What Is a Derecho? The Inland Windstorm That Hits Like a Hurricane

A derecho is not a different kind of cloud. It is a scoring system applied after the fact to a line of thunderstorms that did enough damage over a long enough distance. The Storm Prediction Center sets the bar at a wind-damage swath of at least 250 miles, gusts of at least 58 mph along most of that length, and several well-separated gusts of 75 mph or greater, all from a storm complex racing ahead of the mean wind.1

One detail worth knowing if you read older articles: that 250-mile threshold changed. The criterion sat at 400 miles from 2022 until May 2025, when new research moved it back. Plenty of explainers still quote the old number.

The practical difference from a tornado is the direction of the damage. A tornado twists things; a derecho pushes everything the same way, which is why whole fields of corn lie flat in one direction and why the damage path is measured in hundreds of miles rather than hundreds of yards.

Derechos are a warm-season story. More than 75% happen between April and August, and one of the two main U.S. tracks runs straight through the Corn Belt, from the upper Mississippi Valley southeast into the Ohio Valley. That is why a storm most of the country rarely thinks about is a fact of summer life across Iowa, Illinois, and their neighbors.1

The August 10, 2020 event is the benchmark. It ran about 770 miles in roughly 14 hours, produced a measured gust of 126 mph at Atkins, Iowa, and damaged an estimated 14 million acres of Iowa cropland. The National Weather Service calls it the most expensive thunderstorm event in modern U.S. history.23

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This is the part that catches people. There is no such thing as a "derecho warning." The label is applied afterward, once someone measures the damage path. What you get in the moment is a Severe Thunderstorm Warning, and for the strongest events a rare high-end version of it.

So the useful question is not whether a storm will earn the derecho label. It is how fast the line is moving toward you and how strong the winds are. A derecho-producing line can cover 60 miles in an hour, which means the gap between "storms are a few counties away" and "the power is out" can be under 30 minutes.

• Watch the line, not the dot: Derecho damage comes from a bowed line of storms, so track the whole leading edge and how fast it is advancing.
• Treat it like a tornado for shelter: Get to an interior room on the lowest floor. Straight-line winds of 80 to 100 mph do tornado-grade damage.
• Assume multi-day outages: These storms take down transmission lines across entire regions, not just neighborhood poles. Charge devices while you can.
• Mobile homes and vehicles are not safe: The same rule as tornadoes applies, and a moving line gives you less time than you think.

A fast line of storms is exactly the case where a general forecast is not enough. What matters is where the leading edge is right now and when it reaches your address.

ZoneWatch tracks severe storms on high-resolution radar and sends hyper-local alerts for the places you save, so you can see a line approaching, watch it close the distance, and get everyone inside before the winds arrive rather than after.