What is a Derecho?

Most summer thunderstorms pop-up, produce downpours, gusty winds and maybe a tornado and then gradually diminish. However, a rarer weather event, a line of long-lived gusty thunderstorms called a derecho, can rampage for hundreds of miles, producing tremendous damage in its wake.

Derechos are relatively common across the Plains, but are a fairly rare event in the East. The name "derecho" comes from the Spanish word for "direct," indicative of the storm`s ability to produce very powerful straight line winds. This is comparable to the word "tornado," which comes from the Spanish word "tornar," or "to turn."

The long length of a derecho is one of its hallmarks. By definition, a derecho is a line of thunderstorms that contain winds of 58 mph or higher, lasting for 240 miles or longer. A derecho typically forms as a cluster of thunderstorms congeals into a band of activity. Rain-cooled air around the thunderstorm reaches the ground and spreads horizontally outward from the storm. This forces warmer air to spread above the ground ahead of the storm, feeding the storm`s development even further.

Over time, this development causes strong thunderstorms to build away from the initial band of storms, expanding its profile along the ground. As this growing activity advances, the process continues, with warm and humid air feeding storm development and allowing the thunderstorms to expand outward to the north and south of the initial line of thunderstorms.

Derechos have been compared to hurricanes, in the sense that they are self-supporting. In the case of a hurricane, the warm ocean water feeds and maintains the storm`s development. With derechos, the constant overturning of cold air at the surface and warm air aloft feeds the storm and keeps it going for hours at a time.

As it moves along, the derecho accelerates, reaching forward speeds of 50 to 75 mph. By itself, the thunderstorms are capable of producing 60 to 70 mph winds, and coupled with the forward motion, wind gusts can reach 70 to 90 mph, with occasional gusts in excess of 100 mph.

Typically, the strong, hurricane-force winds will knock over everything in their path, snapping trees like twigs, and tearing apart power lines and power poles. Not surprisingly, massive and prolonged power outages can be expected along a derecho`s path.

A historical derecho occurred in June 2012. This one traveled from Indiana to the Mid-Atlantic, hitting the highly populated I-95 corridor from North Carolina to Philadelphia. This one knocked out power to millions of people, some people lost power for weeks. It caused nearly $3 billion in damage and 22 deaths were blamed on this event.

Image: A cross-section diagram shows the science involved with a mature derecho (Dennis Cain and Stephen Corfidi, Storm Prediction Center and National Weather Service)