December 2021 Overview
Temperatures and precipitation were above the long-term average in Illinois in December. Mean streamflow statewide was above the median for the month. Shallow groundwater levels were above the long-term depths.
Air temperatures averaged 40.0°F, 10.6° above the 1991–2020 normal for December (Figure 1). December average temperatures ranged from the low 30s in northern Illinois to the high 40s in southern Illinois, between 6° and 12° above normal.
Precipitation statewide in December was 2.46 inches, 0.16 inches above the long-term statewide average (Figure 1).
Mean provisional streamflow aggregated statewide was above the long-term median flow for December, about 155% of median (Figure 1). Monthly mean discharge values in December were mostly in the normal range for the month.
Water surface levels at the end of December were below the full pool or target level at 3 of 23 reporting reservoirs. At the end of December, Lake Shelbyville was 7.9 feet above the winter target level, the Carlyle Lake level was 1.5 feet above the winter target level, and Rend Lake was 1.6 feet above the spillway level. Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels statewide were 0.84 feet above the long-term average at the end of December (Figure 1). Levels averaged 0.96 feet above those in November 2021 and 0.94 feet above December 2020 levels.
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 much warmer and slightly wetter than average statewide.
Temperatures averaged 40.0°F, 10.6° above the 1991–2020 normal for December (Figure 1, Table 1a). It was the second warmest December on record statewide.
December average temperatures ranged from the low 30s in northern Illinois to the high 40s in southern Illinois, between 6° and 12° above normal (Figures 1 & 2). High temperatures regularly reached into the 70s last month in southern Illinois, including a 78° high in Randolph County on December 4. Carbondale recorded 74° on December 25, one of the warmest Christmas Day temperatures on record in the state. Although fleeting, we did see very cold temperatures in December, including nighttime lows in the single digits across much of northern Illinois.
December was the warmest on record in Carbondale, second warmest in Quincy, third warmest in St. Louis, Springfield, and Peoria, fourth warmest in Rockford, and fifth warmest in Chicago.
Precipitation statewide in December was 2.46 inches, 0.16 inches above the long-term statewide average (Table 1a). November was drier than normal virtually everywhere in the state, and that dryness persisted in December for the western half of the state. Total December precipitation ranged from around 1 inch in far western Illinois to nearly 6 inches in far southeast Illinois. Most of western Illinois was 0.50 to 1.5 inches drier than normal, whereas eastern and southern Illinois were near to 0.50 inches wetter than normal for the month.
Snow essentially requires two ingredients: (1) cold-enough temperatures to permit the snow to reach the ground and (2) enough moisture in the air to precipitate as snow. In December we generally had enough moisture in the air, but the persistently warm weather ensured most of the month’s precipitation was in liquid form. Consequently, snowfall was 1 to 8 inches below normal in December. Snow totals ranged from 4 inches in far northwest Illinois to 0 inches south of Interstate 72.
Many places did not record their first measurable snow until the last week of the month. Chicago recorded their first measurable snowfall of the season on December 28, eight days later than the previous recorded latest first snowfall (December 20, 2012). Most places in south-central and southern Illinois did not record any measurable snowfall in December, including Springfield. The capital city has made it to January 1 without measurable snowfall only twice in its 120-year record, in 1912 and in 2020.
Table 1a. December 2021 Temperature and Precipitation Summaries
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Table 1b. 2021 Temperature and Precipitation Summaries
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With the liquid water content of snow and rainfall taken together, the preliminary statewide average total December precipitation was 2.70 inches, 0.02 inches above the 1991–2020 average and tied for the 40th wettest on record.
Drought: Although we entered climatological winter with moderate to severe drought in parts of northern Illinois, conditions tend to improve slowly in winter. The near normal precipitation helped eat away at drought a bit in December in northern Illinois, as the extent of both moderate and severe drought was reduced by about 10% by the end of the month, according to the U.S. Drought Monitor. Very warm December weather kept soils unfrozen throughout the month, allowing precipitation to infiltrate, and helping to improve soil moisture conditions in northern Illinois.
