United States

By Alex Sosnowski, Expert Senior Meteorologist

July 26, 2012; 11:50 PM

Share |

This image by Grover C. captures the derecho moving over Cincinnati, Ohio, on July 24, 2012.

The use of the term "derecho" used by the weather media to describe Tuesday's damaging thunderstorms raised a stir on social media.

The system that struck this past Tuesday downed trees and power lines from around Chicago, Ill., to Bristol, Tenn.

Until this past June, the use of the term may have been unknown to many people outside of the Central States. However, you can bet that in the wake of the storms on June 29, 2012, most people in the swath from West Virginia to Washington, D.C., have a good idea what it is and what it can do.

These powerful complexes of thunderstorms have been no stranger to the Plains and Midwest and have been documented for well over 100 years.

In 2009, a derecho slammed portions of Kansas, Missouri, Kentucky and other Midwest states with widespread damage and power outages.

AccuWeather meteorologist Jesse Ferrell gives his take on Tuesday's derecho on his WeatherMatrix blog.

"Derecho" is a Spanish word that means "straight ahead" or "direct." The word was first used to describe powerful straight-line winds produced by thunderstorms in 1888 by University of Iowa physics professor, Dr. Gustavus Hinrichs.

The definition of derecho by the National Weather Service's Storm Prediction Center (SPC) is as follows, "A widespread, long-lived wind storm that is associated with a band of 'rapidly moving' showers or thunderstorms that can produce damage similar in magnitude to a tornado, but from a single direction, rather than a rotation."

More specifically, if the wind damage swath extends for more than 240 miles and includes wind gusts of at least 58 mph or greater along most of its path, the event may be classified as a derecho. A derecho is an amplified, high-speed version of a large thunderstorm complex (mesoscale convective complex, mcc) or a small thunderstorm complex (mesoscale convective system, mcs).

While not as intense, nor as large, as the derecho of June 29, 2012, the event this past Tuesday appears to have met the minimal scientific requirements.

From a technical standpoint, wind gusts were pushing the 60 mph realm from Chicago's southern and eastern side to the Dayton and Cincinnati, Ohio, areas. The distance from Chicago to Dayton is 245 miles and the system moved along at an approximate forward speed of 50 mph.

This map produced by the Storm Prediction Center (SPC) shows storm damage reports from July 24, 2012. Wind damage reports are shown in blue.

Why We Used the Term "Derecho"

There was great concern on behalf of AccuWeather.com that the system could go unnoticed until it had already slammed many communities.

Today's instantaneous communication of weather extremes by way of social media commands the use of attention-grabbing terms.

There is a risk of overuse of terms like derecho or blizzard. However, the use of strong or severe thunderstorms is used so commonly today that we felt many people may disregard a potentially life-threatening event if we stopped there.

Not all derechos, like other storms, are created equal.

Indeed, Tuesday's system was not as far-reaching nor as intense as the June 29 event. However, it produced dozens of wind-damage incidents, covering seven states in about 10 hours. At times it was moving faster than the June 29 event.

According to Severe Weather Expert Henry Margusity, "We began to see derecho-level damage around Chicago and northern Indiana and became very concerned that we had another widespread, fast-moving event unfolding."

Expert Senior Meteorologist Dale Mohler added, "The system was past its peak upon crossing the Ohio River into eastern Kentucky and southern West Virginia. However, significant tree and power-line damage continued into northeastern Tennessee and into northwestern North Carolina."

"When we use the word 'derecho' to describe a thunderstorm event, people should be thinking that impacts will be much more serious than a routine thunderstorm and has the potential to cause damage and power outages over a broad area," Margusity said.

You will always have meteorologists' interpretations spread across the spectrum of possibilities for weather events. This was the case not only within AccuWeather's team of meteorologists, but also throughout the meteorological community for this event.

AccuWeather.com uses a consensus approach when making forecasts for all sorts of weather ranging from pleasant weather patterns to rapidly changing and life-threatening situations.

The advantage of having a large team of meteorologists at our immediate disposal is there is little chance of missing a major weather event.

Multiple times a day there are insightful discussions within AccuWeather on the realms of possibilities of how a particular weather pattern will evolve and the impacts it will bring.

People receiving the message can then decide for themselves whether or not to act, such as ignoring it, keeping an eye on it or taking precautions and getting out of harm's way.