29 Apr 2020

29 Apr 2020

Review status: a revised version of this preprint is currently under review for the journal NHESS.

Atmospheric Conditions Leading to an Exceptional Fatal Flash Flood in the Negev Desert, Israel

Uri Dayan1, Itamar M. Lensky2, and Baruch Ziv3 Uri Dayan et al.
  • 1Department of Geography, The Hebrew University of Jerusalem, Jerusalem, 9070227, Israel
  • 2Department of Geography and Environment, Bar-Ilan University, Ramat-Gan, 5290002, Israel
  • 3Department of Natural Sciences, The Open University of Israel, Raanana, Israel

Abstract. The study deals with an intense rainstorm that hit the Middle–East between 24 and 27 April 2018. The storm reached its peak over Israel in April 26, when it produced a heavy flash flood that took the lives of ten people. The rainfall observed in the southern Negev was comparable to the long-term annual rainfall there. The timing of the storm is also unique, at the end of the rainy season when rain is relatively rare and spotty. The study analyzes the dynamic and thermodynamic conditions that made this rainstorm one of the latest spring severe events in the region during the last 3 decades.

The synoptic background was an upper-level cut-off low that entered the region from west, along 30° N latitude, which is rather exceptional for such systems in the late spring. While approaching the Levant, it slowed its movement from ~ 10 to < 5 ms−1. On the day of maximum intensity, the radius of the cyclone shrank to 275 km. The effect of the small radius was estimated by the measure of its curvature vorticity (MCV), which was the largest among the spring rainstorms during the latest 33 years.

The lower-levels were dominated by a northwesterly wind 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 favorable for thunderstorms (CAPE = 909 J Kg−1, LI = −4.9 K, SI = −2.7 K and MKI = 30 K), and the precipitable water increased from 17 to 30 mm. The latter is explained here by a combined effect of the lower-level moisture advection and a mid-level band of tropical moisture that entered Israel above it.

Three major rain centers were active during April 26, two of them were non-orographic, which is unusual for this type of system. This is explained by the dominance of sub-synoptic features, found in the 0.25 resolution data of ERA5 that were used to derive Omega and MKI maps. The buildup of static instability is explained by a −5 K temperature anomaly over the region, caused by a northerly flow east of the blocking high over North Europe that transported cold air over the Mediterranean water.

The unique 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 several moisture sources, led to extreme dynamic and thermodynamic conditions favorable for this exceptionally severe rain-storm.

Uri Dayan et al.

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Uri Dayan et al.

Uri Dayan et al.


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Short summary
An intense rainstorm affected the Levant in April 2018 producing a heavy flood that took the lives of ten people. The timing was the end of the rainy season when rain is relatively rare. The study inquires what made this storm the most severe in the region during the spring in the latest 3 decades. We found it as the outcome of a deep low-level quai-stationary cyclone and low upper-level temperatures. Several moisture sources, including tropical, contributed also to the intensity of the storm.