Although moderate drought remains largely north of Interstate 88 and severe drought remains in parts of McHenry and Lake Counties, conditions are much improved heading into 2022 than they were at the beginning of fall.
Severe weather, including tornadoes, hail, and strong winds, tends to have the highest frequency in Illinois between April and June. However, unlike Atlantic hurricanes or extreme cold, Illinois does not have a “tornado season” because tornadoes and other severe weather can and do occur all year in the Land of Lincoln. This past month we got an awful reminder of this fact.
The exceptionally warm weather and large-scale dynamics in December made for conducive conditions for severe weather, and indeed we had more than our fair share of severe weather last month. The most noteworthy event was on the night of December 10, when severe thunderstorms moved across the southern Midwest and the mid-South, resulting in multiple very strong tornadoes in Illinois, Missouri, Arkansas, Tennessee, and Kentucky.
The tornado outbreak was responsible for dozens of fatalities across the area, including six deaths in Illinois, many injuries, and damage or destruction to thousands of homes and structures. More details on the Illinois side of the severe weather outbreak are here: https://stateclimatologist.web.illinois.edu/2021/12/16/the-december-2021-tornado-outbreak-in-illinois/.
Overall, the NOAA Storm Prediction Center showed nine tornado reports in Illinois for December. Illinois averaged just over one December tornado per year between 1950 and 2020.
The year 2021 was both warmer and wetter than the 1991–2020 normal. The statewide average annual temperature was 53.7°, 2.1° above normal and tied for the 10th warmest year on record. Statewide total precipitation in 2021 was 41.06 inches, 3.59 inches above normal and the 37th wettest year on record.
2021 temperatures: The mild start to the 2020–2021 winter spilled over into the first month of the year. January average temperatures were nearly 3° above the 1991–2020 normal. This was followed by an extremely cold February that was 10° below the 1991–2020 normal and the 11th coldest on record statewide. March was nearly 5° warmer than normal, followed by April and May, which were both slightly cooler than normal. Although July was slightly cooler than normal, it was outweighed by June and August which were both nearly 2° warmer than normal. September and October were 2.7° and 5.4° above normal, respectively. October was the sixth warmest on record statewide and the warmest since 1971. The year ended with a slightly cooler than normal month of November and an extremely warm December that was 8° above normal. The final month of 2021 was the second warmest December on record statewide.
The year 2021 tied with 1998 as the 4th warmest year on record in Chicago and was the 4th warmest year in Rockford, the 7th warmest year in Moline, the 6th warmest in St. Louis, and the 13th warmest in Peoria. Statewide, 10 of the top 20 warmest years on record have occurred since 1990.
2021 precipitation: Calendar year 2021 began with a slightly wetter than normal January and slightly drier than normal February. March was more than 1 inch wetter than normal statewide, but the onset of drought in northern Illinois in spring resulted in drier than normal months in April and May. All three months of climatological summer were wetter than normal, and July was nearly 1 inch wetter than normal. Fall was a roller coaster, as September was about 1 inch drier than normal, followed by the fifth wettest October on record statewide and the ninth driest November on record statewide.
In 2021, perhaps more than in most years, the statewide precipitation numbers do not properly reflect the variability of precipitation between different parts of the state. One CoCoRaHS observer in Shelby County recorded 61 inches of precipitation in 2021, while several CoCoRaHS observers in Stephenson, Winnebago, Boone, McHenry, and Lake Counties observed less than 26 inches in 2021. The year 2021 was the 5th driest year on record in Rockford with just 24.2 inches (65 percent of normal) and the 10th driest year on record in Freeport.
Meanwhile, 2021 was also the 14th wettest year on record in Peoria, the 13th wettest year in Springfield, the 6th wettest year in Bloomington, the 8th wettest year in Danville, and the 15th wettest year in Jacksonville.
