October 2023 Overview
October 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 56.7 degrees in October, 1.9 degrees above normal (Figure 1). October average temperatures ranged from the low 50s in northern Illinois to the low 60s in southern Illinois, between 1 and 2 degrees above normal in most places.
Precipitation statewide in October was 3.23 inches, 0.03 inches below the 30-year normal (Figure 1). October precipitation ranged from nearly 6 inches in northeast Illinois to less than 2.5 inches in the St. Louis Metro East.
Soil moisture, on average, rose 40 percent at 2 inches to an end-of-month average of 0.32 water fraction by volume (wfv). Increases were seen through the 8-inch depths for all regions, while moisture levels remained steady at depths of 39 inches and greater.
Mean provisional streamflow aggregated statewide was above the long-term median flow for October, about 125 percent of median (Figure 1). Monthly mean discharge values in October ranged mostly from below normal to normal for the month.
Water surface levels at the end of October were below the full pool or target level at 18 of 19 reporting reservoirs. At the end of October, Lake Shelbyville was 0.9 feet below the seasonal target level, Carlyle Lake’s level was 0.2 feet below the seasonal target level, and Rend Lake was 0.6 feet below the spillway level. Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels were 2.32 feet below the long-term average at the end of October (Figure 1). Levels averaged 0.39 feet below September levels and 1.39 feet below last year’s levels.
Weather/Climate Information
The following description of temperatures, precipitation, drought, and snow 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 warmer and drier than normal statewide.
Temperatures averaged 56.7 degrees in October, 1.9 degrees above normal and the 31st warmest on record statewide.
October average temperatures ranged from the low 50s in northern Illinois to the low 60s in southern Illinois, between 1 and 2 degrees above normal in most places (Figure 2, Table 1). Most of the state saw high temperatures in the upper 80s or low 90s in early October, including 94 in Quincy and 90 in Hoopeston. The brief taste of winter in the last few days of the month brought widespread low temperatures in the low 20s, including 21 degrees in Mt. Vernon and 23 in Bloomington.
The warm periods last month broke 31 daily high maximum temperature records and an incredible 66 daily high minimum temperature records. Aledo in Mercer County broke its all-time October high temperature records last month with a high temperature of 93 degrees on October 3. The cold end to the month broke 18 daily low maximum temperature records and 37 daily low minimum temperature records.
Precipitation statewide in October was 3.23 inches, 0.03 inches below the 30-year normal. Because of the long-term wetting trend in Illinois due to climate change, last month was drier than the 30-year normal but still the 45th wettest on record statewide.
The month-end precipitation totals across the state do not tell the entire story of October precipitation. The first two-thirds of the month were somewhat to very dry across the state, as most places were 1 to 2 inches drier than normal through October 24. More active weather brought multiple rounds of rain–and some snow–to Illinois, raising month-end totals near or above the 30-year normals. October precipitation ranged from nearly 6 inches in northeast Illinois to less than 2.5 inches in the St. Louis Metro East. Most of the state north of Interstate 70 was 1 to 2 inches wetter than normal, while areas farther south were just slightly drier, up to 2 inches drier than normal (Figure 3, Table 1).
Drought: Last month wasn’t an extremely wet or extremely dry month anywhere in the state; however, wetter conditions in the northern half of the state helped reduce the drought extent from 23 percent of the state on October 3 to 7 percent on October 31. Small areas of western and southern Illinois remain in moderate drought due to longer-term precipitation deficits. For example, the first 10 months of the year have had the third lowest precipitation total in Quincy with 20.17 inches, around 10 inches below average. Although topsoil moisture in western Illinois has improved from the wetter end of October, deeper layer moisture and water table levels remain less than ideal because of the long-term dryness.
Snow: Halloween snow in northern and central Illinois happens once every four to six years. Measurable snow, with totals exceeding 0.1 inch, was recorded in much of northern Illinois, with totals as high as 1.5 inches in Mundelein and 0.9 inches at O’Hare. For reference, the average first measurable snow comes in the third or fourth week of November for most of northern and central Illinois, so this year’s event came about two to three weeks early. It’s important to note that an early snowfall does not mean we will necessarily have a very snowy winter.
