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Derecho: Understanding the Term

What does the newest weather buzzword really mean?
Related: Derecho, Earth
Derecho

As severe storms moved from the Plains into the Midwest and Ohio Valley Wednesday, strong winds battered houses and knocked down trees and power lines from Minnesota to Illinois. Numerous tornadoes and funnel clouds were reported. In the late evening, the complex barreled toward Chicago, knocking out power to thousands and resulting in hundreds of cancelled flights.

By Thursday morning, severe storms had crossed into the east, plowing forward to heavily populated areas such as Philadelphia, New York City and Atlanta with the same destructive energy.

After a similar, but stronger event, officially declared a derecho, occurred last year, many are wondering what defines the term. Does strict criterion exist, or has it become another buzzword indicative of a strong storm?

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According to the National Weather Service, a derecho is a "widespread, long-lived storm that is associated with a band of rapidly move showers or thunderstorms." Often, according to the NWS, the system will produce damage comparable to that from a tornado, but is typically in one direction along a straight swath. This is called straight-line wind damage.

By the National Weather Service definition, a storm must match certain criteria to be considered a derecho:

1. The wind damage swath must extend more than 240 miles (about 400 kilometers), and also

2. must include wind gusts of at least 58 mph (93 km/h) or greater along most of its length

While some believe both of these criterion were met June 12-13, others think the language of the definition is vague and can be met by mesoscale systems that are not derechos.

"When you look at the definition, it's clear except for one word, 'most.' That leaves it up for interpretation. The one thing that I think is crystal clear, though, is that discussing the possibility of it occurring was obviously appropriate," said AccuWeather.com Expert Meteorologist Bernie Rayno.

"I think the problem was that people were gravitating more toward the characterization of the event instead of focusing on on its impacts," he said.

In a blog post by the Capital Weather Gang, the Washington, D.C.-area weather group, has even gone as far as to call the term a "huge distraction."

"Stop focusing on whether it's a derecho or not," they wrote.

But ultimately, the decision to name a complex a derecho is made by the NWS. And the agency did, on Thursday, officially deem the events that occurred on June 12-13, 2013, a low-end derecho.

The severe weather event met the basic criteria, they said, as it moved over 600 miles, averaging 47 miles per hour and resulted in more than 150 damaging winds reports.

"For many people in the Ohio Valley and mid-Atlantic region, the term 'derecho' brings to mind the June 29, 2012, event and the resulting impacts. This isn't surprising, since past experience (especially in the recent past) strongly influences peoples' perception of risk," according to Bill Bunting, a meteorologist with NOAA's Storm Prediction Center.

"Derechos occur across a spectrum of intensities and impacts, and it's important to know that each event is different. The more important message is to focus on the forecasted impacts from any one severe weather event, and not on its classification."

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