The paper explores the use of a numerical storm model as a pre-processor for a detailed urban runoff model. The proposed storm model generates hydrographs for each subcatchment, thus simulating the spatial and temporal growth and decay of a system of storm cells as they track across the urban catchment system. Traditionally, design storms are developed from statistical analysis of rainfall records; the methodology was originally for flood predictions based on the rational formula. This synthetic temporal distribution is typically applied uniformly across the catchment. The resultant rain distributions and runoff hydrographs are unlike observed rain storms and flows. Intensity-duration-frequency curves obtained from such a model are similar to those derived statistically from the long-term rainfall record. (from authors' abstract)