Mostly Mild January Ends with a Bang

Our first month of 2021 was quite a bit warmer and slightly wetter than average across the state. The preliminary statewide average January temperature was 28.8 degrees, 2.4 degrees above the 1981–2010 normal and tied for the 36th warmest on record going back to 1895. Preliminary statewide average total January precipitation was 2.56 inches, 0.49 inches above the 1981–2010 normal and tied for the 32nd wettest on record going back to 1895.  

Data are provisional and may change slightly over time

Above Average January Temperatures

Following a warmer than average December, temperatures remained persistently above average throughout January. Figure 1 (below) shows January daily average temperature as a departure from average in Peoria. Only 7 out of 31 January days were cooler than average in Peoria. The lack of very cold days and nights this winter season has resulted in below average heating degree days, a metric often used to estimate and predict energy demand for heating. For example, Marengo in McHenry County has accumulated just over 3,600 heating degree days in the winter season to date, compared to a 30-year normal of 4,100 by this time.

Figure 1. Daily temperature in Peoria, expressed as a departure from the long-term average.

Despite the consistency of warmer than average conditions last month, only one daily temperature record was broken. This was the daily high minimum temperature record in Rockford. As the maps below show, January average temperatures ranged from the mid-20s in northern Illinois to the high 30s in southern Illinois, which was between 3 and 5 degrees above the 1981–2010 normal (Figure 2).

Figure 2. January average temperature (left) and departure from the 1981-2010 normal (right).

The preliminary statewide average January temperature was 28.8 degrees, 2.4 degrees above the 1981–2010 normal and the 36th warmest on record going back to 1895.

January Precipitation

Climatologically, January is one of the drier months of the year in Illinois. Last month started with rain and snow for most of the state, followed by two weeks when most of the state received less than half an inch of total precipitation. However, the last week of January brought more active winter weather, including multiple large winter storms that produced heavy precipitation and impressive snow totals across the state.

The map below shows snowfall totals on January 30–31 ranged from less than a tenth of an inch in south-central Illinois to over 10 inches in the Chicagoland area. The National Weather Service Chicago office reported isolated totals of over 12 inches in Romeoville, Peotone, and Thornton. Chicago’s O’Hare Airport reported 10.8 inches, which was the 26th largest two-day snowfall total on record there and the largest since 2015.

Figure 3. Total snowfall (inches) between January 30 and 31.

The late-month snow helped boost January totals above the 1981–2010 normal across most of northern and north-central Illinois, while the southern two-thirds of the state were within 2 inches of normal (Figure 4). January total snowfall ranged from less than half an inch in southern Illinois to over 20 inches in northern Illinois. Most folks south of Interstate 80 have seen below average snowfall for the winter season to date. Total snowfall is 10 to 50 percent of average in southern and central Illinois and above average only in northwest and far northern Illinois.

Figure 4. January snowfall total (left), departure from normal (middle), and total season snowfall as a percent of normal (right).

Overall, total January precipitation ranged from nearly 6 inches in far southern Illinois to just under 2 inches in northwest Illinois, wetter than the 1981–2010 normal virtually everywhere in the state (Figure 5). Preliminary statewide average total January precipitation was 2.56 inches, 0.49 inches above the 1981–2010 normal and tied for the 32nd wettest on record going back to 1895.  

Figure 5. January total precipitation (left) and departure from normal (right).

Outlooks

Short-term 8–14-day outlooks from the NOAA Climate Prediction Center show strongly elevated odds of below normal temperatures in the second week of February as a push of Arctic air makes its way southeast. Short-term precipitation outlooks show slightly elevated odds of near normal precipitation totals over this time period. Climatologically, February is the driest month of the year in Illinois.

Figure 6. 8 to 14 day temperature (left) and precipitation (right) outlooks from the NOAA Climate Prediction Center.

Longer-term outlooks for February as a whole month continue to show elevated odds of below normal temperature, but slightly elevated odds of wetter than normal conditions. This suggests the expectation of a wetter middle to later part of February as we move toward climatological spring.

Figure 7. One-month temperature (left) and precipitation (right) outlooks from the NOAA Climate Prediction Center for February.

Finally, three-month outlooks for February through April lean more toward a long-term warming trends and ongoing La Niña conditions in the Pacific, showing elevated odds of warmer and wetter than normal conditions to transition from winter into spring.

