The study deals with an intense rainstorm that hit the Middle East between 24 and 27 April 2018 and took the lives of 13 people, 10 of them on 26 April during the deadliest flash flood in Tzafit Basin (31.0° N, 35.3° E), the Negev Desert. The rainfall observed in the southern Negev was comparable to the long-term annual rainfall there, with intensities exceeding a 75-year return period. The timing of the storm, at the end of the rainy season when rain is relatively rare and spotty, raises the question of what the atmospheric conditions were that made this rainstorm one of the most severe late-spring storms. The synoptic background was an upper-level cut-off low that formed south of a blocking high which developed over eastern Europe. The cut-off low entered the Levant near 30° N latitude, slowed its movement from ∼ 10 to < 5ms-1 and so extended the duration of the storm over the region. The dynamic potential of the cut-off low, as estimated by its curvature vorticity, was the largest among the 12 latespring rainstorms that occurred during the last 33 years. The lower levels were dominated by a cyclone centred over northwestern Saudi Arabia, producing north-westerly winds that advected moist air from the Mediterranean inland. During the approach of the storm, the atmosphere over Israel became unstable, with instability indices reaching values favourable for thunderstorms (e.g. CAPE > 1500 J kg-1, LI D 4 K) and the precipitable water reaching 30 mm. The latter is explained by lower-level moisture advection from the Mediterranean and an additional contribution of mid-level moist air transport entering the region from the east. Three major rain centres were active over Israel during 26 April, only one of them was orographic and the other two were triggered by instability and mesoscale cyclonic centres. The build-up of the instability is explained by a negative upper-level temperature anomaly over the region caused by a northerly flow east of a blocking high that dominated eastern Europe and ground warming during several hours under clear skies. The intensity of this storm is attributed to an amplification of a mid-latitude disturbance which produced a cut-off low with its implied high relative vorticity, low upper-level temperatures and slow progression. All these, combined with the contribution of moisture supply, led to intense moist convection that prevailed over the region for 3 successive days.
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Acknowledgements. The authors wish to thank Efrat Morin and Yair Rinat from HUJI for the hydrometric data and the radar images. We are grateful to Noam Halfon and Yoav Levi from the Israel Meteorological Service (IMS) for providing the IMS C-band Doppler radar imagery and the integrated rain maps and to Elyakom Vadislavsky for the MSG IR images and for his assistance and scientific inputs while running the IMS-COSMO model and to Michal Kidron and Guy Keren from HUJI for their help in the preparation of the figures. Baruch Ziv thanks the Israeli Science Foundation (ISF; grant no. 1123/17). We also wish to express our gratitude to the three anonymous referees for their most helpful comments and constructive suggestions which led to substantial improvements of the paper.
Financial support. This research has been supported by the Israel
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