Temperatures and precipitation were above the long-term average in Illinois in October. Mean streamflow statewide was above the median for the month. Shallow groundwater levels were above the long-term depths.
Air temperatures averaged 60.2°F, 5.4° above the 1991–2020 normal for October (Figure 1). Monthly average temperatures ranged from the high 50s in northern Illinois to the low to mid-60s in southern Illinois.
Precipitation statewide in October was 6.13 inches, 2.87 inches above the long-term statewide average (Figure 1).
Soil moisture increased at depths from 2 to 20 inches in October. Levels at 2 inches increased 81% on average because of the wet weather across the state. Levels at 39 and 59 inches had little overall changes.
Mean provisional streamflow aggregated statewide was well above the long-term median flow for October, about 615% of median (Figure 1). Monthly mean discharge values in October ranged from normal to much above normal for the month.
Water surface levels at the end of October were below the full pool or target level at 5 of 24 reporting reservoirs. At the end of October, Lake Shelbyville was 7.6 feet above the seasonal target level, Carlyle Lake level was 0.8 feet above the seasonal target level, and Rend Lake was 1.1 feet above the spillway level. Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels statewide were 1.93 feet above the long-term average at the end of October. Levels averaged 1.81 feet above September 2021 and 1.62 feet above October 2020 averages.
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.
October in Illinois was much warmer and wetter than average statewide.
Temperatures averaged 60.2°F, 5.4° above the 1991–2020 normal for October (Table 1, Figure 1). Monthly average temperatures ranged from the high 50s in northern Illinois to the low to mid-60s in southern Illinois. These temperatures were 2° to 7° above normal across most of the state, with the larger departures in northern Illinois (Figure 2). Following a warm start to fall in September, temperatures were consistently 5° to 10° above normal for the first half of October. Nighttime minimum temperatures had the highest departures. The October average minimum temperature (51.1°) was the highest on record statewide.
The highest temperature observed in October was 91° in Congerville in Woodford County, and the lowest temperature was 24° in Galena in Jo Daviess County. The unusually warm October weather broke 13 daily high maximum temperature records and 58 daily high minimum temperature records. No daily low maximum or low minimum temperature records were broken in October.
Modified growing degree days (DD, base 50°, from April 1) ranged from around 3600 DD in northern Illinois to over 4200 DD in southern Illinois. DD accumulation this season (from April 1) has been near normal in southern Illinois and between 200 and 600 DD above normal in central and northern Illinois.
Precipitation statewide in October was 6.13 inches, 2.87 inches above the long-term statewide average (Table 1). Following a very dry September for most of northern Illinois, the taps opened in October and created persistently soggy conditions. For example, Champaign recorded 19 “wet days” in October, those in which at least 0.01 inches of precipitation are observed, which was the third most on record.
The persistently wet conditions resulted in monthly totals ranging from 3.5 inches in southwest and far southeast Illinois to over 9 inches in parts of north-central Illinois. While southern Illinois was 1 to 3 inches wetter than normal last month, central and northern Illinois were closer to 3 to 6 inches wetter than normal. It was the second wettest October on record in Peoria and Quincy and the fifth wettest in Springfield.
Severe weather reports: The NOAA Storm Prediction Center recorded 55 severe weather reports in October for Illinois: 16 for tornadoes, 4 for hail, and 35 for wind. (Multiple reports can be generated for a single event). As of the end of October, 78 tornadoes were confirmed in Illinois this year, compared to 71 in all of 2020, 37 in 2019, and 64 in 2018.
Drought: Although the October rain put harvest on hiatus, it was greatly beneficial in improving the drought conditions in northern Illinois. Soil moisture and streamflow were both boosted by the wet conditions last month. The U.S. Drought Monitor showed improved drought conditions throughout much of northern Illinois in the most recent month. The percentage of the state in at least severe drought (D2) declined from 8.78% on October 1 to 1.22% on November 2.
Most areas between Whiteside County in northwest Illinois and Lake County in northeast Illinois remain in moderate to severe drought, however, as year-to-date precipitation deficits of 5 to 10 inches remain. Wetter weather will need to prevail throughout the winter to terminate the northern Illinois drought, but October was a big step in the right direction.
Illinois Climate Network (ICN) (Jennie Atkins)
The Illinois Climate Network (ICN) consists of 19 stations across the state that collect hourly weather and soil information.
