European Flood Alert System Post-Event Summary Report: The Alpine Floods of August 2005- What Did EFAS Forecast? What Was Observed? Which Feedback Was Recieved From End-Users?

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From the 22nd until 27th August 2005, the northern Alpine region was hit by severe floods. The floods occurred in tributaries of the Rhine River in Switzerland and Austria (Vorarlberg) and in several tributaries of the Danube River in Germany (Iller, Lech, Isar), Austria (Lech, Inn, Mur, Drau/Drava, Raab/Raba) and Hungary (Mur and Raba/Raab), as well as in the German part of the Danube.

The European Flood Alert System (EFAS), under development and running in preoperational mode at the Joint Research Center (JRC) since February 2003 (De Roo et al., 2003), forecasted these events in all river basins and reported in realtime to its partners’ organizations in the Member States. During the period from 16th to 25th August, EFAS forecasted potential flooding in the Upper Danube (tributaries Isar, Iller, Lech) in Germany and Austria, as well as in the upper Rhine (Switzerland and Germany). Furthermore, potential flooding was forecasted in the Mur and Drava rivers in SE-Austria, Hungary, Slovenia and Croatia.

This report describes what was forecasted with the EFAS system, what was communicated to the EFAS partners’ organizations (water authorities in the relevant countries), where and when the floods occurred and which feedback was received from the end-users. Lessons learnt are described at the end of the report.

Flood forecasting based on weather forecasts from ECMWF-Ensemble Prediction System (EPS), one of the key elements of EFAS forecasts, has been implemented since June 2005 pre-operationally in EFAS. For technical reasons (one corrupt input file), the probabilistic forecasts did not run during the August floods. Following a rerun of the EPS immediately after the August floods, it is demonstrated that the availability of the probabilistic forecasts might have increased the flood warning time of 1-2 days. Especially the forecasts of 16th and 19th August show an increased number of EPS above EFAS High Alert Level (HAL).

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