September 2022 Overview
September temperatures were above and precipitation was below the long-term average in Illinois. Mean streamflow statewide was above the median for the month. Shallow groundwater levels were below the long-term depths.
Air temperatures averaged 66.9 degrees F, 0.1 degree above the 1991–2020 normal for September (Figure 1). Monthly average temperatures ranged from the low 60s in northern Illinois to the low 70s in southern Illinois.
Precipitation statewide in September was 2.54 inches, 0.81 inches below the long-term statewide average (Figure 1).
Soil moisture at 2 inches declined 14 percent in September to a state average of 0.21 water fraction by volume (wfv). Levels at 4 and 8 inches declined 13 and 9 percent, respectively. Moisture levels at 20 inches and greater remained steady.
Mean provisional streamflow aggregated statewide was above the long-term median flow for September, about 140 percent of median (Figure 1). Monthly mean discharge values in September ranged mostly from below normal to above normal.
Water surface levels at the end of September were below the full pool or target level at 16 of 23 reporting reservoirs. At the end of September, Lake Shelbyville was 0.6 feet below the seasonal target level, Carlyle Lake was 0.1 foot above the seasonal target level, and Rend Lake was 1.0 foot above the spillway level. Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels were 0.6 feet below the long-term average at the end of September (Figure 1). Levels averaged 0.7 feet below those in August and 0.3 feet below last year’s levels.
Weather/Climate Information (Trent Ford)
The following description of temperatures, precipitation, 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 slightly warmer and much drier than average statewide.
Temperatures averaged 66.9 degrees F, 0.1 degree above the 1991–2020 normal for September (Figure 2, Table 1). Monthly average temperatures ranged from the low 60s in northern Illinois to the low 70s in southern Illinois. The warmest place in the state last month was Bean Ridge in Alexander County with an average September temperature of 72.8 degrees. The coolest place was Stockton in Jo Daviess County with an average September temperature of 63.0 degrees.
A heat wave in the third week of September brought high temperatures in the mid- to upper 90s across the state. Among the very high temperatures was 99 degrees observed in Carbondale on September 21, tied with 1940 and 1955 for the second latest 99+ degree temperature on record in Carbondale. However, the cold front that moved through the state around September 21 brought much cooler, drier air into the region. Nighttime low temperatures in the final week of the month regularly dipped into the upper 30s and low 40s. A few stations even reached the 32-degree mark in the last week of September, including the 19th earliest first fall freeze on record in Normal.
Precipitation statewide in September was 2.54 inches, 0.81 inches below the
long-term statewide average (Figure 3, Table 1). September was much drier than normal for most of southern and northeast Illinois and near normal to slightly wetter than normal in parts of northern and west-central Illinois (Figure 3). Total precipitation last month ranged from over 6 inches in western Illinois to less than 1 inch in far southern Illinois. Among the driest points in the state in September was Morton in Tazewell County, which only had 0.58 inches of rain. In contrast, the wettest point in the state last month was Warsaw in Hancock County with 7.51 inches.
Drought: September dryness added to rainfall deficits from August in southern Illinois and increased fire risk and concerns of low flow in the Mississippi and Ohio Rivers. As of the second week of October, seven southern Illinois counties had mandatory burn bans in place, as 10-hour fuel moisture remained less than 10 percent. One silver lining of the dry weather was that it has allowed a smooth fall harvest so far.
The 2021–2022 Water Year: The transition from summer to fall is typically our hydrologically driest time of the year because of a higher evaporative demand and human demand for water in summer. Therefore, to best capture the seasonal variability of water availability and water balance, we denote the “water year,” which runs from October 1 to September 30. The 2021–2022 water year precipitation in Illinois ranged from nearly 60 inches in southern Illinois to less than 30 inches in far western Illinois. In all, water year precipitation was between 75 and 125 percent of normal statewide, not unusually wet or dry for the state.
Illinois Climate Network (ICN) (Jennie Atkins)
The Illinois Climate Network (ICN) collects hourly weather and soil information from 19 stations across the state.
Wind speeds averaged 4.5 mph in September, 0.2 mph higher than in August, but 0.6 mph lower than the long-term average. ICN Stelle (Ford County) once again had the month’s highest average with 7.1 mph. The highest measured wind gust was 47.7 mph, recorded at ICN Peoria (Tazewell County) on September 18.
