The Effect of the Balcones Escarpment on Forecasting Major South Central Texas Rainfall Events

The location and topography of South Central Texas make the area susceptible to flooding. Many Texas cities are located along the Balcones Escarpment, a fault that separates the hill country from the coastal plains of Texas. Moist air often flows in from the Gulf of Mexico and ascends over the escarpment, leading to deep convection and heavy rainfall. In this study, we seek to determine how important the Balcones Escarpment is in determining the intensity and location of rainfall during heavy rain and flood events in South Central Texas. Two flash flood cases with similar large-scale meteorological patterns involving the low level jet (LLJ) interacting with a midlevel vortex were tested. Convection-permitting forecasts were run using the Weather Research and Forecasting (WRF) model for the events taking place on 8-9 June 2010 and 25 May 2013 and were used as control runs. Then, the Balcones Escarpment was eliminated within the model by moving the terrain gradient to the northwest. Both cases were simulated with the new topography and compared to the original forecasts. The elimination of the Balcones Escarpment surprisingly caused little change to the rainfall distribution and amount. The differences that were found between the two sets of simulations were primarily associated with minor differences in the intensity and location of the low level jet, and associated differences in the location of the convectively generated outflow. Nonetheless, these results show that the meteorological factors play the larger role in determining the location and intensity of precipitation in major South Central Texas rainfall events.