Storm Forecast
Valid: Thu 30 May 2013 06:00 to Fri 31 May 2013 06:00 UTC
Issued: Thu 30 May 2013 05:31
Forecaster: GATZEN

A level 2 was issued for southern Romania and surroundings mainly for large hail.

A level 1 was issued for the surrounding areas of the southern Adriatic Sea mainly for large hail.

A level 1 was issued for eastern Poland and surroundings mainly for tornadoes.

A level 1 was issued for western Poland mainly for excessive precipitation.

A level 1 was issued for the western Urkaine mainly for large hail and severe wind gusts.

A level 1 was issued for western Germany mainly for excessive convective precipitation.

The large digging European cut-off trough will slowly move across the north Mediterranean today. Its southern periphery will be affected by another strong jet streak spreading into east Europe during the period. Another vorticity maxima currently situated across the North Sea will rotate around the trough, moving into western Europe on Friday morning. In the wake of this feature, warm air advection will affect central Europe.

DISCUSSION

Romania and surroundings

The most robust severe weather set-up seems to evolve across southern Romania and surroundings today. Ahead of the nose of the approaching jet streak, a cold front has moved into Bulgaria and Romania and is currently moving northward into Romania, leading to some thunderstorms. In the wake of the cold front, clear conditions are indicated across Bulgaria by latest satellite imagery. The air mass in the wake of the cold front is characterized by relatively steep mid-level lapse rates and dry low levels sampled by the Thessaloniki sounding. Rich low-level moisture is expected across southern Romania near the cold front that will remain quasi-stationary today.

Strong diurnal heating is likely to the south of the cold front today, and a thermal low will develop in response to this heating across Bulgaria as indicated by latest models. This will result in continuing easterly winds across Romania, where the moist boundary will remain. Near the cold front, steep lapse rates evolving across Bulgaria will likely spread northward and can overlap with this low-level moisture. Latest GFS indicates 30 hPa mixed-layer CAPE around 1000 J/kg in the afternoon hours. The low-level convergence along the frontal boundary will assist for storm development as early as noon, and additional storms are expected to follow in the afternoon hours, spreading slowly northward as the frontal boundary starts to move north later on. Vertical wind shear will be moderate to strong near the jet streak exceeding 20 m/s in the lowest 6 km and 15 m/s in the lowest 3 km. Veering will be not too strong and SRH will be about 100 m²/s², but well-developed multicells and a few supercells are expected, capable of producing large hail. The storms moving along the quasi-stationary cold front may also produce excessive rain given rather rich moisture at the surface. Additionally, a tornado is not ruled out near the surface cold front where low-level vertical wind shear is rather strong. With continuing QG forcing, storms are expected to merge to a MCS in the evening hours moving northward with the cold front. Excessive precipitation, locally severe wind gusts, and isolated large hail are not ruled out until the night hours, when the system starts to weaken due to low-level stabilization.

Ukraine into Poland and surroundings

Strong warm air advection affects Poland and the surrounding regions ahead an approaching vort-maxima across Romania. This warm air currently across the Ukraine will spread westward and steep mid-level lapse rates will overlap with the moist boundary-layer further west, creasing CAPE up the nearly 2000 J/kg according the latest GFS.

Storms have already formed near the vort-max across Slovakia and will spread into Poland until noon. Despite rather weak deep layer shear, strong easterly winds reaching more than 15 m/s at 700 hPa will lead to favourably veering profiles with SRH around 150 m²/s² across southern Poland spreading into central Poland later on.

Storm clusters with embedded mesocyclones are forecast today. Large hail is forecast as well as severe wind gusts given the strong low-level wind field. The large overlap of 10 m/s 0-1 km vertical wind shear and CAPE highlights a tornado threat as well, and every isolated supercell forming in this regime may produce a tornado. Across the Ukraine, the deeper boundary layer will enhance the threat of severe wind gusts are mor likely. In the afternoon, storm clusters will spread into western Poland where excessive rain becomes more likely. After sunset, storms are forecast to weaken.

Italy to Greece

In the range of the mid-level jet, remaining low-level moisture can lead to weak CAPE during the day. Given the strong vertical wind shear, isolated large hail and severe wind gusts are not ruled out with stronger storms that form.

Western France

At the cyclonically sheared flank of a jet streak approaching from the north, westerly winds will advect some low-level moisture into western France. Diurnal heating will result in weak CAPE. A convergence zone s expected to develop near the Atlantic coasts in response to QG forcing. Thunderstorms are expected. Given strong low-level vertical wind shear in the range of a northerly low-level jet, these storms may produce severe wind gusts. A tornado is not ruled out.

Northern Europe

A zone of rich low-level moisture extends from the northern Baltic States to southern Norway. Diurnal heating will result in thunderstorms. Although vertical wind shear is rather limited across northern Europe, scattered thunderstorms may be accompanied by large hail and excessive rain. Main focus of this these threats will be near the cut-off low over the northern Baltic States.

Eastern Germany

Continuing convective activity, upslope flow, and rich low-level moisture result in a threat of excessive precipitation today. Storms will merge and weaken after sunset. A brief tornado is also not ruled out given enhanced low-level vertical wind shear.