On the morning of August 10, 2020, meteorologists at the Storm Prediction Center were watching a cluster of ordinary-looking thunderstorms drift east from South Dakota. Their models rated the risk as “slight.” By noon, those same meteorologists had upgraded the threat to “moderate” — because the storm was already upon Iowa, and Cedar Rapids was experiencing sustained winds of an estimated 140 miles per hour (225 km/h) that would not relent for 45 minutes or longer. It was, and remains, the costliest thunderstorm event in US history, and the anniversary this week arrives as new severe storms threaten the same corridor — and as peer-reviewed research confirms that what made 2020 feel like a once-in-a-generation anomaly is becoming a recurring feature of Midwest summers.
Six years after the derecho, communities across Iowa and northern Illinois are still rebuilding. New storms developing from South Dakota swept through eastern Iowa early this morning, August 11, 2026, prompting the National Weather Service to designate the day a First Alert event and earning a Slight Risk classification from the Storm Prediction Center. The atmospheric setup — unstable warm air, high moisture, strong upper-level wind shear — is not quite a replay of 2020. But it is recognizably the same playbook.
What Is a Derecho, and Why Do Scientists Study Its Mechanics So Carefully?
A derecho — from the Spanish for “straight,” in deliberate contrast with the “twisted” tornado — is a widespread windstorm associated with a fast-moving complex of severe thunderstorms called a mesoscale convective system. By the National Weather Service’s current definition, a storm qualifies as a derecho when it produces a swath of damaging winds at least 250 miles (400 km) long, with sustained gusts above 58 miles per hour (93 km/h) and at least three separate wind reports exceeding 75 miles per hour (121 km/h) spaced 80 miles (130 km) apart. That definition was revised in 2025 based on new algorithmic criteria developed by NOAA researchers to eliminate the definitional ambiguity that had led different meteorologists to classify borderline events inconsistently.
The defining difference between a derecho and a tornado is structural: where a tornado’s destruction runs in a narrow, rotating column, a derecho deals damage across a swath hundreds of miles wide in a single, straight-ahead rush. And where a tornado typically takes 10 seconds to a couple of minutes to pass over any given point, a serious derecho can maintain hurricane-force winds over a city for 30 to 60 minutes or longer. That duration is the feature that turns a dramatic weather event into a civilization-disrupting one…