A mid-winter, tropical extreme flood-producing storm in southern Israel: Synoptic scale analysis

Summary

An exceptional rainstorm affected the Negev Desert, the southern part of Israel, during 20–23 December 1993 as a result of an active Red Sea trough (ARST). The latter refers to a low pressure trough that extends from eastern Africa northward, along the Red Sea, accompanied by an upper-level cyclonic system. Rainfall totals in certain parts exceeded December averages by a factor of 5. Major precipitating systems had a typical Meso-scale Convective System structure accompanied by showers, with peak intensities of 60–80 mm h⁻¹, and by hail and flash floods, resulting in loss of life and heavy damage.

The origin of rainfall systems and of the moisture at lower latitudes, as well as the type of precipitation involved, indicate its tropical nature. The timing of the storm around the winter solstice is unusual, since these tropical-like storms are most frequent in the region in October–November.

Unusual synoptic circumstances explain the development and intensity of this storm and this exceptional timing. In contrast to the normal situation in December the mid-level subtropical high over the Sahara was replaced by a cyclone, while the persistent high over the Arabian Peninsula intensified. As a result, a southerly flow persisted over the Red Sea several days prior to the storm initiation and transported tropical moisture and heat northward. The ascending air motion observed along the Red Sea enhanced convection there, which transported lower-level moisture from the water surface and transported it to the surrounding regions. Finally, a moving upper-trough that approached the region from the west transported the tropical moisture further to the Negev and supplied the lifting that transformed this moisture and instability into heavy showers.