Overview
A wet and slightly warmer August for Illinois.
Air temperature averaged 74.4 degrees for the month, 0.8 degrees above the 1991-2020 normal (Figure 1). The month was the 41st warmest August on record.
Precipitation statewide averaged 5.34 inches, 1.78 inches above the long-term statewide average (Figure 1). August 2026 tied for the 9th wettest August on record statewide.
Soil moisture decline 22%, on average, at 2-inch depths and 10% at 4 inches. Levels rose at 8 and 20 inches and remained stable at 39 inches and greater.
Mean provisional streamflow aggregated statewide was above the long-term median flow for August, about 480% of median (Figure 1). Monthly mean discharge values ranged mainly from normal to much above normal for August.
Water surface levels at the end of August were below the full pool or target level at 7 of 19 reporting reservoirs. At the end of August, Lake Shelbyville was 0.7 feet above the summer target level, Carlyle Lake level was 0.5 feet above the summer target level, and Rend Lake was 0.1 foot above the spillway level.
Lake Michigan’s mean level was slightly above its long-term mean for the month.
Shallow groundwater levels were 0.90 feet below the long-term average at the end of August (Figure 1). Levels averaged 0.73 feet below July and 0.33 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.
August in Illinois was warmer and much wetter than average statewide.
Temperatures averaged 74.4 degrees, 0.8 degrees above the 1991-2020 normal for August and the 41st warmest August on record (Table 1a). The month began with temperatures that were a couple of degrees cooler than normal, followed by a prolonged stretch of very warm and very humid weather. A cold front brought in much cooler weather in the third week of August, followed by a rapid warm up to transition into September. Warm and cold periods combined, average temperatures ranged from the low 70s in northern Illinois to the high 70s in southern Illinois, within 1 degree of average in most of northern and central Illinois, and between 2 and 4 degrees above average in southern Illinois (Figure 2). The warmest point in the state last month was Olmstead at 79.0 degrees, and the coolest point was Stockton at 69.5 degrees.
Precipitation statewide in August was 5.34 inches, 1.78 inches above the long-term statewide average and tied for the 9th wettest August on record statewide (Table 1a). Total precipitation ranged from just under 2 inches in southeast Illinois to over 12 inches in central Illinois. Much of the Interstate 74 corridor from the Quad Cities to Danville had 4 to 8 inches above average precipitation in August, while parts of south-central Illinois were 1 to 3 inches drier than normal (Figure 3).
Multiple rounds of thunderstorms ran around a large ridge in the southern Great Plains and moved from northwest to southeast across central Illinois between August 13 and August 15, producing widespread 3-to-6-inch rainfall totals and isolated pockets of 8 to 12 inches. Excess rain caused flooding in central Illinois rivers, reaching moderate flood stage on the Vermilion River near Danville, the Sangamon River at Monticello, and the Mackinaw River near Congerville. Last month was a top 10 wettest August on record in much of central and north-central Illinois and was the wettest August on record in Danville.
Summer: The warmest season of the year started with a mild June that was 1 to 2 degrees cooler than normal in northern and central Illinois. The heat turned on in July and pushed temperatures to 2 to 4 degrees above average in the hottest time of the year. August’s warmer than normal conditions wrapped up the season, ending as a somewhat, but not extremely, warm summer. This summer’s average temperature in Illinois was 74.4 degrees, 0.6 degrees above the 1991-2020 normal and tied for the 40th warmest summer on record statewide (Table 1b).
As is often the case in summer, different parts of the state experience different summers. June was very wet in central and north-central Illinois, while being very dry in southern Illinois. July was extremely dry for most everyone except Chicago and far southern Illinois, and August was exceptionally wet in central Illinois while being slightly to somewhat drier than average for others. This summer was a top five wettest on record for many places around the state, including the wettest summer on record in Danville. Statewide, summer total precipitation was 15.71 inches, 3.44 inches above the 1991-2020 normal and the 10th wettest summer on record (Table 1b).
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.
Winds remained calmer in August averaging 4.2 mph, equal to the July average and 0.3 mph lower than the network’s long-term average. The month’s windiest station was ICN Monmouth (Warren County) with an average of 5.9 mph. The highest recorded wind gust was 52.8 mph, measured at ICN Bondville (Champaign County) on August 11.