Figure 8. Three-month temperature (left) and precipitation (right) outlooks from the NOAA Climate Prediction Center for February through April.

2020 was Warmer and Wetter than Long-term Average

Illinois was both warmer and wetter than average in 2020. The statewide average yearly temperature was 53.3 degrees, 1.7 degrees above average and the 21st warmest year on record. Statewide total precipitation in 2020 was 41.84 inches, 4.37 inches above average and the 23rd wettest year on record.

2020 Temperatures

Calendar year 2020 began very warm, with January average temperatures more than 5 degrees above the 1981–2010 normal. This was followed by warmer than normal months in February and March; however, April and May were both more than 2 degrees below normal. June and July were warmer than normal, followed by three consecutive cooler than normal months in August, September, and October. The year ended with very warm months of November and December, relative to normal conditions. November, for example, was the seventh warmest on record statewide.

The maps below show 2020 average temperatures and departures from normal across the state. Average temperatures ranged from the high 40s in far northern Illinois to the high 50s in the southern half of the state. Most of northern Illinois was 1 to 2 degrees warmer than the 30-year normal, whereas the southern two-thirds of the state was within 1 degree of normal. 2020 was a top 10 warmest year in most counties in northeast Illinois, including the 6th warmest on record in McHenry County.

The table below shows the number of daily weather records broken at Illinois Cooperative Observer stations in each month of 2020. November led with the most high daily maximum and minimum temperature records broken, with 112 and 85, respectively. Daily low maximum and low minimum temperature records were broken at over 70 stations in September and October. However, over the entire year, twice as many low maximum temperature records were broken as low minimum temperature records. Illinois’ climate has warmed over the past 100 years, but daily minimum temperatures have increased at a larger rate than daily maximum temperatures. This disproportionate change results in fewer daily low minimum temperature records broken.

DailyHigh Maximum TemperatureHigh Minimum TemperatureLow Maximum TemperatureLow Minimum TemperatureHigh Precipitation
January213710163
February368243321
March152061
April133203342
May06423760
June20371430
July133093
August111744725
September0073451
October118701964
November112850343
December3412026
Total260210282140679

There were also 679 daily total precipitation records broken across Illinois in 2020. This includes 163 records in January, most of which were caused by a large system that brought heavy rain over multiple days to a large area of the state from the St. Louis Metro-East to Champaign-Urbana.

2020 Precipitation

Calendar year 2020 began much as 2019 ended: wet. Statewide total precipitation was more than 2 inches above the 1981–2010 normal in January, followed by wetter than normal months in March, April, and May. However, a switch to a very dry pattern in most of central and northern Illinois resulted in drier than normal conditions statewide in four of the past six months of the year. 

The plot below shows the 2020 monthly precipitation departure from normal in Chicago, Decatur, and Belleville. The first seven months of the year were wetter than normal in Decatur, followed by five consecutive drier than normal months. The year was more mixed in northern and southern Illinois. Both July and August were more than 4 inches wetter than normal in Belleville, leading to the second wettest climatological summer on record there.

The maps below show spatial variability of 2020 precipitation in more detail. Total annual precipitation ranged from less than 35 inches in western Illinois to nearly 60 inches in southeast Illinois. A large area of western Illinois was 2 to 4 inches drier than average, while most of central Illinois was within 1 inch of the long-term average. Much of northern and southern Illinois was 1 to 6 inches wetter than average, and a small area of southeast Illinois was 8 to 12 inches wetter than average. White County in southeast Illinois was nearly 12 inches wetter than average in 2020, making it the 8th wettest year on record there.

2020 Consistent with Long-term Trends

Both temperature and preciptiation have increased across the state over the past century, which define climate change in Illinois. The plot below shows the number of years in each decade between 1901 and 2020 in which the statewide average temperature and total precipitation were above and below the 1901–2020 average. Recent decades have tended to have more warmer and wetter than average years, and the most recent decade (2011–2020) had the highest incidence of both warmer than average and wetter than average years, eighth out of ten each.

Warm, Dry End to 2020

December was much warmer and drier than average across Illinois. The preliminary statewide average December temperature was 32.8 degrees, 2.9 degrees above the 30-year normal and tied for the 35th warmest on record. Preliminary statewide average total precipitation for December was 1.78 inches, 0.91 inches less than the 30-year normal, and the 43rd driest on record.

