Well, Blow Me Down! Inside a Derecho
In 2012, a massive straight-line storm system spanning more than 700 miles devastated large sections of the Midwest and Mid-Atlantic regions, and added a new word to the weather vocabulary of many Americans.
This type of storm system, called a “derecho,” is relatively rare when compared to other types of destructive weather events, such as tornadoes and hurricanes. Only a small area of the United States, centered on the Oklahoma, Missouri, Iowa tri-state region, sees more than one derecho event per year, on average. A surrounding region encompassing most of the Eastern U.S. sees a gradually lessening frequency of the storms, based on distance from this epicenter, with outlying areas such as Vermont and West Texas and the Southeastern Coast seeing no more than one derecho every four years.
A derecho is defined as “a widespread, long-lived, straight-line windstorm that is associated with a fast-moving band of severe thunderstorms.” The name “derecho” comes from the Spanish word for “straight,” as opposed to other destructive wind events, such as tornadoes, that tend to be cyclonic in nature.
To be classified as a derecho, a storm must meet all of the following qualifications:
1) There must be a concentrated area of wind gusts greater than 50 knots, or 60 miles per hour.
2) The storm must have a major axis length of 248 miles or more.
3) Wind must occur in a continuous and nonrandom pattern.
4) There must be at least three reports, separated by 40 miles or more, of wind gusts greater than 64 knots, or 74 mph.
5) No more than three hours can elapse between successive wind damage events.
Derechos are generally a warm-weather phenomenon, occurring mostly in summer, usually between May and August in North America, with peak activity during June and July.
Derechos are made even more dangerous by their relative silence. While most major thunderstorms can be heard approaching from several miles away, derechos can strike suddenly, with little to no warning. A derecho’s violent winds can uproot trees and damage structures in a very short period of time, leaving a wake of destruction before anyone even knows what happened.
Have you ever experienced a derecho? Share your stories below!