Derechos: straight-line windstorms that run for hundreds of miles

What counts as a derecho

The Storm Prediction Center's derecho facts page gives the current test. An event may be classified as a derecho if its swath of wind damage extends at least 250 miles, includes gusts of at least 58 mph along its length with several well-separated gusts of 75 mph or more, and comes from a storm complex that moves faster than the mean wind. The system should also show radar features such as bow echoes and rear-inflow jets. SPC notes those requirements were added in revisions made in 2016 and 2025.

The name is old. According to the same SPC page, Gustavus Hinrichs, a University of Iowa physics professor, coined "derecho" in 1888 to separate straight-line thunderstorm winds from the rotating winds of tornadoes; the Spanish word means straight ahead.

How derecho wind behaves

Derecho winds are not steady. SPC explains that the strongest winds come from downbursts, which arrive in irregular clusters along the path, with smaller microbursts and burst swaths embedded in them. Burst swaths run about 50 to 150 yards long, and SPC says the damage they leave may resemble a tornado's.

Speeds can be extreme. The facts page notes that stronger derechos can exceed 100 mph, citing a measured 128 mph gust in eastern Wisconsin on May 31, 1998. The Weather Service office in Des Moines puts it in tornado terms: derecho winds above 100 mph are equivalent to an EF-1 tornado, spread over a far larger area.

They also move fast. SPC says derecho-producing systems typically travel at 50 mph or more, a few have been clocked at 70 mph, and the warning they give someone outdoors is often too short to act on.

When and where they happen

According to the SPC climatology section, derechos in the United States are most common from May through August, with more than 75% occurring between April and August. They favor two corridors: one following the Corn Belt from the upper Mississippi Valley into the Ohio Valley, the other running from the southern Plains into the mid-Mississippi Valley. In the cool season they are rarer and most likely from east Texas into the Southeast.

June 29, 2012: Chicago to the Atlantic

SPC's case page for the June 29, 2012 derecho describes a system that traveled about 700 miles in twelve hours, averaging nearly 60 mph. It set all-time June and July wind-gust records at several official stations; a measured 91 mph gust at Fort Wayne was the strongest in 61 years of record there for those months.

For roofs, the case page is specific: scattered 80 mph gusts blew roofs off homes, businesses and schools and overturned tractor-trailers as the line crossed Ohio. Five million people lost power between Chicago and the Mid-Atlantic coast, and 22 people were killed.

August 10, 2020: across Iowa and beyond

NOAA's satellite service, citing SPC, reports that the August 10, 2020 derecho traveled about 770 miles from far southeastern South Dakota into Ohio in 14 hours. The same page relays the Des Moines Weather Service office's assessment that it caused more than $11 billion in estimated damage, the costliest thunderstorm event in U.S. history.

The worst of it fell on eastern Iowa. The Quad Cities Weather Service office found damage consistent with intermittent 100 to 130 mph winds from Benton County through Linn, Jones, Cedar and Clinton counties, with maximum estimated winds around 140 mph at an apartment complex in southwest Cedar Rapids. It recorded an unofficial measured gust of 126 mph at Atkins. The office singled out duration: many places had damaging winds for 30 to 60 minutes, compared with the 10 to 20 minutes more common in derechos.

Farther west, the Des Moines office logged a highest official gust of 99 mph at the Marshalltown Airport, with the strongest winds between the Highway 30 and Interstate 80 corridors, including the Des Moines metro.

The county record shows the same storm. NOAA's Storm Events Database entry for Linn County on August 10, 2020 lists an estimated 122-knot gust, one death and 100 injuries, and describes winds lasting around an hour at any one location. The database records 26 thunderstorm wind events of 65 knots or more in Linn County from 2016 through 2025.

What derecho damage looks like on a roof

The official descriptions point to a few recurring patterns:

• Damage laid out in one direction. SPC notes that derecho damage typically runs along a relatively straight path, which is why it is called straight-line wind damage.
• Trees through the roof. SPC's hazard section describes falling trees and branches crashing through walls and roofs, and the removal of a roof as a direct danger to people inside.
• Openings that fail first. A NOAA database entry for a downburst in Linn County on June 30, 2014 records garage doors blowing in, unroofing the garage and the front half of the house roof.
• Long exposure. A roof that survives the first gust still has to survive the rest; in 2020, the Quad Cities office said the long duration of the high winds is what made that event so extreme.

After a derecho, the National Roofing Directory's storm damage roof repair guide covers the first steps, and its entry on documenting roof damage for a claim explains how public weather records can support a claim.

Frequently asked questions

How is a derecho different from a tornado?

A tornado's winds rotate in a narrow path; a derecho's winds blow in one direction across a swath at least 250 miles long, according to the SPC definition. Tornadoes can form inside a derecho, but the derecho itself is the widespread straight-line wind.

Can a derecho happen in winter?

Yes, though it is rare. The Des Moines Weather Service office describes the December 15, 2021 derecho as the first December derecho anywhere in the United States since known records.

How much warning does a derecho give?

Often very little for someone outdoors. SPC notes that derecho-producing systems typically move at 50 mph or more, so dark skies and shelf clouds can arrive on very short notice.

Where can I read about past Iowa derechos?

The Des Moines office keeps a list of derechos that have affected Iowa, with links to SPC summaries and event pages.