Illinois Water & Climate Summary

September 2023 Overview

September air temperatures were above average, and precipitation was below the long-term average in Illinois. The mean streamflow statewide was below the median for the month. Shallow groundwater levels were below the long-term average.

Air temperatures averaged 68.4 degrees in September, 1.6° above normal (Figure 1). September average temperatures ranged from the mid-60s in northern Illinois to the low 70s in southern Illinois.

Precipitation statewide in September was 2.32 inches, 1.03 inches below the 30-year normal (Figure 1). Total rainfall ranged from less than half an inch in western Illinois to over 7 inches in northeast Illinois.   

Soil moisture, on average, rose 20% at 2 inches to an end-of-month average of 0.23 water fraction by volume (wfv).  Increases were seen through the 8-inch depths.  However, there were significant differences between the regions. 

Mean provisional streamflow aggregated statewide was below the long-term median flow for September, about 75% of the median (Figure 1). Monthly mean discharge values in September ranged mostly from below normal to normal for the month.

Water surface levels at the end of September were below the full pool or target level at 19 of 22 reporting reservoirs. At the end of September, Lake Shelbyville was 0.7 feet below the seasonal target level, Carlyle Lake level was 0.2 feet above the seasonal target level, and Rend Lake was 0.1 foot above the spillway level.

Lake Michigan’s mean level was above its long-term mean for the month.

Shallow groundwater levels were 2.16’ below the long-term average at the end of September.  Levels averaged 1.25’ below August and 1.72’ below last year.

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Weather/Climate Information 

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. 

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Temperatures averaged 68.4 degrees in September, 1.6 degrees above normal and 35^th warmest on record statewide. 

September average temperatures ranged from the mid-60s in northern Illinois to the low 70s in southern Illinois, between 1 and 4 degrees above normal (Table 1, Figure 2). Several places hit highs in the low 90s in the latter half of the month, including 94 degrees in Charleston and Aurora. Meanwhile, cooler weather in the first part of the month pushed nighttime low temperatures into the 30s, including 37 degrees in Joliet, and 39 degrees in Stockton. 

Altogether, 10 daily high maximum temperature records and 10 daily high minimum temperature records were broken in September. Cooler weather in the first half of the month broke 3 daily low maximum temperature records last month.  

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Precipitation statewide in September was 2.32 inches, 1.03 inches below the 30-year normal and the 32^nd driest on record statewide. 

September precipitation ranged from less than half an inch in western Illinois to over 7 inches in northeast Illinois. The northeast quarter of the state was 1 to 4 inches wetter than normal in September, while much of the rest of the state was 1 to 3 inches drier than normal (Table 1, Figure 3). Last month was the eighth wettest September on record in Joliet, with 6.76 inches. Last month was the fourth driest September on record in Quincy, with just 0.35 inches. 

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Drought: The very dry September conditions added to longer-term drought in western Illinois, particularly around the Quincy area. Much of western Illinois has moved in and out of moderate to extreme drought throughout the past 18 months, culminating in significant drought impacts on river and stream levels, crop yields, water quality, and pasture conditions. The 2022-23 water year, which spans October 1, 2022, to September 30, 2023, was the 3^rd driest on record in Quincy and nearly 15 inches below the 30-year normal. The dryness in western Illinois is a regional manifestation of a larger drought event that spans from central Kansas to northern Wisconsin. The extensiveness of this year’s drought has resulted in another year of near-record low flow on the Mississippi River south of Cairo. Precipitation deficits over 12- to 18-month periods cannot be made up in a month or two, but instead require extended wetter weather over multiple seasons. A wet winter would be beneficial for improving soil, stream, and groundwater conditions – especially in places with only ephemerally frozen soils – but we will likely continue to see moisture deficits in parts of the Midwest into spring 2024. 

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 4.1 mph in September, 0.3 mph lower than in August, and 0.9 mph lower than ICN’s long-term average.  ICN Bondville (Champaign County) had the windiest month with a monthly average of 6.4 mph.  The highest recorded wind gust was 37.5 mph, measured at ICN Snicarte (Mason County) on September 16.

