Derechos: Widespread, Curved Windstorms

February 22, 2014
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Map of the Right Turn Derecho of  July 12-13, 1995. US Government photo/Public Domain. Derechos are windstorms generally associated with thunderstorms that assume a curved, or bowed shape, producing waves of winds with deadly force!

Wind: A blessing and a curse for sailors.

Harnessing the wind is the goal of every sailor. It is a complicated task solely responsible for the thrill of sailboat racing. It requires great skill and a tremendous amount of practice.

Wind and weather was on the minds of many Texas boaters as they left their homes early in the morning of May 17, 1986, prepared for fishing contests or races. They hoped to reach Lakes Livingston, Steinhagen, Sam Rayburn and Houston, Galveston Bay, Matagorda Bay, and Corpus Christi Bay, with plenty of time to get their boats in the water before the start of the scheduled regattas.

Unfortunately, most of the Texas boaters were completely unaware that their precious wind had transformed into a dangerous line of thunderstorms alongside a south-moving cold front in Central Texas. The blessing had become a curse.

The Texas Boaters’ Derecho

The “Texas Boaters’ Derecho ” began around mid-day as a series of storms in curved, or bowed, shapes moved across the state, creating deadly waves for the Texas boaters as well as damaging winds that blasted participants in fishing competitions and other family events taking place throughout eastern and southern Texas.

According to “About Derechos,” part of the NOAA-NWS-NCEP Storm Prediction Center website, hundreds of boats capsized, were damaged or destroyed. Five boaters drowned on Lake Livingston and one on Lake Steinhagen. Over 100 boaters were rescued and dozens were injured. Rescuers were unable to reach boaters stranded on Galveston Bay until the following morning.

An article in The New York Times reported that the thunderstorms spawned numerous tornadoes in Central Texas and as far as Arkansas, but most of the damage was created by the derecho’s winds.

Understanding Derechos and Bow Echoes

According to the NOAA’s “Derecho Facts Page,” the term Derecho, a Spanish word pronounced “deh-REY-cho,” means “direct,” or straight ahead.” Derechos were named by Dr. Gustavus Hinrichs, a physics professor at the University of Iowa who defined the weather phenomenon in an 1888 study published in The American Meteorological Journal.

The Derecho is a wide-spread and long-lived windstorm most often connected to a line of thunderstorms, such as those that formed across Texas on May 17, 1986. Although the Texas Boaters’ Derecho formed in the South, Derechos are also common in the Corn Belt, the section of the United States that runs from the Upper Mississippi Valley to the Ohio Valley. Derechos generally occur in the late spring or early summer.

In a Derecho, the thunderstorms take on an odd shape, referred to as a convection. Visually, they appear as a curved, or bowed shape. The clouds are referred to as “bow echoes,” and there can be one large bow echo, or numerous bow echoes involved in a Derecho event..

Derechos are not tornadoes, and are less common than tornadoes, though the winds in a derecho can reach speeds equivalent to an EF0 or EF1 tornado–between 65 and 95 mph. The speed of the winds logically determine the amount of rain that will fall in any given area. Generally, during a derecho, the rainfall totals are low as the wind moves the rain along too quickly for it to accumulate in one place.

This, then, is the simplest definition of a derecho–sudden, straight forward, persistent, and shockingly fast winds racing across the land. Everything about a derecho involves speed, including safety issues, which is why they are particularly dangerous to those who participate in outdoor activities such as during the Texas Boater’s Derecho. The sky turns dark, the clouds start to race across the sky, and the weather changes so quickly that there is little time to reach safety, particularly for boaters on a lake.

The Four Known Types of Derechos

There are four known types of derechos, including progressive derechos, serial derechos, hybrid derechos and low-dewpoint derechos. Progressive Derechos begin with a short line of thunderstorms–between 40 and 250 miles long–that may have a single bow echo in the beginning, but eventually evolve into a squall line with multiple bow echoes traveling a long, narrow path. They are often found with weak, low pressure systems.

Progressive_derecho_diagram

 

 

 

 

 

 

 

 

 

Progressive Derecho diagram. NOAA/Public Domain.

Serial derechos could be considered the most ferocious of the beasts as multiple bow echoes form in a long squall line that is often hundreds of miles long. The sweep of a Serial Derecho can be shockingly wide. A hybrid derecho is exactly what it sounds like–a combination. They can be both progressive and serial, though not generally a Low Dewpoint Derecho as discussed below.

The low-dewpoint derecho occurs in the opposite conditions of those listed above. There are no wide-spread bands of moist air. Although it resembles a serial derecho in behavior, low-dewpoint derechos are connected to low pressure systems with limited amounts of moisture. These derechos often occur in late fall, or even early spring.

