Monday storms that produced damaging winds and tornadoes have been classified as a Derecho

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What is a derecho? The National Weather Service explains:

1.) Derecho development is necessarily tied to the formation of bow echoes. A bow echo usually arises from a cluster of thunderstorms but also may evolve from a single strong storm. Bow echoes most frequently occur when atmospheric winds are relatively strong and unidirectional (i.e., they vary little in direction with height but increase in speed). As the rain-cooled downdraft of a thunderstorm reaches the earth’s surface, it spreads horizontally, most rapidly in the direction of the mean atmospheric flow. As the cool, dense air spreads outward, it forces the lighter, warm and moist air surrounding the storm up along the leading edge of the outflow, or gust front (see figure below, with mean flow assumed to be from left to right). Gust fronts often are marked by a band of ominous, low clouds known as “arcus.” 

2.) The rain produced by the newer storms reinforces the cold pool, strengthening the inflow of air from the back side of the developing storm complex and encouraging the downward transport of higher momentum winds from aloft. These processes can enable the system to attain a nearly steady-state condition. This increases the longevity and strength of the entire system and is what allows the storm to travel such a large area over a short amount of time. At this point, the convective system typically exhibits a pronounced bow shape on radar 

3.) As the thunderstorms continue to increase in coverage, even more rain-cooled air reinforces the cold pool (pictured below). The line of storms continue to accelerate either as one large bow echo or multiple smaller bow echoes within an overall line. At this point, widespread and persistent wind damage has been occurring for a prolonged period of time.

Credit: National Weather Service

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WGN-TV Tom Skilling Weather Center

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