Temperatures fell 8° from August to a network average of 68°, 2° higher than the long-term average.  Station highs ranged from the high 80s to the mid 90s and were 10 – 15° warmer than average.   Lows were in the 40s at most stations, 8 – 13° cooler than average.  The highest temperature was 95°, recorded at ICN Big Bend (Whiteside County) on September 4.   ICN DeKalb (DeKalb County) had the month’s lowest, reporting 39° on September 14.

Soil temperatures fell 5 to 7° from August to averages in the low 70s.  Under bare soils, temperatures at 2 inches ranged from 51 to 101° and from 52 to 94° at 4 inches. Temperature under sod ranged from 60 to 90° at 4 inches and 62 to 84° at 8 inches.

Precipitation was below normal for the month with a network average of 2.42”, 0.55” lower than the long-term average.  Many southern and west central Illinois stations received less than normal rainfall for the month with two stations, ICN Brownstown (Fayette County) and ICN Snicarte (Mason County), having totals of less than one inch.   However, totals were higher in the north and east central regions where ICN Bondville (Champaign County) received 4.64”, the month’s highest total.

On average soil moisture at 2 inches rose 20% in September.  However,  moisture levels varied significantly by region with large increases in the north and east and drying in the south and west.

Soil moisture in northern Illinois increased 123% to an end-of-month average of 0.28 water fraction by volume (wfv).  Significant increases occurred through the 8-inch depths.  There were similar patterns at the eastern stations though the overall increases were smaller.

Drying was seen at southern and western stations with both regions averaging a 7% decrease in moisture levels at 2 inches with drying continuing through the 8-inch depths.

Soil moisture declined in all regions at depths from 20 to 39 inches but remained steady and higher at 59 inches. 

Surface Water Information (Bill Saylor)

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Provisional monthly mean flows for this month for 26 stream gaging stations located throughout Illinois are shown in Table 2, compared to statistics of past records 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 is 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 75 percent of the median) and below the mean for September (approximately 35 percent of the mean). Monthly mean discharge values in September ranged mostly from below normal to normal. Monthly mean discharge of the Embarras River at Ste. Marie was much below normal for the month.

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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 the seasonal target level or from the 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 22 reservoirs for which levels were reported last month and this month, reported end-of-September water levels were lower at all reporting reservoirs. For the 22 reservoirs with measurements reported for the end of September, water levels were below the normal target pool or spillway level at 19 reservoirs, and above the normal target pool or spillway level at 3 reservoirs. Carlinville's supply continues to be from its second lake.

Major Reservoirs. Compared to water levels at the end of August, at the end of September the water level at Lake Shelbyville was 0.5 feet lower, Carlyle Lake was 0.3 feet lower, and Rend Lake was 0.5 feet lower. At the end of September, Lake Shelbyville was 0.7 feet below the seasonal target level, Carlyle Lake level was 0.2 feet above the seasonal target level, and Rend Lake was 0.1 foot 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 2023 mean level for Lake Michigan was 579.5 feet. The monthly mean level one year ago (September 2022) was 579.8 feet. The long-term average lake level for September is 579.2 feet, based on 1918-2022 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 2020 at 582.0 feet. The month-end level of Lake Michigan was 579.4 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.

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Groundwater Information (Jennie Atkins)

Water tables declined at all monitored wells in September, averaging 1.25’ lower than last month.

Wells in western Illinois continued to see the largest decreases.  The Perry well (Pike County) was 2.55’ lower at the end of August, 11.81’ lower than September 2022.  The water level at the site has been steadily dropping since March.   A similar pattern is seen at Belleville (St Clair County) where the well was 2.37’ lower at the end of September and was 6.70’ lower than one year ago.

Wells averaged 1.72’ below September 2022, 2.94’ below the 15-year average, and 2.16’ below the long-term record.