December 2023 Overview
December temperatures were above, and precipitation was below the long-term average in Illinois. Mean streamflow statewide was below the median for the month. Shallow groundwater levels were below the long-term depths.
Air temperatures averaged 39.7 degrees, 8.1 degrees above the 1991–2020 normal for December (Figure 1). December average temperatures ranged from the low 30s in northern Illinois to the low 40s in southern Illinois, between 6 and 10 degrees above normal.
Precipitation statewide in December was 2.71 inches, 0.28 inch below the 1991–2020 normal (Figure 1). Total precipitation ranged from just over 4 inches in parts of northern Illinois to less than 1 inch in parts of far southern Illinois.
Mean provisional streamflow aggregated statewide was below the long-term median flow for December, about 80 percent of median (Figure 1). Monthly mean discharge values in December ranged mostly from much below normal to normal for the month.
Water surface levels at the end of December were below the full pool or target level at 11 of 20 reporting reservoirs. At the end of December, Lake Shelbyville was 5.4 feet above the winter target level, Carlyle Lake was 2.1 feet above the winter target level, and Rend Lake was 1.5 feet below the spillway level. Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels were 3.30 feet below the long-term average at the end of December (Figure 1). Levels averaged 0.84 feet above those in November and 2.40 feet below last year’s levels.
Weather/Climate Information
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 much warmer and somewhat wetter than normal.
Temperatures averaged 39.7 degrees, 8.1 degrees above the 1991–2020 normal for December (Table 1a). It was the third warmest December on record statewide.
For the second time in three years Illinois has experienced an extremely mild December. Last month was the second warmest December in Rockford, the third warmest in Peoria, and the fourth warmest in Chicago and Moline. None of the more than 120 National Weather Service stations in Illinois recorded a low temperature in the single digits last month, and many places hit the freezing mark only a handful of nights.
When taken altogether, December average temperatures ranged from the low 30s in northern Illinois to the low 40s in southern Illinois, between 6 and 10 degrees above normal (Figure 2, Table 1a). The warmest place in the state last month was Du Quoin with an average December temperature of 46.1 degrees. The coolest place in the state was Stockton in Jo Daviess County with an average December temperature of 33.9 degrees.
It is important to note that the nighttime low temperatures last month were much higher than normal. The preliminary average December minimum temperature in Illinois was 32.2 degrees, which would be the second highest on record statewide.
Precipitation statewide in December was 2.71 inches, 0.28 inch above the 1991–2020 normal (Figure 3, Table 1a). The first month of climatological winter brought a more active storm track to the Midwest than for most of fall. December total precipitation ranged from just over 4 inches in parts of northern Illinois to less than 1 inch in parts of far southern Illinois. Most of northern Illinois was around 1 inch wetter than normal, while most of the state south of Interstate 64 was 1 to 3 inches drier than normal in December (Figure 3, Table 1a).
Overall, the statewide average total December precipitation was 2.71 inches, 0.28 inches above the 1991–2020 average and 40th wettest on record statewide.
Snow: December total snowfall ranged from around 4 inches in far northwest Illinois to less than a tenth of an inch in southern Illinois, between 1 and 8 inches below normal.
2023 Temperatures
The past year was very warm in Illinois, but the largest temperature departures from normal were during the climatologically coldest months. January, February, and December 2023 were all at least 5 degrees warmer than normal, while June, July, and August were near to slightly cooler than normal. Average temperatures ranged from the low 50s in northern Illinois to the high 50s in southern Illinois. The entire state was between 1 and 2 degrees above normal last year (Figure 4, Table 1b).
The year 2023 was the 2nd warmest on record in St. Louis, 3rd warmest on record in Chicago and Moline, the 7th warmest in Champaign-Urbana, the 8th warmest on record in Carbondale, the 9th warmest in Rockford, the 12th warmest in Springfield, and the 14th warmest in Quincy. The warmest point in the state last year was Du Quoin with an average temperature of 60.3 degrees. The coolest point in the state was Stockton in Jo Daviess County with an average temperature of 49.3 degrees.
Last year also tied with 1931 for warmest on record in Peoria with an average temperature of 55.6 degrees. The year 1900 had a higher annual average temperature in Peoria but had almost a month’s worth of missing daily weather observations and is therefore not considered for annual records.
2023 Precipitation
Calendar year 2023 began with wetter than normal months in January, February, and March. The combination of wetter spring conditions and a large snowpack that rapidly melted in the Upper Midwest led to brief flooding along the upper Mississippi River, including the fourth highest crest on record at Davenport.
April, May, and June were all somewhat to very dry across the state. Dry weather and high evaporation in late April quickly dried out topsoil and–along with strong winds and farming activity–contributed to a severe dust storm on May 1 in south-central Illinois. The near zero visibility caused a 72-vehicle pileup on Interstate 55 that killed 7 and injured another 37 (https://www.weather.gov/ilx/01may2023-dust).
