The south central Texas heavy rain event of October 1998: an MM5 simulation and diagnosis of convective initiation

Abstract

During the weekend of 17-18 October 1998, extremely heavy rainfall over south central Texas resulted in widespread flash flooding and numerous river floods. Southern Hays County received 760 mm of rainfall, and an area of 18,000 km² recorded over 250 mm. The convection began in a weakly forced environment well ahead of a cold front that was forecast to trigger the storms. The Penn State University/NCAR Mesoscale Model version 5 (MM5) was used to diagnose the extent and magnitude of upward motion, the convective potential of the environment, and the causes of the upward motion that contributed to the convective initiation. A rainfall analysis constructed from all available observations and radar estimates was used for a quantitative comparision with the MM5-simulated rainfall. The MM5's success in simulating many aspects of the rainfall suggested that the atmospheric processes that brought about this heavy rain event were also present in the model simulation. Using a 48-km model grid, upward motion was found to be more than sufficient to cause deep convection in the conditionally unstable atmosphere of south Texas. The cause of the upward motion was attributed to differential warm advection focused by a low-level jet centered over the region where convection began.