At times, a pronounced trough of low barometric pressure extends from equatorial Africa northward, over the Red Sea and the eastern Mediterranean countries, i.e., the Red Sea Trough. The associated weather is usually hot and dry, and consequently the atmosphere becomes conditionally unstable. In cases in which additional moisture is supplied and dynamic conditions become supportive, as the case analyzed here, intense thunderstorms occur, with extreme rain rates, hail and floods. The storm herein analyzed caused extensive damage both in casualties and property and evolved in two main consecutive phases: In the first a Mesoscale Convective System that moved from Sinai northward over Israel dominated, and in the second deep convection was organized mainly along a cold front. Data analysis indicates several synoptic-scale factors that had a supportive effect on the storm formation and intensification: Conditional instability established by the Red Sea trough, mid-level moisture transport from Northern Africa, and upper-level divergence imparted by both polar and subtropical jet streams over the Middle-East. Mesoscale features were further investigated by means of a hydro-meteorological observational analysis with high spatio-temporal resolution using raingauge and radar data, and satellite imagery. It is shown that local factors, particularly topographic effects, play a major role in the evolution, intensity and spatial organization of the convective activity. Our findings support results of a numerical study of another autumn rainstorm associated with the Red Sea trough. In the present case we identify an additional contributing factor, i.e., a mid-latitude upper-level trough that further intensified the storm as it was approaching the Middle-East.

Intense rainstorms cause debris flows on escarpments in hyperarid environments. In contrast with more temperate environments, there have been no direct observations on rainfall intensities and durations required for initiating debris flows in hyperarid environments. Here, we report rainfall volume and intensities, acquired by gauge and radar measurements, for two successive storms along the hyperarid (\textless50 mm/yr) western escarpment of the Dead Sea basin. These rainfall data were analyzed in conjunction with detailed mapping of debris flows that occurred during these storms to determine values of rainfall intensity and duration required to generate debris flows on the Dead Sea western escarpment. The first of the two analyzed storms occurred on 2 November 1995. During this storm, two convective cells rained sequentially within a 5 It period at the lower reaches of the Nahal David and the Nahal ’Arugot that dissects the western escarpment of the Dead Sea, Israel. This storm triggered debris flows in 38 small (\textless3 km(2)) and high-gradient drainage basins along the escarpment. Total rainfall volume and spatial distribution were determined by 10 cumulative rain gauges that were also used to calibrate rainfall-intensity distributions from radar data. For this storm, region, and landscape, rainfall intensities exceeding 30 mm/h for a duration of I h were required to initiate debris flows. A second storm in the same area on 1718 October 1997 allowed the evaluation of the results determined from the 1995 storm. In this second, more regional storm, maximum rainfall intensities were 19-27 mm/h for a duration of 45 min. These values, lower than the 30 mm/h minimal threshold defined in the previous storm, are consistent with the occurrence of only three debris flows. The small number of debris flows resulted from the concentration of the highest intensities of rainfall on the desert plateau and not directly on top of the canyon walls. Most first- to third-order basins draining the Dead Sea escarpment contain evidence of zero to three late Holocene (\textless3000 yr) debris flows. From analysis of the two storms, we propose that most of these debris flows were triggered by storms similar to the 2 November 1995 event in which localized convective cells had rainfall intensities of \textgreater30 mm/h and durations of at least 1 h. The small number of debris flows that has occurred during the late Holocene indicates that such events are rare at the scale of individual drainage basins.

The last decade has witnessed the development of methodologies for the post-flood documentation of both hydrogeomorphological and social response to extreme precipitation. These investigations are particularly interesting for the case of flash floods, whose space–time scales make their observations by conventional hydrometeorological monitoring networks particularly challenging. Effective flash flood documentation requires post-flood survey strategies encompassing accurate radar estimation of rainfall, field and remote-sensing observations of the geomorphic processes, indirect reconstruction of peak discharges—as well eyewitness interviews. These latter can give valuable information on both flood dynamics and the related individual and collective responses. This study describes methods for post-flood surveys based on interdisciplinary collaborations between natural and social scientists. These surveys may help to better understand the links between hydrometeorological dynamics and geomorphic processes as well as the relationship between flood dynamics and behavioral response in the context of fast space–time changes of flooding conditions. This article is categorized under: Science of Water > Methods Science of Water > Hydrological Processes A flash flood and its forensic analysis.

This work analyses the prominent characteristics of extreme storms and flash-flood regimes in two main areas of the Mediterranean region: the North-Western (comprising Spain, France and Italy) and South-Eastern region (Israel). The two areas are chosen to represent the two end members of variation in flash-flood regimes in the Mediterranean basin. Data from 99 events collected in the two areas (69 from the North-Western region and 30 from the South-Eastern region), for which occurrence date, catchment area and flood peak are available, were used to provide a detailed description the flash-flood seasonality patterns, the synoptic and mesoscale atmospheric controls, and flood envelope relationship. Results show that the flood envelope curve for the South-Eastern region exhibits a more pronounced decreasing with catchment size with respect to the curve of the North-Western region. The differences between the two relationships reflect variations in the fractional storm coverage of the basin and hydrological characteristics between the two regions. Seasonality analysis shows that the events in the North-Western region tend to occur between August and November, whereas those in the South-Eastern area tend to occur in the period between October and May, reflecting the relevant patterns in the synoptic conditions controlling the generation of intense precipitation events.