As no forecast is complete without a description of its uncertainty (National Research Council of the National Academies 2006), it is necessary, for both atmospheric and hydrologic predictions, to quantify and propagate uncertainty from various sources in the forecasting system. For informed riskbased decision making, such integrated uncertainty information needs to be communicated to forecasters and users effectively. In an operational environment, ensembles are an effective means of producing uncertainty- quantified forecasts. Ensemble forecasts can be ingested in a user's downstream application (e.g., reservoir management decision support system) and used to derive probability statements about the likelihood of specific future events (e.g., probability of exceeding a flood threshold). Atmospheric ensemble forecasts have been routinely produced by operational Numerical Weather Prediction (NWP) centers for two decades. Hydrologic ensemble forecasts for long ranges have been initially based on historical observations of precipitation and temperature as plausible future inputs (e.g., Day 1985) in an attempt to account for the uncertainty at the climate time scales. Ensemble forecasts generated in this fashion were considered viable beyond 30 days where the climatic uncertainty would dominate other uncertainty sources. More recently, as the needs for risk-based management of water resources and hazards across weather and climate scales have increased, the research and operational communities have been actively working on integration of the NWP ensembles into hydrologic ensemble prediction systems and quantification of all major sources of uncertainty in such systems. In particular, the Hydrological Ensemble Prediction Experiment (HEPEX; www.hepex.org/), launched in 2004, has facilitated communications and collaborations among the atmospheric community, the hydrologic community, and the forecast users toward improving ensemble forecasts and demonstrating their utility in decision making in water management (Schaake et al. 2007b; Thielen et al. 2008; Schaake et al. 2010).