Data are provisional and may change slightly over time

December Temperatures

The atmospheric conditions that gave us a very warm November continued most of last month, resulting in above average December temperatures statewide. The maps below show December average temperatures and departures across the state. Average temperatures ranged from the high 20s in northern Illinois to the high 30s in southern Illinois, between 1 and 6 degrees above average. Rockford recorded its 10th warmest December on record, following its 3rd warmest November. In fact, seven of the twelve months of 2020 were in the top 10 warmest on record in Rockford, and 2020 as an entire year was fourth warmest on record there.

December average temperature (left), and departure from long-term average (right).

Daily maximum temperature records were broken at 34 stations last month. Overall, the statewide average December temperature was 32.8 degrees, 2.9 degrees above the 30-year normal and tied for 35th warmest on record.

Another Dry December

As the maps below show, December total precipitation ranged from just under 3 inches in northwest Illinois to just over 1 inch in much of central and southern Illinois. The entire southern two-thirds of Illinois was drier than average in December, with one-month precipitation departures between 2 and 3 inches in southern Illinois.

December precipitation expressed as a total (left) and departure from long-term average (right).

In terms of hydrology, December is considered a recharge month because precipitation is typically much larger than evaporation. This allows soil moisture to recharge prior to soils freezing. However, statewide, four out of the past five and seven out of the past ten Decembers have been drier than the current 30-year normal (see figure below).

Statewide December total precipitation, expressed as a departure from the 1981-2010 normal.

Statewide total December precipitation was 1.78 inches, approximately 0.91 inches below the 30-year normal and the 43rd driest on record.

Central Illinois Drought

December continued a dry latter half of 2020 for central and northern Illinois. The maps below show precipitation departure from average over the past 3, 6, and 12 months. A broad area of the state from Pike County to Cook County has experienced a 3-inch precipitation deficit since the start of July, with some areas of western and central Illinois experiencing 6- to 7-inch deficits. These deficits represent between 65 percent and 75 percent of normal over the past 6 months. The 12-month departures are more moderate due to the very wet start to 2020.

Precipitation over the last 3-, 6-, and 12-months, expressed as a departure from the long-term average. Values are shown for periods ending on December 31, 2020.

The December 29 version of the U.S. Drought Monitor showed continued moderate drought across western and central Illinois, and severe drought persisting along the Interstate 72 corridor between Springfield and Decatur. These areas have significantly depleted soil moisture, and a wet end to winter and start to spring will be necessary to return soil moisture to near normal conditions.

Most of the state experienced below normal snowfall in December, following largely snow-free months of October and November. As the maps below show, all but the northwest corner of Illinois had less than 5 inches of snowfall in December. These totals were between 1 and 10 inches below average, with the largest departures in northeast Illinois.

December total snowfall (left) and departure from long-term average (right).

Outlooks

After an active start to January, the Climate Prediction Center’s 8- to 14-day outlooks indicate the highest odds of a change to warmer and drier than normal conditions across the state into the second week of January.  

Climate Prediction Center 8 to 14 day outlooks for temperature (left) and precipitation (right).

The outlooks for the entire month of January lean toward warmer and wetter than normal conditions, suggesting a change to more active winter weather around the middle of the month.

Climate Prediction Center outlooks for the entire month of January. Temperature is on the left, precipitation on the right.

As La Niña conditions are expected to persist into early spring, forecasts from the North American Multi-model Ensemble for February through April favor warmer and wetter conditions through the winter-spring transition.

Temperature and precipitation forecasts for February through April from the North American Multi-Model Ensemble (NMME).

November brings a warm end to Fall

November was much warmer and slightly drier than average across Illinois. The preliminary statewide average November temperature was 46.6 degrees, 4.1 degrees above the 30-year normal and tied for the 9th warmest on record. Preliminary statewide average total precipitation for November was 2.94 inches, 0.53 inches less than the 30-year normal, and the 70th driest on record.

Data are provisional and may change slightly over time

November Temperatures

Following cooler than normal months in September and October, a broad upper-level ridge established over the central U.S. and brought warm air into the Midwest. This resulted in well above average temperatures across Illinois. The plot below shows daily average temperatures in Rockford between November 3rd and 10th were 5 to 25 degrees above average. The station in Rockford recorded its highest November daily average temperature on record last month, 69 degrees on November 9th. This temperature is close to the climatological average for late September!

