A derecho is a widespread, long-lived, straight-line windstorm that is associated with a fast-moving group of severe thunderstorms. The term "derecho" comes from the Spanish word meaning "straight ahead" or "direct," which is fitting because these storms often leave a swath of destruction in a relatively straight line.
Derechos were first identified and named by Dr. Gustavus Hinrichs, a physics professor at the University of Iowa, in a paper published in 1888. He used the term to distinguish these straight-line wind events from the more common circular winds of tornadoes.
For a storm system to be classified as a derecho, it must include a band of storms that produces a swath of particularly damaging winds, extending more than 240 miles and including wind gusts of at least 58 mph along most of its length. These severe winds can sometimes exceed 100 mph, causing significant damage equivalent to that of a strong tornado.
Derechos are most common in the late spring and summer months, particularly from May through August. They are more likely to occur in the Midwestern United States, especially in an area known as "derecho alley," which includes parts of Oklahoma, Kansas, Nebraska, Iowa, Illinois, Indiana, and Ohio. However, derechos can occur in other parts of the United States and the world.
These storms are typically caused by a combination of very hot and humid conditions, along with strong upper-level winds. They often form along the leading edge of a strong cold front or in the warm sector of a low-pressure system. The most severe derechos are usually associated with a curved band of rapidly moving showers or thunderstorms called a bow echo, which can sometimes take on a comma-like shape.
One of the most destructive derechos in recent history was the June 2012 Mid-Atlantic and Midwest derecho. This storm traveled over 600 miles in just 10 hours, causing widespread damage across several states and leaving millions without power. Wind speeds in excess of 90 mph were recorded, and the storm was responsible for 22 deaths and millions of dollars in damage.
Other notable derechos include the July 1995 derecho that affected parts of Canada and the northeastern United States, the May 2009 "Super Derecho" that traveled over 1,000 miles from Kansas to Kentucky, and the August 2020 Midwest derecho that caused widespread damage in Iowa and Illinois.
In addition to the damaging winds, derechos can also bring heavy rain, large hail, and even tornadoes. They can be particularly dangerous because they often occur at night when people are sleeping and may not be aware of the approaching storm.
While derechos are relatively rare compared to other types of severe weather, they can be extremely dangerous and destructive. It's important for people living in areas prone to these storms to stay informed about weather conditions and have a plan in place to seek shelter if a derecho warning is issued.