne city, in southeastern Illinois, was zero cfs.
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-September water levels at 19 reservoirs for which levels were reported last month and this month, reported end-of-October water levels were lower at 17 reservoirs and at about the same level as at the end of September at 2 reservoirs. For the 19 reservoirs with measurements reported for the end of October, water levels were below normal target pool or spillway level at 17 reservoirs, and slightly above the nominal seasonal target level at 2 reservoirs.
Major Reservoirs. Compared to water levels at the end of September, at the end of October the water level at Lake Shelbyville was 0.5 feet lower, Carlyle Lake was 0.3 feet lower, and Rend Lake was 0.8 feet lower. At the end of October, Lake Shelbyville was 0.4 feet below the seasonal target level, Carlyle Lake level was 0.1 foot below the seasonal target level, and Rend Lake was 1.4 feet below 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 October 2026 mean level for Lake Michigan was 578.8 feet. The monthly mean level one year ago (October 2025) was 578.3 feet. The long-term average lake level for October is 578.9 feet, based on 1918-2025 data. In this period of record, the lowest mean level for Lake Michigan for October occurred in 1964 at 576.4 feet, and the highest mean level for October occurred in 1986 at 582.4 feet. The month-end level of Lake Michigan was 578.6 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Water tables continued to decline in October. Twenty-one of the 25 monitored wells were lower at the end of October.
Wells averaged 0.66 feet lower than September. Declines occurred throughout the state with most less than one foot. The DeKalb (DeKalb County) well fell 2.47 feet as the area dried out from a wet September.
The largest drop was at the Perry (Pike County) well which was 2.62 feet lower at the end of the month. The ICN station at the site recorded only 1.27 inches of precipitation in October and a 21% decline in the 2-inch soil moisture.
The Monmouth (Warren County) well rose 0.62 feet, the network’s largest increase. The co-located ICN station received more than 3 inches of precipitation with soil moisture remaining higher for most of the month.
Improvements were seen when comparing October 2026 to October 2025, a time when 79% of the state was listed as in drought. Thirteen wells ended the month higher than one year ago. Most notably, The Fermi Lab (DuPage County) well was 7.49 feet higher than last year
Wells averaged 1.18 feet above October 2026, 0.89 feet below the 15-year average, and 0.81 feet below the long-term record (Table 4).
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