Summary
Temperatures were above and precipitation was below the long-term average in Illinois in September. Mean streamflow statewide was above the median for the month. Shallow groundwater levels were slightly below the long-term depths.
Air temperatures averaged 69.6°F, 2.8° above the 1991–2020 normal for September (Figure 1). Monthly average temperatures ranged from the high 60s in northern Illinois to the low to mid-70s in southern Illinois.
Precipitation statewide in September was 2.29 inches, 1.06 inches below the long-term statewide average (Figure 1). Precipitation totals ranged from less than an inch in northwest Illinois to over 5 inches in east-central and southern Illinois.
Soil moisture at 2 inches declined 20% on average in September. Levels also decreased at the 4- and 8-inch depths but remained relatively steady at 39 and 59 inches.
Mean provisional streamflow aggregated statewide was above the long-term median flow for September, about 185% of median (Figure 1). Monthly mean discharge values in September ranged primarily from normal to above normal for the month.
Water surface levels at the end of September were below the full pool or target level at 11 of 22 reporting reservoirs. At the end of September, Lake Shelbyville was 0.8 feet above the seasonal target level, Carlyle Lake was 0.4 feet below the seasonal target level, and Rend Lake was 1.5 feet above the spillway level. Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels statewide were slightly below the long-term average this month with an average departure of 0.30 feet below the period of record (Figure 1). Levels averaged 0.96 feet below August 2021 and 0.48 feet below September 2020 levels.
Weather/Climate Information (Trent Ford)
The following description of temperatures, modified growing degree days, 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 average statewide.
Temperatures averaged 69.6°F, 2.8° above the 1991–2020 normal for September (Table 1a, Figure 2). Monthly average temperatures ranged from the high 60s in northern Illinois to the low to mid-70s in southern Illinois. These temperatures were 1° to 5° above normal across most of the state, with the larger departures in northern Illinois (Figure 2). Apart from the first few days of the month, September temperatures were largely above normal across the state. Most of the northern half of the state experienced 5 to 10 days last month with temperatures that were 5° to 10° above normal. The highest temperature observation of the month, 94°, was recorded in Alton (Madison County) on September 28. However, there were also a few places where temperatures reached into the 30s, including 36° in Galena (Jo Daviess County).
Warmer weather in September broke 15 high daily maximum temperature records and 4 high daily minimum temperature records across the state, including a record-breaking 77°F minimum temperature in Alton on September 19. In contrast, only two low daily maximum temperature records and two low daily minimum temperature records were broken last month.
Modified growing degree days (DD, base 50°F, from April 1) ranged from around 3500 DD in northern Illinois to over 4000 DD in southern Illinois. DD accumulation this season so far (from April 1) has been near normal to 200 DD above normal in southern Illinois and between 200 and 600 DD above normal in central and northern Illinois.
Precipitation statewide in September was 2.29 inches, 1.06 inches below the long-term statewide average (Table 1, Figure 2). September precipitation patterns followed those of previous months, with a large north-to-south gradient. Precipitation totals ranged from less than an inch in northwest Illinois to over 5 inches in east-central and southern Illinois. September was 2 to 3 inches drier than normal throughout northern Illinois and near normal to about 1inch wetter than normal in central and southern Illinois.
Dry conditions in September added to very dry spring and summer seasons in northern Illinois. The 2020–2021 water year, which runs from October 1 to September 30, was the third driest on record in Rockford. Total water year precipitation in Rockford was 21.69 inches, approximately 60% of average and 4 inches less than the 2011–2012 water year.
Severe weather reports: The NOAA Storm Prediction Center recorded 44 severe weather reports in September for Illinois: 1 tornado, 13 hail, and 30 wind reports. (Multiple reports can be generated for a single event). As of the end of September there were 62 tornadoes confirmed in Illinois this year, compared to 71 in all of 2020, 37 in 2019, and 64 in 2018.
Drought: Very dry conditions in September added to existing precipitation deficits in northern Illinois. In response, the latest U.S. Drought Monitor expanded severe and moderate drought across the northern third of the state and expanded extreme drought in McHenry County. The percentage of the state in severe drought (8.78%) reached its highest since fall of 2013.
Agricultural and ecological impacts have mostly run their course by this time of year. However, dry conditions have left substantial soil moisture deficits across northern Illinois, and a near to wetter than normal winter will be necessary to recharge dry soils. A healthy, persistent snowpack across northern Illinois would also go a long way to help improve soil moisture conditions as we move into next spring.
