August 2025 Overview
August 2025 was dry for Illinois. Precipitation, soil moisture, and water tables were all below normal.
Temperatures averaged 73.2°F, 0.4° below normal. August was the 60th coolest on record.
Precipitation averaged 1.75 inches, 1.81 inches below normal. The month was the 6th driest on record. High rainfall totals were reported in northern Illinois with parts of the south recording less than 0.10 inches.
Soil moisture at 2-inch depths averaged 0.17 water fraction by volume (wfv) at the end of August, 41% lower than the beginning of the month. Moisture levels declined were observed to depths of 5 feet.
Shallow groundwater levels were 1.28 feet below the long-term average at the end of August Levels averaged 1.54 feet below July and 0.40 feet below last year.
Mean streamflow statewide was above the median for the month.
Mean provisional streamflow aggregated statewide was above the long-term median flow for August, about 145% of median (Figure 1). Monthly mean discharge values ranged mainly from below normal to above normal for August.
Water surface levels at the end of August were below the full pool or target level at 13 of 16 reporting reservoirs. At the end of August, Lake Shelbyville was 1.4 feet below the summer target level, Carlyle Lake level was 0.9 feet below the summer target level, and Rend Lake was 0.9 feet above the spillway level.
Lake Michigan’s mean level was below its long-term mean for the month.
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.
August in Illinois was cooler and much drier than average statewide.
Temperatures averaged 73.2°F 0.4° below the 1991-2020 normal for August and the 60th coolest on record (Table 1a). The month began with temperatures that were 2 to 5 degrees below normal, followed by a prolonged stretch of very warm and humid weather. A series of cold fronts in the last third of the month parked cooler Canadian air over the state, pushing some days 10 to 12 degrees below normal in late August. When the warm periods and cold periods in August are averaged together, temperatures ranged from the low 70s in northern Illinois to the high 70s in southern Illinois, around within 2 degrees of normal virtually everywhere (Figure 2). The warmest point in the state last month was Olmstead at 77.8 degrees, and the coolest point was Stockton at 68.8 degrees.
Precipitation statewide in August was 1.75 inches, 1.81 inches below the long-term statewide average and the 6th driest August on record statewide (Table 1a). August rainfall was much more plentiful north of Interstate-80, and virtually non-existent in some parts of southern Illinois. August totals ranged from just over 10 inches in Will County (~300% normal) to less than a tenth of an inch in parts of south-central Illinois (Figure 3).
Carbondale only had 0.02 inches of rain the entire month making it the driest August on record there and the third driest of any month since the late 1800s. It was also the driest August on record in Mt Vernon (0.09 inches), Centralia (0.09 inches), and Charleston (0.03 inches), and the second driest in Springfield (0.16 inches) and Effingham (0.32 inches). In contrast, parts of northeast Illinois experienced a few heavy rainfall events that pushed August totals above 8 inches, including 10.5 inches in Oswego and 9.9 inches in Lake Forest.
Summer: Climatological summer encompasses June, July, and August, and the season often brings more than its fair share of intense weather. This past season was hot and humid to say the least. Average temperatures in June, July, and August ranged from the low 70s in northern Illinois to the high 70s in southern Illinois, between 1 and 3 degrees above normal statewide. Overall, the statewide summer average temperature was 75.2 degrees, 1.5 degrees above normal and the 24th warmest summer on record. The persistently high humidity last season kept nighttime temperatures especially high, and the statewide average summer minimum temperature was nearly 3 degrees warmer than normal, and 3rd highest on record (Table 1b).
Week to week and month to month precipitation variability is usually at its highest in summer, but this past season was extreme even for summer. June was extremely wet in far southern Illinois and very dry in most of northern Illinois. July was extremely wet in northwest and western Illinois, and near normal elsewhere. August was bone dry in southern Illinois and extremely wet in northeast Illinois. The final summer total was not in the top 10 wettest or top 10 driest at any of Illinois’ long-term weather stations, but precipitation extremes were nonetheless impactful last season. Those included intense rainfall and flooding in Chicago and rapid onset or “flash” drought in southern Illinois. Overall, the statewide average summer total precipitation was 12.08 inches, 0.19 inches below normal and the 48th wettest summer on record (Table 1b).
