Parts of the Midwest are cleaning up after severe thunderstorms barreled through the region on the night of Monday, July 15, 2024, bringing hurricane-force winds and multiple tornadoes around Chicago. More than 600 reports of damaging winds from Iowa to Michigan left over 350,000 customers without power Tuesday morning.
The storm complex was a derecho, or a widespread, long-lived wind storm that is associated with a band of fast-moving showers or thunderstorms. Storms first erupted during the mid-evening hours in central and eastern Iowa, bringing tennis-ball-size hail in Crawford. Initial storms began as rotating supercells, including one that spawned a tornado near Des Moines, then continued through northeastern Indiana along a 500-mile-long path before dissipating around 2 a.m. Tuesday. It toppled hundreds of trees, downed wires and power poles, ripped off roofs and damaged vehicles.
In Chicago, forecasters at the National Weather Service had to abandon their posts as a tornado approached, transferring warnings to a sister office in Gaylord, Michigan. These storms then merged into a cluster, reaching 55,000 feet in height. From there, the storms moved east along a stalled frontal boundary. Cooler air was present to the north, with very warm, moist air to the south. Storms tend to straddle boundaries, in this case, steering them into northern and central Illinois and then Chicago.
Chicago was particularly prone to tornadoes due to the presence of a “northern bookend vortex,” where a part of the northern end of the squall line curled back on itself, acquired counterclockwise rotation, and produced intermittent tornadoes. At 9:04 p.m. CT, three tornadoes were simultaneously on the ground in the southwestern Chicago metro, detected by doppler radar.
As the storms moved through the metro area, numerous tornadic circulations prompted meteorologists to issue blanket warnings for entire segments of the squall line rather than individual areas of rotation. A local news station reported “possible tornadoes almost everywhere in Chicagoland” as multiple confirmed tornadoes were ongoing.
The powerful winds were not only limited to the Chicago area. Extensive damage was reported across the Midwest from Iowa to Indiana, with hundreds of thousands of power outages reported. The Weather Service said a tornado also touched down near Sugar Grove, Ill., about 40 miles west of Chicago. A 105 mph wind gust, which is as strong as a Category 2 hurricane, was reported in Camp Grove, Illinois, and a 101 mph wind gust was reported in Davis Junction. Other areas, including Lena, Ill., and Aurora, IA also reported hurricane-force gusts. An EF-1 tornado touched down in Des Moines and another was reported in Davenport, Iowa, with winds of 100 mph and that was on the ground for nearly 8 miles. Water also overtopped a dam near Nashville, Illinois, leading to evacuations.
Summertime windstorms and derechos aren’t unusual; they typically occur across the Corn Belt, Great Lakes and Midwest roughly once per summer. They form on the northern edge of “heat domes,” or sprawling ridges of high pressure, utilizing hot, humid air, and the jet stream to move eastward.
By Tuesday morning, the storm had moved out, leaving significant damage. The Weather Service was surveying damage across Northern Illinois and Northwest Indiana on Tuesday, investigating at least 29 paths of potential tornado tracks.
In all, the Chicago area, which was under a "moderate" risk of severe weather—a level that ranks four of five on the Storm Prediction Center’s Severe Weather Scale—saw as many as 16 tornado warnings Monday evening, from the suburbs to the city.