Derechos: Rare and Serious

On August 10 a severe storm rolled across Iowa, Wisconsin, Illinois and Indiana that generated winds up to 130 miles per hour. Winds of 130 mph are found in EF2 tornadoes and category 3 hurricanes. Several people were injured, two died, and more than 1 million homes and businesses were without power in different states. Neither tornado nor hurricane, the storm that caused this is a derecho. It travelled more than 700 miles in fourteen hours, downing fifty-foot trees in some areas. Northern Indiana and Illinois are known as a corridor of enhanced derecho activity.

So what exactly is a derecho? It is a large group of storms producing winds of 57.5 miles an hour or greater along most of its length, extending over a path at least 250 miles long, with separated gusts up to 75 mph or greater, and with no more than three hours separating individual severe wind reports. It’s a wall of storms basically, or sometimes called an inland hurricane, rapidly moving across the country in a straight line. Unlike a tornado the derechos move in a straight line causing straight line wind damage. Downbursts often occur along the path of the derecho causing variations in wind speeds along the length of the storm; a derecho can be considered to be made up of families of downburst clusters that extend continuously for 250 miles or more. It is important to remember that these strong winds cover a much broader area than a tornado, leading to significant damage over a large area.