Flash floods are among the most devastating and lethal natural hazards. In 2018, three flash-flood episodes resulted in 46 casualties in the deserts of Israel and Jordan alone. This paper presents the hydrometeorological analysis and forecasting of a substantial storm (25-27 April 2018) that hit an arid desert basin (Zin, <span classCombining double low line"inline-formula" g 1/41400 km2</span , southern Israel) claiming 12 human lives. This paper aims to (a) spatially assess the severity of the storm, (b) quantify the timescale of the hydrological response, and (c) evaluate the available operational precipitation forecasting. Return periods of the storm's maximal rain intensities were derived locally at 1 <span classCombining double low line"inline-formula" km2</span resolution using weather radar data and a novel statistical methodology. A high-resolution grid-based hydrological model was used to study the intra-basin flash-flood magnitudes which were consistent with direct information from witnesses. The model was further used to examine the hydrological response to different forecast scenarios. A small portion of the basin (1 %-20 %) experienced extreme precipitation intensities (75-To 100-year return period), resulting in a local hydrological response of a high magnitude (10-To 50-year return period). Hillslope runoff, initiated minutes after the intense rainfall occurred, reached the streams and resulted in peak discharge within tens of minutes. Available deterministic operational precipitation forecasts poorly predicted the hydrological response in the studied basins (tens to hundreds of square kilometers) mostly due to location inaccuracy. There was no gain from assimilating radar estimates in the numerical weather prediction model. Therefore, we suggest using deterministic forecasts with caution as it might lead to fatal decision making. To cope with such errors, a novel cost-effective methodology is applied by spatially shifting the forecasted precipitation fields. In this way, flash-flood occurrences were captured in most of the subbasins, resulting in few false alarms.
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Acknowledgements. The authors wish to thank the editor, Maria-Carmen Llasat, Lorenzo Marchi, and one anonymous reviewer for their useful comments that helped improve the paper. We wish to thank Tamir Grodek, Alon Ronen, and the Soil Erosion Research Station for their help in post-event analysis peak discharge estimations. This study was funded by the Israel Science Foundation (grant no. 1069/18), a NSF–BSF grant (grant no. BSF 2016953), and a Google gift grant. This study is a contribution to the PALEX project “Paleohydrology and Extreme Floods from the Dead Sea ICDP Core” funded by the DFG (BR2208/13-1/-2) and is a contribution to the HyMeX program. We are also grateful for the detailed direct rainfall and flood observations made by Ari Matmon and Nathalie Neagu, and for sharing the information with us.
Financial support. This research has been supported by the Israel
Science Foundation (grant no. 1069/18), the NSF–BSF grant (grant no. BSF 2016953), and the Paleohydrology and Extreme Floods from the Dead Sea ICDP Core (grant no. DFG BR2208/13-1/-2).
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