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Derechos are relatively uncommon, and uncommonly intense

An archived radar view, with storm reports overlaid, of the derecho moving through eastern Iowa on Monday, August 10, 2020.
An archived radar view, with storm reports overlaid, of the derecho moving through eastern Iowa on Monday, August 10, 2020.(KCRG)
Published: Aug. 11, 2020 at 10:34 AM CDT
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CEDAR RAPIDS, Iowa (KCRG) - Monday’s line of severe storms will easily be classified using a term that you may not have heard of before.

Those storms were a well-organized system called a “derecho.” It’s a Spanish word that can be translated as “straight ahead” or “right.” Derechos are long-lasting, very intense straight-line wind storms. A technical definition includes requirements of a path length of at least 250 miles and multiple wind gust reports of 75 mph or more spaced out along the path. Some researchers define that last requirement as at least three individual reports spaced apart.

An outline of storm reports associated with a derecho in the Midwest on Monday, August 10, 2020.
An outline of storm reports associated with a derecho in the Midwest on Monday, August 10, 2020.(KCRG)

The term actually dates back to 1878, and it has its roots here in eastern Iowa. Dr. Gustavus Hinrichs, a physics professor at the University of Iowa, coined the phrase in a research paper. He intended to use the word to classify the damage caused by such storms compared to a tornado. The paper was based on a strong line of storms that cut a path across Iowa from northwest to southeast, inspiring him to analyze it.

The intense wind gusts produced by a derecho can match the wind speeds of a tornado or hurricane, but are caused by different processes so it’s tough to directly compare them. One notable feature of derechos is the spatial extent of the damage they cause; you can get tornado-like damage over a much wider area than a tornado can strike.

Derechos generally require a few things to occur: large amounts of energy (we also call this instability), wind shear, and a disturbance to initiate the thunderstorms. We had plenty of energy on Monday due to the several hours of sunshine in the morning, coupled with the very high dew points. As the derecho becomes fully organized and mature, its structure at the mid and upper levels allows higher wind speeds that are found at those layers to move to the surface. The most-favored location for those intense wind gusts are right at the “apex” of the bow-shaped radar signature, or the furthest-forward part of the area of heavy rain associated with it.

Property and crop damage are significant risks in derechos, which can total in the millions in insured losses. You can also be seriously injured or killed if caught outside in one of these storms, particularly in an area where you would be prone to flying debris or falling trees. For this reason, you should always seek shelter indoors when severe thunderstorms approach. Windows can be blown out, and we even saw walls and roofs destroyed on Monday, which really hammers home the point that you should try to shelter in the lowest floor of your home in an interior room.

Derechos are relatively uncommon in any one location, though the eastern half of the United States experiences at least a few a year somewhere. May through July are the most common months to see them, but they have been observed in every month of the year in the U.S. Some notable derechos in the recent past in eastern Iowa were the July 11, 2011 system that struck the Vinton area particularly hard in the early morning hours, and the June 29, 1998 event that followed a very similar path to Monday’s line of storms. Its worst damage was focused toward the Interstate 80 corridor down toward Washington.

Want to know more about derechos? The Storm Prediction Center has a good page with all kinds of information, both casual and technical.

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