Focusing Mechanisms in the Texas Hill Country Flash FIOMS of 1978

Abstract

During the early morning of 2 August 1978, a stationary thunderstorm complex drenched the Balcones Escarpment of Texas and unleashed flash floods in the Hill Country which killed 27 people and produced extensive damage. After the storm, an unofficial 24 h total of at 1east 79 cm of rain was observed near the town of Bluff (about 90 km northwest of San Antonio). Five distinct mechanisms or meteorological features interacted to focus and anchor the storm complex: 1) a east-west oriented mesohigh outflow boundary was situated just south of the storm area; 2) a rapidly moving ribbon of extremely moist boundary-layer air was flowing toward the storm are from the southeast 3) an elevated warm, dry layer of air extended over the area east of the Hill Country and capped the southeasterly low level inflow; 4) a deep vertical motion field associated with a midtropospheric short-wave trough was advancing toward the Hill Country from Mexico; and 5) the diurnal heating cycle with its associated production of thermally forced convective clouds was ending. The juxtaposition of the first tour mechanisms over the storm are in combination with terrain lifting and the termination of convection initiated by boundary layer heating resulted in a forced, stationary storm complex that produced catastrophic flash floods in the Hill Country.