December 2024 Overview
Illinois was warm in December.
Temperatures averaged 34°F, 2.4° above the 1991-2020 normal for December (Figure 1). It was the 26th warmest December on record statewide.
Precipitation statewide averaged 2.58 inches, 0.15 inches above the 30-year normal (Figure 1). It was the 47th wettest December on record.
Mean provisional streamflow data was sufficiently limited to preclude estimating an aggregate mean streamflow condition statewide for December. Available data suggests that monthly mean discharge values in December ranged mostly from below normal to normal.
Water surface levels at the end of December were below the full pool or target level at 5 of 18 reporting reservoirs. At the end of December, Lake Shelbyville was 6.4 feet above the January 1 target level, Carlyle Lake level was 1.6 feet above the January 1 target level, and Rend Lake was 2.8 feet above the spillway level.
Lake Michigan’s mean level was below its long-term mean for the month.
Shallow groundwater levels were 0.30 feet above the long-term average at the end of December (Figure 1). Levels averaged 1.79 feet above November and 3.62 feet above last year.
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.
December in Illinois was warmer and somewhat wetter than normal.
Temperatures averaged 34°F, 2.4° above the 1991-2020 normal for December (Table 1a). It was the 26th warmest December on record statewide.
When taken altogether, December average temperatures ranged from high 20s in northern Illinois to the low 40s in southern Illinois, between 1 and 5 degrees above normal (Figure 2). The warmest place in the state last month was Olmstead in Pulaski County with an average December temperature of 41.4 degrees. The coolest place in the state was Stockton in Jo Daviess County with an average December temperature of 26.6 degrees.
Precipitation: December wasn’t a washout anywhere but brought enough precipitation to continue improving water conditions from peak fall drought. December total precipitation ranged from over 6 inches in far southern Illinois to around 1.5 inches in northern Illinois. Most areas of the state south of Interstate 64 were 1 to 3 inches wetter than normal last month, while parts of northern Illinois were around 1 inch drier than normal (Figure 3, Table 1a).
The wettest part of the state was Olmstead in Pulaski County, which picked up just over 10 inches in the final month of the year. The driest place in the state in December was Freeport, with only 0.89 inches for the month.
As is often the case in milder Decembers, snowfall was a little harder to come by across Illinois last month. December total snowfall ranged from around 5 inches in far northern Illinois to less than a tenth of an inch in far southern Illinois, between 1 and 6 inches below normal. Snowfall so far this season has also been below normal across Illinois, to the tune of 1 to 10 inches.
2024 Temperatures
Average temperatures in Illinois in 2024 ranged from the low 50s in northern Illinois to the low 60s in southern Illinois, between 1 and 6 degrees above normal (Figure 4). The year 2024 was the warmest year on record in Chicago, Peoria, St. Louis, and Paducah, and it was a top 5 warmest year on record in the Quad Cities, Champaign-Urbana, Quincy, and Carbondale, among other places. The warmest point in the state last year was Dixon Springs in Pope County with an average temperature of 60.2 degrees. The coolest point in the state was Stockton in Jo Daviess County with an average temperature of 50.2 degrees.
Overall, 2024 was the 2nd warmest year on record with an average temperature of 55.5 degrees, only less than 2012 (Table 1b).
2024 Precipitation
Calendar year 2024 had quite variable precipitation, with five months of wetter than normal conditions (January, April, July, November, December) and seven months that were drier than normal. Illinois experienced the 2nd warmest and 2nd driest February on record statewide, with only 0.48 inches of statewide average February precipitation, about 1.5 inches below normal. A wetter than normal April and May helped refill soils ahead of the growing season, but also delayed crop planting and critical fieldwork in many parts of the state.
July brought multiple rounds of heavy rain to the state, including events that caused widespread damaging flooding in the St. Louis Metro East area: https://storymaps.arcgis.com/stories/df7ac819701a445399d7fe87328b4f77, and the Rockford area: https://www.weather.gov/lot/2024_07_13-14_Flooding. Nashville, Illinois experienced over 6 inches of rain in less than 8 hours on July 16th, forcing overflow of a local reservoir and flooding of several homes.
For the third consecutive year Illinois fell back into drought in the fall as the months of September and October were 1 to 3 inches drier than normal across the state. Total precipitation in September and October statewide was only 3.48 inches, about 60% of normal and the 12th driest 2-month period on record. Once again, the region’s largest rivers approached low stage, including along the Illinois, Mississippi, and Ohio Rivers, causing some moderate challenges to shipping. November and December were both wetter than normal to end the year.
