At a Glance
- Derechos are widespread, damaging wind events from clusters of severe thunderstorms.
- Two of them struck parts of the nation's mid-section, first on Saturday night, then again Monday.
- They're capable of downing trees, damaging structures and knocking out power.
- They can also spawn tornadoes along their leading edges.
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Two derechos struck parts of the Midwest and Northern Plains over the past three days, including the Chicago metro area, illustrating the danger of these severe thunderstorm events.
What is a derecho? A derecho is a large cluster of thunderstorms that most commonly forms in late spring and summer. From the Spanish word for "straight," these windstorms leave widespread swaths of straight-line wind damage. The winds can be as strong as 100 mph or higher in extreme cases.
A derecho's power and impact is from its size. While a typical tornado can leave a damage path up to a mile wide and tens of miles long, a derecho can do so over a swath more than 60 miles wide and hundreds of miles long over multiple states.
Monday's Midwest derecho: A line of severe thunderstorms formed in central and eastern Iowa in the afternoon, then raced east-southeast into extreme southern Wisconsin and Illinois through the evening before ending up in northern and central Indiana late Monday night.
There were well over 100 reports of high wind gusts and wind damage, including two gusts over 100 mph near Camp Grove (105 mph) and Davis Junction, Illinois (101 mph). Other notable gusts included 97 mph in Holy Cross, Iowa; 84 mph in Footville, Wisconsin; and 75 mph at both Chicago's O'Hare Airport and on the campus of Purdue University in West Lafayette, Indiana.
(MORE: Impacts, News From Chicago, Midwest Derecho)
Saturday night's Plains derecho: Just two days before the Midwest event, a derecho swept through parts of eastern Montana into extreme southwestern North Dakota and South Dakota Saturday night.
Among the more than 100 reports of high winds and wind damage were three separate gusts over 100 mph, including a 109 mph gust in Stanley County, South Dakota, and an 84 mph wind gust at Pierre Regional Airport.
(MORE: Further beef up your forecast with our detailed, hour-by-hour breakdown for the next 8 days – only available on our Premium Pro experience.)
A Derecho's Dangers
A 2005 study by Walker Ashley and Thomas Mote found derechos claimed 153 lives in the U.S. during an 18-year period from 1986 through 2003.
Almost 70% of derecho fatalities occurred in areas other than a permanent building, including in vehicles (30%), in boats (19%), under trees (11%) and camping (9%).
Downed Trees
Derecho winds are strong enough to down or even uproot trees, which can then fall onto homes, vehicles, tents or RVs and block roads.
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In the July 15 Midwest derecho, a tree fell onto a home in northwest Indiana, killing one person. Three people were injured by falling trees in Miles City, Montana, in the July 13 derecho.
Structural Damage
As we alluded to earlier, peak wind gusts in derechos can match those in EF1, even EF2 tornadoes. Winds that high are capable of damaging buildings and structures, even without knocking trees down. Large hail whipped by high winds can smash windows and obliterate a home's siding.
Saturday night's derecho blew a roof off a home near Miles City, Montana, and bent a hotel sign to the ground in central South Dakota.
Tornadoes
Tornadoes can form quickly along the leading edge of a derecho or squall line of severe thunderstorms. Monday's derecho spawned over 30 tornadoes from Iowa to northern Indiana.
While it may not have been a tornado, you can visualize the potential in a spectacular timelapse (shown below) posted by Walker Ashley as the derecho blew through Northern Illinois University in DeKalb, Illinois, Monday night. Note the rotation near the cloud base toward the end of the timelapse.
Power Outages
If you're lucky enough not to suffer any damage from a derecho, chances are you'll at least lose electricity.
These widespread, damaging winds can not only down trees onto power lines, but can also snap power poles and even down high-transmission towers.
That can knock out power to hundreds of thousands, if not a million customers or more. And when a derecho happens in summer, that can leave those customers without power during lingering heat.
In the worst derecho events, relief workers from other states sometimes are needed to aid in these efforts due to the scope of damage and outages.
The Aug. 10, 2020, derecho in the Midwest caused an estimated $13.3 billion in damage as it raked across parts of eight states. Power was knocked out to almost 2 million homes and businesses. Some didn't have their power restored for weeks.
Here's what you can do: As with any severe weather situation, planning ahead and staying informed are key.
- Know where you'll seek safe shelter if a National Weather Service warning is issued. This should be on the lowest floor, in a basem*nt, if possible, away from doors and windows.
- If you live in a mobile or manufactured home, find either a sturdy building or community shelter ahead of time and know how to reach it quickly. A derecho's strongest winds can heavily damage or blow over a mobile home.
- Have multiple ways of receiving NWS warnings, including by smartphone and from NOAA weather radio. These are two sources that can also wake you if a warning is issued while you're sleeping, as derechos can happen at night. Make sure each is charged fully before the storms arrive.
- Move to shelter immediately when a warning is issued, including both severe thunderstorm and tornado warnings. Treat a severe thunderstorm warning as seriously as a tornado warning, especially one that mentions "damaging winds".
Jonathan Erdman is a senior meteorologist at weather.com and has been covering national and international weather since 1996. His lifelong love of meteorology began with a close encounter with a tornado as a child in Wisconsin. He completed a Bachelor's degree in physics at the University of Wisconsin-Madison, then a Master's degree working with dual-polarization radar and lightning data at Colorado State University. Extreme and bizarre weather are his favorite topics. Reach out to him on X (formerly Twitter), Threads, Facebook and Bluesky.
