After a powerful storm system moved across the Midwest and Mid-Atlantic states last week, many people were introduced to a new vocabulary word: Derecho.
The mechanics of most derechos are familiar to Oklahomans, but the size of the storm is what distinguishes it, said Karen Hatfield, a meteorologist with the National Weather Service office in Tulsa.
"It's a squall line on steroids," Hatfield said. "In order for a squall to be termed a derecho, it has to meet a strict set of criteria."
Squall lines occur frequently during Oklahoma's storm season, Hatfield said. Storms of derecho size are rare but not completely uncommon. Four derechos occur in the tri-state area of Oklahoma, Arkansas and Missouri every three years, according to the National Weather Service Storm Prediction Center.
To be classified as a derecho, a storm must produce a damage swath that extends at least 240 miles and must have wind speeds in excess of 58 mph, in the threshold that would call for a severe thunderstorm warning, Hatfield said.
According to the Storm Prediction Center, the term derecho comes from the Spanish language and means "direct" or "straight ahead." It was first proposed as a term to describe these large, powerful storms in 1888 by a physics professor at the University of Iowa who observed a storm in 1877.
Hatfield said a derecho is similar to a squall line in that it is a cluster of storms that develops into a line of organized storms pushed by winds in the upper atmosphere.
The mechanics of the storm create a cyclical pattern of downbursts from the storm with cool air forcing warm, moist air up and out in the storm. On radar, the line of storms creates what is termed a bow echo.
Derechos are most common during the late spring and summer months after the jet stream makes its annual shift north, which is also to blame for Oklahoma's typically hot and dry summers, Hatfield said.
A memorable derecho from her early career working at the Norman office occurred over Memorial Day weekend in 2001, Hatfield said. At one point, the storm went nearly from one side of the state to the other, with wind gusts in excess of 90 mph recorded in Oklahoma City, Ardmore and Garden City, Kan.
"I was working in the office when it happened," Hatfield said. "I just remember being really busy taking a whole lot of wind reports at the time."
Another more recent derecho occurred on May 7 and 8, 2009. That storm blew across more than 1,000 miles in 24 hours, from the high plains of eastern Colorado to the Appalachians.
The storm caused some damage in far northeast Oklahoma in Ottawa County and brushed the Tulsa area.
"Most of the focus of that was when it got closer to southern Illinois," Hatfield said. "It turned into almost a hurricane over land."
So, why are most people just becoming familiar with this term? Hatfield said it has to do with the location of the recent storm.
"It did go over such a populated area, therefore there was more impacts on infrastructure," Hatfield said. "If something happens over there, that's where the population is and it gets the national media attention."
The widespread storm in the eastern U.S. last weekend knocked out power to more than 2 million people and killed 26.
Jerry Wofford 918-581-8310
jerry.wofford@tulsaworld.com
Original Print Headline: Derechos unusual but have a familiar feel
Weather
During the past two days, an Oklahoma Mesonet site on the northeast Tulsa Community College campus has recorded 0.1 of an inch, as of 9:20 a.m. today.
After a month of no recorded rainfall, the Tulsa area is expected to have showers and thunderstorms this morning.
