Storm Forecast
Valid: Tue 12 Mar 2013 06:00 to Wed 13 Mar 2013 06:00 UTC
Issued: Mon 11 Mar 2013 19:25
Forecaster: PISTOTNIK

A level 1 was issued for parts of Turkey mainly for severe wind gusts and large hail, and to a lesser degree for excessive precipitation and tornadoes.

A level 1 was issued for parts of Southern Italy mainly for excessive precipitation.

A level 1 was issued for parts of Southern Portugal and Southern Spain for severe wind gusts.

SYNOPSIS

The dominant feature of the European weather is a sharp zonal frontal boundary from the Bay of Biscay eastward across France and the Alpine region into Romania. It separates temperature air to the South from very cold air to the North and slowly slips southward, though its motion is slowed down and at times even reversed by shallow wave developments.
At upper levels, this outbreak of arctic air is supported by a positively tilted long-wave trough from the Barents Sea via Finland and the Baltic Sea to the Netherlands. Lapse rates are steep in the range of the trough but both the boundary layer moisture and the temperature level are too low to support electrified convection. Nonetheless, shallow maritime convection over the Norwegian Sea and later on over the North Sea may be accompanied by heavy snow showers.
Elongating the main trough axis, an upper-level low slowly wobbles from the Bay of Biscay into France while it successfully struggles to maintain its independence. Late in the forecast period, a vorticity lobe from its back side also ejects into Spain. Apart from that, the corridor along and to the South of the frontal boundary is characterized by weak pressure gradients throughout the troposphere, by mild, moderately humid low levels and rather cold upper levels. This combination results in low to moderate instability under weak vertical wind shear.
Rounding off the journey from high towards low latitudes, it finally needs to be mentioned that the well-established subtropical jet still runs straight from West to East across the Mediterranean region, which results in better kinematic conditions towards the Southern rim of the belt of instability. A fairly convincing CAPE-and-shear overlap is only present in Turkey, though.

DISCUSSION

... Turkey ...

The most robust severe weather threat evolves over parts of Turkey, where a slowly advancing cold front gets activated by a powerful short-wave trough in the late morning hours. Rising motions ahead of the trough axis and diurnal heating over land will probably result in the build-up of a few hundred J/kg of CAPE. With values up to 35 m/s, deep-layer shear is more than sufficient for organized convection. Scattered thunderstorms with a chance for severe wind gusts and large hail are expected. Along the Turkish West coast, backbuilding storms may also cause one or two excessive rain events and an isolated tornado (due to low condensation levels and slightly enhanced low-level shear), though these threats are rather subordinate.
Towards the South, the air mass will possibly be capped and the lift support from the short-wave trough diminishes. This may reduce the coverage of storms in the strongest shear regime and is the reason which precludes a level 2 despite the multiple severe weather threat.
Storms should gradually decay after the passage of the short-wave trough in the afternoon to evening hours while moving inland.

... Central Mediterranean, Italy, Balkans ...

Uncapped CAPE on the order of 200 to 500 J/kg will be available beneath cool upper levels in the Central Mediterranean region and will also develop on the Balkans, supported by diurnal heating. In either case, scattered to widespread thunderstorms are expected to develop again with the maximum activity in the afternoon hours, tied to the strongest insolation over the Balkans and to the passage of another short-wave trough over the Central Mediterranean region and Italy.
Due to the weak vertical wind shear, severe weather is generally unlikely. The only exception is Southern Italy, where the stronger Westerly background flow and orographic lift may result in a few heavy rain events, reflected by a small level 1 area. Apart from that, an isolated case of large hail is not ruled out especially over land, in case meso- or small-scale circulations can create slightly better wind profiles.

... Iberian Peninsula ...

Interesting things will also happen near the Southwestern corner of the forecast map. A wave depression will enter Southern Portugal and Andalusia from the West in the noon to afternoon hours. Forecast models simulate a blob of low CAPE under very strong deep-layer shear (~30 m/s), indicating that a period of rainfall along the warm front may be finished by convective developments along the trailing cold front. A low-end level 1 was issued for the possibility of a convective line with severe wind gusts. Limiting factor here is the unimpressive dynamic lift support.
In the meantime, the rest of the Iberian Peninsula will see another round of diurnal cool-air convection under weak vertical wind shear, triggered by diabatic heating and local wind systems in the vicinity of orographic features. The arrival of a pronounced vorticity lobe from the Northwest may keep these storms locally going on well into the night, though they should gradually decrease in coverage after sunset and shift to areas off the Spanish East coast, where a new cyclone in the wake of the Pyrenees will rapidly deepen towards the end of the forecast period.