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Variable precip, derecho mark 2020 weather

Variable precip, derecho mark 2020 weather

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Editor’s note: The following was written by Trent Ford, Illinois State Climatologist, for the University of Illinois farmdoc daily website.


Although perhaps not as challenging as 2019, the 2020 Illinois growing season weather was remarkable. From late spring and early fall freezes, to wet-dry-wet swings, to drought that visited most of the state, and even a derecho, in this article I will review the 2020 growing season weather from a climatological perspective.

Wet spring — but not 2019

Coming off of the fifth wettest calendar year on record statewide in 2019, most folks were concerned about a repeat of exceedingly wet conditions this spring.

The first three months of the year together were wetter than average for virtually the entire state, and even wetter than January-March 2019 in southern Illinois. National Weather Service outlooks at the end of February showed most gauges along the Mississippi River in Illinois were likely to reach major flood stage between March and May.

Although April and May were wetter than average across Illinois, both months had two- to three-week dry spells. These two time periods, the first three weeks of April and the first two weeks of May, allowed planting to progress much ahead of the 2019 growing season.

The dry spells were broken up by heavy rainfall events toward the end of April and middle to end of May which forced several producers to replant early-planted corn and beans.

However, at the state scale the beginning of the 2020 growing season did not bring the widespread challenges related to excess moisture that characterized 2019.

Hot, humid June & July

The end of spring and beginning of summer marked a noticeable change in weather, with a shift to warmer, more humid conditions across most of the state. Statewide average temperatures in both June and July were approximately 2 degrees above the 30-year normals for those months.

The persistence of warmer than normal temperatures in June and July was noteworthy. Nearly 80% of June and July days in Rockford were warmer than normal, including a stretch in late June and early July of 13 consecutive days with daily average temperature more than 4 degrees above the 30-year daily normals.

The warm weather in June and July helped accumulate growing degree days after degree day deficits formed due to cold weather in April and May.

Summer precipitation

Precipitation between June 1 and Sept. 30 was within 2 inches of the long-term average across most of the state. However, the wide-view perspective of summer rainfall is somewhat misleading given the remarkable precipitation variability during the 2020 growing season.

The central and northern thirds of the state spent the majority of June through September in moisture deficit. Multiple, prolonged dry spells were broken up by extreme precipitation that in some cases flooded crops.

For example, following the 11th driest June on record in Peoria County, the station at the Peoria airport recorded 5.15 inches in just six hours on July 15, marking the highest six-hour rainfall total on record in Peoria.

The station in Minonk in Woodford County, for example, recorded 6.5 inches on July 15. This day, along with a 9-inch rainfall observation in September of last year, are the only two days in Minonk’s 125-year record with more than 6 inches of rainfall observed. July ended as the eighth wettest on record in Tazewell County, following the county’s 7th driest June.

While drought appeared in northern and central Illinois in July, the southern third of the state remained wet. July was the second wettest on record in Belleville in St. Clair County, followed by the wettest August on record in Belleville.

However, as late August rains helped alleviate drought in northern Illinois, most of southern Illinois remained dry through most of September. As a result, the wettest August on record in Belleville was followed by the location’s seventh driest September.

The rapid change from very dry to very wet likely impacted yields this season.

Most of the state experienced some form of drought and/or drought-related agricultural impacts during the 2020 growing season. However, multiple, timely precipitation events prevented drought conditions from persisting, as they did in western and central Iowa.

The lack of June rain resulted in moderate drought in central and southeast Illinois; this was the first week with moderate drought since October 2019. Intense rainfall in mid-July alleviated the dry conditions in these areas, and the state was drought free in the beginning of August.

Multiple dry weeks combined with very high temperatures in late August resulted in rapid onset or “flash” drought in northern Illinois by the end of the summer. September brought much-needed rain and drought alleviation in northern Illinois.

However, September was very dry in southern Illinois, resulting in re-introduction of drought and abnormally dry conditions in the southern half of the state.

August derecho

The most noteworthy weather event in the Midwest during the 2020 growing season was undoubtedly the August derecho. On Aug. 10, a strong mesoscale convective system moved across the Upper Midwest. The system intensified in the eastern Dakotas and caused a derecho — a widespread, long-lived windstorm — that impacted areas of Iowa, Illinois, Wisconsin and Indiana.

A derecho is characterized by strong straight-line winds that can exceed 75 mph and often affect areas between 250 and 500 miles. The Aug. 10 derecho produced observed winds exceeding 100 mph and estimated (from damage) wind gusts up to 140 mph across east-central Iowa.

Based on initial reports and satellite imagery analysis, the derecho damaged between 6 and 10 million acres of crops across Iowa and northern Illinois. Crop damage from straight-line winds and hail were reported in northern Illinois, although the damage was not nearly as extensive as that in eastern Iowa.

Early frost

A strong cold front moved across the Midwest in early October, bringing very cold air into the region. Many areas of the state saw their first widespread frost on the morning of Oct. 5, bringing an end to the 2020 growing season.

The combination of a late spring freeze in May and early fall freeze in October in eastern Illinois resulted in an unusually short growing season. For example, only 147 days elapsed between the last spring freeze and first fall freeze in Olney this year, 36 days shorter than the long-term average, and the second shortest growing season on record in Olney going back to 1897.

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