Ag Weather Forum

One of the many extreme weather events so far in this 2025 crop season is a very hot spell in the northern United States growing areas during the middle of May. On May 11, the high temperature in Grand Forks, North Dakota reached 99 degrees Fahrenheit -- 29 degrees above normal.

This weather happening also calls to mind a temperature signpost, so to speak, which is watched by University of Nebraska-Lincoln Extension Ag Meteorologist Eric Hunt. Hunt has referred to the "Grand Forks Rule" in presentations about the implication of the mid-May heat wave for crop weather later in the season.

"Years when Grand Forks hits 95 (F) before Memorial Day tend to be warm and dry," Hunt said on a recent Great Plains Drought Outlook webinar. "That early 95 is a symptom of the atmosphere that's putting a lot of ridging over the north-central portion of the (North America) continent. And historically, if we get ridging over the north-central U.S. in May, we tend to have it showing up again during mid to late summer."

In his look at Grand Forks temperatures, Hunt used recorded data from the Grand Forks National Weather Service office and assumed an average date of May 28 for the Memorial Day holiday. He noted that this occurrence happened several times in the 1980s and again in 2006. Harsh corn production years of 1980 and 1988 saw the Grand Forks high temperatures match or exceed 95 F in May -- in 1980, Grand Forks hit 100 F on April 21; and in 1988, Grand Forks reached 95 on May 18. The U.S. corn yield in 1980 was 18 bushels per acre (bpa) less than the previous year; and in 1988, the corn yield was almost 35 bpa below the previous year. "July had a stronger hot/dry signal than August, which could certainly be an issue for corn," Hunt said in an email.

The Grand Forks Rule does not catch every crop-damaging heat wave in a growing season. One can look at the 2012 crop year, for an example.

Weather records show that Grand Forks did not even reach 95 F during 2012. Yet, overall U.S. corn crop yields swooned to the lowest level of this century, finishing at 123.4 bpa.

However, atmospheric conditions which led to the early hot spells in northeastern North Dakota do show the potential of redeveloping later in the season. This prospect of repeat heat during corn grain-fill and soybean pod-fill is the primary message in the "Grand Forks 95 Rule".

See more about the U.S. heatwave forecast from DTN Ag Meteorologist John Baranick in his latest Ag Weather Forum blog, "Heatwave to Move Through the US Into Next Week," https://www.dtnpf.com/….

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Bryce Anderson can be reached at bryce.anderson@dtn.com

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