8:37am

Mon July 2, 2012
The Two-Way

Word Of The Day: 'Derecho'

Originally published on Mon July 2, 2012 5:35 pm

We learned a new word on Saturday, thanks to Korva's post about the devastating storm that has left millions without power from Ohio east through the mid-Atlantic states:

Derecho.

Now, we'd never heard that word before so we went in search of more about it. According to the National Oceanic and Atmospheric Administration's Derecho Facts Page:

-- "A derecho (pronounced similar to "deh-REY-cho" in English ... ) is a widespread, long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms. Although a derecho can produce destruction similar to that of tornadoes, the damage typically is directed in one direction along a relatively straight swath. As a result, the term 'straight-line wind damage' sometimes is used to describe derecho damage. By definition, if the wind damage swath extends more than 240 miles ... and includes wind gusts of at least 58 mph ... or greater along most of its length, then the event may be classified as a derecho. "

-- "The word 'derecho' was coined by Dr. Gustavus Hinrichs, a physics professor at the University of Iowa, in a paper published in the American Meteorological Journal in 1888."

-- "Derechos are associated with bands of showers or thunderstorms (collectively referred to as 'convection') that assume a curved or bowed shape. The bow-shaped storms are called bow echoes. ... Derecho winds are the product of what meteorologists call 'downbursts.' A downburst is a concentrated area of strong wind produced by a convective downdraft. Downbursts have horizontal dimensions of about 4 to 6 miles (8 to 10 kilometers), and may last for several minutes."

-- "Derechos in the United States are most common in the late spring and summer (May through August), with more than 75% occurring between April and August. ... [They] most commonly occur along two axes. One extends along the 'Corn Belt' from the upper Mississippi Valley southeast into the Ohio Valley, and the other from the southern Plains northeast into the mid Mississippi Valley."

As for the term itself, according to a paper written by retired National Weather Service forecaster Robert Johns, the University of Iowa's Hinrichs "decided to use the term derecho (Spanish for 'direct or straight ahead') to define these non-tornadic events since this term could be considered as an analog to the term tornado which is also of Spanish origin."

Update at 2:45 p.m. ET. Like Having "30-plus Weak Tornadoes":

Jeff Masters, director of meteorology for Wunderground.com, a weather website, just told All Things Considered host Melissa Block that during the derecho, "we had over 30 thunderstorms with wind gusts of 80 miles per hour. ... It's kind of the equivalent to having 30-plus weak tornadoes on the ground."

As for how a derecho forms, he said that if enough thunderstorms develop "in the same location ... they have the opportunity to work together and form a complex or a greater whole."

More from his conversation with Melissa is due on All Things Considered later today. Click here to find an NPR station that broadcasts or streams the show. Later, we'll add the as-broadcast version of the interview to this post.

Copyright 2013 NPR. To see more, visit http://www.npr.org/.

Transcript

MELISSA BLOCK, HOST:

Familiar as we are with tornadoes, we confess we'd never heard the name of this kind of storm system before. The derecho, from the Spanish word meaning straight ahead. So tornadoes are turning winds while derechos are moving in one direction fast.

Jeff Masters is a meteorologist with the weather website, Weather Underground, and he's going to tell us more about them.

Jeff, welcome to the program.

JEFF MASTERS: Thank you.

BLOCK: And, first, what causes a derecho?

MASTERS: Thunderstorms can pop up randomly during the summer and, if you get enough of these popping up in the same location, they have the opportunity to kind of work together and form a complex - kind of a greater whole among a bunch of separate thunderstorms. And, when they get organized like this, they form one of these - they're called mesoscale convective systems or, sometimes, they're called derechos if they happen to move in a straight line and stay together as one mass.

BLOCK: I've seen this described as having a bow echo shape and, on radar, it looks sort of like concentric semicircles, almost like a wave moving in one direction.

MASTERS: Yes. A lot of times, a derecho will have a squall line that has a bow shape to it and right at the point of that bow is where you don't want to be. That's where the most violent winds are.

BLOCK: This storm over the weekend caused damage across a huge swath of the country, from Indiana across Ohio, West Virginia into Maryland, Virginia, Washington, D.C. here and up to New Jersey. Is it typical for the path of a derecho to be that long?

MASTERS: The definition of a derecho is it does have to hold together for at least 240 miles, so they're very long-traveling types of thunderstorm systems and they're also defined as being able to generate winds in excess of 58 miles per hour, which is the threshold for a severe thunderstorm.

BLOCK: And I think, at least around here, the winds were much, much stronger than that, even.

MASTERS: We had over 30 of these thunderstorms that had wind gusts in excess of 80 miles per hour and that's equivalent to a weak tornado. This is kind of equivalent to having 30-plus tornadoes of the weak variety on the ground, so no wonder they caused so much damage.

BLOCK: Yeah. It was also moving at what seemed to be an extremely fast speed, at 60 miles per hour traveling across the country. Is that unusual?

MASTERS: That's pretty typical for a derecho. They do move quickly and they cover a lot of territory.

BLOCK: Are there certain parts of the country where these storms are likely to happen?

MASTERS: They're most likely in the Midwest and, sometimes, in the northern tier states from, say, Minnesota across into southern Canada, as well. They tend to form right along the northern edge of a intense heat wave, which exactly is what we had on Friday.

BLOCK: As you were watching this storm develop, Jeff, were you predicting that it would be as bad and as catastrophic as it has been?

MASTERS: No, because it's very unusual to have a derecho of this intensity. I don't think I've ever seen that sort of a death toll in a derecho event. That's more like what you'd get from a tornado outbreak. And the last time we had anything close to this was back in 2009, when we had a derecho that killed six people from Kansas to Kentucky. So, very unusual, very hard to predict and forecast these sorts of intense rare events.

BLOCK: Well, Jeff Masters, thanks very much for talking with us.

MASTERS: You're welcome, Melissa.

BLOCK: That's Jeff Masters, a meteorologist with the website, Weather Underground, telling us about derechos. Well, we were also curious where that name, derecho, comes from and it turns out that Dr. Gustavus Hinrichs penned the term in the 19th century. Hinrichs was a physics professor at the University of Iowa. The state hired him to study local severe weather and, in 1888, he published a paper in the American Meteorological Journal. He defined a new term to describe these severe windstorms.

Since tornado was derived from Spanish, Professor Hinrichs chose another Spanish word to describe these storms and he landed on the word, derecho, meaning straight ahead. It took nearly a century for the term to be used in the modern meteorological lexicon. Transcript provided by NPR, Copyright NPR.

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