Storm Forecast
Valid: Fri 11 Jul 2014 06:00 to Sat 12 Jul 2014 06:00 UTC
Issued: Fri 11 Jul 2014 09:28
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for Bulgaria, E Romania and Moldova for large hail, severe wind gusts and to a lesser extent for tornadoes and excessive precipitation.

A level 1 was issued for parts of the Ukraine and Belarus mainly for excessive precipitation and to a lesser extent for large hail and severe wind gusts.

A level 1 and level 2 were issued for North-Central Europe mainly for excessive precipitation and to a lesser extent for severe wind gusts.

A level 1 was issued for N Italy for large hail, excessive precipitation and to a lesser extent for severe wind gusts.

A level 1 was issued for a small area in N Sweden and N Finland for large hail and severe wind gusts.

SYNOPSIS

A complex upper-level low still governs the weather over much of Europe. Its individual centers move from the Channel into Southern France, from Bosnia into Western Romania and from Lithuania into Northern Poland, respectively. The former two also induce weak surface cyclogeneses ahead of their paths in response to travelling vorticity maxima aloft.
While cool air is present in the range of these upper-level low centers, very warm air from Turkey spreads over the Black Sea, the Ukraine and Belarus. A moderate mid-level jet surrounds the upper-level low to the South and stretches from Southern Itay across the Eastern Mediterranean to the Black Sea.
Anticyclonic influence dominates from Iberia across the British Isles into Scandinavia.

DISCUSSION

... Bulgaria, E Romania, Moldova, Ukraine, Belarus ...

The dominant surface cyclone will move from Central Romania into the Western Ukraine during the forecast period. Latest surface observations indicate particularly rich low-level moisture in its warm sector over Eastern Romania and Eastern Bulgaria, where Southeasterly onshore flow with 5-10 m/s has advected dewpoints between 19 and 23°C well inland. With regionally steeper lapse rates aloft (refer to the 00z Chernivtsi sounding), CAPE on the order of 1000 to 2000 J/kg can build in response to daytime heating. To the West, a more or less pronounced convergence zone has formed along the advancing cold front, which will likely further be intensified as it merges with sea-breeze circulations.
Widespread initiation is expected in the next couple of hours, and with deep-layer shear between 15 and 25 m/s and favourably veering profiles beneath the Southwesterly mid-level jet, a quick organization into multi- and supercells and possibly a longer convective line can be expected. Large hail and severe wind gusts are the dominant risks, and a tornado is not ruled out in coastal areas where low-level shear will be maximized and cloud bases are lowest. A level 2 was issued for the areas around Eastern Romania which seem at the highest risk. A level 1 suffices further North, where vertical wind shear is weaker, and further South, where initiation becomes more doubtful due to a stronger cap and lack of large-scale lift.

Veering wind profiles and moderate deep-layer shear (10-15 m/s) extend into the Ukraine and Belarus, associated with increasing warm air advection from the South. CAPE magnitude is lower than to the South due to weaker lapse and slightly lower near-surface moisture, but at least around 500 J/kg are expected to build as soon as the leftover cloudiness from overnight's storm clears again. Scattered to widespread thunderstorms are expected to form in the afternoon. Multicells will be the dominant mode which pose a threat of large hail and severe wind gusts in their developing stages. Later on, they will likely merge into several large clusters which continue to move north- to northwestward in the evening. The main risk will then shift to excessive precipitation. Activity will gradually become elevated and weaken but will go on through much of the night, spreading into Belarus and possibly Easternmost Poland.

... Central Europe ...

Setup is very similar to the day before in the enclave of warm and moist air that is centered over the Netherlands and much of Germany. CAPE up to 1000 J/kg will quickly build again in response to diurnal heating, and widespread thunderstorms will form in the afternoon and evening. Weak vertical wind shear will mostly preclude a better organization, but upscale growth into numerous large clusters is likely. With very slow motion and precipitable water in excess of 30 mm, another round of quite numerous flash floods is suspected. Cold pool formation in large clusters may also result in a few swaths of severe wind gusts.
Highest coverage of storms can be expected along the main convergence zone which runs from the Central Netherlands across North-Central Germany into Bohemia and the bordering region between Austria and Slovakia, only very slowly moving towards the Southwest. Especially along this convergence, also one or two non-mesocyclonic tornadoes may spin up.
The advection of drier and cooler air will gradually start to quench activity from the Northeast towards evening.

... N Italy ...

The approach of the French upper-level low core keeps a weak lee cyclogenesis going over the Ligurian Sea. Southeasterly onshore flow advects Adriatic moisture into Northern Italy and improves vertical wind shear in the wake of the Appennines, where deep-layer shear increases to 15 m/s with CAPE values in excess of 1000 J/kg. Scattered to widespread afternoon storms can turn into multicells and isolated supercells. Large hail, heavy precipitation and isolated severe wind gusts are possible.

... Scandinavia ...

A tongue of particularly warm air is present to the West and North of the anticyclone centered over Southern Finland. Low-level moisture is maximized along a convergence zone that aligns along the Norwegian Alps and extends eastward into Lapland and Karelia. CAPE up to 1000 J/kg is predicted, and at least scattered afternoon storms will likely form. The Northern rim of the robust CAPE belt overlaps with increasing deep-layer shear up to 20 m/s over Lapland, where some storms can organize well and pose a threat of large hail and isolated severe wind gusts.