What Is a Derecho? — the Straight-Line Wind Storm

A derecho is a widespread, long-lived wind storm. It is linked to a band of rapidly moving showers or thunderstorms. That band can be called a squall line, a bow echo, or a quasi-linear convective system.

A derecho can cause destruction similar to that of a tornado. But the damage usually happens in one direction, along a relatively straight swath. That is why the term “straight-line wind damage” is used to describe derecho damage.

The numbers that make it a derecho

Under the Storm Prediction Center’s definition, a storm must meet clear size and wind rules to count as a derecho:

• The wind damage swath must extend, continuously or intermittently, more than 250 miles (about 400 km).
• The storm system must have a line-width of at least 60 miles (about 100 km).
• It must include wind gusts of at least 58 mph (50 knots or 93 km/h) or greater along most of its length.

Derecho winds typically are less than 100 mph. But gusts as high as 130 mph have been recorded. That is equivalent to those with strong EF2 tornadoes.

Where the word came from

The word “derecho” was coined by Dr. Gustavus Hinrichs, a physics professor at the University of Iowa. He used it in a paper published in the American Meteorological Journal in 1888.

“Derecho” is a Spanish word meaning “direct” or “straight ahead.” Hinrichs coined it to tell straight-line wind damage apart from damage produced by tornadoes.

The term then fell out of use for nearly 100 years. Severe weather meteorologists brought it back in the mid-1980s.

What it can look like

Many derechos are preceded by ominous-looking “shelf” clouds. These form on the leading edge of the storm system’s outflow winds.

Derecho-producing storms also tend to appear darker than most, especially late in the afternoon or early evening.

There is no single look that reliably identifies one. The appearance varies from event to event.

When and where they happen

70% of all derechos occur between May and August, in the warm season. The other 30% occur during the cool season.

In the United States, they most commonly occur along two axes. One runs along the “Corn Belt,” from the upper Mississippi Valley southeast into the Ohio Valley. The other runs from the southern Plains northeast into the mid Mississippi Valley.

During the cool season, from September through April, derechos are relatively infrequent. They are most likely from east Texas into the southeastern states.

Derechos are extremely rare west of the Great Plains.

One example: The June 29, 2012 derecho swept from the upper Midwest to the central Atlantic coast in a matter of hours. Almost 1,200 reports of damaging winds were received. There were numerous reports of gusts at hurricane force or greater (74 mph).

The danger

Those outdoors are most at risk. That includes:

• Campers or hikers in forested areas, because of falling trees.
• People at sea, because winds and waves can overturn boats.
• Occupants of cars and trucks, because of falling trees and utility poles.
• People at outdoor events, because temporary structures can collapse and debris can fly.

High-profile vehicles such as semi-trailer trucks, buses, and SUVs may be blown over.

Even indoors there is risk. Mobile homes in particular may be overturned or destroyed. Falling trees and branches can crash through walls and roofs.

In a city, damage to overhead electrical facilities sometimes results in massive, long-lasting power outages affecting hundreds of thousands of people. In the worst events, power may not be restored for days.

Getting ready, caught outside, and afterward

Getting ready

If you are planning to be outdoors for a significant length of time, be aware of the weather forecast, especially if you will be far from sturdy shelter.

Stay “connected” via television, radio, NOAA Weather Radio, or social media.

Derechos rarely occur without warning, although warning lead times may be comparatively limited early on.

Because protracted electrical and communication disruptions may occur, set aside emergency water and food supplies, can openers, batteries, and flashlights.

If you are caught out-of-doors

Falling debris is the most serious hazard. Cloud-to-ground lightning is an additional hazard.

Lie flat and face-down on low ground, protecting the back of your head with your arms. If possible, avoid trees.

Afterward

If you suffer a power outage, do not assume the utility is aware of it. If possible, contact your utility.

Avoid any downed power lines, especially contact with lines in wet areas. The wires could still be “live.”

Watch for broken glass, upstanding nails, partially fallen tree limbs, and other hazardous debris.

A sizable number of derecho injuries and deaths occur during clean-up operations, long after the storm has passed. Take care during clean-up.