Storm Forecast
Valid: Thu 09 May 2013 06:00 to Fri 10 May 2013 06:00 UTC
Issued: Wed 08 May 2013 22:46
Forecaster: GATZEN

A level 1 was issued for central northern Poland and Lithuania mainly for large hail.

SYNOPSIS

Two high pressure areas across eastern Europe and the west Mediterranean are separated by a trough that extends across Italy and the Balkans. This trough digs south and forms a cut off centred over the south Aegean Sea on Thursday. As a consequence, mid-level sinking forms a weak ridge across Central Europe. To the north of this ridge, a negatively tilted Atlantic trough expands into western Europe. Winds will turn to south-west leading to increasing cold air advection across Central Europe.

DISCUSSION

Central Poland to Baltic States and southern Finland

On Wednesday, 21 UTC, a tongue of rich low-level moisture extends from the northern Balkans to the southern Baltic Sea. This moisture overlaps with steep low-level lapse rates, allowing for CAPE below 1000 J/kg. On Thursday, low-level flow rapidly turns to south-west across this region, advecting the moisture northward into the Baltic States. A convergence zone will develop ahead of the east European high during the day separating the rather cool, moist air to the west from warmer air masses to the east.

Current soundings indicate that the warm air across eastern Europe is limiting the chance of initiation given the strong capping inversion. Thunderstorms seem to be most intense just along the convergence line, where low-level moisture pooling can overlap with steep low- to mid-level lapse rates. Across central and northern Poland, CAPE may exceed 1500 J/kg locally, allowing for severe hail even in the rather weakly-sheared environment. Storms will likely cluster rapidly given the slow movement of the low-level convergence that seems to be the dominant forcing source.

Further north, thunderstorms will become gradually weaker, given weaker instability. However, a few large hail reports are not ruled out across the Balkan States. Along the northern flank of the moist air mass regime, southern Finland may see weak CAPE as well. If indeed storms can form, low-level veering profiles and about 10 m/s 0-1 km bulk shear can be supportive for isolated tornadoes. Main limiting factor in the northern regions will be the influence of the cold low-level Baltic Sea air mass that advects eastward behind the convergence, reducing instability rapidly.

Central and eastern Germany, western Poland, Czech Republic, and southern Sweden

Weak vertical wind shear, weak instability, but also weak CIN are expected in this region. Ahead of the approaching trough, small vort-maxima embedded in the south-westerly flow will travel across the region. This will likely allow for showers and thunderstorms that travel north-eastward. Bulk shear will become stronger during the day, exceeding 10 m/s in the lowest 3 km in the afternoon. