32 tornadoes made July derecho record-breaker, with DeKalb, Ogle counties bearing brunt of storm

DeKALB – DeKalb, Ogle and Winnebago counties experienced the strongest straight-line winds during the July 15 derecho that swept across northern Illinois, according to analysis of the event by the National Weather Service.

Wind as fast as 90 mph blew through northern DeKalb, northern Ogle and southern Winnebago counties about 8 p.m. July 15, felling trees and damaging property throughout the area.

Brett Borchardt, a senior meteorologist at the National Weather Service Chicago, said the line of severe thunderstorms in northern Illinois that night was a derecho.

“A derecho – it’s kind of a funny name, but it’s a word that’s used to describe a long-lived windstorm. And you look at a meteorological definition, there are specific thresholds, like a certain distance it has to travel and a certain wind speed, but generally speaking, it’s [a] long-lived windstorm,” Borchardt said.

Derechos are most common during July and August across the Midwest and the lower Great Lakes region, especially northern Illinois, Borchardt said.

“Smack dab in the middle of northern Illinois, DeKalb County, we’re actually in a highway for derechos,” Borchardt said. “We typically average a derecho every year, but we don’t typically see the derechos as strong as the one that went through on July 15 every year. That’s typically once every five to 10 years.”

With 32 tornadoes – including one in eastern DeKalb County, two in Kane County and three that went through Kendall and Will counties – confirmed as a part of the derecho, July 15 broke the record for tornadoes in a single calendar day in the NWS Chicago forecast area, according to the agency. A double derecho event June 30, 2014, and a tornado outbreak March 31, 2023, previously tied for the record with 22 tornadoes each.

For all but 15 minutes between 8:25 and 10:30 p.m. July 15, a tornado was on the ground somewhere within the Chicago forecast area. Simultaneous tornadoes occurred for about 70% of that almost two-hour window, according to the NWS.

Borchardt said derechos and lines of storms can produce tornadoes, but some do so more than others.

“The derecho that came through here on July 15 was a prolific tornado producer,” he said. “Across northern Illinois and northwestern Indiana, there were more than 30 tornadoes, and in the whole life cycle of the storm – so this is from Iowa into Indiana, Wisconsin, Michigan – over 50 tornadoes. And for one storm to produce more than 50 tornadoes, that is definitely very unusual. That’s ... probably the top tornado event with a line of storms in our general area in recent history.”