Storm Forecast
Valid: Mon 07 May 2012 06:00 to Tue 08 May 2012 06:00 UTC
Issued: Sun 06 May 2012 20:46
Forecaster: PISTOTNIK/ SCHLENCZEK

A level 1 was issued for SE-ern Poland, parts of Belarus and W-ernmost Ukraine mainly for excessive precipitation, large hail and to a lesser extent for tornadoes.

A level 1 was issued for parts of Russia, central Ukraine, Moldova and NE-ern Romania mainly for large hail, excessive precipitation and to a lesser extent for severe wind gusts.

A level 1 was issued for the central Adriatic, Bosnia, Montenegro, Serbia and W-ern Romania mainly for large hail and severe wind gusts.

A level 1 was issued for Wales mainly for excessive precipitation and to a lesser extent for tornadoes.

SYNOPSIS

The transition to a more zonal weather pattern over Europe is in progress. Ahead of a stationary long-wave trough over the Northern Atlantic, a broad Westerly to Southwesterly flow covers most of the continent. While the polar jet runs over cool and mostly stably stratified air masses over the Northern half of Europe, the subtropical jet curves from Southwestern Europe via the central Mediterranean to Eastern Europe before fanning out over warm air with moderate instability. A short-wave trough initially situated over Italy travels northeastward while decelerating and closing into an - albeit shallow - upper level low over Slovakia during the forecast period.
Near the surface, a long frontal zone of varying sharpness runs from Western Russia via Poland to the Alpine region and continues its slow eastward movement, replacing the subtropical warm air by cooler maritime air masses. An offshoot of the cooler air also spreads into the central Mediterranean. Various shallow lows and wave disturbances form and decay along this frontal zone.
The strengthening cyclonic activity over the Northern Atlantic drops a first mature low pressure system over Ireland during the forecast period. Further and more juvenile lows stay offshore yet but start to influence the Iberian Peninsula with increasing warm air advection.

DISCUSSION

...Russia via central Ukraine S-ward...

Along the axis of the warmest air, numerous thunderstorms are again expected to form during the day, in particular over orographic features (mainly the Carpathian mountains) and along old outflow boundaries. Vertical wind shear remains very weak throughout most of the region, therefore single cells will be the dominant mode. However, moderate instability around 1000 J/kg suggests that a few strong pulse storms can be associated with large hail and/or locally excessive precipitation. The deep and well-developed convective boundary layer may also support a few severe winds gusts. A level 1 was issued for the regions with highest instability and expected highest storm coverage in order to account for these various possibilities, even though no concentrated areas of severe weather are expected. Storm coverage will generally decrease further South towards Bulgaria, Macedonia and Greece due to less instability and weaker dynamic support.
Storms further North over parts of Russia may benefit from stronger vertical wind shear in the warm sector of one of the wave disturbances along the frontal zone, whose center will be situated near St. Petersburg at the beginning of the forecast period and travel northeastward to the White Sea afterwards. Deep layer shear increases to 15 m/s and may support at least multicell storms with an enhanced risk of large hail.

...Slovakia, Poland, Belarus, W-ern Ukraine...

A complex scenario unfolds along the wavy frontal zone. Parts of Poland might be affected by a mesoscale area of moderate to heavy precipitation at the start of the day, which is the leftover of a shallow lee cyclogenesis to the Northeast of the Alps the evening before. However, this precipitation is thought to have lost its initially convective character overnight, and it will weaken along its way northeastward to Belarus.
With the arrival of the short-wave trough's vorticity lobe, the shallow low will re-intensify or form anew in the late morning over Southeastern Poland. Converging winds and insolation on the warm side of the frontal zone will strongly intensify the temperature gradient and probably create CAPE on the order of 500 to 1000 J/kg in a narrow band ahead of the front. Storms are expected to form around noon the latest. Even though most of the vertical wind shear will be confined to the cold side of the boundary, deep layer shear around 15 m/s will overlap with the unstable regions and is thought sufficient to shift the dominant convection mode to multicells. Low-level buoyancy is maximized immediately ahead of the front, which in combination with favourable interaction of outflow boundaries and/or the front itself might also support rotating updrafts and a slightly enhanced tornado threat. All in all, the setup seems somewhat marginal, tough, and large hail with stronger multicell storms will pose the major threat.
The strongly forced ascent along the front may concentrate the storms to a terminatory line with a tendency to propagate towards the warm side (i.e., to the East), even though in general the steering level winds run almost parallel to the front from the Southwest to the Northeast. This suggests that the threat of severe wind gusts is limited (though still worth mentioning as a side note), whereas localized flooding may become a distinct possibility, given that areas along the front may be exposed to heavy and predominantly convective precipitation for several hours. Convection will continue well into the evening, while spreading into Belarus and adjacent parts of Western Ukraine.