Air temperatures averaged 74.6 degrees in August, 2.3 degrees lower than July but 1.4 degrees lower than the long-term average. Station highs ranged from the high 80s to high 90s and lows were in the 50s. The highest network temperature was 99.6 degrees, recorded at ICN Olney (Richland County) on August 31 while the lowest was 51.1 degrees, measured at ICN St Charles (Kane County) on August 24.
Soil temperatures were 2 -4 degrees lower than July as averages were in the high 70s. Under bare soil, temperatures ranged from 50.6 to 108.8 degrees at 2 inches and 44.5 to 103.1 degrees at 4 inches. Temperatures under sod ranged from 68.3 to 94.1 degrees at 4 inches and 68.5 to 87.9 degrees at 8 inches.
ICN received 5.23 inches of rain in August, 1.27 inches more than July and 1.93 inches greater than the long-term average. However, there was a wide range between the stations. Eight stations recorded more than 5 inches of rain. ICN Stelle (Ford County) received 9.84 inches, 8.71 inches more than July and the month’s highest total.
The driest stations were in the south where ICN Olney (Richland County) measured 1.60 inches, the month’s lowest.
Soil moisture declined in the first 4 inches. Levels at 2 inches were 22% lower, on average at the end of the month. The largest differences were at the northern stations which ended the month 38% lower. The central stations, in comparison, saw only a 5% average decline.
Moisture levels, like rainfall, varied greatly across the network. The central stations rose 14% at 4 inches and increased through 59 inches, the entire measured soil column. Southern stations, however, saw decreases throughout the measured 59 inches. And northern stations declined at 4 and 8 inches but rose or stayed stable at deeper depths.
On average, soil moisture declined 10% at 4 inches across the state but rose at 8- and 20-inch depths. Levels remained stable at 39 and 59 inches.
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 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 480 percent of the median) and above the mean for August (approximately 295 percent of the mean). In August, monthly mean discharge values ranged mainly from normal to much above normal for the month. Monthly mean streamflows of Macoupin Creek near Kane and the Big Muddy River at Plumfield were below normal for August.
The August 2026 monthly mean streamflows of the Sangamon River at Monticello and the Vermilion River near Danville, both in east central Illinois, were the highest reported for the calendar month of August in the period of record of those gages.
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 19 reservoirs for which levels were reported last month and this month, reported end-of-August water levels were lower at 12 reservoirs, higher at 3 reservoirs, and about the same as at the end of July at 4 reservoirs. For the 19 reservoirs with measurements reported for the end of August, water levels were below normal target pool or spillway level at 7 reservoirs, above normal target pool or spillway level at 8 reservoirs, and at about the target level or spillway level at 4 reservoirs. Sparta Reservoir inflow has been supplemented by pumping from the Kaskaskia River. Carlinville water system supply was from its second lake, where the lake level was approximately 2 feet 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 4.0 feet lower, Carlyle Lake was 0.1 foot higher, and Rend Lake was 0.3 feet lower. At the end of August, Lake Shelbyville was 0.7 feet above the summer target level, Carlyle Lake level was 0.5 feet above the summer target level, and Rend Lake was 0.1 foot above the spillway level.
Great Lakes. Current month’s mean and end-of-month values are provisional and are relative to International Great Lakes Datum 1985. The August 2026 mean level for Lake Michigan was 579.4 feet. The monthly mean level one year ago (August 2025) was 578.9 feet. The long-term average lake level for August is 579.3 feet, based on 1918-2025 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 579.3 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Water tables declined at most monitored wells as 23 out of 26 wells were lower at the end of August.
Most decreases were less than a foot, but four wells dropped by more than two feet. The Carbondale (Jackson County) well fell 2.82 feet, the month’s largest decline. The area around the well experienced a drier month as soil moisture at 2 inches fell 40%. Smaller declines in moisture levels were seen through the top 40 inches.
The largest increase was in northern Illinois at the DeKalb (DeKalb County) well which rose 1.76 feet.
Wells averaged 0.33 feet above August 2025, 1.09 feet below the 15-year average, and 0.90 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