This Preliminary Viability Assessment (PVA) was initiated by the Prado Dam Forecast Informed Reservoir Operations (FIRO) multi-agency Steering Committee in 2017 and follows the workplan completed in 2019. The PVA provides an initial evaluation of the viability of FIRO as a strategy for improving water supply reliability for Orange County Water District (OCWD), while not impairing and possibly enhancing environmental conditions and flood risk management. Constructed and operated by the U.S. Army Corps of Engineers (USACE), Prado Dam was primarily designed for flood risk management. Stormwater capture for downstream groundwater recharge into OCWD’s water supply is a secondary purpose. The PVA demonstrated that FIRO is viable, and it lays the groundwork for a Final Viability Assessment (FVA) and recommendations for updating USACE’s Water Control Manual (WCM), which governs operation of Prado Dam.