Illinois Climate Network (ICN) (Jennie Atkins)
The Illinois Climate Network (ICN) collects hourly weather and soil information from 19 stations across the state.
Winds averaged 6.0 mph in October, 1.9 mph higher than in September but 0.5 mph lower than the long-term average. ICN Bondville (Champaign County) had the windiest month again with a monthly average of 10.2 mph. The highest recorded wind gust was 36.8 mph, measured at ICN Big Bend (Whiteside County) on October 24.
Temperatures fell 12 degrees from in September to a network average of 56 degrees, 1 degree higher than average. Monthly highs were in the mid- to high 80s at most stations with lows in the 20s. The highest temperature was 90 degrees, recorded at ICN Snicarte (Mason County) on October 3. The lowest was 21 degrees, measured at ICN Stelle (Ford County) on October 31.
Soil temperatures fell 10 to 14 degrees from in September to averages in the low 60s and high 50s. Under bare soils, temperatures at 2 inches ranged from 33 to 92 degrees and from 36 to 88 degrees at 4 inches. Temperatures under sod ranged from 40 to 83 degrees at 4 inches and 45 to 78 degrees at 8 inches.
Precipitation was near normal for October, averaging 3.25 inches for the network or 0.03 inches below the long-term average. The lowest totals were at the southern stations. ICN Belleville (St. Clair County) recorded 1.70 inches for the month, 1.30 inches below its October normal and the lowest for the month. ICN Springfield had the month’s highest total, recording 4.53 inches or 1.34 higher than its long-term average.
Soil moisture at 2 inches was 40 percent higher at the end of October, thanks partially to rain in the last week of the month. Increases were seen in all regions of the state. In southern Illinois, soils were drying for most of the month before an 80 percent rise in the last four days.
Similar increases occurred at 4 and 8 inches, while at 20 inches moisture levels increased in the north and east central regions while remaining steady in the south and west central areas. Soil moisture remained steady at depths of 39 inches and greater.
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 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 the 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 October (approximately 125 percent of the median) and below the mean for October (approximately 55 percent of the mean). Monthly mean discharge values in October ranged mostly from below normal to normal. Monthly mean discharge of the Kaskaskia River at Vandalia was much below normal for the month.
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 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-September water levels at 19 reservoirs for which levels were reported last month and this month, reported end-of-October water levels were lower at 13 reservoirs, higher at 3 reservoirs, and about the same as at the end of September at 3 reservoirs. For the 19 reservoirs with measurements reported for the end of October, water levels were below normal target pool or spillway level at 18 reservoirs and above normal target pool or spillway level at 1 reservoir, which is not a primary water supply.
Major Reservoirs. Compared to water levels at the end of September, at the end of October the water level at Lake Shelbyville was 0.2 feet lower, Carlyle Lake was 0.4 feet lower, and Rend Lake was 0.7 feet lower. At the end of October, Lake Shelbyville was 0.9 feet below the seasonal target level, Carlyle Lake’s level was 0.2 feet below the seasonal target level, and Rend Lake was 0.6 feet below 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 2023 mean level for Lake Michigan was 579.3 feet. The monthly mean level one year ago (October 2022) was 579.4 feet. The long-term average lake level for October is 579.0 feet, based on 1918–2022 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 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 in October, averaging 0.39 feet lower than last month.
Western Illinois well levels continued to drop. The Belleville well (St. Clair County) was 1.70 feet lower at the end of October, 8.36 feet lower than the 15-year average, and 7.39 feet lower than in October 2022. The Perry well (Pike County) ended the month 10.54 feet lower than in 2022 and 9.76 feet below the 15-year average.
However, there were improvements in 20 percent of the wells. The SE College well (Saline County) rose 0.56 feet during October but was still 2.04 feet lower than the 15-year average. Wells averaged 1.39 feet below those in October 2022, 2.92 feet below the 15-year average, and 2.32 feet below the long-term 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