High Wind Derecho Event of May 2, 2007

Above: Regional radar mosaic at 7:12 pm on May 2

A high wind event, or derecho, struck north Texas during the evening hours of May 2, 2007. Damaging winds marched across much of north Texas, with several reports of 80 mph winds from the Metroplex eastward. In the metro area alone, over 300,000 people lost power as powerlines succumbed to the fierce winds.  90-100 mph winds were estimated as the storms moved through Kaufman County. Across north Texas, numerous trees were downed, roofs and metal buildings sustained damages, and several trailers were rolled.

 What is a derecho?

A derecho is a widespread and long-lived windstorm that is associated with a fast-moving band of showers and thunderstorms. The name is a Spanish word which means "direct" or "straight".

For more information on derechos, visit http://www.spc.noaa.gov/misc/AbtDerechos/derechofacts

Atmospheric Conditions...

A moist and unstable atmosphere had been in place across the region for several days. By the afternoon hours, temperatures warmed into the mid to upper 70s, with dewpoints ranging from 67-69F. A potent upper level storm system lifted out of New Mexico and into west Texas, providing large scale lift across the region.

Ahead of the upper storm system, two areas of enhanced lift helped trigger clusters of strong to severe thunderstorms during the afternoon.

Below: NAM80 00hr Depiction of 500mb Heights at 7 pm May3. An upper level storm system, marked by the "L", was centered over eastern New Mexico and the Texas Panhandle.

Severe Weather Marches Across Texas...

Two complexes of severe thunderstorms developed across western and south central Texas. These complexes, also known as Mesoscale Convective Systems, were likely driven, in part, by bands of stronger winds aloft in the mid levels of the atmosphere.

 A small line of thunderstorms, oriented west to east, developed out ahead of the thunderstorm complex approaching Dallas/Fort Worth. These small thunderstorms quickly became mini-supercells, or rotating storms capable of producing tornadoes. Several reports of funnel clouds, as well as a few reports of brief, weak tornadoes were reported in Johnson County, near Cleburne, as well as northern portions of the county, south of Burleson. However, no damage was reported with these tornadoes.

Right: KFWS 0.5 degree radar reflectivity from 5:27 pm on May 2.

As the evening progressed, the approaching severe thunderstorm complexes took on a bow echo shape in radar reflectivity. Bow echoes occur when strong winds aloft are driven downward toward the surface and push precipitation ahead of the main line of thunderstorms. In the center of the bow, or apex, the strongest winds typically reach the surface. 

Above: KFWS 0.5 degree radar reflectivity from 7:57 pm on May 2. 

Note the observation in Collin County, at the McKinney airport reading a wind gust of 62 kts (71 mph) on the 8 pm observation.

Above: KFWS 0.5 degree radar base velocity  from 6:59 pm on May 2. Note the bright red shading (outbound velocities) in west central Dallas County...to Desoto...to near Ferris in Ellis County. The brightest red just southwest of Dallas indicates velocities of 90 kts  (104 mph) above the surface. However, a broad area of 75-80 kts (86-92 mph) extends from southwest Dallas County into Northeast Ellis County.

Storm Reports...

Above: Map of storm reports from May 2 courtesy of the Storm Prediction Center. Notice all the high wind reports (annotated by black squares) were located in north Texas. The high wind reports are those with speeds in excess of 65kts (75 mph). Blue circles indicate wind reports greater than 58 mph and less than 75 mph. The bulk of these reports are located across Texas.

 Here is a list of wind reports from north Texas:

Comanche County:
423 PM     Estimated 60 mph    8 miles west of Comanche
436 PM    Estimated 60 mph    Comanche

Bosque County:
523 PM    Estimated 80 mph    Meridian

Hood County:
548 PM    Measured 58 mph    Granbury

Johnson County:
548 PM    Estimated 80+ mph    3 miles south of Godley
605 PM    Estimated 80 mph    Cleburne
625 PM    Measured 60 mph    Cleburne

Tarrant County:
620 PM    Estimated 75 mph    2 miles south of Benbrook
645 PM    Estimated 60 mph    3 miles south of Keller
651 PM    Measured 60 mph    Arlington Airport

Dallas County:
654 PM    Measured 66 mph    Lockheed Martin (Grand Prairie)
714 PM    Measured 61 mph    Dallas Executive Airport

Collin County:
730 PM    Estimated 75, Gust to 80 mph    Plano
731 PM    Measured 68 mph     Plano
803 PM    Measured 71 mph     McKinney Airport

Kaufman County:
730 PM    Estimated 90-100 mph    Terrell

Henderson County:
740 PM    Estimated 80-100 mph     1 mile southwest of Payne Springs

Hopkins County:
846 PM    Measured 59 mph       Sulphur Springs

Milam County:
900 PM    Estimated 70+ mph     Rockdale