What Is a Derecho?
After the powerful storm system that moved through on August 10th, 2020, it was clear that many have never heard of the phrase Derecho to describe the event that unfolded. In fact, a lot of you believed it was a new term that was created by the media. But did you the word “derecho” was actually coined in 1887 by Dr. Gustavus Hinrichs, a professor of physics at the University of Iowa? Hinrichs used the term in a paper published by the American Meteorological Journal to distinguish thunderstorm-induced straight-line winds from the damaging, rotary winds of tornadoes. “Derecho” is a Spanish word meaning “right,” “direct,” or “straight ahead.”
It wasn’t until a century later that meteorologists decided to use the term derecho to describe long-lived convective straight-line wind events. So by official definition, 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. Although a derecho can produce destruction similar to the strength of tornadoes, the damage typically is directed in one direction along a relatively straight swath. By definition, if the wind damage swath extends more than 240 miles and includes wind gusts of at least 58-mph or greater along most of its length, then the event may be classified as a derecho.
This storm system produced a complex of storms across South Dakota and Nebraska. It continued to grow in length as it moved to the east across Iowa. As it did, the wind speed continued to increase. By the time the line had reached the I-35 corridor, wind gusts were ranging from 70 to 80-mph. As storms moved east of I-35, this is when wind gusts increased to 80 to 100+ mph, and when the National Weather Service issued a PDS (particularly dangerous situation) severe thunderstorm watch. This line of storms continued to produce severe winds through Illinois and Indiana before eventually dissipating in strength over Ohio.
At Midway, Iowa, which is located north of Cedar Rapids, winds were measured at 112-mph. A wind gust of 100-mph was recorded nearby in Hiawatha, Iowa. Winds of this magnitude are comparable to a strong EF-1 to a low-end EF-2 tornado. This is why a good portion of the state of Iowa saw the damage that it did on this day.
Derechos in the United States most commonly occur along two axes. One extends along the “Corn Belt” from the upper Mississippi Valley southeast into the Ohio Valley, and the other from the southern Plains northeast into the mid-Mississippi Valley. According to derecho climatology, Iowa typically experiences a derecho every couple of years.
