Overview
December was cool and dry for Illinois.
Temperatures averaged 29.7 degrees, 1.9 degrees below the 1991–2020 average (Figure 1). December tied for 53rd coolest on record going back to 1895.
Precipitation averaged 1.49 inches, 0.94 inches less than the 1991-2020 average (Figure 1). December was the fifth consecutive month with lower than normal precipitation.
Mean provisional streamflow data was sufficiently limited to preclude estimating an aggregate mean streamflow condition statewide for December. Available monthly mean discharge values in December ranged from much below normal to normal.
Water surface levels at the end of December were below the full pool or target level at 18 of 21 reporting reservoirs. At the end of December, Lake Shelbyville was 1.4 feet above the winter target level, Carlyle Lake level was 1.1 feet above the winter target level, and Rend Lake was 1.1 feet below the spillway level.
Lake Michigan’s mean level was below its long-term mean for the month.
Shallow groundwater levels were 2.59 feet below the long-term average at the end of December (Figure 1). Levels averaged 0.41 feet above November and 2.64 feet below 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 cooler and drier than normal.
Temperatures averaged 29.7 degrees, 1.9 degrees below the 1991–2020 average and tied for 53rd coolest on record going back to 1895 (Table 1a).
Day to day temperature variability in Illinois is highest in climatological winter, and December followed suit with huge dips and jumps in daily temperature. Daily average temperatures were 25 to 40 degrees below normal in the second week of the month and were 15 to 25 degrees above normal days around Christmas. Decatur broke its daily low temperature record with a -14-degree low on December 14th and broke a daily high temperature record with a 69-degree high on December 28th.
When taken altogether, December average temperatures ranged from low 20s in northwest Illinois to low 40s in far southern Illinois. December was 1 to 5 degrees cooler than normal in the northern half of the state, and near to around 1 degree warmer than normal in southern Illinois (Figure 2). The warmest place in the state last month was Olmstead in Pulaski County with an average December temperature of 38.4 degrees. The coolest place in the state was Stockton in Jo Daviess County with an average December temperature of 22.3 degrees.
Precipitation: December reminded us of an important difference between total precipitation and total snowfall. Northern and central Illinois had higher than normal snowfall last month, with totals ranging from 3 to 20 inches. Parts of central Illinois received over 400% of normal December snowfall, including 15.5 inches in Fisher and 12.8 inches in Bloomington. However, one inch of snowfall in Illinois usually yields between 0.05 inches and 0.10 inches of liquid water, meaning that even the highest snowfall totals last month equated to only around 1 to 2 inches of liquid precipitation.
Overall, December total precipitation ranged from around 4 inches in northeast Illinois to less than half an inch in southwest Illinois (Figure 3, Table 1a). Most of the northern half of the state was near to slightly wetter than normal last month, while the southern half of the state was 1 to 3 inches drier than normal. Morrisonville had its driest December on record (going back to 1948) with only 0.60 inches last month.
2025 Temperatures: The past year was very warm in Illinois, especially outside of climatological winter. March led the way with average temperatures that were over 6 degrees warmer than normal, and July, September, and October were all at least 2 degrees warmer than normal. Meanwhile, January and December were both 2 degrees colder than normal statewide, and below normal temperatures in February, May, and August as well.
Average temperatures in Illinois last year ranged from the high 40s in northern Illinois to the high 50s in southern Illinois, between 1 and 3 degrees above normal (Figure 4). The year 2025 was the 15th warmest on record in Chicago, and the 20th warmest on record in Peoria. The warmest point in the state last year was Bean Ridge in Alexander County with an average temperature of 60.5 degrees. The coolest point in the state was Stockton in Jo Daviess County with an average temperature of 48 degrees.
Overall, the statewide average annual temperature was 53.5 degrees, 0.9 degrees above normal and tied for the 20th warmest year on record statewide (Table 1b).
2025 Precipitation: Calendar year 2025 was very dry in Illinois, as all but 2 months last year were drier than normal. Only April and July were wetter than normal statewide. Some of the more extreme precipitation months included the 6th driest August on record statewide (1.74 inches), the 11th driest September on record (1.36 inches), and the 9th wettest July on record (6.23 inches).
As is seemingly a perennial occurrence, June and July brought multiple rounds of extremely heavy rainfall in Illinois. Most of far southern Illinois had 8 to 10 inches of precipitation in June, following an extremely wet late spring in the region. Following the very wet month of June, parts of Fayette County in south-central Illinois experienced more than 12 inches of rain in a single day. Never to be outdone, the Chicagoland was subsequently impacted by extremely heavy rainfall in July, Including over 5 inches of rain in less than 90 minutes around the United Center on the west side.
For the fourth consecutive year Illinois fell back into drought in the late summer and fall as each of the last five months of the year were at least 0.75 inches drier than normal statewide. Total precipitation in August and September statewide was only 3.1 inches, less than 50% of normal and the third driest August to September period on record in Illinois. Once again, the region’s largest rivers approached low stage, including along the Illinois, Mississippi, and Ohio Rivers, causing some moderate challenges to shipping. The dry fall weather was beneficial to harvest, but depleted moisture throughout the soil and caused near-record low flow conditions in several Illinois streams including the Sangamon and Vermilion Rivers. Dry conditions persisted through the rest of fall and into the first month of winter, causing a significant expansion of moderate to extreme drought across Illinois.
