The Woodward News

July 28, 2013

Derechos are not uncommon

Rowynn Ricks
Woodward News

Woodward, Okla. — Derecho (dā-rā-chō) is the Spanish word for "straight."

It is also, fittingly, a term used by meteorologists to label a certain type of windstorm that produces widespread and sustained straight-line damaging winds.

One of these windstorms struck the northeast part of the state earlier this week, sweeping through with wind speeds up to 80 mph, which led to more than 100,000 power outages in Tulsa County alone as it snapped power lines and felled trees.

While derecho type windstorms are not frequent, they are also not that uncommon for this time of year or in this part of the country.

"They're fairly common especially in the late summer months across the northern plains," said Ryan Barnes, a meteorologist with the National Weather Service in Norman.  "But they can come as far south as here and can develop almost anywhere.  But generally it's in the plains where they are common.  It has to do with storms systems and the patterns that are set up."

Matt Lehenbauer, director of Woodward County Emergency Management, agreed that derechos are common across the Great Plains during the months of June, July and August.

"We usually see one or 2 sweep through the region every summer," Lehenbauer said.

Barnes explained that a common derecho-producing weather pattern will be a system that moves out of Canada and south across the plains to combine with the moisture "that's usually in place across the plains," resulting in the formation of large storm complexes.

"Basically a derecho is due to very heavy rainfall associated with a large storm complex," he said.  "It's a little more complicated than that, but basically air rushes very quickly to the ground and then spreads out rapidly from the complex."

Just how fast are these rushing winds?

"Usually you'll see with derechos the wind can be anywhere from 70 mph to 90 mph.  They can even exceed 100 mph as well," Barnes said.  "They can be just as dangerous as tornadoes, the small or weaker tornadoes."

Lehenbauer said that sometimes a derecho storm can even spawn some lower level EF-0 or EF-1 tornadoes.

"However, it's often hard to see these types of tornados on radar because of all other wind activity going on and they're hard to prove, unless someone sees them, because it's hard to separate the damage from that caused by the straight-line winds alone," Lehenbauer said.



DIFFERENT THAN OTHER WIND EVENTS

Barnes noted that derechos are "included in a family of downburst clusters," with a downburst referring to a strong downward current of air usually associated with thunderstorms.

Lehenbauer said "it can get kind of confusing sometimes when you're talking about downbursts, microbursts, and derechos.  And I think last year we even had a heat burst in the area, which is not even storm related, but all of a sudden warm air falls out of the atmosphere and you can get winds from that which can cause damage.  They're all wind, but just caused by different things."

Both Barnes and Lehenbauer said that the biggest difference between a derecho and other wind events is the scope of the associated wind.

"It's a little bit different than a normal downburst or a microburst.  Those are high winds as well but they're in isolated pockets that go away pretty quickly," Lehenbauer said.  "With a derecho there are sustained winds.  You may get hit with that gust front, but behind that there's heavy winds that usually last for several hours."

Barnes used distance, rather than time to describe the scope of a derecho.

"A derecho is caused by a big cluster of storms and can produce damaging straight-line winds over hundreds of miles and is sometimes more than 100-miles wide," he said.  "That's the main difference from other wind events, because if you're talking about just one storm, then it may just cause damage over a 5-mile or 10-mile area."



AN UNFAMILIAR TERM

If you find derecho to be an unfamiliar term, that's because it is not often used when releasing weather warnings to the public.

There are many reasons why meteorologists don't use the derecho term when warning the public about these wind events.

One is simply because a storm might not be classified as a derecho until after the storm system has already passed, such as with the recent storm that did damage in Northeast Oklahoma this past week.  

Since a derecho involves strong winds that are sustained for hundreds of miles, the storm must first cross hundreds of miles before it can be deemed a derecho.  So there is a lot of analysis and data collection involved before meteorologists will label a storm as a derecho.

"It's more of a term that's used after the fact or a term used by meteorologists," Lehenbauer said.

He said that's because is "more of a technical term. So normally you would not hear the word used in news broadcasts.  Instead you'll hear terms like bow echo or heavy winds along a front."

Also, since derechos are often associated with thunderstorms, the National Weather Service will usually issue a broader thunderstorm warning and then include strong wind advisories within that warning.

"When we expect real strong straight-line winds, if you'll pay close attention to National Weather Service storm warnings, you'll hear specifics about wind speeds and the precautions you need to take," Barnes said.

Both he and Lehenbauer said the precautions are similar for any other strong wind events.

"With a derecho, winds are expected to be around 70 mph to 80 mph, so you'll want to be in the in the interior of your home away from any windows and not be driving around," Barnes said.



MAY 2001 DERECHO SWEEPS THROUGH NORTHWEST OKLAHOMA

Neither Barnes nor Lehenbauer knew when the last time was that a derecho hit Northwest Oklahoma.

However, information on the National Weather Service's Storm Prediction Center (SPC) website (spc.noaa.gov) lists that a significant derecho impacted the area over Memorial Day Weekend in May 2001.  The following are some details from that storm, according to the SPC website.

Over the course of more than 9 hours overnight between May 27 and May 28, 2001, a group of supercell storms moved through Southwest Kansas, much of Oklahoma and into northeast Texas.  

Oklahoma Mesonet stations registered a wide swath of maximum wind speeds over 50 mph and in some instances over 70 mph, associated with this 2001 derecho.  One station even measured a max wind speed of 93 mph in south central Oklahoma.

Just as with the recent storm in Tulsa, the 2001 storm snapped many power poles and lines, causing over 160,000 residents in the Oklahoma City metro area alone to lose power.  The storm even overturned several mobile homes and blew over semi-trucks as they traveled along Oklahoma highways.

The storm also led to several injuries, including 4 people who were injured when a mobile home overturned at Canton Lake.  One person in Lawton was killed by a falling utility pole.

So the next time the National Weather Service warns about strong winds, whether or not they use the term derecho, you might want to pay attention and take the appropriate safety precautions.

Since some derechos can pack winds that are close to or at tornado speeds, Lehenbauer said that sometimes just getting indoors isn't enough.  Like with tornadoes, he said when there are heavy winds expected over 70 mph, it is best to be inside sturdier structures and not take shelter in travel trailers or single-wide mobile homes.

"If we detect winds at 70 mph or more, we start to worry because that 70 mph point is when we start seeing overturned vehicles and travel trailers.  Those winds can even blow over single-wides that are unsecured," he said.