Total precipitation last year ranged from over 60 inches in far southern Illinois and the St. Louis Metro East area to less than 35 inches in parts of northern and north-central Illinois (Figure 5). Areas along the Wisconsin and Iowa borders were 2 to 6 inches wetter than normal last year, and areas around St. Louis and along the Interstate 70 corridor were 6 to 12 inches wetter than normal.
A CoCoRaHS citizen science observer in Metropolis in Massac County had 62.92 inches of precipitation in 2024, making the birth of Superman the wettest point in the state. Meanwhile, another CoCoRaHS observer in El Paso in Woodford County had only 28.22 inches of precipitation in 2024.
Overall, statewide average total precipitation in 2024 was 40.69 inches, 0.05 inches above normal and the 43rd wettest year on record (Table 1b).
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 7.6 mph in December, 0.1 mph higher than November but 0.2 mph lower than the long-term average. ICN Monmouth (Warren County) had the windiest month again having the network’s highest average, 12.4 mph, and the highest recorded wind gust, 53.3 mph measured on December 4.
Temperatures fell 12° from November to an average of 35°, 3° higher than the long-term average. Station highs ranged from the mid-50 to mid-60s with lows from the negatives to teens. The month’s highest temperature was 65°, recorded at ICN Belleville (St Clair County) on December 16. The lowest temperature was -1°, measured at ICN Freeport (Stephenson County) on December 12.
Soil temperatures fell 12 to 13° to averages in the high 30s to low 40s. Under bare soils, temperatures ranged from 19 to 60° at 2 inches and 23 to 55° at 4 inches. Temperatures under sod ranged from 32 to 53° at 4 inches and 33 to 55° at 8 inches.
Precipitation averaged 3.35 inches, 0.78 inches lower than November but 0.84 inches higher than the long-term average. All but the northern stations recorded higher than normal totals for the month. Five southern stations had monthly totals greater than 5 inches with ICN Dixon Springs (Pope County) recording 7.56 inches, 2.99 inches greater than the station’s long-term average.
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.)
Sufficient data was not available to estimate an aggregate mean streamflow condition statewide for December, due to winter conditions or equipment outages for extended periods at several gaging stations. Available data suggests that monthly mean discharge values in December ranged mostly from below normal to normal. Flow of the Illinois River at Valley City was much below normal for December.
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 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-November water levels at 18 reservoirs for which levels were reported last month and this month, reported end-of-December water levels were lower at 6 reservoirs, higher at 11 reservoirs, and about the same as at the end of November at 1 reservoir. For the 18 reservoirs with measurements reported for the end of December, water levels were below normal target pool or spillway level at 5 reservoirs, above normal target pool or spillway level at 11 reservoirs, and at about the target level or spillway level at 2 reservoirs.
Major Reservoirs. Compared to water levels at the end of November, at the end of December the water level at Lake Shelbyville was 0.3 feet higher, Carlyle Lake was 0.6 feet lower, and Rend Lake was 2.2 feet higher. At the end of December, Lake Shelbyville was 6.4 feet above the January 1 target level, Carlyle Lake level was 1.6 feet above the January 1 target level, and Rend Lake was 2.8 feet above the spillway level. (Target pool levels of Lake Shelbyville and Carlyle Lake decrease during December.)
Great Lakes. Current month mean and end-of-month values are provisional and are relative to International Great Lakes Datum 1985. The December 2024 mean level for Lake Michigan was 578.2 feet. The monthly mean level one year ago (December 2023) was 578.9 feet. The long-term average lake level for December is 578.6 feet, based on 1918-2023 data. In this period of record, the lowest mean level for Lake Michigan for December occurred in 2012 at 576.2 feet, and the highest mean level for December occurred in 1986 at 581.6 feet. The month-end level of Lake Michigan was 578.2 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Water tables rose again in December, averaging 1.79 feet higher than November. Sixteen of twenty-two monitored wells recorded increases at the end of the month.
Ten of the wells rose more than two feet over the month. Most of the highest increases occurred in southern Illinois, a region that received from 2 to more than 5 inches of precipitation during December. The Carbondale well (Jackson County) was 4.70 feet higher, increasing 4.1 feet between December 16 and 19 when the weather station associated with the well recorded 1.75 inches of rain.
However, the several northern wells recorded decreases. The Galena well (Jo Daviess County) declined 1.07 feet, 1.67 feet below the well’s 15-year average and 1.2 feet below the long-term average.
Wells averaged 3.62 feet above December 2023, 0.31 feet below the 15-year average, and 0.30 feet below the long-term record (Table 4).
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://warm.isws.illinois.edu/warm/