Temperatures fell 7 degrees from in August to an average of 66 degrees, or 1 degree above the long-term average. Station highs were in the 90s with temperatures up to 6 degrees warmer than in August. Two stations, ICN Carbondale (Jackson County) and ICN Rend Lake (Jefferson County), recorded highs of 100 degrees on September 21, the highest of the month.
Low temperatures fell to the 40s and 30s. ICN DeKalb (Dekalb County) had the network’s lowest temperature, measuring 31 degrees on September 29.
Soil temperatures were 5 to 8 degrees cooler in August with averages in the mid-70s. Under bare soils, temperatures ranged from 47 to 95 degrees at 4 inches. Temperatures under sod ranged from 52 to 88 degrees at 4 inches and 57 to 84 degrees at 8 inches.
Precipitation averaged 2.35 inches for the month, 2.09 inches less than in August and 0.58 inches below the historical average. ICN Carbondale (Jackson County) had the driest month, recording only 0.69 inches for the month, 2.32 inches less than normal. Two other stations, ICN Dixon Springs (Pope County) and ICN Stelle (Ford County), also had totals of less than 1 inch. ICN Champaign (Champaign County) recorded 4.64 inches of rain, the highest for the month.
The drier weather led to lower soil moisture for most of the state. Two-inch soil moisture declined 14 percent in September to an end-of-month average of 0.21 water fraction by volume (wfv). Only the western stations saw improvements with levels increasing 13 percent on average.
Declines continued through 8 inches as levels stabilized at depths of 20 inches and greater. Moisture levels at 39 and 59 inches remained high.
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 records of monthly mean flows at those stations for the same month. Both recent and long-term data are retrieved from U.S. Geological Survey (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 above the median value for September (approximately 140 percent of the median) and below the mean for September (approximately 70 percent of the mean). Monthly mean discharge values in September ranged mostly from below normal to above normal. The monthly mean streamflow of the Kankakee River at Momence was much below normal for September.
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 an 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 19 reservoirs, higher at 1 reservoir, and about the same as at the end of August at 2 reservoirs. For the 23 reservoirs with measurements reported at 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 5 reservoirs, and at about full pool level at 2 reservoirs. The supply at Carlinville was pumped from their Lake 2, which was 6 inches below full pool.
Major Reservoirs. Compared to water levels at the end of August, at the end of September the water level at Lake Shelbyville was 0.2 feet lower, Carlyle Lake was 1.1 feet lower, and Rend Lake was 0.9 feet lower. At the end of September, Lake Shelbyville was 0.6 feet below the seasonal target level, Carlyle Lake was 0.1 foot above the seasonal target level, and Rend Lake was 1.0 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 2022 mean level for Lake Michigan was 579.8 feet. The monthly mean level one year ago (September 2021) was 580.5 feet. The long-term average lake level for September is 579.2 feet, based on 1918–2021 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 579.6 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Water table levels declined throughout the state in September. Twenty-one stations reported lower levels at the end of the month. The largest drop occurred at the DeKalb well (DeKalb County), which fell 1.5 feet to an end-of-month level of 5.8 feet. Declines continued at the Good Hope well (McDonough County), which ended the month 1.3 feet lower than at the end of last month. Well levels have dropped 5 feet since July.
Overall, well levels averaged 0.7 feet below those in August, ranging from 1.5 feet below to 1.4 feet above. Water tables averaged 0.3 feet less than in September 2021 and ranged from 4.8 feet below to 4.7 feet above. Levels averaged 1.1 feet below the 15-year average and 0.6 feet below the period of record.
Data sources for the IWCS include the following:
CPC - Climate Prediction Center, https://www.cpc.ncep.noaa.gov/index.php
ISWS - Illinois State Water Survey, https://www.isws.illinois.edu
MRCC - Midwestern Regional Climate Center, https://mrcc.purdue.edu/
NCEI - National Centers for Environmental Information, https://www.ncei.noaa.gov
NWS - National Weather Service, https://www.nws.noaa.gov
SPC - Storm Prediction Center, https://www.spc.noaa.gov
USACE - U.S. Army Corps of Engineers, https://www.lre.usace.army.mil
USDM - U.S. Drought Monitor, https://droughtmonitor.unl.edu
USGS - U.S. Geological Survey, https://waterdata.usgs.gov/il/nwis
WARM - Water and Atmospheric Resources Monitoring Program, https://www.isws.illinois.edu/warm
