A severe storm swept over the eastern Mediterranean between 10-13 December 2010, bringing extremely strong winds, high waves and substantial amounts of precipitation to northern and central Israel.
By Oren Davidoff (Israel Meteorological Service)
While, in the past Israel has experienced severe weather, the extent and severity of this event was out of the ordinary. It left behind a trail of destruction to property and landscapes, including the coastline where many of Israel's ports and docked marine vessels were damaged (Photo courtesy of Lilach Lev (IMS).
The entire trail of destruction was caused by a combination of an unusual amount of precipitation, that almost exceeded a quarter of the annual amount of precipitation, mainly in the northern and central regions of Israel; strong winds, both at sea and inland, and the destructive effect of the high waves that struck the Israeli coast.
The genesis of this destructive outcome, both in anomalous weather conditions and in extraordinary marine conditions, originated in a substantial deepening of a low high in the atmosphere at the 500 hPa pressure level, with the 540 gph line originally situated north of northern Greece, eventually touching base with the northern part of Israel.
This low was accompanied by a substantial deepening low at ground level, reaching an outstanding pressure value of 991 hPa in its centre, positioning itself between Crete and Cyprus (Figure 2).
The Meteosat-9 satellite imagery clearly shows this outstanding storm, in several channels, which individually delineate a deeper understanding of the severe extent of the storm.
Severe Convection RGB
Water Vapour, 6.2 µm
In the Airmass RGB image, 11 December 12:30 UTC (Figure 1, top right, click to expand), it is easy to see the descending stratospheric air protruding and eventually sweeping over Israel (red hues), followed by the cold air mass accompanied by a low tropopause (purple hues).
In the Severe Convection image of the same date and time (Figure 3), well-developed ice clouds (represented by orange-yellowish hues) can be seen passing over the region. These were responsible for the massive amounts of precipitation, as was recorded in Israeli meteorological stations.
In the Water Vapour image (Figure 4), again on the same date and time, the location of the jet axes can be clearly identified at the rear of the frontal cloud band, approximately perpendicular to the cloud band portrayed by a black stripe.
The dry and dark areas in the image are a distinct portrayal of the cyclogenetic regions, coinciding with the deep MSLP low mentioned earlier, as dry stratospheric air protrudes downward into the troposphere. While the black stripes indicate the location of the jet axes, they are also indicative, in this case, of the unstable development at the boundary between dry and moist air.
The High Resolution Visible (HRV) image (Figure 5), gives an actual sense of the multi-layered cloud coverage reigning the region.
In addition to the mass devastation to property and near-shore landscapes, and a huge amount of precipitation in northern and central Israel, extreme significant wave heights were measured.
Two offshore buoys situated 2 km off from the port cities of Haifa, located off the northern coastline, and Ashdod, off the southern Israeli coastline (buoy locations) recorded ~7 m significant wave heights, with maximal wave height of 10.9–11.9 m (Figure 6).
The progressive growth in these wave heights were well predicted by the ECMWF forecast wave charts preceding the actual storm event, remaining consistent with the forecast wave heights for each time interval for each model run throughout the duration of the storm, see Figure 7.
Another wave forecasting model, used by Israeli forecasters, is the regional SWAN model, based on ECMWF's input in the outer boundaries of the SWAN forecast region.
This model also exhibited a reasonable forecast, although slightly overestimating the wave heights by 0.5–1 m. It was consistent in both the -48 hrs (10 Dec 00:00 UTC) run and the -24 hrs (11th Dec 00:00 UTC), to the highest significant peaks of waves which were measured both in Ashdod and Haifa around 03:00 UTC on 12 Dec. The forecasts for -51 hours and -27 hours, also coincided with the actual highest significant peaks measurements.
A civilian cargo ship that had been patrolling the area (off the west coast of Crete), provided a SYNOP observation that was received at the Israeli Meteorological Service, which included very important measurements of the state of the sea, winds (direction and speed), significant wave heights and swell (height and period).
These measurements concurred (for the exact timing and location) with the forecast variables and state of sea by all the available predicting products produced by the ECMWF model runs. This was a usefulbe service for the national Israeli forecasters who, using information and the available forecast state of sea, were able to issue optimal state of sea warnings to all relevant clientele.
Another affirmation of the severity of the event for the local forecasters, was the measurement of surface wind speeds exceeding 50 kts. These measurements were provided by a Metop-A ASCAT descending pass over the region, on 11 Dec 03:52 UTC (Figure 8).
These recorded wind speeds were underestimated to some extent by many of the used weather models (including ECMWF), which forecast averaged wind speeds (at their peaks) at 35-40 kts.
Nevertheless, ECMWF winds are underestimated by a model which only captures 50% of wind variability on the meridional component and 75% on the zonal component for scales smaller than 200 km (for example, for local downbursts.