The Quad City Derechos of July 20 and 21, 2008

It was on a Sunday afternoon, July 20, 2008, when the thunderstorms that eventually became the Quad City Derecho developed over Wyoming. Late in the evening, these storms moved into northern Nebraska and by Monday morning, July 21, they had reached Northwest Iowa.

The storms grew stronger, and suddenly, widespread straight-line winds were shooting across Iowa and Illinois. They reached the Quad Cities–five cities straddling the Mississippi River on the border of Iowa and Illinois, including Davenport, Bettendorf, Rock Island, Moline, and East Moline. One wind gust in Moline, Illinois, was clocked at 94 mph, according to the NOAA.

Strong, living trees were torn up from their roots, power lines were knocked to the ground and 130,000 residents in the Quad Cities and surrounding areas lost power. The damage path was between 40 and 60 miles wide, stretching from Omaha to Chicago.

In a July 21 news interview with the Quad-City Times, meteorologist Linda Engebretson compared the storm to a “60 mile-wide train barreling across the state.” Engebretson estimated the wind speeds in the storm ranged from 65 to 95 mph.

When conditions are ripe, Engebretson explained, the storms seem to explode forward at super-high speeds that hold the bow shape of the storm. The front of the bow has the strongest winds. The winds weaken slightly toward the end points of the bow, but they are still strong enough to create serious damage.

The Super Derecho of May 8, 2009

Conditions were ripe for the Quad City Derecho. The storms were as typical as a derecho can be, but this was not the case for the “Super Derecho of May 8, 2009” that raced across the plains of Western Kansas into the Appalachian Mountains of East Kentucky.

According to NOAA, the “Super Derecho” was one of the most “intense and unusual derechos ever observed,” and this storm was anything but typical–the storm included numerous small bow echoes, an exceptionally large bow, and small, spinning, turbulent flows that spawned tornadoes. It also began on a deceptively calm evening.

The storm was actually born on May 7, 2009, starting over the plains of northeast Colorado and southwest Nebraska with a stationary wave, protected by the Rocky Mountains and fed by a disturbance from the west-northwest. The plains slowed the wind in the disturbance and left it a bit dry, so only a few light showers appeared–hardly typical for the beginnings of a derecho.

These small rain showers cooled the leading edge of the storm as it moved east-southeast. In the meantime, Texas and Oklahoma contributed their own low-level moisture pushing north into the system, and as thunderstorms developed they took on convective, or curved shapes–they became bow echoes.

The thunderstorms grew stronger as they moved across Kansas, pummeling homes and businesses with golf ball-sized hail, in wind gusts clocked at 67 mph. These thunderstorms joined with a band of new thunderstorms forming over south central Kansas. At this point, the winds became so severe they rolled a car on the Kansas turnpike. The heavy rainfall also stranded motorists in their vehicles, as they were trapped by high water in Coffeyville, Kansas.

The storm moved into Missouri on May 8, 2009 with its one large bow and numerous smaller bows firmly in place, and 80 to 100 mph winds slammed into trees and power lines, knocking them to the ground. Wind gusts also knocked down the transmission tower for KSN-TV in Joplin, Missouri. Six EF2 tornadoes and one EF3 tornado were later confirmed to have touched down in Missouri.

The leading edge of the bow moved into Arkansas. According to “About Derechos,” the storm began to resemble a tropical storm complete with an “eye.” The winds were relentless, blasting through towns with gusts lasting as long as 45 minutes and flattening 80% of the trees in one forested area.

The storm continued its march forward into Illinois and Kentucky. Over 100 windows were blown out in the residence halls of Southern Illinois University and 63,000 Illinois residents lost power to their homes. As the larger bow echo began to fall apart, smaller storms continued to form over Kentucky, Tennessee, Virginia, and West Virginia.

The Super Derecho of May 8, 2009, traveled over a thousand miles in 24 hours, caused numerous deaths and injuries, and left millions of residents without power, some for as long as a week. The storm is now the focus of workshops and conferences as meteorologists continue their quest to learn more about the fascinating, terrifying, and deadly power of the derecho.

Sources:

  • Brecht, Tory. “Rare ‘Derecho’ leaves swath of damage across Iowa, Q-C region.” Quad City Times. Posted Monday, July 21, 2008. Retrieved September 24, 2011.
  • Corfidi, Stephen F., Johns, Robert H., Evans, Jeffry S. “About Derechos.” NOAA-NWS-NCEP Storm Prediction Center Website. Retrieved September 20, 2011.
  • “July 21, 2008 Derecho.” Quad Cities, IA/IL. National Weather Service Weather Forecast Office.
  • “Storms Cut Through Midwest Killing Five.” The New York Times. Published May 9, 2009. Retrieved November 4, 2011.

“Texas Lake Searched for 18 Missing in Storm.” The New York Times. Published May 19, 1986. Retrieved November 4, 2011.

 

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