Models presently used in NWS operational flash flood forecasting are spatially lumped and were originally developed to represent hydrologic processes in humid watersheds, with time steps too coarse to provide adequate simulation of semiarid hydrographs that can peak in 15 min or less. These models include the Sacramento Soil Moisture Accounting model [Burnash, 1995] and the Continuous Antecedent Precipitation Index model [Anderson, 1993]