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 to an average of 7.7 mph in December, 0.8 mph higher than in November and 0.1 mph lower than the network’s long-term average. ICN Bondville (Champaign County) had the windiest month with an average of 11.8 mph. The highest wind gust was 61.2 mph, recorded at ICN Monmouth (Warren County) on December 15.
Air temperatures averaged 41° for the month, the same as in November and 10° warmer than the long-term average. All stations reported higher than normal temperatures, averaging 9° above their monthly long-term averages.
Highs reached the 70s at 11 stations with the rest having highs in the mid- to high 60s. The month’s highest reported temperature was 77°, measured at ICN Belleville (St. Clair County) on December 3. Lows ranged from the single digits to the low 20s. The lowest temperature was 6°, reported at ICN DeKalb (DeKalb County) on December 7.
Soil temperatures declined 2° to 5° from in November to averages in the low 40s. Under bare soil, temperatures ranged from 26° to 67° at 2 inches and 29° to 62° at 4 inches. Temperatures under sod ranged from 33° to 58° at 4 inches and 34° to 55° at 8 inches.
Precipitation increased across the network in December as totals ranged from 1.43 inches at ICN Monmouth (Warren County) to 6.60 inches at ICN Dixon Springs (Pope County). Totals averaged 3.03 inches, 0.61 inches higher than the long-term average.
Soil moisture data will return to the IWCS in Spring 2022.
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 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 above the median value for December (approximately 155 percent of the median) and slightly below the mean for December (approximately 95 percent of the mean). Monthly mean discharge values in December were mostly in the normal range for the month, except at a few Table 2 stations where monthly mean flows were above normal.
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-November water levels at 22 reservoirs for which levels were reported last month and this month, reported end-of-December water levels were lower at 3 reservoirs, higher at 14 reservoirs, and about the same as at the end of November at 5 reservoirs. For the 23 reservoirs with measurements reported at the end of December, water levels were below normal target pool or spillway level at 3 reservoirs, above normal target pool or spillway level at 13 reservoirs, and at about full pool level at 7 reservoirs. Kinkaid Lake level had recently been drawn down intentionally.
In this issue of the Illinois Water and Climate Summary, Table 3 includes reported pumpage totals for both November 2021 and December 2021. Future issues of IWCS will include pumpage reports for the current reporting month in Table 3 rather than the previous month.
Major Reservoirs. Compared to water levels at the end of November, at the end of December the water level at Lake Shelbyville was 2.4 feet lower, Carlyle Lake was 0.7 feet lower, and Rend Lake was 1.1 feet higher. At the end of December, Lake Shelbyville was 7.9 feet above the winter target level, the Carlyle Lake level was 1.5 feet above the winter target level, and Rend Lake was 1.6 feet above the spillway level. (Seasonal target operating levels of Lake Shelbyville and Carlyle Lake decrease in mid-December.)
Great Lakes. Current month mean and end-of-month values are provisional and are relative to International Great Lakes Datum 1985. The December 2021 mean level for Lake Michigan was 579.7 feet. The monthly mean level one year ago (December 2020) was 581.2 feet. The long-term average lake level for December is 578.6 feet, based on 1918–2020 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 579.6 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
End-of-month level data were collected at 24 wells for December. Water table levels increased at most monitoring wells in December. On average, wells were 0.96 feet higher than in November, with individual well changes ranging from 0.92 feet below to 3.15 feet above last month’s levels. The declines were seen primarily in northern and western Illinois as the Good Hope well (McDonough County) fell 0.92 feet in December and the Mt. Morris well (Ogle County) declined 0.72 feet.
Levels averaged 0.94 feet higher than in December 2020. The largest increase was in western Illinois where the Perry well (Pike County) level was 10.73 feet higher than that of last year as the area recovered from the drought it was experiencing at the end of 2020. However, the continuing dryness in the north is reflected in the declining levels. The Freeport well (Stephenson County) was 4.54 feet lower and the Galena well (JoDaviess County) was 1.16 feet lower than 2020.
Wells averaged 0.38 feet above the 15-year average and 0.84 feet above 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, 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