Thunderstorm Safety and Preparedness

What Is A Derecho? The Science Behind This Widespread Damaging Thunderstorm Danger

At a Glance

Derechos are widespread damaging wind events from clusters of severe thunderstorms.
They are most common in late spring and summer.
Parts of the Great Plains and Midwest experience derechos most often.

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A derecho is a large cluster of thunderstorms that most commonly forms in late spring and summer and causes widespread destruction to trees, power lines and sometimes structures.

Here's what this term means: From the Spanish word for "straight," these windstorms leave wide, long areas of straight-line wind damage. The winds can be as strong as 60 to 100 mph or higher in extreme cases.

To meteorologists, they have a distinct look on radar, usually as one or more curved lines of thunderstorms known as a bow echo.

This is a radar sequence of the Aug. 10, 2020, Midwest derecho from 8 a.m. through 7 p.m. CDT.
(NWS-Chicago)

Their damage is widespread. A single severe thunderstorm may produce an area of damaging winds only a mile or two wide and perhaps a few miles long, but derechos can produce damage tens of miles wide and hundreds of miles long.

To be called a derecho, the thunderstorm-generated high winds and wind damage should cover a distance of at least 400 miles long and be at least 60 miles wide, according to a 2016 proposal led by Stephen Corfidi.

The cleanup and recovery can take days or weeks. In the worst derecho events, relief workers from other states are sometimes 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 $12.8 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.

LUTHER, IA - AUGUST 11: In this aerial image from a drone, damaged grain bins are shown at the Heartland Co-Op grain elevator on August 11, 2020 in Luther, Iowa. Iowa Gov. Kim Reynolds said early estimates indicate 10 million acres, nearly a 1/3 of the states land used for crops, were damaged when a powerful storm battered the region a day earlier. (Photo by Daniel Acker/Getty Images) — In this aerial image from a drone, damaged grain bins are shown at the Heartland Co-Op grain elevator on Aug. 11, 2020, in Luther, Iowa.
(Daniel Acker/Getty Images)

Derechos in the U.S. are most numerous during the late spring and summer. A 2005 study from Walker Ashley and Thomas Mote found that both May and July average over four derechos per year. June is nearly as active with three to four derechos per year.

More than 75% of them occur between April and August, according to NOAA's Storm Prediction Center.

Derechos are a potential threat for many east of the Rockies. But they're particularly a threat in parts of the Great Plains and Midwest.

According to a 2016 study, a distinct corridor from the upper Mississippi Valley through the Ohio Valley is most susceptible to these from May through August.

"Northeastern Illinois is ground zero for warm-season progressive derechos," said Lance Bosart, professor emeritus at the University at Albany/SUNY and co-author of the April 2016 study with Corey Guastini.

Parts of Minnesota, Wisconsin, Iowa, Illinois, Indiana, southern Michigan, Ohio, Kentucky and the Appalachians averaged one to two progressive derechos each May-August over the 18-year period from 1996 through 2013, consistent with prior studies.

They can occur during the daytime or in the overnight and early-morning hours when most people are sleeping.

The number of progressive derecho events from May through August 1996-2013 passing through a 100 kilometer by 100 kilometer grid box is plotted in this map above.
(Guastini and Bosart 2016)

They can even be deadly. The Ashley and Mote study found derechos claimed 153 lives in the 18-year period of their study. Three northern states – New York (23), Michigan (17) and Ohio (16) – comprised over a third of all derecho deaths.

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%).

Here's what you can do: As with any severe weather situation, planning ahead and staying informed are key.

First, know where you'll seek safe shelter if a National Weather Service warning is issued. 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.

Next, 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. 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.

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