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 increased in September to an average of 5.2 mph, 1.0 mph higher than in August and 0.1 mph higher than the network’s long-term average. ICN Bondville (Champaign County) had the month’s highest average at 8.1 mph and the highest measured wind gust with 39.9 mph recorded on September 7.
Air temperatures declined 9°F to a September average of 69°, 4° higher than the long-term average. Highs remained in the 90s for most stations. ICN Brownstown (Fayette County) had the month’s highest temperature, recording 93° on September 18. Cooler weather in the last half of the month brought temperatures in the low to mid-40s. The network’s lowest temperature was 41°, measured at ICN Bondville (Champaign County) on September 24.
Soil temperatures fell 5° to 7°F from in August to averages in the low to mid-70s, 2° to 3° above the long-term averages. Under bare soil, temperatures ranged from 49° to 106° at 2 inches and 51° to 95° at 4 inches. Temperatures under sod ranged from 56° to 88° at 4 inches and 61° to 84° at 8 inches.
Precipitation averaged 2.19 inches for the network, 0.91 inches less than in August and 0.74 inches below the long-term average. Five stations in western and northern Illinois received less than half of their normal rainfall for the month. ICN Freeport (Stephenson County) received only 0.65 inches, 21% of its long-term average for September. ICN Carbondale (Jackson County) was the month’s wettest station with 5.47 inches or 181% of its long-term average.
Soil moisture at 2 inches declined 20% on average to a September 30 average of 0.21 water fraction by volume (wfv). Northern stations remained dry, ending the month at 0.12 wfv, which is the wilting point for most soils monitored. Dry conditions persisted in the northern soils at the 4- and 8-inch depths, but soils at 20 inches and greater retained their moisture. Moisture levels in the north averaged 0.25 wfv at 20 inches, a decline of 11% during September but still well above the wilting points. Drying was also observed in other regions at 4 and 8 inches, but soils remained moist 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. 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 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 185% of the median) and below the mean for September (approximately 90% of the mean). Monthly mean discharge values in September ranged primarily from normal to above normal for the month. Monthly mean streamflow of the LaMoine River at Ripley was below normal for September.
Water-Supply Lakes and Major Reservoirs. Table 3 lists reservoirs in Illinois, their normal pool or target water surface elevation, and other data related to observed variations in water surface elevations. Current reservoir levels are obtained from a network of cooperating reservoir operators who are contacted each month by ISWS staff. Reservoir levels are reported in terms of their difference from normal pool (or target level). The average of the month-end readings for the period of record is reported in terms of the difference from normal pool or target level (column 6 of Table 3), and the number of years of record for each reservoir also is given (column 7). Most reservoirs serve as public water supplies, with the exceptions noted in the last column.
Compared to end-of-August water levels at 21 reservoirs for which levels were reported last month and this month, reported end-of-September water levels were lower at 15 reservoirs, higher at 2 reservoirs, and about the same as at the end of August at 4 reservoirs. For the 22 reservoirs with measurements reported at the end of September, water levels were below the normal target pool or spillway level at 11 reservoirs, above the normal target pool or spillway level at 5 reservoirs, and at about the full pool level at 6 reservoirs.
Major Reservoirs. Compared to water levels at the end of August, at the end of September the water level at Lake Shelbyville was 4.1 feet lower, Carlyle Lake was 0.4 feet lower, and Rend Lake was 1.1 feet lower. At the end of September, Lake Shelbyville was 0.8 feet above the seasonal target level, Carlyle Lake was 0.4 feet below the seasonal target level, and Rend Lake was 1.5 feet 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 2021 mean level for Lake Michigan was 580.5 feet. The monthly mean level one year ago (September 2020) was 581.8 feet. The long-term average lake level for September is 579.2 feet, based on 1918–2020 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 580.3 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Shallow groundwater levels fell 0.96 feet from last month as 22 out of 25 wells reported declines (Table 4). The largest drop was at Brownstown in Fayette County where the well was 4 feet lower than at the end of August. At the Sparta well, however, levels increased 1.02 feet, the month’s highest level.
On average, wells were 0.71 feet below the 15-year average at the end of September. Levels ranged from 6.36 feet below to 2.82 feet above average levels. Some of the largest differences occurred in northern Illinois where three wells (Freeport in Stephenson County, Mt. Morris in Ogle County, and St. Charles in Kane County) had end-of-month levels that were 5 feet or more below the 15-year averages.
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, http://rivergages.com, 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
