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Who ordered winter? The cold season came with a vengeance in February, bringing bitter cold and snow across the state. The preliminary statewide average February temperature was 20.0 degrees, 11.1 degrees below the 1991–2020 average and the 10^th coldest on record going back to 1895. Preliminary statewide average total February precipitation was 2.01 inches, 0.10 inches below the 1991–2020 average and tied for the 51^st wettest on record going back to 1895.  

Data are provisional and may change slightly over time

February Can Still Be Cold

Winter average temperatures in Illinois have increased by 0.20 degrees per decade since 1895 and by 0.80 degrees per decade since 1970. Two thirds of the 2020-21 winter season followed this pattern, but February did not abide. The plot in Fig. 1 shows daily February temperatures as a departure of the 1981–2010 normal in Monmouth. Average temperatures on all but one day between February 7 and February 18 in Monmouth were at least 25 degrees below the 30-year normal, and many were more than 30 degrees below normal. Most places in Illinois experienced at least 20 days of below normal temperatures in February, following fewer than 20 days with below normal temperatures in January and December combined.

The extreme cold broke 218 daily low maximum temperature records and 81 daily low minimum temperature records at stations around Illinois, from Galena to Cairo. No February low maximum and minimum temperature records were broken last month. However, among stations with at least 30-year records, last month was the coldest February on record in Dixon, Du Quoin, Olmsted, Carmi, and Mt. Vernon. Some of the most extreme observed temperatures include -21 degrees in Altona and Mt. Carroll, -17 in Aurora, and -15 in Moline.

As the maps below show, February average temperatures ranged from the low teens in northwest Illinois to the high 20s in southern Illinois, which was between 8 and 14 degrees below the 1981–2010 normal (Figure 2). The two-week stretch during February 6^–20 exhibited temperatures between 18 and 25 degrees below the 1981-2010 normal across the state.

The preliminary statewide average February temperature was 20.0 degrees, 11.1 degrees below the 1991-2020 average and 10^th coldest on record going back to 1895. February propelled the 2020-21 winter season temperatures below the 1981-2010 normal, despite a warmer than normal January and December. Winter average temperatures ranged from the high teens in northern Illinois to the high 30s in southern Illinois, between 0 and 4 degrees below normal (Fig. 3).

More Snow Than Rain

February was noteworthy not only for extremely cold weather, but also for exceedingly high snowfall totals. The position of the subpolar jet stream created an active winter storm track across the state, bringing multiple heavy snowfall events. One storm in particular produced 10 to 20 inches of snow in a 24-hour period in the northeast part of the state. This included 13 inches observed in Kankakee on February 16 alone, which was the largest single day snowfall total on record in Kankakee. February snowfall totals ranged from just under 34 inches in the northern Chicago suburbs to about 3 inches in Mt. Vernon.

The maps below show February total snowfall ranged from 8 inches in southwest Illinois to over 30 inches in northeast Illinois, between 5 and 15 inches above the 1981–2010 normal (Fig. 4). The incredibly snowy month pushed winter season totals above normal across most of the state.

Because February snow fell from very cold air, it did not produce a large amount of equivalent liquid precipitation. February total precipitation ranged from just over an inch in far northern Illinois to just over six inches in southeast Illinois. All areas except for the southeast quadrant were a tenth to one inch drier than the 1981–2010 normal in February. Combined with drier than average months in January and December, most parts of the state experienced a winter season that was one-half inch to three inches drier than normal (Fig. 5).

Overall, preliminary statewide average total February precipitation was 2.01 inches, 0.10 inches below the 1991–2020 average and tied for the 51^st wettest on record going back to 1895.  

Outlooks

Both the short-term 8- to 14-day outlooks and the one-month March outlooks from the Climate Prediction Center show strongly elevated odds of warmer than normal conditions this month. The outlooks also indicate the highest odds of wetter than normal conditions in the first half and through the entirety of March (Fig. 6). Lastly, the newest three-month outlooks for climatological spring (March–May) also indicate the highest odds of warmer and wetter than normal conditions.

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 36^th 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 32^nd 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.

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).

The preliminary statewide average January temperature was 28.8 degrees, 2.4 degrees above the 1981–2010 normal and the 36^th 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 26^th largest two-day snowfall total on record there and the largest since 2015.

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.

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 32^nd wettest on record going back to 1895.  

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.

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.

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.

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 21^st warmest year on record. Statewide total precipitation in 2020 was 41.84 inches, 4.37 inches above average and the 23^rd 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 6^th 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.

Daily	High Maximum Temperature	High Minimum Temperature	Low Maximum Temperature	Low Minimum Temperature	High Precipitation
January	21	37	1	0	163
February	36	8	24	33	21
March	1	5	2	0	61
April	13	3	20	33	42
May	0	6	42	37	60
June	20	37	1	4	30
July	1	3	3	0	93
August	11	17	44	7	25
September	0	0	73	4	51
October	11	8	70	19	64
November	112	85	0	3	43
December	34	1	2	0	26
Total	260	210	282	140	679

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 8^th 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.

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 35^th 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 43^rd 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 10^th warmest December on record, following its 3^rd 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.

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 35^th 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.

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 total December precipitation was 1.78 inches, approximately 0.91 inches below the 30-year normal and the 43^rd 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.

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.

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.  

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.

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.