Hello everybody and happy #WeatherWednesday! Last week we discussed the concept of the monsoon. Today we will talk about a type of storm called a “derecho”. The word is Spanish for straight and it has to do with straight line winds!
A derecho isn’t the most common storm, but it’s definitely dangerous. These storms occur several times per year in the USA. They are storm systems associated with high surface winds from storm outflow. The longevity and elevated forward speed of the damaging storm are primary defining factors. This means more area liable to be damaged by a storm moving faster than normal. Many derechos propagate at 35-65mph.
Storms can produce locally high winds and damage anytime, but derechos produce high winds for hours over a great length and width. A squall line on the other hand usually produces sporadic wind damage at various parts of the storm. A squall is similar in structure but is only considered a derecho if there have been 60mph+ winds on the storm front for at least 6 hours. A serial derecho can commonly be identified as a bow echo or backward C on radar reflectivity during its lifetime. Visually, a shelf cloud normally develops on the leading gust front where the temperature gradient from colder outflow and warm inflow exists.
Derechos occur in the summer most of the time when warm air advection and rich moisture content is widespread enough for the storm to maintain itself for a long distance. A shortwave trough or “disturbance” creating relatively high velocity and divergence of winds aloft is typically the lifting mechanism. Since the jet stream is at a higher latitude in the summer and is the source of vorticity vital to storm development, derechos favor the Midwest on the periphery of the continental ridge over the southern plains.
The dangers of derechos are the straight line winds, and occasionally tornadoes and flash flooding. These storms occur often in the Midwest which mean they occur over higher populations of trees that can fall down. A derecho in June 2012 that started in the Midwest and ended in the Mid-Atlantic and featured 90mph wind gusts and 3.7 million power outages is the big derecho event of recent times.
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