D is for Derecho

Just a couple of months after a devastating tornado tore across Nashville, Mother Nature had another blow for Music City. Late in the evening on Saturday, May 1, a complex of severe thunderstorms developed across southern Kansas. They continued tracking east through Missouri and western Kentucky over the next morning before finally reaching central Tennessee by the afternoon.

Shelf cloud approaching Hwy109 at I-40 near Lebanon, TN on May 3, 2020
photo by NWS Nashville meteorologist Brendan Schaper

The storms produced widespread straight-line wind damage, downing numerous trees and power lines and damaging several buildings. Winds averaged between 60 and 80 miles per hour across the Nashville metro area, and the Nashville Airport recorded a peak wind gust of 71 miles per hour (the fifth highest wind gust ever recorded there!). The strong winds resulted in over 130,000 customers losing power – the worst power outage the city has ever experienced. At least one man was killed and three others were injured as a result falling trees from the strong winds.

OHX radar image at 4:40 PM CDT on May 3, 2020

Citizens in Smith, Dickson, Lewis, Lawrence, Maury, Bedford, and Coffee Counties also experienced severe wind damage, and tens of thousands lost power. So, what was this phenomena that wreaked such a havoc?

Damage in Lavergne, TN (via #tSpotter on Twitter)

“I’ll take fun weather words that start with D for $1,000, Alex.” — it was a derecho! Derechos are defined by the American Meteorological Society as widespread convectively induced straight-line windstorms. Additionally, the wind damage swath typically needs to extend more than 240 miles with wind gusts of at least 58 miles per hour to be considered a derecho. Their name is derived from a Spanish word that can be interpreted as “straight ahead” or “direct” and was chosen to discriminate between wind damage caused by tornadoes, which have rotating flow, from straight-line winds.

Storm Reports from the Storm Prediction Center, 5/3/20

Derechos are most common in the United States in the late spring and summer, with over 75% occurring between April and August. They also occur most commonly in certain areas of the country, as shown in the map below. Although derechos are rare west of the Great Plains, derechos occasionally do occur over interior portions of the western United States, especially during spring and early summer. Additionally, although it is relatively uncommon, derechos can occur outside North America.

Image courtesy SPC, illustration by Dennis Cain

Due to the warm season in which they occur, those involved in outdoor activities are thought to be especially at risk. For example, campers or hikers in heavily forested areas are vulnerable to being injured or killed by falling trees, and those on the water risk injury or drowning from winds and high waves that can overturn boats. Those outdoors are especially vulnerable due to the rapid movement of the system, as derecho-producing storm systems are typically traveling at speeds of 50 miles per hour or greater. This is why it is especially important to have multiple ways to monitor the weather and to receive weather warnings, no matter where you are.

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