Ag Weather Forum

Hail up to 2.5 inches in diameter and wind gusts near 100 miles per hour accompanied some of the storms that hit Iowa on Monday. The storms didn't stop there though. Places across northern Illinois, southern Wisconsin, and northern Indiana also reported large hail and severe winds as storms advanced east Monday night into early Tuesday morning. These storms came at a time when crops are entering critical stages -- corn is silking and soybeans are setting pods.

The storms that hit Iowa, Wisconsin, Indiana, and Illinois on Monday can be classified as a derecho. A derecho is a fast-moving windstorm where the wind damage must extend more than 240 miles and wind gusts in the damage path must be at least 58 mph. This derecho left thousands of people without power by Tuesday morning while causing extensive tree damage and even some damage to buildings.

Around the middle of Monday afternoon, isolated severe thunderstorms began forming in central Iowa. By 4 p.m. CDT, many of the storms had started clustering together and were beginning to form bowing segments. These bowing segments will appear as comma-shaped storms on radar and meteorologists call these bow echoes. Bow echoes are one of the first steps to forming a derecho.

Around 6 p.m. CDT, a large line of storms extended from southwest Wisconsin into southeast Iowa and headed east for northern Illinois. In a matter of just a few hours, the line of storms was impacting northeast Illinois and exiting eastern Iowa as it raced across the Corn Belt. Throughout the rest of the evening, storms entered western Indiana and southern Illinois.

This derecho packed a punch from its high winds to heavy rainfall. The National Weather Service out of the Quad Cities listed a maximum wind gust near Holy Cross, Iowa, of 97 mph. Lena, Illinois, and Aurora, Iowa, reported maximum wind gusts of 93 mph and 86 mph, respectively. In addition to the high winds, hail up to 2.5 inches in diameter, or tennis ball-sized, was reported across a few towns in Iowa as storms were strengthening Monday afternoon. Rainfall in parts of eastern Iowa approached 1.5 to 2.5 inches while some areas in southern Wisconsin saw 2 to 3 inches of rain. There was even an isolated area in west-central Illinois that reported 4 to 5 inches of rain from late Monday to Tuesday morning.

No matter the stage crops are in, the damaging straight-line winds from derechos will always leave behind a swath of damage. In the latest crop progress report released by the United States Department of Agriculture on July 15, 62% of corn was silking in Illinois while Indiana was at 42% and Iowa had 44%. Soybeans are just getting started with setting pods as Iowa only had 12% of their soybean crop setting pods as of July 15. Illinois was a bit higher with 28%.

Over half the corn crops in states like Illinois, Indiana, and Iowa were rated in good condition in the July 15 report. Soybeans showed similar to even slightly higher percentages. For example, Illinois reported 64% of the soybean crop in good condition. By next Monday's report, these percentages may not be as high given the widespread damage caused by the derecho. For many producers across eastern Iowa, northern Illinois, southern Wisconsin, and Indiana, the corn and soybean yields may have taken a hit, and it may be difficult for some of these crops to bounce back after the damage.

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Teresa Wells can be reached at teresa.wells@dtn.com

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