It’s a violent storm system that can produce widespread wind damage, usually associated with a rapidly moving band of showers and thunderstorms. The strong-to-violent winds typically move ahead of the main system, as the outflow from the storms becomes more concentrated. Wind damage is typically directed into one direction and can create its vast damage along a relatively straight path. 

How does a derecho develop? Typically, a cluster of storms develops during the late spring and summer time period. These clusters of storms can eventually evolve into a single strong storm. Have you ever felt a cool wind prior to a thunderstorm approaching your area? If you have, you are feeling the outflow winds from the storm. In other words, a rain-cooled downdraft (cool winds, moving down) occurs in the thunderstorm and hits the Earth’s surface, spreading horizontally and pushing outward. The cool, dense air spreads out and the warm airmass ahead of the system typically moves along the leading edge of the outflow as an updraft. The winds in the troposphere, or the layer of the atmosphere where our weather occurs, typically become relatively strong and unidirectional.

In some systems, the downdrafts in the storms can weaken the systems as cooler air pushes towards the surface and stabilizes the atmosphere. However, in this case, warm air ahead of the system can actually refuel and energize the thunderstorm complex. The downdraft winds can create a cool pool along the surface. As more storms develop, it can help strengthen and elongate the cold pool at the surface. As this happens, the cold pool induces a inflow of air known as the rear-inflow jet that helps the updraft (winds moving up) of the thunderstorm to expand and further intensify the cold pool.

It is the cold pool and the winds ahead of the system that begin to strengthen and straight line winds become more of an issue.

As the system continues to organize, the squall line can bow out and eventually become a derecho. Of course, for it to be classified as a derecho, it has to meet the above-mentioned criteria. Some derechos can form small, quick spin-up tornadoes – sometimes called gustnadoes – that only last a minute or two. These tornadoes are generally weak and are below EF-2 strength. However, there have been stronger tornadoes associated with derechos in some instances. Regardless, derechos tend to produce widespread damage, unlike tornadoes, which produce isolated damage.

With this is mind, derechos are serious storms. If you hear one is coming, consider taking shelter immediately, as falling trees and power lines may be likely.


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