Ag Weather Forum

As feared, significant storms developed across portions of the North-Central United States on Monday, July 28, and continued into the early morning hours of Tuesday, July 29. The storms fired on the northern edge of a hot and humid air mass centered over the middle of the country and were able to tap into the extreme heat and humidity to produce rapid intensification and growth into a pair of mesoscale convective systems (MCSs) in the evening.

One developed in northeast South Dakota, cruising through central Minnesota before curving southeast along the Mississippi River on the border of Minnesota and Wisconsin. Another formed in southeast South Dakota and plowed through northern Iowa before taking a turn southeast across eastern Iowa overnight. Both MCSs produced widespread wind damage. But the southern MCS produced some intense wind speeds over a long enough period to be labelled as a derecho.

A derecho is a long-duration windstorm that is produced by thunderstorms, most often an MCS, that lasts for at least 240 miles with continuous severe wind gusts over 58 mph along its track and multiple gusts of hurricane force over 74 mph. This particular derecho had 11 wind gusts in the 75-100 mph range, with the highest recorded gust of 99 mph in Sioux Center, Iowa. The storm did not look the prettiest as it moved across northern Iowa, then briefly weakened near Mason City before it took a turn southeast and strengthened for a couple of hours overnight. Additional severe wind gusts, and four in the 75-85 mph range were recorded. Since there was a break, the official declaration will probably end the derecho around Mason City. But if added together, it was a 400-mile path of severe winds across some of the best-looking farmland in the country.

The USDA NASS Crop Progress report noted that Iowa had an 87% good-to-excellent rating on corn, and an 82% good-to-excellent rating on soybeans. Limited heat and frequent rainfall have led to good crop conditions in this area of the country for most of the season. Reports of wind damage to crops are still filling in, with some users on X indicating damage mostly to infrastructure and buildings, downed trees, and powerlines more than snapping of corn stalks and of lodging of soybeans. This damage will need to be properly assessed though, as any significant and widespread damage could lead to a decline in production for what looks to be a good corn and soybean crop otherwise.

Farther north, the MCS produced some significant damage as it started to make its turn near the Minneapolis/St. Paul metro area down along the Mississippi River Valley before reaching Iowa where it fizzled out. Widespread wind gusts of more than 60 mph were noted, but only one gust over 74 mph was reported. With the consistent wind damage recorded over a span of approximately 200 miles, this MCS did not meet the qualifications of a derecho, but still may have led to wind damage that would have affected crop production.

Additional thunderstorms have developed in the same areas of northwestern Iowa on Tuesday morning, July 29. More rain and potential thunderstorms will move through these areas into Wednesday as well, which could lead to more damage and heavy flooding rain that could complicate recovery efforts. This is all in part of a strong cold front that is slowly pushing south through the U.S. this week. Additional areas of severe weather and heavy rain are forecast along the front throughout the week.

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John Baranick can be reached at john.baranick@dtn.com

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