This study focuses on the effect of projected changes in rainfall, snow accumulation and snowmelt, and consequent changes in the timing of runoff on NYC water supply system storage and operation as simulated by the NYC reservoir system Operational Analysis and Simulation of Integrated Systems (OASIS) model. The Generalized Watershed Loading Functions—Variable Source Area (GWLF-VSA)—watershed model is used with future climate scenarios derived from different General Circulation Models (GCMs) to simulate future inflows to reservoirs that are part of the New York City Water Supply System (NYCWSS). Future scenarios that use current system operation rules and demands, but changed reservoir inflows, suggest that changes in precipitation and snowmelt will affect regional water availability on a seasonal basis. The combined effect of projected increases in winter air temperatures, increased winter rain, and earlier snowmelt may result in more runoff during winter. This will cause reservoir storage levels, water releases and spills to increase during the winter and earlier reservoir refill in the spring. An overall increase in precipitation will result in a reduction in number of days the system is under drought conditions, despite increased evapotranspiration later in the year. Copyright © 2011 John Wiley & Sons, Ltd.