Between November 9th and 12th, 103 Illinois stations broke their daily high maximum temperature records, and 79 stations broke their daily high minimum temperature records. The Streator station in Livingston County broke its 126-year daily high minimum temperature record by 14 degrees on November 10th. Impressively, 19 stations in Illinois broke their all-time November high minimum temperature records last month. This included in Rock Island, where the 65-degree minimum temperature on November 10th was the highest minimum temperature recorded since observations began there in 1872.

Although temperatures moderated in the second and third weeks of November, temperatures mostly remained above normal throughout the rest of the month. The maps below show November average temperatures and departures across the state. Average temperatures ranged from the low 40s in northern Illinois to the low 50s in southern Illinois, between 4 and 8 degrees above average. The statewide average November temperature was 46.6 degrees, 4.1 degrees above the 30-year normal and tied for 9th warmest on record.

November was the temperature outlier this climatological fall season, as both September and October were cooler than normal statewide. The maps below show monthly average temperature departures in each of the three fall months. Overall, the preliminary 2020 climatological fall season average temperature was 54.7 degrees, which was 0.3 degrees above the 30-year normal and tied for 46th warmest fall on record.

November Precipitation

Last month began with the end to daylight savings time. The ensuing onslaught of 4:30 p.m. sunsets has me quoting the great philosopher Axl Rose: “So never mind the darkness, we can still find a way. ‘Cause nothin’ lasts forever, even a cold November rain.”

As the maps below show, November total precipitation ranged from less than an inch in Iroquois and Kankakee counties in eastern Illinois to nearly 8 inches in southern Illinois. A broad area of the state from Jacksonville to Chicago was 1 to 2 inches drier than average last month, while the Interstate 64 corridor was 1 to 2 inches wetter than average.  Statewide total November precipitation was 2.94 inches, approximately 0.53 inches below the 30-year normal and 70th driest on record.

As the map below shows, measurable snowfall stayed north of Interstate 72 last month, with the exception of small accumulation in Hancock and Adams counties. Areas in northwest Illinois and the far western Chicago suburbs recorded over 3 inches of November snowfall, while totals farther south remained less than an inch. A snow-free November is not unusual in central and southern Illinois, but the very high temperatures last month certainly played a role in keeping the white stuff in the northern third of the state.

Precipitation this climatological fall season has been quite variable in northern and southern Illinois, as the maps below show. Northwest Illinois was 4 to 6 inches wetter than normal in September, while most of southern Illinois was 1 to 3 inches drier than normal. This pattern flipped in October, and far southern Illinois observed 4 to 6 inches of additional precipitation.

Overall, the preliminary 2020 climatological fall season total precipitation was 9.07 inches, which was 0.87 inches below the 30-year normal and 65th driest fall on record.

Drought Continues in Central Illinois

Climatological summer was drier than average for most of central Illinois, and this pattern continued throughout fall. The plot below shows that Springfield has experienced four consecutive drier than average months, with a cumulative precipitation departure over 5 inches since Aug. 1.

The maps below show 6-month (June–November) precipitation departure from average and percent of average statewide. An area from Fulton County to DeWitt County in central Illinois has experienced a 7 to 9 inch precipitation deficit since June 1, representing between 65% and 75% of normal 6-month precipitation.

The most recent (Nov. 24) version of the U.S. Drought Monitor showed widespread moderate drought from Sangamon County to Kankakee County and severe drought in the driest area of central Illinois. Most of the state between Interstates 70 and 80 is considered abnormally dry, following below average precipitation in October and November.

Soil moisture across the central part of the state is near normal to well below normal in response to precipitation deficit. However, despite the prolonged dryness, the figure below shows the total estimated amount of water in the top 40-inch soil column in Springfield is right at the 18-year median level. The soil water estimates are based on observations from the Illinois Climate Network.

Although both top- and sub-layer soil moisture are less than in each of the last two years in central Illinois, soil moisture levels are still near to slightly below average. Therefore, a normal to wetter than normal winter season could help recharge soil moisture and reduce drought concern moving into next spring.

Outlooks

The Climate Prediction Center’s 8- to 14-day outlooks indicate highest odds of a continuation of warmer and drier than normal conditions across the state in the second week of December.

The outlooks for the entire month of December are similar, with elevated odds of above normal temperatures and below normal precipitation statewide.

Finally, the most recent three-month outlooks for climatological winter, December–February, lean into La Niña conditions with highest odds for wetter than normal weather and equal chances of warmer or colder than normal weather in Illinois.