The state accumulated a nearly 6-inch precipitation deficit from April and June, which plunged most of Illinois into a significant drought. The June 27 U.S. Drought Monitor showed over 90 percent of the state in at least moderate drought and nearly 60 percent of the state in at least severe drought. Pasture conditions deteriorated rapidly in the early summer with little regrowth after the first hay cutting. Mature trees in central and northern Illinois showed significant stress and many municipalities reported substantial young tree mortality across the state.
July and August brought enough rain to limit agricultural and water resource impacts from the drought. Very heavy rain in Chicago in both July and August caused widespread flooding of hundreds of homes. Berwyn, Cicero, and Garfield Park all reported over 8 inches of rain in less than a day (https://www.weather.gov/lot/2023_07_02_Flooding).
For the second consecutive year, Illinois fell back into drought in the fall as the months of September, October, and November were all drier than normal statewide. After reaching its fourth highest crest at Davenport, the Mississippi River once again dipped below low stage in St. Louis and Memphis, forcing active dredging throughout the fall.
Overall, the preliminary statewide average total precipitation in 2023 was 34.63 inches, 6.12 inches below normal and the 30th driest year on record (Figure 5, Table 1b). Much like past years, the statewide precipitation statistics are not representative of all places in Illinois. Calendar year 2023 had close to normal precipitation in parts of northern and southern Illinois but was somewhat to much drier than normal in parts of central Illinois.
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.
Winds averaged 6.9 mph in December, 0.7 mph higher than in November but 0.9 mph lower than the network’s long-term average. ICN Bondville (Champaign County) continued its streak as the windiest station with a monthly average of 10.4 mph. The highest recorded wind gust was 41.3 mph, measured at ICN Snicarte (Mason County) on December 18.
Temperatures averaged 4 degrees lower than in November with a network average of 39 degrees. Temperatures were 7 degrees warmer than the long-term average. Station highs were in the 50s and 60s with lows in the teens. The highest temperature was 67 degrees at ICN Rend Lake (Jefferson County) on December 24. The lowest was 11 degrees, recorded at ICN Freeport (Stephenson County) on December 19.
Soil temperatures were 5 to 6 degrees lower than in November with averages in the low to mid-40s. Under bare soils, temperatures at 2 inches ranged from 28 to 61 degrees and from 31 to 58 degrees at 4 inches. Temperatures under sod ranged from 34 to 55 degrees at 4 inches and 35 to 57 degrees at 8 inches.
Precipitation increased in December to a network average of 2.98 inches, 2.11 inches more than in November and 0.47 inches higher than the long-term average. Northern and central stations were wetter than normal with totals 3 to 65 percent higher than their long-term averages. ICN Brownstown (Fayette County) recorded 4.02 inches in November, 58 percent higher than normal and December’s highest total.
The southern stations had a drier month as totals were 10 to 50 percent lower than normal. ICN Carbondale (Jackson County) received 1.69 inches, the month’s lowest total. However, despite the lower totals in the south, all ICN stations recorded more precipitation than in November.
The soil moisture summary will return in spring 2024.
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 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 below the median value for December (approximately 80 percent of the median) and below the mean for December (approximately 50 percent of the mean). Monthly mean discharge values in December ranged mostly from much below normal to normal.
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-November water levels at 20 reservoirs for which levels were reported last month and this month, reported end-of-December water levels were lower at 5 reservoirs, higher at 14 reservoirs, and about the same as at the end of November at 1 reservoir. For the 20 reservoirs with measurements reported for the end of December, water levels were below the normal target pool or spillway level at 11 reservoirs, above the normal target pool or spillway level at 7 reservoirs, and at about the target level or spillway level at 2 reservoirs. Kinkaid Lake’s level remains drawn down intentionally for annual maintenance. Inflow to Salem Lake was supplemented by pumping from Carlyle Lake during part of the month.
Major Reservoirs. Compared to water levels at the end of November, at the end of December the water level at Lake Shelbyville was 0.7 feet higher, Carlyle Lake was 0.5 feet higher, and Rend Lake was 0.2 feet lower. At the end of December, Lake Shelbyville was 5.4 feet above the seasonal target level, Carlyle Lake was 2.1 feet above the seasonal target level, and Rend Lake was 1.5 feet below 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 2023 mean level for Lake Michigan was 578.9 feet. The monthly mean level one year ago (December 2022) was 579.0 feet. The long-term average lake level for December is 578.6 feet, based on 1918–2022 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 578.9 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Water tables rose at wells across the state in December, ending the month 0.84 feet above the November average.
Levels increased at 20 of the 25 monitored wells. Although most wells had increases of less than a foot, two wells in the northeast were more than 3 feet higher at the end of December. The Dekalb well (Dekalb County) and the Fermi Lab well (DuPage County) rose 3.65 feet and 3.79 feet, respectively. However, both wells ended the month lower than last year and below their 15-year averages.
Similar patterns were seen at wells throughout Illinois. Despite December’s increases, most wells ended the year lower than in 2022 and all monitoring wells were lower than their 15-year average.
Wells averaged 2.40 feet below December 2022 levels, 3.66 feet below the 15-year average, and 3.30 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