Synthesizing interdisciplinary modeling for precipitation estimation, hydrologic and geomorphologic response, this research examines the potential change in flash flood occurrence frequency in the Southern California mountains region under a moderate–emissions climate change scenario. The modeling approach employs a geomorphologically-based precipitation threshold indicator for bankfull flow, downscaling of climate model output using a simplified numerical model of topographically-driven precipitation to yield high resolution (hourly and ~10 km2 resolution) precipitation estimates, and detailed hydrologic modeling of soil moisture. Atmospheric state output from the state-of-the-science climate model CCSM drives the downscaling and precipitation estimation modeling component for two periods: the historical control climate (1970-1999) and a moderate-emission-scenario future climate (2070-2099). The resulting precipitation estimates are input to the hydrologic modeling and flash flood occurrence simulation components to estimate flash flood occurrence frequency under the control and future climates. The results of this modeling indicate an increase in flash flood occurrence frequency under the climate change scenario. The causal changes in precipitation and simulated soil moisture are explored, along with implications of uncertainty in the modeling approach.