Storm Forecast
Valid: Fri 17 May 2013 06:00 to Sat 18 May 2013 06:00 UTC
Issued: Thu 16 May 2013 22:12
Forecaster: PISTOTNIK

A level 1 was issued for parts of Russia, Latvia, Lithuania, Belarus and the Ukraine mainly for large hail and excessive precipitation.

A level 1 and level 2 were issued for Montenegro, Albania, Southern Serbia, Macedonia, and parts of Bulgaria and Greece mainly for large hail and severe wind gusts.

A level 1 was issued for parts of Italy, for Slovenia, Croatia, Bosnia, Northern Serbia, and parts of Hungary and Romania mainly for large hail and excessive precipitation.

A level 1 was issued for parts of Germany mainly for excessive precipitation and large hail.

SYNOPSIS

The Western European long-wave trough has finally cut off into a big upper-level low centered over the Bay of Biscay and Northern Spain. The main trough axis gets an increasing negative tilt, while it accelerates from Sardegna and Tunisia at the beginning of the forecast period northeastward across Italy and into the Balkan States.
The second main player on the weather maps is a large and stationary anticyclone over Northwestern Russia and Finland. Between these two steering centers, the flow has turned Southeasterly and advects warm and partly humid air, allowing an active convection day over much of the continent.
Meanwhile, cool maritime air continues to settle across Western Europe and pushes further into the Iberian Peninsula and the Western Mediterranean region. However, also these regions will see some convection, supported by diurnal heating and further passing vorticity lobes.

DISCUSSION

...Russia, the Baltics, Belarus, Ukraine...

Over the horseshoe-shaped area sketched by the level 1 and the high probability thunder area, very warm air continues to move westward. It is characterized by steep lapse rates and rich boundary layer moisture which re-forms in a daily cycle of rich insolation in the morning and widespread thunderstorm activity in the afternoon and evening. On Friday, again moderate to strong instability release with peak CAPE values around 2000 J/kg (refer to Thu 12 UTC's Smolensk and Bologoe soundings) will result in numerous thunderstorms. Deep-layer shear is as weak as it can be with less than 10 m/s, but the sheer number of storms means that a few hail events can be expected with strong pulses. One or two severe downbursts may occur in the far East, where the boundary layer becomes deeper with the follow-up of even hotter and very dry prairie air, but at the same time this also decreases the overall thunderstorm chances.
Later towards evening, clustering into one or several MCSes may also result in excessive rainfalls in some places. Activity will continue into the evening and night hours, while it slowly moves westward into the Baltic States before it finally becomes elevated over the cold Baltic Sea.

...Italy, the Balkans, Hungary, Romania...

A long history of warm air advection over the Balkan Peninsula and the arrival of the pronounced main trough from the Southwest provide an interesting but also very complex setup.
Starting with a diagnosis of Thursday's state, the boundary layer was uniformly warm across the Balkans, but big dew point variations (e.g. from 4°C to 18°C over Bulgaria and Southern Romania) and the complex terrain cause some headache when it comes to the judgement of the amount and depth of the low-level moisture. However, in general the latest dew point readings were in the range of or even higher than the model forecasts, and a plot with passing mid-level clouds with a little rain overnight followed by some hours of insolation on Friday morning may indeed result in a quick moisture build-up.
Staying on the safe side, at least between 500 and 1000 J/kg (or locally more) of CAPE can be expected to build up on Friday. The first round of thunderstorms will fire up around noon over orographic features across the Balkans and may become quite widespread in the course of the afternoon, as plentiful QG lift ahead of the trough will facilitate convective initiation. In the Northern half of the level 1 area, deep-layer shear is weak with 10-15 m/s, hence large hail with stronger pulse storms and heavy rain (precipitable water is forecast to be around 30 mm) will be the main threat in Slovenia, Croatia, Bosnia and Northern Serbia. Further South, deep-layer shear increases to 20-25 m/s, but also the capping becomes stronger. This means that a somewhat lower storm coverage but a better organization into multi- and supercells with a risk of large hail and severe wind gusts can be expected. A small level 2 area was issued around Macedonia, where strong vertical wind shear and confidence in convective initiation sufficiently overlap. A higher coverage of hail events and even one or two very large hail falls (~5 cm) are possible in this region.
The second round of thunderstorms can be expected further upstream in the range of the trough axis, which also marks the transition to cold air advection and significant drying. Convective activity may start quite early over Central Italy and move northeastward. If there is enough insolation and if patches of a milder and moister boundary layer can hold their ground in the wake of the Apennin mountains beneath cooling and drying mid-levels, a few hundred J/kg of CAPE could build up under increasing deep-layer shear (15-20 m/s). Large hail seems to be an option, and with very favourable mesoscale developments also a severe wind gust (if the drying mid-levels sufficiently enhance evaporative cooling) or a tornado (if the boundary layer moisture resists the mixing processes long enough) cannot be ruled out. However, all in all the whole setup is too conditional to take any measures beyond an expansion of the Western edge of the level 1 area over Italian grounds.
Towards evening and into the night, convection over the Adriatic Sea and the Balkan states will gradually merge and continue to move northeastward into Hungary and Romania, while it starts to weaken.

...Germany...

A mixture of forcing along a stationary front (meridionally aligned from Hamburg to Munich) and lee effects in the wake of the Alps results in the formation of a shallow surface low, which then travels northward along the sharpening boundary. Convergent low-level winds will increase the depth of the low-level moisture, as it was already observed on Thursday with rising dew points up to 16°C. Scattered thunderstorms are expected to form in the afternoon on the warm side of the boundary, moving north- to northwestward and slowly crossing it. Moderate CAPE release (500-1000 J/kg) and moderate deep-layer shear (~15 m/s) in the Southerly background flow may result in multicells with a threat of medium-sized hail. Later, upscale growth into one or two MCSes may also locally bring excessive rain. A tornado is not ruled out if the cyclogenesis manages to favourably modify the low-level wind field to Easterly directions. However, storms will become elevated as soon as they enter the cool side of the boundary, hence they will not be able to fully consume the then rising shear.

...Western Europe...

Portugal, Spain, France, Wales and Ireland will see scattered, diurnally driven thunderstorms in the range of the big upper-level low or under pivoting vorticity maxima. Severe weather is unlikely due to limited CAPE release and weak vertical wind shear.

...beneath the Mediterranean jet...

An unseasonably strong and Southward displaced jet streak runs from Southern Spain to Southern Italy, before it gradually fans out towards Greece and Turkey. Despite some CAPE signals in the forecast models, current thinking is that barely any convection will be able to benefit from this impressive kinematic setup for various reasons.
For example, the strong cap in the warm air advection regime over Southern Greece and the Aegean Sea suggests that the CAPE and precipitation signals in these regions should rather be interpreted as warm front rain with possibly a few embedded elevated thunderstorms, than any form of surface-based convection. The transition to pronounced cold air advection over the Western Mediterranean erodes the cap, but large-scale subsidence ahead of a short-wave ridge will likely keep convection quite shallow between Italy and the Spanish East coast. The same applies to Southern Spain and Southern Portugal, where finally another pronounced short-wave trough arrives in the afternoon, but forecast cloud top temperatures do not raise the belief that the convection will be able to punch into the strong upper-level winds.