Storm Forecast
Valid: Fri 18 Jun 2010 06:00 to Sat 19 Jun 2010 06:00 UTC
Issued: Thu 17 Jun 2010 16:37
Forecaster: TUSCHY

A level 2 was issued for Macedonia, parts of Serbia, parts of Bulgaria, parts of Romania and Hungary mainly for large hail (significant events possible), severe wind gusts and to a lesser extent for tornadoes.

A level 1 surrounds the level 2 mainly for large hail, an isolated significant event over Greece possible and severe wind gusts.

SYNOPSIS

A broad trough over W-Europe gets reinforced by another one, which moves in from the north. WAA downstream affects most parts of SE-Europe with an active severe thunderstorm day in store.

DISCUSSION

... Greece, Macedonia, Bulgaria, Romania, Serbia, Hungary, parts of Bosnia and Herzegovina and Croatia ...

Quasi-stationary, flat trough over west/central Europe becomes reinforced by another, strong trough, which moves in from the north. This causes a continued period of a diffluent upper streamline pattern over SE-Europe with overall weak wind fields present. Numerous short waves move into this diffluent pattern with a constant weakening trend, so forcing is not impressive but it is available.

BL airmass features an high moisture content with latest synop data revealing surface dewpoints in the upper tens to lower twenties as a well structured EML approaches from the SW, so lapse rates undergo a continuously steepening trend throughout the forecast. This may help to boost SBCAPE values to 2000 - 3000 J/kg over Greece with a gradual decrease further north. Confidence is high that explosive thunderstorm development occurs, once thunderstorms initiate.

A more obstructive point is aforementioned weaker wind field, however a modest streak at roughly 3km moves in over Hungary and W-Romania. Anvil-layer shear is also augmented over those areas, so given abundant CAPE and modest shear (speed/directional), well organized multicells and a few supercells are forecast. Further south, over Greece and Macedonia, shear relaxes to less than 10 m/s 0-6km bulk shear, which limits potential degree of storm organization.

To summarize all that, we expect widespread initiation over the highlighted regions, as weakening forcing moves in from the west around noon. Large to very large hail (locally greater 5 cm in diameter) accompanies most vigorous thunderstorms over Hungary, E-Croatia, Bosnia and Herzegovina, Serbia and W-Romania next to severe wind gusts, especially if storms tend to line-up betimes, as strong cold pools evolve. LL shear is forecast to remain weak, but LCLs remain low, sub-3km layer very buoyant and keeping potential deviant storm motion in mind, a few storms may become tornadic. Storms tend to grow upscale into a large mesoscale convective system, moving eastwards during the night with locally excessive rainfall amounts.

Further to the south, over Macedonia and Greece, initition is forecast to occur a bit later and more isolated in nature in a weakly sheared environment. Nevertheless, explosive thunderstorm development is forecast with intense multicell thunderstorms. Despite bad kinematics, intense updrafts may produce large to very large hail, especially during the initiation stage before clustering. Also, environmental conditions are supportive for intense downdrafts with locally severe wind gusts reports. As clustering takes place, locally excessive rainfall is possible with that activity. Despite the fact that an isolated significant hail event is possible, no level 2 was issued, as coverage may be too isolated.

... N-Italy, Switzerland and Austria ...

An active day in store with numerous showers/thunderstorms forecast. Shear remains weak but 500-800 J/kg MLCAPE may localy support a stronger thunderstorm event. Thunderstorm activiy decreases after sunset.