Flash Flooding in San Angelo on August 13, 2011

Rain Relief Across West Central Texas

A shift of the upper pattern was the pre-cursor to the rain event that started on Thursday, August 11, 2011.  A developing northwest flow pattern allowed thunderstorms from the north to propagate into the region.

On Thursday evening, scattered showers and thunderstorms formed along an old outflow boundary which was situated across the northern Big Country.  In addition, a few strong thunderstorms developed across the Northwest Hill Country and Northern Edwards Plateau, beneath the weakening upper-level ridge.

On Friday, the upper level ridge began to build across the Rockies and a trough to the east deepened as it reached the Great Lakes by Friday night. Storms formed along a very slow moving cold front across the southern plains and the associated cold pool (outflow) moved south overnight (Figure 1).

Click on images to enlarge. 

West Central Texas had the necessary ingredients for thunderstorms to produce much needed rainfall across the region. At the surface, a trough of low pressure  was centered along the New Mexico and Texas border near the Permian Basin and this increased the low level moisture as surface winds were south to southeast across West Central Texas. The surface convergence along the cold pool (outflow boundary) was sufficient to initiate storms across the eastern Big Country (Figure 2).

With low and mid-level moisture in abundance, a slow moving line of thunderstorms developed and extended along an area from the Dallas/Fort Worth metroplex west to Sweetwater. Between 4 and 5 a.m. Saturday, rain began in Abilene and then drifted southward. The system reached San Angelo between 8 and 9 a.m., but rain did not begin until almost 10 a.m. at Mathis Field. The system had become stationary at this time, extending along an area from the southern Concho Valley to the Heartland counties of Coleman and Brownwood (Figure 3).  The San Angelo Doppler Radar Three Hour Rainfall Esimate graphics from Saturday are shown in Figures 4 and 5.

The system began to dissipate by early afternoon.  However, with the combination of the boundary still draped across West Central Texas and abundant moisture available (with surface dew points in the mid 60s to low 70s) across the region, a few thunderstorms would develop again on Sunday.

Figure 6: Three Day Storm Total Rainfall Across San Angelo

Concho River floodwaters flowing across Irving St in downtown San Angelo	Concho River floodwaters flowing across Irving St in downtown San Angelo	Vehicles swept off the road by floodwaters near the Loop 306 and Southwest Blvd	Vehicle submerged in floodwaters in San Angelo
Approx. 2 feet of water running through a residential area	Stalled car in a residential area.	Floodwaters several feet deep in San Angelo	The Red Arroyo