December 2022 Overview
December temperatures and precipitation were 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 30.3 degrees F, 1.3 degrees below the 1991–2020 normal for December (Figure 1). December average temperatures ranged from the low 20s in northern Illinois to the high 30s in southern Illinois.
Precipitation statewide in December averaged 2.42 inches, 0.01 inches below the 1991–2020 normal (Figure 1). December total precipitation ranged from just under 2 inches in western Illinois to over 5 inches in 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 below normal to above normal.
Water surface levels at the end of December were below the full pool or target level at 8 of 22 reporting reservoirs. At the end of December, Lake Shelbyville was 5.8 feet above the winter target level, Carlyle Lake’s level was 0.8 feet above the winter target level, and the Rend Lake water level was even with the spillway level. Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels were 1.1 feet below the long-term average at the end of December (Figure 1). Levels averaged 1.6 feet above November levels and 1.9 feet below last year’s levels.
The following description of temperatures, precipitation, 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.
December in Illinois was cooler and slightly drier than normal statewide.
Temperatures averaged 30.3 degrees F, 1.3 degrees below the 1991–2020 normal for December (Table 1a). It was the 62nd coldest December on record statewide. A strong winter storm moved through the Midwest in late December, bringing extremely cold Arctic air into the region. Actual temperatures recorded in Illinois in the final two weeks of the month included -15 degrees in Knox and Mercer Counties, -12 degrees in Lee and Cook Counties, and -11 in McLean and Champaign Counties. Combined with 25+ mph sustained winds and 45+ mph wind gusts, wind chill values were in the -25 to -40-degree range. The extreme temperatures and wind chill values are very unusual for late December, only occurring in northern and central Illinois once every 20 to 25 years. As often happens, a strong ridge followed the intense trough in the jet stream, and the state warmed up quickly in the last week of the month. High temperatures ranged from the low 50s to mid-60s, between 10 and 25 degrees above normal.
December average temperatures ranged from the low 20s in northern Illinois to the high 30s in southern Illinois, within 1 degree of normal (F). The warmest place in the state last month was Olmsted in Pulaski County with an average December temperature of 37.1 degrees. The coldest place in the state was Elizabeth in Jo Daviess County with an average December temperature of 22.4 degrees.
Precipitation statewide in December was 2.42 inches, 0.01 inches below the 1991–2020 normal (Table 1a). Most of the state entered the first month of climatological winter drier than normal with some parts of southern Illinois having precipitation deficits of 8 to 10 inches going back to the summer. Although December precipitation didn’t eliminate drought issues, the near-normal precipitation last month put us on the road to drought improvement.
December total precipitation ranged from just under 2 inches in western Illinois to over 5 inches in far southern Illinois (Figure 3). Most of central and south-central Illinois was 0.5 to 1.5 inches drier than normal last month, while northern and southern Illinois were 0.5 to 1.5 inches wetter than normal. December was the first wetter than normal month since July in parts of far southern Illinois.
Snow: Although the extreme temperatures and wind were the main storylines of the winter storm in late December, the storm did bring modest snowfall to the northern two-thirds of the state. A weaker storm system moved through the mid-south in early December, making for a snowier than normal December in southern Illinois, while snowfall totals were 1 to 5 inches below normal north of Interstate 64. The same snowfall pattern exists for the winter season to date, with areas north of Interstate 64 having 1 to 5 inches below normal snowfall and the areas south having 1 to 5 inches above normal snowfall.
2022 Temperatures: Following the second warmest December on record statewide, temperatures moderated quite a bit to start 2022. January and February average temperatures were 4.5 degrees and 3.2 degrees below the 1991–2020 normals, respectively. Spring was a mixed bag, with a very warm March and May split by an April that was nearly 3 degrees colder than normal. A hot start to summer pushed June 1.3 degrees above normal statewide, while July was only 0.3 degrees warmer than normal and August’s average temperature equaled the normal. Fall was very pleasant, as September and November were both just slightly warmer than normal, and October was 1.3 degrees colder than normal statewide. A December full of wild temperature swings ended just over 1 degree colder than normal to cap off 2022.
Overall, the statewide average annual temperature was 52.0 degrees, 0.6 degrees below normal (Table 1b). However, because Illinois has experienced a 100+ year warming trend as part of human-caused climate change, the statewide average annual temperature was still 0.4 degrees above the 20th century average, and 2022 tied for the 51st warmest on record statewide (Figure 4).
2022 Precipitation: Calendar year 2022 began with a very dry January, with less than 50 percent of normal precipitation statewide. February and March were 1.04 inches and 1.42 inches wetter than normal, respectively, making for wet soils heading into the spring. Although April was slightly drier than normal statewide, many places in the state had a double-digit number of days with measurable rain. Macomb, for example, had 25 out of 30 April days with measurable rainfall, but was still drier than normal for the month. May was slightly drier than normal, and June was nearly 2 inches drier than normal statewide. Meanwhile, July and August were both wetter than normal, followed by all three fall months with below normal precipitation. December wrapped up 2022 just slightly drier than normal statewide (Figure 5).
