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Hydrological ensemble prediction systems (HEPS) have in recent years been increasingly used for the operational forecasting of floods by European hydrometeorological agencies. The most obvious advantage of HEPS is that more of the uncertainty in the modelling system can be assessed. In addition, ensemble prediction systems generally have better skill than deterministic systems both in the terms of the mean forecast performance and the potential forecasting of extreme events. Research efforts have so far mostly been devoted to the improvement of the physical and technical aspects of the model systems, such as increased resolution in time and space and better description of physical processes. Developments like these are certainly needed; however, in this paper we argue that there are other areas of HEPS that need urgent attention. This was also the result from a group exercise and a survey conducted to operational forecasters within the European Flood Awareness System (EFAS) to identify the top priorities of improvement regarding their own system. They turned out to span a range of areas, the most popular being to include verification of an assessment of past forecast performance, a multi-model approach for hydrological modelling, to increase the forecast skill on the medium range (>3 days) and more focus on education and training on the interpretation of forecasts. In light of limited resources, we suggest a simple model to classify the identified priorities in terms of their cost and complexity to decide in which order to tackle them. This model is then used to create an action plan of short-, medium- and long-term research priorities with the ultimate goal of an optimal improvement of EFAS in particular and to spur the development of operational HEPS in general.

Hydrological ensemble prediction systems (HEPS) have in recent years been increasingly used for the operational forecasting of floods by hydrometeorological agencies. The most obvious advantages of HEPS are that more of the uncertainty in the modelling system can be assessed; and that ensemble prediction systems generally have better skill than deterministic systems both in the terms of the mean forecast performance and the potential forecasting of extreme events. Research efforts have so far mostly been devoted to the improvement of the technical aspects of the model systems themselves. However, in this paper we argue that there are other areas of HEPS that need urgent attention; such as assessment of the full uncertainty in the forecast chain, multimodel approaches, robust forecast skill assessment and further collaboration and knowledge exchange between operational forecasters and the model development community. In light of limited resources we suggest a simple model to classify the identified priorities in terms of their cost and complexity to decide in which order to tackle them This model is then used to create an action plan of short-, medium- and long-term research priorities with the ultimate goal of an optimal improvement in operational HEPS.

Europe repeatedly observes flood events that affect several countries at the same time and which require the coordination of assistance at the European level. The European Flood Awareness System (EFAS) has been developed specifically to respond to the need for forecasting transnational floods with sufficient lead time to allow coordination of aid at the European level in case the national capacities for emergency management are exceeded. In order to achieve robust and reliable flood forecasting at the continental scale with lead times up to 10 days, EFAS promotes probabilistic forecasting techniques based on multiple numerical weather prediction inputs including ensemble prediction systems. Its aim is to complement existing national flood forecasting services with added value information and to provide European decision makers with coherent overviews on ongoing and upcoming floods in Europe for better planning and coordination of aid. To date, EFAS is a unique system providing daily, probabilistic flood forecast information for the entire of Europe on a single platform. Being a complementary system to national ones, EFAS predicts the probabilities for exceeding critical flood thresholds rather than quantitative information on stream flows. By maintaining a dedicated, multinational partner network of EFAS users, novel research could be transferred directly to the operational flood forecasting centers in Europe. EFAS development started in 2003, and the system has become fully operational under the umbrella of Emergency Management Service of the European Copernicus Space Program in 2011.