NOAA’s GOES-East satellite captured severe thunderstorms moving across the Midwest in the United States on August 10, 2020. These thunderstorms triggered a derecho, a widespread, long-lived, straight-line wind storm.
A derecho, by definition, must have wind gusts of at least 58 mph (50 knots or 93 km/h) throughout the majority of its length.
Derechos can cause devastation comparable to tornadoes, the damage is usually concentrated in one direction over a relatively straight path.
The derecho moved from southeastern South Dakota into Ohio. This wind storm traveled 770 miles over the course of 14 hours. Wind gusts reached up to 74 mph (65 kt) at times and was recorded at 90 mph in central Iowa.
Where is the Name Derecho From?
Dr. Gustavus Hinrichs, a physics professor at the University of Iowa, created the term “derecho” in a study published in the American Meteorological Journal in 1888, and it originates from the Spanish word for “direct” or “straight ahead.”
How Do Derechos Form?
As the wet air in a thunderstorm collides with the drier air around it, the water in the air evaporates. When water evaporates, it cools the air it comes into contact with. Because chilly air is denser, it sinks quickly to the ground, causing downbursts of intense winds.
The downburst can actually draw in more dry air into the storm, resulting in even bigger downbursts or clusters. When the ideal conditions for downbursts occur across a large area, derechos occur. A derecho’s path of destruction is at least 240 miles long.
Bow echoes occur when the storm expands in size, and they are vast curving packs of thunderstorms that speed forward in one direction. Because the downbursts are stronger near the storm’s center, bow echoes form. Faster winds are associated with stronger downbursts. The storm’s faster winds create a bow by racing ahead of it.
Bow echoes, which occur when atmospheric winds are relatively strong and unidirectional, are linked to the development of derechos. A bow echo is usually caused by a group of thunderstorms, but it can also be caused by a single severe storm.
This series of figures created by the National Weather Service shows how atmospheric conditions can lead to the formation of derechos.
Where do Derechos Form in the United States?
There are two main areas in the United States where the geography is favorable to the formation of derechos.
These two areas are along the “Corn Belt” which spans from the upper Mississippi Valley southeast into the Ohio Valley, and the “Southern Plains” which runs from the southern Plains northeast into the mid Mississippi Valley.
Although derechos are uncommon west of the Great Plains, sporadic derechos have occurred in the interior of the United States, particularly in the spring and early summer.
When Do Derechos Occur?
The vast majority of derechos occur during the warm season (May-August). Only about 30% of derechos occur during the cooler months (August – April).
Derecho. (n.d.). National Weather Service. https://www.weather.gov/lmk/derecho
Derechos FAQ page. (n.d.). NOAA/NWS Storm Prediction Center. https://www.spc.noaa.gov/misc/AbtDerechos/derechofaq.htm
Hinrichs, G. D. (1888). Tornadoes and derechos. Amer. Meteor. J., 5, 306–317, 341–349. Available online at www.spc.noaa.gov/misc/AbtDerechos/hinrichs/amj_hinrichs.pdf.
What is a Derecho? (n.d.). NOAA SciJinks – All About Weather. https://scijinks.gov/derechos/