Storm Forecast
Valid: Sun 20 Jun 2010 06:00 to Mon 21 Jun 2010 06:00 UTC
Issued: Sun 20 Jun 2010 05:46
Forecaster: SCHLENCZEK

A level 2 was issued for most of the Balkans mainly for isolated very large hail and severe wind gusts and to a lesser extent for excessive precipitation.

A level 1 was issued for parts of Italy mainly for large hail and severe wind gusts.

A level 1 was issued for the Dinaric Mountains towards Romania and W Belarus mainly for excessive precipitation and isolated large hail.

SYNOPSIS

Between an upper ridge west of Ireland and an upper trough over N-central Europe, cold polar air is advected southward. A cut-off process is expected at the southern tip of the trough over the Gulf of Genoa. Very warm and moist air is present ahead of the trough. The left exit region of a 35 m/s upper jet streak should be centered over S Greece at 06 UTC. The threat of severe thunderstorms should be maximised along this jet streak where moderate CAPE and strong vertical shear tend to overlap. Further north, weak pressure gradients and weak upper level winds are expected in a region with 1000 - 1500 J/kg CAPE.

DISCUSSION

...S Greece, Aegean Sea to SW Bulgaria from 06 UTC until 21 UTC...

Ahead of an upper shortwave trough over Greece, unstable air with 1000 - 1500 J/kg MLCAPE is present in a region with favourable shear as indicated by 16622 00 UTC sounding. Winds tend to veer with height with a local speed maximum around 8 km AGL (22 m/s). South / southeast of Thessaloniki, shear and instability should be similar. The airmass is strongly capped with CIN in a range between -100 and -200 J/kg but the approaching vort-max at the left exit should provide enough forcing for convective initiation. As the strength of the cap has a strong meridional gradient from the south to the north, the chance for isolated supercells should be best in the southern parts of Greece. A bit further north, storms will tend to merge into one or two large mesoscale convective systems. In the southern half of Greece towards the Bosporus region, the main threat should be isolated large to very large hail and some isolated severe wind gusts but an isolated tornado is not ruled out. The rapid storm motion around 20 m/s should preclude widespread flooding at first but later in the evening, another vort-max will arrive in this region.

...S-central Italy, Adriatic Sea, Balkans from 15 UTC until 06 UTC...

Another upper vort-max is located at the southern tip of the Italian cut-off low, moving eastward during the second half of the forecast period. Most CAPE should be present along the cold front which separates warm and moist subtropical air from rather cool polar air. MLCAPE should be in order of 500 J/kg and deep layer shear is expected around 25 - 30 m/s. Current thinking is that storms will tend to merge into a linear system, probably a LEWP line or some isolated bow echoes. Storms that stay discrete will likely produce large hail but the main threat should be severe wind gusts. The severe wind gust threat should be maximized over the southern Balkans whereas LL winds are almost parallel to the frontal boundary over the Dinaric Mountains and training storms will likely lead to excessive rainfall.

...Pannonian Plain to Danube delta...

In the warm subtropical airmass with approx. 1000 - 1500 J/kg CAPE, vertical wind shear is quite weak with 10 - 15 m/s. As the airmass is weakly capped, diurnal heating and local convergence zones will allow storm initiation, leading to briefly organised multicells and some pulsating storms. Some storms will tend to produce isolated large hail and slow storm motion may allow an isolated heavy rainfall event with flash floods possible. After sunset, CIN is expected to increase and storms will tend to dissipate.

...E Poland, W Ukraine, W Belarus...

Moderate instability, weak vertical shear and high BL moisture are forecast for this region which may lead to excessive rainfall and flooding as storm motion is very slow.

...N Germany...

Near the upper cold core of the Scandinavian trough, diurnal heating will lead to a few hundred J/kg CAPE over N Germany with showers and weakly electrified thunderstorms expected. Vertical shear should be below 15 m/s in most places but weak LL winds and locally steep LL lapse rates may allow an isolated tornado.