...Hungary, Balkan States...

Further Southwest, the frontal zone becomes increasingly diffuse while it is overrun by the Italien short-wave trough. The complex topography and the lack of a distinct air mass boundary make it hard to predict where and when further shallow lows will form, either dynamically due to lee effects in the wake of the Alps and the Apennin mountains, or thermally due to strong insolation. The left exit of a moderate jet streak which rounds the tip of the short-wave trough may be an additional player.
Current thinking is that the day will start with a cluster of rain and embedded convection affecting the Northern Adriatic Sea, coastal Croatia and maybe Bosnia in the morning hours. The same cluster may spread extensive convective debris and thick clouds over most of Hungary. Limited insolation and a possible shallow surge of cooler air from the Northwest make convective initiation and the severe weather threat somewhat conditional there. Higher probabilities for storm formation can be seen over Croatia, Bosnia and Serbia, where also the chances for the formation of a mesoscale (thermal) low and accompanying moisture piling at low levels seem to be best. CAPE will be around 500 J/kg, while the best deep-layer shear of 15-20 m/s is displaced to the South. A level 1 centered over Bosnia and Serbia covers the best overlap of vertical wind shear (which is mainly unidirectional) and instability. Large hail and severe wind gusts will be the main threat with these storms. Storm coverage is expected to be lower further South over Montenegro and Southern Serbia, but any storm may become multicellular or even supercellular there due to the stronger vertical wind shear. Convection will continue into the evening and spread northeastward into Transsylvania then, but decreasing boundary layer quality due to incipient nighttime cooling and probable antecedent air mass convection in the afternoon will gradually limit the severe weather threat. Besides, also the dynamic lifting support of the decelerating short-wave trough should decrease notably by then.

...Italy...

Central and Southern Italy will see similar conditions like Montenegro and Serbia, with a still unclear overlap between moderate instability (500 to 1000 J/kg) and moderate to good deep-layer shear (15-20 m/s). Chances for organized convection with a threat of large hail and severe wind gusts seem to be best over the East Coast of Calabria, where the convergence of up-valley winds and/or sea breezes from the Adriatic Sea with the Southwesterly background flow provides the most convincing initiation mechanism. Overall, the displacement of the axis of maximum shear to the South and the unfavourable timing with the arrival of the cold front in the late morning hours are limiting the threat potential.
Patches of CAPE are also forecast over Sicily and the mountains of Northern Algeria and Tunisia. Any storms forming there could benefit from deep-layer shear around 20 m/s beneath the subtropical jet branch. Confidence in convective initiation, however, is too low for issuing any threat level due to the lack of dynamic support.
Behind the cold front, convection will also go on or re-develop over Northern Italy, where the air mass exchange remains incomplete and lingering patches of augmented boundary layer moisture create modest instability in a weakly-sheared environment. Isolated small hail and gusty winds may occur with stronger storms, but severe weather is unlikely.

...Central Europe, Sweden...

A few weak storms may form in a cool maritime air mass with weak vertical wind shear. Severe weather is not expected, and the majority of convection will be non-electrified and quickly diminish after sunset.

...British Isles...

The frontal system of a mature cyclone, which parks itself just off the Irish West coast, crosses these areas from the Southwest and looks capable of sparking a round of maritime convection. Insolation and diurnal warming are expected to create one or two hundred J/kg of CAPE in the warm sector, where also deep layer shear is enhanced (15 to 20 m/s) underneath the polar jet. While the bulk of a strongly helical flow will be placed ahead of the warm front, still veering wind profiles (100-200 m^2/s^2 of 0-3 km SRH) and some low-level shear (~10 m/s between 0 and 1 km) will probably persist into the warm sector. Showers and thunderstorms in this environment may turn into multicells and low-topped supercells with a chance of small hail, marginally severe wind gusts and maybe an isolated tornado. The threat is maximized over Southern England and in particular over Wales, where persistent upslope flow and a possible convective line along the trailing cold front may additionally cause excessive precipitation. A level 1 was drawn in order to reflect this combined threat.
Vertical wind shear is much weaker in Ireland in the vicinity of the vertically stacked low pressure system, which might result not only in locally excessive precipitation but also in the formation of a few funnel clouds or even landspouts. A threat level does not seem to be necessary, in particular as the center of the low pressure system remains offshore.