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August 17, 2020

Derecho: What is it and does it happen in Virginia?

People who lived in or were visiting Virginia on June 29th, 2012 know what it feels like to be caught in a derecho. So what exactly is a derecho?

Image of 2012 Derecho Radar Impressions on Weather Map

A derecho, pronounced deh-RAY-cho, comes from the Spanish word for “straight” but can also mean “direct” or “straight ahead.” A derecho is a large cluster of thunderstorms with high winds that can be tens of miles wide and hundreds of miles long. They occur in many states east of the Rockies, but are most common in parts of the South, Midwest, and Plains states.

Put another way, a derecho is a long-lived, widespread damaging wind event created by a line of severe storms. The Storm Prediction Center defines a derecho as a swath of thunderstorm-related wind damage that is at least 250 miles long, in sequence, and contains three instances of 75 mph or greater winds. The term was coined by a scientist, Gustavus Hinrichs, in 1899 to differentiate straight-line windstorms from tornadoes. The term wasn’t widely adopted until the 1980s when scientists began classifying storms for the research community.

Virginia experiences a derecho, on average, once every four years and it usually occurs in June or July. All derechos begin somewhere in the Midwest and move east. Mountains help to disrupt them and stop them from proceeding to Central Virgina. Consequently, the western part of Virginia is more likely to experience a derecho than the eastern side.

A derecho is not easy to forecast, but with today’s technology, they won’t sneak up on us. Storms coming from the west or northwest that damage the Roanoke or Shenandoah Valleys can make it over the mountains and hit central Virginia with a few hours to prepare, unlike a tornado which can appear without warning.

The Derecho of 2012

According to a report in the Richmond Times-Dispatch, a line of thunderstorms that had formed near Chicago in the afternoon of June 29th raced across Ohio and the Appalachian Mountains, bringing damaging winds to almost all of Virginia.

Winds gusted to 81 mph in Roanoke and over 60 mph across the rest of the state. Falling trees and limbs left over one million Virginians without power, more than any other state affected by the storm. High heat and humidity combined with a ridge of high pressure and northwest winds above that created the perfect test tube conditions for this derecho. Forecasters and computer models predicted that the storm line would weaken once it hit the mountains, but conditions were simply too good for it to quit.

Five Virginians were killed during the storm, and several more succumbed to the heat that followed in the storm’s wake as power crews worked to restore electricity to homes.

The differences between a derecho and a tornado

A tornado can devastate a small community or farm, but it fairly isolated in its scope. A derecho, on the other hand, can cause damage similar to an EF-0 or EF-1 tornado over a wide area. Here are some other differences:

Chart with charachteristic comparisons of derecho versus a tornado
Sources:  Richmond Times-DispatchThe Weather Channel