Hydrometeorological prediction involves the forecasting of the state and variation of hydrometeorological elements -- including precipitation, temperature, humidity, soil moisture, river discharge, groundwater, etc.-- at different space and time scales. Such forecasts form an important scientific basis for informing public of natural hazards such as cyclones, heat waves, frosts, droughts and floods. Traditionally, and at most currently operational centers, hydrometeorological forecasts are deterministic, "single-valued" outlooks: i.e., the weather and hydrological models provide a single best guess of the magnitude and timing of the impending events. These forecasts suffer the obvious drawback of lacking uncertainty information that would help decision-makers assess the risks of forecast use. Recently, hydrometeorological ensemble forecast approaches have begun to be developed and used by operational hydrometeorological services. In contrast to deterministic forecasts, ensemble forecasts are a multiple forecasts of the same events. The ensemble forecasts are generated by perturbing uncertain factors such as model forcings, initial conditions, and/or model physics. Ensemble techniques are attractive because they not only offer an estimate of the most probable future state of the hydrometeorological system, but also quantify the predictive uncertainty of a catastrophic hydrometeorological event occurring. The Hydrological Ensemble Prediction Experiment (HEPEX), initiated in 2004, has signaled a new era of collaboration toward the development of hydrometeorological ensemble forecasts. By bringing meteorologists, hydrologists and hydrometeorological forecast users together, HEPEX aims to improve operational hydrometeorological forecast approaches to a standard that can be used with confidence by emergencies and water resources managers. HEPEX advocates a hydrometeorological ensemble prediction system (HEPS) framework that consists of several basic building blocks. These components include: (a) an approach (typically statistical) for addressing uncertainty in meteorological inputs and generating statistically consistent space/time meteorological inputs for hydrological applications; (b) a land data assimilation approach for leveraging observation to reduce uncertainties in the initial and boundary conditions of the hydrological system; (c) approaches that address uncertainty in model parameters (also called 'calibration'); (d) a hydrologic model or other approach for converting meteorological inputs into hydrological outputs; and finally (e) approaches for characterizing hydrological model output uncertainty. Also integral to HEPS is a verification system that can be used to evaluate the performance of all of its components. HEPS frameworks are being increasingly adopted by operational hydrometeorological agencies around the world to support risk management related to flash flooding, river and coastal flooding, drought, and water management. Real benefits of ensemble forecasts have been demonstrated in water emergence management decision making, optimization of reservoir operation, and other applications.

Qingyun Duan (EIC): National Chair Professor and Chief Scientist of Faculty of Geographical Science at Beijing Normal University, author of >120 peer reviewed scientific papers. He is currently on editorial boards of several scientific journals and was the lead editor of the American Geophysical Union Water Science and Applications Monograph series "Calibration of Watershed Models". He is a Fellow of American Geophysical Union and American Meteorological Society.

Florian Pappenberger (Co-EIC): Director of Forecasts at the European Centre For Medium-Range Weather Forecasts. The Forecast Department at ECMWF has a strong user focus and undertakes production of forecasts, forecast evaluation and diagnostics, development of forecast products and applications, software development, catalogue and data services and outreach and training. Florian has a scientific background in the forecasting of weather driven natural hazards including floods, droughts, windstorms, forest fires and impacts on human health. He has over 10 years of expertise in operational probabilistic forecasting, extreme value statistics and numerical model system development at ECMWF. He was responsible for the development and implementation of the operational centre of the Copernicus Emergency Service - Early Warning Systems (floods). Florian is the author of over 150 publications, has won several scientific awards and is visiting Professor at the University of Bristol. He is an elected fellow of the Royal Geographical Society and the Royal Meteorological Society and a member of several other professional bodies including HEPEX, British Hydrological Society, EGU, AGU, EMS, AMS. He is on the editorial board of several international scientific journals and regularly advises on international committees including WMO.

Andy Wood (Co-EIC) a Development and Operations Hydrologist for the U.S. National Weather Service, Dr. Wood is also the Chair of the Hydrology Committee of the American Meteorological Society (AMS), an associate editor of the AMS Journal of Hydrometeorology, and a co-leader of the Hydrologic Ensemble Prediction Experiment (HEPEX). He previously worked as a research professor at the University of Washington Department of Civil and Environmental Engineering, and as the lead Scientist of a private renewable energy prediction firm, and has co-authored over 40 scientific publications focusing on hydrologic prediction, downscaling and climate change impact assessment, drought and other topics.

Hannah L. Cloke (co-EIC): a hydrologist and physical geographer specializing in land surface modeling, flood forecasting, applications of Numerical Weather Predictions and catchment hydrology. Her current research focuses on the theoretical and practical development of early warning systems for natural hazards, particularly for floods and droughts and disaster risk management.

Hannah is Visiting Professor in the Department of Earth Sciences at Uppsala University, Sweden. She is an elected fellow of the Royal Geographical Society and the Royal Meteorological Society. She works closely with the Environment Agency, the European Centre for Medium-Range Weather Forecasts (ECMWF) and the UK Met Office, as well as a wide range of other national and international partners. Hannah advises government on national and international flooding incidents and provides expert commentary in the media. She is Executive editor of Hydrology and Earth System Sciences and an active member of the HEPEX initiative.

John Schaake (co-EIC): formerly Chief Scientist with Office of Hydrologic Development, NOAA National Weather Service. He retired from National Weather Service after over 20 years of service in 2001. In retirement, he initiated the Hydrologic Ensemble Prediction Experiment (HEPEX) in 2004 and served as the inaugural co-chair for HEPEX scientific steering committee. Dr. Schaake is an internationally recognized expert on hydrometeorological ensemble forecasting and published extensively on hydrometeorological ensemble forecasting. Prior to his service in NOAA, he was an Associate Professor at Massachusetts Institute of Technology and University of Florida. He has served on editorial boards of various hydrology and water resources journals and various leadership posts of international organizations. He has been elected a Fellow of American Geophysical Union and American Meteorological Society.