Wind speeds increased 0.7 mph in October to an average of 5.9 mph, 0.7 mph lower than the network’s long-term average. ICN Bondville (Champaign County) had the month’s highest average at 9.4 mph. The highest measured wind gust was 42.5 mph, recorded at ICN Perry (Pike County) on October 7.
Air temperatures averaged 60° in October, 9° cooler than in September and 6° above the long-term average. The weather was warm across the state. Monthly highs were in the mid- to high 80s at all ICN stations, ranging from 17° to 27° higher than the stations’ normals. ICN Freeport (Stephenson County) had the network’s highest temperature of 88° on October 1.
Despite the warm start, however, the network saw cooler weather the last third of October. Monthly lows were below normal at every station with temperatures 6° to 12° lower than normal. The northern stations had temperatures at or below freezing as ICN DeKalb (Dekalb County) recorded 28° on October 23, the month’s lowest temperature.
Soil temperatures continued to decline in October. Monthly averages were in the low to mid-60s, 8° to 12° cooler than in September but 5° to 6° warmer than the long-term averages. Under bare soil, temperatures ranged from 38° to 90° at 2 inches and 40° to 88° at 4 inches. Temperatures under sod ranged from 49° to 80° at 4 inches and 50° to 77° at 8 inches.
Precipitation was high across Illinois in October. ICN averaged 6.45 inches for the month, 4.26 inches more than in September and 3.50 inches higher than the long-term average. All stations recorded monthly totals higher than normal. ICN Perry (Pike) recorded 13.64 inches during the month, 12.58 inches more than in September and the network’s highest monthly total.
Wet conditions caused soil moisture to rise across Illinois. Soil moisture levels at 2 inches averaged 0.37 water fraction by volume (wfv) on October 30, an increase of 81% from the end of September. The largest increases were at the northern stations where 2-inch levels increased 184% from wilting point levels to 0.32 wfv.
Soil moisture increased from the 4- to 20-inch depths in all regions. Levels at 39 inches and 59 inches showed little change statewide, though small increases were seen in central Illinois. Despite increases at the shallower depths, northern soil moisture remained lower than the other regions 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 the 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 after 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 well above the median value for October (approximately 615 percent of the median) and above the mean for October (approximately 280 percent of the mean). Monthly mean discharge values in October ranged from normal to much above normal for the month. The October 2021 monthly mean streamflow of the Vermilion River at Pontiac was the highest for the month of October in 80 years of the station’s record.
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. Reservoir levels are obtained from a network of cooperating reservoir operators who are contacted each month by ISWS staff for the current water levels. 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-September water levels at 24 reservoirs for which levels were reported last month and this month, reported end-of-October water levels were lower at 4 reservoirs, higher at 16 reservoirs, and about the same as at the end of September at 4 reservoirs. For the 24 reservoirs with measurements reported at the end of October, water levels were below normal target pool or spillway level at 5 reservoirs, above normal target pool or spillway level at 12 reservoirs, and at about full pool level at 7 reservoirs.
Major Reservoirs. Compared to water levels at the end of September, at the end of October the water level at Lake Shelbyville was 6.8 feet higher, Carlyle Lake was 1.2 feet higher, and Rend Lake was 0.5 feet lower. At the end of October, Lake Shelbyville was 7.6 feet above the seasonal target level, Carlyle Lake level was 0.8 feet above the seasonal target level, and Rend Lake was 1.1 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 October 2021 mean level for Lake Michigan was 580.3 feet. The monthly mean level one year ago (October 2020) was 581.5 feet. The long-term average lake level for October is 578.9 feet, based on 1918–2020 data. In this period of record, the lowest mean level for Lake Michigan for October occurred in 1964 at 576.4 feet, and the highest mean level for October occurred in 1986 at 582.4 feet. The month-end level of Lake Michigan was 580.2 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Wet weather caused water table levels to rise at most monitoring wells in October. Only four wells (Crystal Lake in McHenry County, Mt. Morris in Ogle County, Snicarte in Mason County, and Sparta in Randolph County) reported end-of-month levels lower than in September. The Perry well in Pike County rose 6.09 feet, the highest of the month. On average, levels rose 1.81 feet from in September.
Levels averaged 1.62 feet above last year, 1.43 feet above the 15-year average, and 1.93 feet above the long-term average. Although improvements were seen at wells in northern Illinois, regional levels remained lower. At Freeport in Stephenson County the well level was 21.72 feet at the end of October, 4.42 feet below last year and 4.56 feet lower than the 15-year average.
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