Understanding Derecho Storms
The Frightening Power of Strong, Long-Lasting Straight Line Winds
What is a derecho? What are some examples of this fearsome storm and what damages can it cause?
Derechos Explained
Derechos are rare but extremely dangerous thunderstorms. A derecho can appear similar to a regular line of thunderstorms on a radar map. A bow echo of intense thunderstorms is generally what appears on the radar screen. Bow echos are lines of thunderstorms that make a convex shape, bulging outward in the direction that the storm is moving. Many thunderstorms can transform into a bow echo shape, an indication that the winds in this portion of a line of thunderstorms are severe. But a bow echo is not a derecho unless its so strong that it becomes a long-lived squall line, rampaging across states and country borders for hours before dissipating.
Derecho, which comes from a Spanish word for "straight," is a dangerous straight-line wind that can cause as much destruction as swirling tornado.
Derchos can persist for 24 hours, incur winds of more than 100 miles per hour, race forward at speeds of up to 70 miles per hour, and cause destruction that extends over an area of 250 miles at a minimum and can continue for thousands of miles. A derecho is truly a monstrous thunderstorm unleashed.
When and Where Do Derechos Occur?
The most common part of the world for derechos to form is over North America. Both the United States and Canada are targets of these monster wind storms. They often first form over the Midwest and charge eastward before weakening over the cooler waters of the Atlantic Ocean.
July is the most dangerous month for a derecho. Warm, muggy afternoons and evenings can give rise to these violent thunderstorms. High temperatures and high dew points can turn what was otherwise a regular thunderstorm into a huge raging wall of wind.
Examples of Derechos and Damages Incurred
On the Fourth of July in 1999, 2,500 vacationers in the Boundary Waters Canoe Area in the arrowhead of Minnesota were enjoying their holiday. In the early afternoon on this hot muggy day, when campers were out in their boats, hiking, and enjoying the outdoors, a derecho raging at more than 130 miles per hour, equal to a category 4 hurricane, swept across the area. People were dumped into the lake, hit by falling trees, and thrown through the air. Fortunately, no one was killed. Four hundred thousand acres of forest were damaged and the storm continued through the day and overnight, moving across Ontario, Quebec, and Maine before finally submitting to the Atlantic. This event has now become known as the Boundary Water Blowdown.
While these severe storms can damage a lot of woodland, they are just as destructive when smashing into large cities. On July 22, 2003, a derecho ravaged parts of Arkansas, Mississippi ,and Tennessee, including the city of Memphis. This time the straight-line winds occurred early in the morning, before 7 am, as they ravished the area with winds topping 100 miles per hour. Houses were destroyed, windows were broken, trees were knocked down, and residents were without power for two weeks. A few fatalities resulted from this storm, nicknamed Hurricane Elvis, due to its category-2 winds.
