The preliminary statewide average August temperature was 73.7 degrees, 0.1 degrees above 1991-2020 average and tied for the 59th warmest on record going back to 1895. The preliminary statewide average total August precipitation was 1.87 inches, 1.69 inches below the 1991-2020 average and tied for the 9th driest on record statewide.
Data are provisional and may change slightly over time.
It’s Hot Then It’s Cold…
The final month of climatological summer brought both summer and fall weather to Illinois. Daily average temperatures and temperature departures from normal in Aurora show the roller coaster of August weather. The month began with temperatures that were 2 to 5 degrees below normal, followed by a prolonged stretch of very warm and humid weather. A series of cold fronts in the last third of the month parked cooler Canadian air over the state, pushing some days 10 to 12 degrees below normal in late August (Figure 1).
When we average the warm periods and cold periods in August together, average temperatures ranged from the low 70s in northern Illinois to the high 70s in southern Illinois, around within 2 degrees of normal virtually everywhere (Figure 2).
Most parts of the state pushed into the mid to upper 90s in the middle of the month, including during the State Fair. The intense heat was followed by a wonderful cooldown, and many places saw nighttime temperatures dip into the high 40s, including 44 degrees in Springfield. The warmest point in the state last month was Olmstead at 77.8 degrees, and the coolest point was Stockton at 68.8 degrees.
Overall, the preliminary statewide average August temperature was 73.7 degrees, 0.1 degrees above 1991-2020 average and tied for the 59th warmest on record going back to 1895.
August Was Wet North and Dry South
August rainfall was much more plentiful north of Interstate-80, and virtually non-existent in some parts of southern Illinois. August totals ranged from just over 10 inches in Will County (~300% of normal) to less than a tenth of an inch in parts of south-central Illinois (Figure 3).
Carbondale only had 0.02 inches of rain the entire month making it the driest August on record there and the third driest of any month since the late 1800s. It was also the driest August on record in Mt Vernon (0.09 inches), Centralia (0.09 inches), and Charleston (0.03 inches), and the second driest in Springfield (0.16 inches) and Effingham (0.32 inches). In contrast, parts of northeast Illinois experienced a few heavy rainfall events that pushed August totals above 8 inches, including 10.5 inches in Oswego and 9.9 inches in Lake Forest.
An Extremely Humid Summer in Illinois
As a cold season lover, I’m not usually remorseful for the end of summer, and this year particularly so. This past season was hot and humid to say the least. Average temperatures in June, July, and August ranged from the low 70s in northern Illinois to the high 70s in southern Illinois, between 1 and 3 degrees above normal statewide (Figure 4).
The hallmark of this summer, though, was not a plethora of extremely hot days, but instead unrelenting humidity. The dew point temperature is a good indicator of how we experience the uncomfortable effects of high humidity. In general, Illinoisans begin to feel humidity when the dew point is above 60 degrees, begin to fell uncomfortable with a dew point above 65 degrees, and are very uncomfortable when the dew point is above 70 degrees. Much of southern Illinois spent well over half of all summertime hours with a dew point at or above 70 degrees.
Very high humidity tends to limit daytime heating but also keeps nighttime temperatures very high. Indeed, this summer’s average nighttime minimum temperatures were among the highest on record in many places, including 2nd highest on record (after 2010) in Peoria (Figure 5). Higher humidity and extreme overnight temperatures are part of longer trends in our summer climate in Illinois, as evidenced by the clear trends in summer average minimum temperatures in Peoria, where the average minimum temperature over the past 25 years is a full 2 degrees higher than the 20th Century average.
Overall, the preliminary summer average temperature in Illinois was 75.9 degrees, 2.1 degrees warmer than normal and tied for 11th warmest summer on record statewide.
Flooding and a Flash Drought
Week to week and month to month precipitation variability is usually at its highest in summer, but this past season was extreme even for summer. June was extremely wet in far southern Illinois and very dry in most of northern Illinois. July was extremely wet in northwest and western Illinois, and near normal elsewhere. August was bone dry in southern Illinois and extremely wet in northeast Illinois (Figure 6). The final summer total was not in the top 10 wettest or top 10 driest at any of our long-term weather stations, but precipitation extremes were nonetheless impactful last season. Those included intense rainfall and flooding in Chicago and rapid onset or “flash” drought in southern Illinois.
Overall, the preliminary statewide summer precipitation total was 12.99 inches, 0.72 inches above normal and the 31st wettest summer on record.
Outlooks
September brings in what is undoubtedly the best season in Illinois, and we have already experienced some fall-like weather in August. The outlooks for the entire month of September, though, don’t give us much useful information, with equal chances of warmer and colder and drier and wetter than normal weather to kick off fall (Figure 9a). Meanwhile, guidance for climatological fall (September through November) shows stronger signs of above normal temperatures, with mostly equal chances of above and below normal fall precipitation.