Figure 5 shows the spatial variability of 2025 precipitation in more detail. Total precipitation last year ranged from nearly 60 inches in far southern Illinois to less than 30 inches in east-central Illinois. Much of southern and southeast Illinois were 1 to 5 inches wetter than normal in 2025, while most of central and northern Illinois were 1 to 10 inches drier than normal (Figure 5, Table 1b).
2025 Severe Weather: Following the most tornadoes on record statewide in 2024, Illinois had another very active severe weather year… at least until July. Overall, the NOAA Storm Prediction Center reported a preliminary 146 tornadoes in 2025, but that total will likely decrease before the official count is released. March through June had well above average tornado frequency, followed by a large dropoff in tornadoes, and storms more generally, between August and November. It is likely the 2025 tornado total will be adjusted, but irrespective of changes, the year was another extremely active tornado year in Illinois.
The NOAA Storm Prediction Center also had 167 severe hail reports and 656 severe wind reports in Illinois in 2025. Some of the wind reports came from strong, non-thunderstorm winds that caused a dust storm in central and northern Illinois, including the city of Chicago in mid-May.
Illinois Climate Network (ICN) (Jennie Atkins)
The Illinois Climate Network (ICN) consists of 20 stations across the state which collects hourly weather and soil information.
Winds increased again in December, rising to an average of 7.9 mph, 0.9 mph higher than November and 0.1 mph higher than the network’s long-term monthly average. ICN Bondville (Champaign County) averaged 12.0 mph, the highest for the month. The highest recorded wind speed was 59.5 mph, measured at ICN Snicarte (Mason County) and December 18.
Temperatures varied greatly across December as the state experienced extremes on both ends of the scale. Mid-month saw cold weather across the state with temperatures falling into the negatives at the northern and central stations. However, the same stations warmed 60 to 80° at the end of the month as temperatures rose to the 60s and 70s.
Overall, temperatures averaged 31.0°F, 13.0° cooler than November and 1.1° below the long-term average. The highest recorded temperature was 76.9° F, measured at ICN Belleville (St Clair County) on December 28. The lowest was -15.1° which was recorded at ICN Bondville (Champaign County) on December 14.
Soil temperatures were 9 to 10° lower in December with averages in the high 30s to low 40s.
Under bare soils, temperatures ranged from 18.2 to 67.2° at 2 inches and 20.5 to 61.7° at 4 inches. Temperatures under sod ranged from 26.5 to 62.8° at 4 inches and 32.1 to 58.3° at 8 inches.
The network was drier in December with precipitation averaging 1.61 inches, 0.43 inches less than November and 0.90 inches below the long-term average. It was particularly dry month at ICN Belleville (St Clair County) that recorded only 0.65 inches, 1.96 inches below its long-term average. ICN St Charles (Kane County), however, received 2.36 inches, 0.39 inches above its long-term average and the month’s highest total.
Soil moisture will return to the IWCS in Spring 2026.
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 weigh 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 monthly mean discharge values in December ranged from much below normal to normal. Monthly mean flow of the Illinois River at Valley City was the lowest recorded for the calendar month of December in the period of record of the station (since 1938).
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 19 reservoirs for which levels were reported last month and this month, reported end-of-December water levels were lower at 9 reservoirs, higher at 7 reservoirs, and about the same as at the end of November at 3 reservoirs. For the 21 reservoirs with measurements reported for the end of December, water levels were below normal target pool or spillway level at 18 reservoirs, and above normal target pool or spillway level at 3 reservoirs.
The water level of the Sparta reservoir continues to be maintained by supplemental pumping from the Kaskaskia River. Supplemental pumping from Carlyle Lake to Salem Lake was resumed by the end 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 1.6 feet lower, Carlyle Lake was 0.2 feet lower, and Rend Lake was 0.2 feet lower. At the end of December, Lake Shelbyville was 1.4 feet above the winter target level, Carlyle Lake level was 1.1 feet above the winter target level, and Rend Lake was 1.1 feet below the spillway level. (Target pool levels of Lake Shelbyville and Carlyle Lake decrease during December.)
Great Lakes. December’s mean and end-of-month values are provisional and are relative to International Great Lakes Datum 1985. The December 2025 mean level for Lake Michigan was 577.5 feet. The monthly mean level one year ago (December 2024) was 578.2 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 577.5 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Water tables rose slightly in December, ending the month with an average 0.41 feet higher than November.
The average was significantly impacted by large increases at three wells – DeKalb (DeKalb County), Good Hope (McDonough County), and Brownstown (Fayette County) – that reported increases of more than two feet. The DeKalb well rose 5.91 feet in December, much of the increase occurring in the second half of the month when the areas received 1.55 inches of precipitation.
Twelve of the twenty-two reporting wells ended December lower than November.
Wells averaged 2.64 feet below December 2024, 2.42 feet below the 15-year average, and 2.59 feet below the long-term record (Table 4). Water tables have averaged lower than the long-term record for every month of 2025.
Data sources for the IWCS include the following:
CPC - Climate Prediction Center
ISWS - Illinois State Water Survey
MRCC - Midwestern Regional Climate Center
NCEI - National Centers for Environmental Information
NWS – National Weather Service
SPC – Storm Prediction Center
USACE – U.S. Army Corps of Engineers
USDM – US Drought Monitor
USGS – US Geological Survey
WARM – Water and Atmospheric Resources Monitoring Program