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The destructive power of derechos

Alexandra Pope, staff writer

July 24, 2011 — Most people are familiar with the terms 'tornado' and 'straight-line winds,' but meteorologists occasionally talk about another type of destructive wind event: the derecho.

Derechos occur along squall lines of thunderstorms called bow echoes
Derechos occur along squall lines of thunderstorms called bow echoes

Derechos are widespread, long-lived straight-line winds that can reach hurricane strength, explains Mark Robinson, a Weather Network meteorologist and storm chaser.

“A derecho is basically a very violent form of straight-line wind,” he says.

The word derecho is Spanish for “straight ahead.”

Derechos are characterized by sustained winds of more than 93 kilometres per hour. They occur along squall lines of thunderstorms -- also called bow echoes because of their distinctive archer's bow shape on radar -- more than 380 kilometres in length.

Derechos can occur along a single bow echo or multiple bow echoes.

“I've been in some serious derechos and it's actually shocking how strong the winds can get,” Robinson says. “There are branches flying around and dust that's being kicked up into these huge plumes. The car gets bounced around all over the place.”

Damage from a derecho that hit Detroit in 1998 (Courtesy National Weather Service)
Damage from a derecho that hit Detroit in 1998 (Courtesy National Weather Service)

Their power and longevity means derechos can leave massive trails of destruction.

They are most common in the United States during the summer and fall, but have occasionally occurred in Canada.

One of the longest and most devastating derechos in history affected three states and two provinces.

The so-called “Boundary Waters/Canadian Derecho” originated in North Dakota on July 4th, 1999 and travelled more than 2,000 kilometres before finally fizzling out 22 hours later.

In North Dakota, wind gusts of more than 90 kilometres per hour overturned airplanes and ripped roofs off of houses, injuring several people.

The derecho raced into northeastern Minnesota, where hundreds of people were enjoying Independence Day camping in Superior National Forest. Winds clocking in at up to 160 kilometres per hour -- equivalent to a category 2 hurricane -- knocked down tens of millions of trees.

Boaters who were on the water in the Thunder Bay area that day struggled to keep control of their craft as the front rushed over Lake Superior, whipping up large waves. One sailboat was overturned, tossing two people into the cold water. Fortunately, they were rescued by a nearby boater.

Large swaths of forest around Thunder Bay and Timmins were completely flattened by the derecho winds.

The derecho would go on to cause major property damage in Quebec and cross the border a second time before finally dissipating over New England.

All told, the storm killed two people, injured nearly 70 and caused US$100 million in damage.

With files from the National Oceanic and Atmospheric Administration

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