Behind the Forecast: Why derechos are dangerous

Science Behind the Forecast: Derechos
Published: Jun. 4, 2018 at 12:19 AM EDT|Updated: Jun. 15, 2018 at 5:33 AM EDT
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Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.

LOUISVILLE, KY (WAVE) - Derechos are a typically a late spring and summer phenomenon. While the name is reminiscent of a certain tasty corn chip snack, they pack a much more dangerous punch.

The word derecho was coined by Dr. Gustavus Hinrichs, a University of Iowa professor, in 1888. "Derecho" is a Spanish word that means right, straight ahead or direct.

A derecho is a severe wind event that forms as the result of a persistent outflow from what meteorologists call a Mesoscale Convective System or MCS. A MCS is a group of thunderstorms that act as a singular system. It can stretch across an entire state and last for half a day!

A derecho is defined by wind gusts of at least 58 mph and a swath of damage of more than 240 miles, according to the National Weather Service. 
Damage from a derecho can be similar to that of a tornado but this damage is mainly from straight-line winds.

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A derecho's formation is tied to the formation of bow echoes. A bow echo usually is made up of a group of thunderstorms but could evolve from a single storm. These occur when winds are strong and uni-directional throughout the lower atmosphere. Cool air from a thunderstorm's downdraft reaches the surface and spreads horizontally in the direction of the atmosphere's winds. As the cooler, dense air spreads out, warm and moist air is forced up along a gust front, or the leading edge of a storm's outflow. The rising air feeds the formation of other storms. As these storms grow and mature they feed the existing pool of rain-cooled air, which keeps the gust front strong.

What's called a rear inflow jet may develop on the backside of the group of thunderstorms. This causes the storm's updraft to tilt, which lets the thunderstorm expand, increasing the swath of rainfall and in turn strengthens the gust front by adding to the pool of cool air under the storm. All of this allows the storm to stay strong over a long distance. Each new storm that forms repeats this process so the derecho can travel for hundreds of miles.

Learn more about derechos from the National Weather Service by clicking or tapping here.

The continental United States typically sees derechos from May through August. Climatologically, WAVE Country sees one derecho every year.

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