Extreme wind storms: What is a derecho?

Many people may have never heard of a derecho.

A derecho (pronounced similar to “deh-REY-cho”) is a widespread, long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms, according to the National Oceanic and Atmospheric Administration.

What is a derecho?

There are multiple types of storms that come under the derecho umbrella:

  • Serial derechos usually happen in the fall. They are made by “multiple bow echoes embedded in an extensive squall line,” NOAA said. The squall line can be hundreds of miles long. They are easier to forecast than other types, the National Weather Service said.
  • Progressive derechos are a short line of storms, between 40 and 250 miles long. They are usually a single bow echo and can be somewhat narrow, NOAA explained. They are unstable and difficult to forecast, the NWS said.
  • Hybrid derechos have characteristics of both serial and progressive derechos.
  • Low-dewpoint derechos happen when there are surface dewpoints between 40- and low 50-degree dewpoints. They normally happen in the late fall or early spring and are considered a type of hybrid derecho, NOAA said. They are groups of dry downbursts or microbursts that hit one after another, NOAA said.

Dangers in derechos

There can be dangers that come with strong storms. Tornados can occur in the same system as a derecho sometimes preceding the derecho-producing line. A tornado could also form within the squall line, NOAA said.

Another danger are mesovortices or embedded circulations, NOAA said.

A horizontal shear instability could also form. Those are small circulations that stretch a mile or two.

If a system that brings derechos is long-lasting and large, they are called a mesoscale convective vortex or MCV and are when localized pressure falls happen because of warming that comes with developing some types of clouds, NOAA said.

Where and when do derechos happen?

Most of the time derechos happen in late spring through summer. They happen in two areas most frequently -- one paralleling the “Corn Belt” from the upper Mississippi Valley to the Ohio Valley. The other happens through the southern Plains into the mid-Mississippi valley. From September through April, you may see them from east Texas through the southeastern states.

The storms can happen outside of North America but it is not as frequent.

For more on derechos click here and here.

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