Warmer fall temperatures may not eliminate risk of early fall freeze

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CHAMPAIGN, Ill. (WCIA) — Temperatures in September and October have risen for decades in the Midwest; however, early fall freeze events have occurred despite the increasing temperatures, according to Illinois State Climatologist Trent Ford at the University of Illinois’ Illinois State Water Survey.

The most recent Climate Prediction Center outlook for August through October calls for increased odds of above normal temperatures with equal chances of above normal, normal, or below-normal rainfall.

In the past 40 years, the earliest fall freeze dates have ranged from late September in northwestern and central Illinois to early October in southern and eastern Illinois. Median first fall freeze dates range from mid- to late October in northwestern and central Illinois to early November in southern and northeastern Illinois.

Statewide average temperatures in September to November have increased at a rate of 0.08 degrees per decade between 1895 and 2019. However, fall warming has accelerated in recent decades. Over the past 30 years, the statewide average rate of fall temperature changes is 0.6 degrees per decade.

September and October temperatures have increased over the past 30 years, whereas November temperatures have decreased slightly. The largest amount of change is in September. Over the past 30 years, September temperatures in Illinois have increased at a rate of 1.5 degrees per decade.

The statewide average September temperature has been above the 30-year normal in 13 of the past 20 years and in each of the past 5 years. The average September temperature departure over the past five years is 4.2 degrees, and in four of the past five years, the September statewide average temperature has been closer to the August 30-year normal than the September normal.

Higher September temperatures also increase atmospheric evaporative demand, which can intensify dry conditions.

The importance of temperature and evaporative demand for drought was evident in southern Illinois last year. Most counties south of Interstate 64 received 10 to 50 percent of normal September precipitation that combined with very high September temperatures to dry soil rapidly.

A “flash drought” occurred last fall in southern Illinois, when many counties deteriorated from near normal conditions to severe drought in only three weeks. It is likely the drought in southern Illinois would not have intensified as quickly without the higher September temperatures. 

Recent climate assessments indicate that increasing temperatures will exacerbate drought conditions, similar to what occurred last year in southern Illinois.

“An early fall freeze is still very possible despite positive fall temperature trends,” said Ford. “Thankfully, crops this year are much further along in their maturity than last year, so irrespective of fall freeze, overall there is less risk of crop damage.”

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