What is a derecho?

A freak storm took the East by surprise, leaving a 600-mile path of destruction across seven states. It killed more than a dozen people and left millions without power. Chip Reid reports.

An unusual weather phenomenon in the United States resulted in a derecho warning from meteorologists Wednesday. So what exactly is a derecho?

"It's a cluster of thunderstorms that congeal into a line, and that line will then start moving east or southeast, and as it progresses the winds increase and produce damage along that path," said Jim Keeney, weather program manager at the National Weather Service's office in Kansas City, Mo.

The warning occurred after an unusually enormous line of storms carrying hail, lightning and fierce winds with the potential to effect more than one in five Americans moved from Iowa to Maryland.

"Basically from eastern Iowa to northern Illinois into northern Indiana, we have a high risk of severe weather potential this afternoon and into this evening. The greatest threat from that severe weather looks to be high wind probabilities and possible damage. And what looks to be setting up is a derecho," said Keeney.

Derechos differ to tornadoes in scale. Tornadoes are more isolated events - a single parent storm or a single thunderstorm that hits a certain area. A derecho is a very large weather event that covers a much greater area.

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"A tornado, when it does occur, may be on the magnitude of a mile or two wide; a derecho could go for hundreds of miles producing significant damage so winds in excess of 75 miles or greater and could run for hundreds of miles," said Keeney. "A lot of people can be impacted by derechos; tornadoes are more smaller in scale."

Keeney warned that although the derecho can seem similar to a large thunderstorm with its rain and potential hail, the biggest threats are the wind.

"If they do form winds in excess of 75 miles an hour, we're talking significant tree damage. Some derechos can be in excess of 90 miles an hour, this is getting into more structural damage, roof damage and things of that nature, so it's something people have to be more watchful for in these areas this afternoon and tonight," said Keeney.

Being prepared and taking shelter at the lowest level of a building is the best thing to do during a storm of this nature.

"Be mindful of weather situation, don't be caught off guard; we've had several days of the potential here, so stay tuned to local media ... and when a warning is issued for your area, take shelter wherever you are until the storm has passed," said Keeney.

"It doesn't happen that often so people don't hear about it much, but when they do occur, they're pretty noteworthy because they produce so much damage in such a large area," said Keeney.

  • Jessica Hartogs On Twitter»

    Jessica Hartogs is a news editor for CBSNews.com. You can find her on Twitter: @jessicahartogs

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