A round of severe thunderstorms packed a serious wallop as it moved through the Dallas-Fort Worth metro area on Saturday afternoon. Numerous reports of severe hail were received across the region, along with straight line winds. Funnel clouds and possible tornadoes occurred to the northeast of Dallas. The most notable damage occurred just after 3:00 PM local time on Saturday afternoon when a microburst moved over the city of Irving, home of Texas Stadium and the Dallas Cowboys football team. The severe straight line winds of the microburst collapsed the roof at the team's practice facility in Irving, resulting in several serious injuries as well as significant damage to the building itself.
A microburst is a phenomenon associated with severe thunderstorms that was elucidated by Ted Fujita (he of the famous Fujita Scale for tornadoes). A microburst is a very small-scale version of a downburst, which occurs when a mass of rain-cooled air held aloft in a mature thunderstorm suddenly comes crashing down to Earth. When the rapidly descending air reaches the ground, it spreads out radially in all directions from the point of contact, often with great ferocity. In extreme cases, the straight line winds from a microburst have reached 150 mph.
The Dallas-Fort Worth area is no stranger to microbursts. On August 2, 1985, Delta Air Lines Flight 191 was attempting to land at the DFW Airport when it encountered a microburst. The extreme vertical motions and wind shear caused the plane to crash before reaching the runway, killing a total of 135 people on the plane and on the ground.
At left is a radar image from the Doppler radar serving the Dallas-Fort Worth area, taken as the storms were exiting Fort Worth (on the left side of the image) and heading toward the Dallas side of the metroplex at 3:01 PM local (click to open a larger version in a new window). This image shows what is a fairly typical configuration for severe thunderstorms, with the most intense parts of the storms lined up from Fort Worth off to the northeast. The area of interest, which would go on to produce the microburst, is the segment of the line just east of Fort Worth, straddling I-20 and I-30 (the two roads shown on this map running east-west between Dallas and Fort Worth).
The series of three images at left shows the Doppler velocities at the time of the microburst. The first image was captured at 2:53 PM, the middle image at 3:01 PM, and the third image at 3:14 PM when the strongest winds were affecting Irving. For orientation, Fort Worth is on the left, Dallas is on the upper right. Irving is approximately located by the small red dot (which is most easily located from the third image). In these images, the radar is located in the lower left, marked by the blue dot labeled "KFWS." These maps of Doppler velocities show us how the winds are blowing in relation to the radar. The green areas have winds blowing toward the radar, and brighter greens indicate higher inbound velocities. Red areas are where the winds are blowing away from the radar. Stronger outbound velocities are indicated by brighter reds and pinks.
Note the area of brighter reds that first appears to the east of the radar site in the leftmost image. These are the strong winds generated by the severe thunderstorms. The outbound velocities in the first image show where the Doppler radar was able to measure winds of 50 mph to nearly 65 mph. By the time of the middle image, taken just eight minutes later, the area of strong winds has rocketed quickly northeast and is affecting the cities of Arlington and Euless, approaching the DFW Airport. In the third image, the thunderstorm winds slam into Irving. The Doppler radar at this time was able to measure winds of approximately 65 mph.
One of the limitations of Doppler radar is that it only senses winds in the radial sense; that is, it can only measure that part of the wind that is oriented directly toward or directly away from the radar site. If the wind we want to measure is blowing perpendicular to the radar's beam, the Doppler cannot sense it. This is likely why the reported wind speeds on the ground (estimated in the 70 mph range) were greater than what the Doppler was sensing - because there was a component of the wind perpendicular to the radar beam that the Doppler could not detect.
In any event, the microburst in Irving will be another unfortunate chapter in the phenomenon's history in the region. Although there were no deaths reported from Saturday's microburst, the impact on the region's beloved Cowboys was severe. Several injuries were reported among the players and staff who were at the facility at the time of the microburst. The same cluster of severe thunderstorms would go on to leave a swath of wind damage across northeastern Texas after departing from the Dallas-Fort Worth area. Emergency management in Anderson County (in the Palestine vicinity) reported widespread significant tree damage all around the county, with "more trees down than with Hurricane Ike."
Much more information on this weather event is available through the mainstream news media. For another meteorological analysis, the CIMSS Satellite Blog has posted an excellent summary from a satellite's perspective.
