A gust-front shelf cloud (or "arcus") looms on the leading edge of a derecho-producing convective system, as seen in Hampshire, Ill., on the evening of July 10, 2008.
They're definitely not tornadoes, but the straight-line windstorms known as derechos can be just as damaging, due to gusts that can reach hurricane force. And they could make their appearance during the bout of severe weather sweeping over the Midwest on Wednesday.
It's been almost a year since a derecho (pronounced "deh-RAY-cho") was last in the headlines: That's when a powerful storm system blasted from Indiana to Maryland — killing more than a dozen people, leaving millions in the dark and shutting down Netflix as well as other online services that relied on Amazon's Cloud servers.
Last June's "Historic Derecho" sparked an assessment by the National Weather Service, focusing on whether more could have been done to anticipate the damage. This time around, forecasters are spreading the word well in advance — although they're using such terms as "localized downdraft/damaging wind threats" instead of the D-word.
Here's what you should know about derechos:
How do derechos differ from tornadoes?
"Derecho" is a Spanish word, meaning "right" or "straight." That's not the kind of word you'd use to describe a tornado, which whirls into action from a spinning storm system. Derechos arise when huge downbursts of cold air hit the ground, spawning winds that spread out in straight lines from the point of impact.
"Imagine taking a water balloon and dropping it, where you see the balloon break and splatter on the ground. That's basically how a downburst works. And you can think of a derecho as a large cluster of those downbursts all happening simultaneously," said Ken Pryor, a research meteorologist with the National Oceanic and Atmospheric Administration's National Environmental Satellite, Data and Information Service.
The term "derecho" was coined in 1888 to describe a severe straight-wind storm, in contrast to a tornado (which plays off the Spanish word for a "turning" storm). To be classified as a derecho, the swath of wind damage should extend more than 240 miles (400 kilometers), and the winds should meet the National Weather Service's criterion for severe wind gusts (greater than 57 mph, or 92 kilometers per hour). Derecho winds can range well beyond 100 mph.
What causes a derecho?
Derechos are associated with bands of showers or thunderstorms that assume a curved or bowed shape. The classic atmospheric conditions call for very warm temperatures and a lot of moisture near the ground, contrasted with much colder and drier air higher up. "It's the interaction of the heavy precipitation within a thunderstorm complex with that very dry air aloft that causes very large downdraft energy," Pryor said. "With a large thunderstorm system, the interaction of that dry air with that precipitation will result in numerous downdrafts."
Those pockets of the colder, denser air sink rapidly and hit the ground like a bomb, sparking outward bursts of wind. Within the individual downbursts, there may be intense microbursts that can pose extreme hazards for airplanes.
This map shows the number of derechos recorded from May through August over the 1980-2001 time period.
How often do derechos happen, and where?
The prime season for derechos runs from May to August. "They're typically favored over the Southern Plains and the Lower Mississippi Valley early in the season, and the activity moves north later in the summer," Pryor said. Early-season hot spots are in the Tornado Alley states of Oklahoma, Kansas, Missouri and Arkansas. Later in the season, the action shifts to southern Minnesota, the Great Lakes and the Ohio Valley region. The states in the "bull's-eye" regions might get more than one significant derecho in the course of a year, Pryor said.
Roughly every four years, a derecho breaks out of the Midwest, crosses the Appalachian Mountains and heads for the Atlantic without dissipating, Pryor said. That's what happened last year.
How much warning time can we get?
The National Weather Service issued Wednesday's alerts several hours before the expected onset of strong windstorms. That compares with an average lead time of 13 minutes for tornado warnings. "The lead time with a derecho should be much longer ... because these storms are so much larger and have a much longer lifetime," Pryor said. "There's no reason why you couldn't warn a particular area two to several hours in advance, unless it's the area where the storm is developing." Then the lead time might be an hour or less, he said.
The alerts are generally issued in the form of severe weather watches or warnings. One of the things to watch for is the possibility of "widespread damaging winds."
Last June's derecho was a special case, in that it didn't follow the predicted path. Most forecasters expected the derecho to break up when it hit the Appalachians. They were caught off guard when it didn't, but nevertheless, they "generally did an excellent job issuing warnings," according to the weather service's post-storm assessment. Overall lead times were greater than 30 minutes.
What should be done if there's a derecho threat?
The response should be pretty much what you'd do about approaching tornadoes or other types of severe storms. If there's enough time, "secure loose items outside, bring in furniture and other equipment that could become a missile hazard," Pryor said. Seek shelter in a sturdy structure, and stay away from windows.
"With tornadoes, what you see more in terms of structural damage are homes and other types of structures that are twisted and blown off their foundations. [With derechos] there can be roof damage, window damage, but for the most part the home remains on its foundation," Pryor said. "Straight-line winds have more of an impact on vegetation. They've been known to take down large areas of deciduous trees — that's known as a blowdown."
Keep an eye on the National Weather Service's Storm Prediction Center website as well as The Weather Channel and NBC News' weather coverage for updates as the Midwest storms progress.
Desmond Boylan / Reuters
Get a look at a sun dog, a haboob, mammatus clouds, dust devils, a derecho and other weird atmospheric phenomena.
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Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.