Experiences of dealing with flash floods using an ensemble hydrological nowcasting chain: implications of communication, accessibility and distribution of the results

The forecast of flash floods is sometimes impossible. In the last two decades, Numerical Weather Prediction Systems have become increasingly reliable with significant improvements in terms of quantitative precipitation forecasts. However, despite the application of modern probabilistic hydrometeorological chains, some significant flood events remain unpredicted. This was also the case with an event that occurred on the 8th and 9th of June, 2011 in the eastern part of the Liguria Region, Italy. This event affected in particular the Entella basin, a small watershed that flows into the Mediterranean Sea. The application of a hydrological nowcasting chain as a tool for predicting flash floods in small basins with an anticipation time of a few hours (2-5) is presented here. This work investigated the 'behaviour' of the chain in the cited event as well as in other verification cases and showed how the chain could be exploited for operational purposes. The results were encouraging. However, the analysis provided evidence that the difficulties in using operational hydrometeorological tools are not always and only dependent on the performance of such systems, but also on the way the results are made available to forecasters and on the efficiency of communication with the civil protection officials.