Mild March Was Quite a Contrast to February

The preliminary statewide average March temperature was 45.9 degrees, 4.5 degrees above the 1991–2020 average and the 13th warmest on record going back to 1895. Preliminary statewide average total March precipitation was 4.10 inches, 1.16 inches above the 1991–2020 average and the 30th wettest on record going back to 1895.  

Data are provisional and may change slightly over time

Warm Start to Spring

Our suffering in February was rewarded by persistent mild temperatures in March. Figure 1 shows 23 out of 31 March days were warmer than the 1991–2020 average in the village of Normal. 

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

As the maps below show, March average temperatures ranged from the low 40s in northern Illinois to low 50s in southern Illinois, which was between 3 and 5 degrees above the 1981–2010 normal (Figure 2). Last month, 20 daily high maximum temperature records and 16 daily high minimum temperature records were broken statewide. Chicago’s O’Hare Airport recorded a minimum temperature of 57 degrees on March 10, which broke the previous daily high minimum temperature record by 11 degrees.

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

The persistent warmth last month accelerated growing degree day accumulation and prompted an earlier than normal spring greening, the result of which was earlier stone fruit blooms. Unfortunately, an early bloom increases the risk of significant freeze damage to vulnerable crops such as peaches and cherries. This pattern is consistent with longer trends in early spring growing degree day accumulation. Figure 3 shows base 50 growing degree day accumulation between January 1 and April 1 each year and the date of the last spring 28-degree freeze between 1949 and 2020 in Belleville.

Figure 3. (Left) Accumulated base 50-degree growing degree day accumulation between January 1 and April 1 in Belleville. (Right) the date of last spring’s 28-degree freeze in Belleville. Trends are calculated over the period 1949 to 2020.

Growing degree day accumulation in the first three months of the year in Belleville shows a strong increase over the past seven decades, with a trend of 15 additional growing degree days per decade. Concurrently, the date of the last spring freeze has not changed significantly over that same time period and overall exhibits quite a bit of year-to-year variability. The combination of increasing heat accumulation in the first three months of the year with no appreciable change in the last spring freeze results in an overall increased risk of freeze damage to tender perennials and fruit trees.

The preliminary statewide average March temperature was 45.9 degrees, 4.5 degrees above the 1991–2020 average and the 13th warmest on record going back to 1895. In fact, last month was the warmest March statewide since 2012; however, the statewide average temperature last month was still nearly 10 degrees below that in 2012, demonstrating how unusually warm March 2012 was.

Have and Have Nots of March Rain

The first third of last month was very dry across the state. As the maps in Figure 4 show, most of the state north of Interstate 64 received less than 0.05 inches of precipitation in the first 10 days of March. A pattern change around the middle of the month brought several rounds of storms and heavier rain to southern and central Illinois. One CoCoRaHS observer south of Carbondale in Jackson County observed 3.98 inches of rain on March 12. In all, March total precipitation ranged from just over 1 inch in northeast Illinois to over 5 inches throughout most of southern Illinois. March precipitation departures ranged from nearly 3 inches above the 1981–2010 normal in southwest Illinois to nearly 2 inches below the 1981–2010 normal in northeast Illinois.

Figure 4. Maps show (left) total precipitation between March 1 and 10, (middle) total precipitation for the entire month, and (right) March total precipitation departure form 1981-2010 normal.

The heavier precipitation was received either well or poorly depending on the part of the state. Southern Illinois soils were previously at or above normal moisture levels coming into March, so the additional precipitation resulted in standing water in fields and minor to moderate flooding along the Big Muddy River and Wabash River, among others in the region. However, the rain was welcome on drier central Illinois soils. In response to the improvement in moisture conditions in central Illinois, the U.S. Drought Monitor removed all moderate drought in its March 16 map; the first time Illinois was free of drought since August 2020.

March isn’t usually an exceptionally snowy month, but most of the state will typically see some flakes in the first month of spring. Snowfall last month was well below normal statewide, attributable to the mild temperatures. Total March snowfall ranged from less than a quarter of an inch in north-central Illinois to just over 4 inches in northwest Illinois. March snowfall departures ranged from less than an inch below normal in far southern Illinois to nearly 5 inches below normal in northeast Illinois.

Figure 5. Maps show (left) total March snowfall and (right) snowfall departures from 1981-2010 normal.

Overall, the preliminary statewide average total March precipitation was 4.10 inches, 1.16 inches above the 1991–2020 average and the 30th wettest on record going back to 1895.  

Outlooks

Once past these first, cooler days in April, the Climate Prediction Center outlooks suggest warmer conditions are ahead. The 6–10-day outlook for the second week of April shows strongly elevated odds (70% to 80%) of warmer than normal conditions, with only slightly elevated odds of wetter than normal conditions. The warm, dry pattern to start the month will certainly help further progress spring greening.

Looking at the month of April as a whole, the 1-month Climate Prediction Center outlooks also show elevated odds of warmer than normal conditions, with an equal chance of above and below normal precipitation in April.

Figure 6. maps show temperature (left) and precipitation (right) outlooks for the second week of April (top maps) and the month of April as a whole (bottom maps).

A Wild February Ended an Otherwise Mild Winter

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 10th 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 51st 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.



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

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 620 exhibited temperatures between 18 and 25 degrees below the 1981-2010 normal across the state.

Figure 2. February average temperature (left) and departure from the 1981-2010 normal (middle). Right map shows average temperature departures between February 6 and 20.

The preliminary statewide average February temperature was 20.0 degrees, 11.1 degrees below the 1991-2020 average and 10th 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).

Figure 3. Winter season (December–February) average temperatures (left) and departures from 1981–2010 normal (right).

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.

Figure 4. Maps show total February snowfall (left) and departure from 1981-2010 normal (middle). The right map shows total winter season snowfall departures from normal. All maps are in units of inches.

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).

Figure 5. February total precipitation (left) and departure from the 1981–2010 normal (middle). The right map shows winter season total precipitation departure from normal.

Overall, preliminary statewide average total February precipitation was 2.01 inches, 0.10 inches below the 1991–2020 average and tied for the 51st 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.

Figure 6. Temperature (left) and precipitation (right) outlooks from the Climate Prediction Center for the period March 9th to 15th.
Figure 7. Temperature (left) and precipitation (right) outlooks from the Climate Prediction Center for the entire month of March.

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.