Much like past years, the statewide precipitation statistics are not representative of all places in Illinois. Calendar year 2022 was somewhat to much drier than normal in most of central and southern Illinois, but wetter than normal in areas that experienced extreme rainfall, such as the St. Louis Metro East and far northern Illinois. Last year was the 14th driest on record in Mt. Vernon, the 21st driest on record in Peoria, and the 41st driest on record in Moline; however, it was also the 30th wettest on record in Edwardsville and the 9th wettest on record in Freeport. The long-term Cooperative Observer station in Effingham recorded 56 inches of precipitation in 2022, making Effingham the wettest point in the state last year. Meanwhile, CoCoRaHS citizen science observers in LeRoy in McLean County and Mansfield in Champaign County recorded just 26 inches of precipitation in 2022.
Overall, statewide average total precipitation in 2022 was 37.24 inches, 3.51 inches below normal and the 61st driest year on record (Table 1b).
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 8.2 mph, 0.4 mph higher than in November and the network’s long-term average. ICN Stelle (Ford County) was the month’s windiest station having both the highest average at 12.3 mph and highest recorded wind gust at 51.4 mph, measured on December 3.
Temperatures averaged 31 degrees, 11 degrees warmer than in November and equal to the long-term average. However, the average hides the extremes experienced during the month. A winter storm in the third week of December brought extremely cold temperatures to the state. All stations had lows in the negatives with wind chills in the -20s and -30s. The lowest temperature was -12 degrees, recorded at ICN Monmouth (Warren County) on December 22. Illinois saw much warmer weather in the last week of December with station highs in the 50s and 60s. The highest temperature was 70 degrees, measured at ICN Perry (Pike County) on December 29.
Soil temperatures averaged in the high to mid-30s, 8 to 9 degrees lower than in November and 0 to 1 degree lower than the long-term average. Under bare soils, temperatures ranged from 8 to 62 degrees at 2 inches and 18 to 59 degrees at 4 inches. Temperatures under sod ranged from 25 to 54 degrees at 4 inches and 21 to 63 degrees at 8 inches.
Precipitation was slightly higher with an average of 2.82 inches across the network, 0.76 inches higher than in November and 0.40 inches higher than the long-term average. The lowest totals were in western Illinois where ICN Monmouth (Warren County) recorded 1.15 inches, the lowest for the month. ICN Dixon Springs (Pope County) in southern Illinois received 5.57 inches, the month’s highest.
The soil moisture data summary will return in the spring.
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 the 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 below normal to above normal. Monthly mean streamflow of the Kaskaskia River at Vandalia was much below normal in December. Note: December 2022 mean streamflow data are substantially unreliable or incomplete at many streamgage stations because of weather conditions late in the month affecting flow, equipment, or both.
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 4 reservoirs, higher at 15 reservoirs, and about the same as at the end of November at 1 reservoir. For the 22 reservoirs with measurements reported at the end of December, water levels were below the normal target pool or spillway level at 8 reservoirs, above the normal target pool or spillway level at 12 reservoirs, and at about the full pool level at 2 reservoirs. Salem Lake was supplemented by pumpage from Carlyle Lake during part of December.
Major Reservoirs. Compared to water levels at the end of November, at the end of December the water level at Lake Shelbyville was 0.9 feet higher, Carlyle Lake was 1.0 foot lower, and Rend Lake was 0.2 feet higher. At the end of December, Lake Shelbyville was 5.8 feet above the winter target level, Carlyle Lake’s level was 0.8 feet above the winter target level, and the Rend Lake water level was even with the spillway level. (Note: The target operational levels at 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 2022 mean level for Lake Michigan was 579.0 feet. The monthly mean level one year ago (December 2021) was 579.7 feet. The long-term average lake level for December is 578.6 feet, based on 1918–2021 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 578.8 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Water tables rose at most monitoring wells in December, ending the month averaging 1.6 feet above November levels (Table 4). The highest gains were in southern Illinois with the Olney (Richland County), Brownstown (Fayette County), and Boyleston (Wayne County) wells all having increases of 3.95 feet or greater.
Declines were seen at the western wells with Perry (Pike County) levels 1.28 feet lower at the end of the month.
Well levels averaged 1.9 feet lower than last year’s levels. Improvements continued in the north as the Mt. Morris (Ogle County) and Freeport (Stephenson County) wells were 5.36 feet and 5.62 feet, respectively, higher than in December 2021.
Wells averaged 1.5 feet below the 15-year average and 1.1 feet below the period of 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