Storm Forecast
Valid: Fri 30 Aug 2013 06:00 to Sat 31 Aug 2013 06:00 UTC
Issued: Thu 29 Aug 2013 20:44
Forecaster: TUSCHY

A level 2 was issued for NE Algeria, N-Tunisia, Sardinia, the Tyrrhenian Sea and Sicily mainly for large to very large hail (hail diameter in excess of 5 cm is possible), severe wind gusts and excessive rain. An isolated tornado event is possible.

A level 1 surrounds the level 2 with similar risks but a lesser probability for extreme events.

A level 1 was issued for SE Spain mainly for an isolated large hail and excessive rain event.

SYNOPSIS

Multiple upper troughs affect the European weather. A broad trough enters the Ukraine from the west with a slow motion to the east. Another trough is situated over the W-Mediterranean with an autumn-like deep depression approaching Scotland from the NW.

Numerous progressive fronts enter the forecast area from the NW, affecting areas from Irland to W-Norway. Further south over the Mediterranean, an E-W aligned quasi-stationary front plays a role for DMC activity.

DISCUSSION

... N-Algeria, N-Tunisia, Corsica, Sardinia, Sicily and parts of S-Italy ...

The positive tilt of the upper trough over the W-Mediterranean supports an active period with numerous short-waves rounding that feature and moving to the E. Those waves cross a wavy surface boundary, which separates anomalous richt BL moisture to its south from more tempered maritime air to its north. Models agree in falling surface pressure over the W-Mediterranean, which causes the boundary to acquire some warm and cold front characteristics (due to the development of a weak surface depression). A confined but very unstable warm sector evolves just off the coasts of NE-Algeria and N-Tunisia. During the overnight hours, both, the approach of the strongest mid-layer wave of the day and the gradual approach of the upper trough itself insert abundant forcing for increased DMC probabilities.

A 15-20 m/s mid-level jet affects the regions with a repeated regeneration expected during the forecast (enhanced by passing mid-layer waves). 15-20 m/s 0-6 km bulk shear and roughly 15 m/s 3 km shear are forecast all day long.

A very moist maritime layer with BL dewpoints in the lower to mid twenties remains in place. With steep mid-level lapse rates of 7-8 K/km overspreading that air mass, 2-3 kJ/kg MLCAPE are expected. The region just N of Tunisia could see MLCAPE values in excess of 3000 J/kg due to a very anomalous rich BL moisture content.

During the daytime hours, the combination of strong capping and weak mid-layer impulses supports only isolated initiation over Corsica/Sardinia, the Tyrrhenian Sea and S-Italy, Sicily and the Atlas mountain range. However any thunderstorm, which manages to become long-lived will gain rapid organization and supercells are forecast. Very large hail, severe wind gusts and excessive rainfall amounts are possible.
Beyond 21Z, environmental conditions become more supportive for a rapid increase of well organized convection from NE-Algeria towards the Tyrrhenian Sea. Numerous supercells are forecast, which grow upscale into a severe mesoscale convective system. Widespread large to very large hail (hail diameter well in excess of 5 cm), severe to damaging wind gusts and excessive rain are expected. With the diffluent upper-level streamline pattern aloft, the slow approach of the main upper trough from the west, the well structured surface boundary and the extreme CAPE magntiude in place, a major MCC event is possible, which affects parts of the Tyrrhenian Sea, Sicily and S-Italy during the second part of the night. Damaging flash flooding is a major concern. Due to expected internal dynamics, the level and lightning areas were expanded far east.

Despite weak LL shear and strong cap in the lower layer, an isolated tornado can't be ruled out with mature supercells. The risk might be slightly enhanced along the N-fringe of the strongest cap, which would include Sardinia and the coastal areas of SW-Italy.

... SE Spain ...

The departing upper trough with lingering BL moisture create a favorable set-up for ongoing thunderstorms from the overnight hours. Shear and CAPE parameters both increase towards the coast, where 15 m/s DLS and up to 1000 J/kg MLCAPE overlap. Rapid clustering of storms and a gradual weakening trend of shear and instability during the noon and afternoon hours keep the overall severe risk limited. Expect a few severe hail and wind gust events with an isolated multicell event. Clustering storms and slow storm motion could also enhance the heavy rainfall risk along the coast. During the late afternoon hours however, thunderstorm coverage and intensity decreases from NW to SE.