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 3.7 mph in August, 0.2 mph lower than July and 0.8 mph lower than the long-term average. August was the fifth month in which Illinois had lower than normal wind speeds. ICN Bondville (Champaign County) had the windiest month, averaging 5.6 mph. The highest recorded wind gust was 43.1 mph, measured at ICN Olney (Richland County) on August 19.
Temperatures were 5° lower than July with an average of 72.8° or 0.4° lower than the long-term average. Station highs ranged from the high 80s to the low 100s with lows in the 40s and 50s. ICN Rend Lake (Jefferson County) had the highest August temperature, recording 100.2° on August 19. The lowest was 43.9°, measured at ICN Stelle (Ford County) on August 27.
Soils were 2 to 3° cooler in August with averages in the mid to high 70s. Under bare soils, temperatures ranged from 52.2 to 114.4° at 2 inches and 61.1 to 102.9° at 4 inches. Temperatures under sod ranged from 63.4 to 93.0° at 4 inches and 64.9 to 88.2° at 8 inches.
August was dry for the network, especially for the southern and central stations. Six stations recorded less than 0.50 inches and three - ICN Brownstown (Fayette County), ICN Perry (Pike County), and ICN Rend Lake (Jefferson County) – had monthly totals less than 0.10 inches.
Overall, the network averaged 1.51 inches, 4.49 inches less than July and 1.79 inches less than the long-term average.
The drier weather caused soil moisture to fall in all regions. At 2-inch depths, moisture levels declined 41% in August to an average of 0.17 water fraction by volume (wfv). Several stations ended the month near the wilting point for their soil type.
Soil moisture decreases were seen through the entire measured soil column. Moisture levels at 5 feet were 13% lower across the network at the end of the month with declines in all regions.
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 weigh 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 August (approximately 145 percent of the median) and below the mean for August (approximately 90 percent of the mean). In August, monthly mean discharge values were above normal on some northern Illinois rivers, below normal in some east-central Illinois watersheds, and generally normal elsewhere.
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-July water levels at 16 reservoirs for which levels were reported last month and this month, reported end-of-August water levels were lower at 14 reservoirs, and about the same as at the end of July at 2 reservoirs (in systems where supplemental supply was in use). For the 16 reservoirs with measurements reported for the end of August, water levels were below normal target pool or spillway level at 13 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 seasonal pumping from Carlyle Lake. Carlinville water system continues to pump from its second lake, where the lake level was approximately 20 inches below full pool.
Major Reservoirs. Compared to water levels at the end of July, at the end of August the water level at Lake Shelbyville was 0.9 feet lower, Carlyle Lake was 3.1 feet lower, and Rend Lake was 1.3 feet lower. At the end of August, Lake Shelbyville was 1.4 feet below the summer target level, Carlyle Lake level was 0.9 feet below the summer target level, and Rend Lake was 0.9 feet above the spillway level.
Great Lakes. Current monthly mean and end-of-month values are provisional and are relative to International Great Lakes Datum 1985. The August 2025 mean level for Lake Michigan was 578.9 feet. The monthly mean level one year ago (August 2024) was 579.7 feet. The long-term average lake level for August is 579.3 feet, based on 1918-2023 data. In this period of record, the lowest mean level for Lake Michigan for August occurred in 1964 at 576.7 feet, and the highest mean level for August occurred in 2020 at 582.1 feet. The month-end level of Lake Michigan was 578.8 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Dry weather and falling soil moisture led to water tables falling across the state in August. Levels declined, on average, 1.54 feet with all but one of the wells reporting lower at the end of the month.
Most declines were between 1 and 3 feet. The Perry (Pike County) well had the largest decrease of the month, ending August 3.65 feet lower than July.
Only the Mt Morris (Ogle County) well rose in August with a 0.98 foot increase over July.
Seventy percent of wells were lower than August 2024. The Freeport (Stephenson County) had the largest difference, reporting 6.62 feet below last year. However, there was a significant increase in the Perry well over last year which reported end of the month values that were 14.25 feet higher despite the decrease seen in August 2025.
Wells averaged 0.40 feet below August 2024, 1.50 feet below the 15-year average, and 1.28 feet below the long-term record (Table 4). August was the eighth 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