Atmospheric Conditions that Led to the Roma, Texas Floods of August 2008

During the week of 17-24 August, 2008, torrential rains fell on multiple occasions in a small area along the Rio Grande River in Deep South Texas, including southern Zapata and southwestern Starr Counties. Bias-corrected Doppler radar estimates showed more than 15 inches of rain fell over the City of Roma. Similar rainfall was estimated for the nearby communities of Garceño and Escobares, immediately to the east. Significant flash flooding affected nearly 1,000 homes, required more than 200 evacuations, and caused more than $1 million in property damage. For some areas, this was considered a 100-year flood event.

Through the period, a series of upper level disturbances moved across northern Mexico and the southwest United States. As each disturbance approached South Texas, a low level jet developed. Each jet likely transported very moist air across the higher terrain of the Rio Grande Plains toward the eastern slopes of the Sierra Madre Oriental mountains. The entire border region from Del Rio to McAllen, Texas, received heavy rainfall. In particular, extreme southern Zapata and southwestern Starr County, along the Rio Grande, experienced torrential rain-producing thunderstorms on five days.

The combination of synoptic and mesoscale features which produced this memorable flood will be discussed. Results from the analysis will help forecasters better predict the potential for localized significant flooding prior to future events in this portion of Deep South Texas.