Quantitative precipitation estimates (QPE) derived from radar are useful in runoff simulation in urban drainage. Simulation experiments using radar data sampled at various resolutions identify the limits to predictability for various basin sizes. Spatial resolution of radar rainfall used as input to a distributed model affects prediction error and scales with drainage area. Radar data used in this analysis are derived from both S-band (NEXRAD) and X-band radars. Using radar QPE derived from the existing WSR-88D (KHGX) as input to a physics-based hydrologic model of Brays Bayou (260 km2) provides a baseline for comparison and guides design of future radar networks. Results of experiments using historical radar events, including the tropical storm Allison, indicate that accurate rainfall-runoff predictions in realtime are possible and useful for site-specific forecasts. Radar and distributed hydrologic model provide accurate rainfall and runoff data supporting site-specific flood information.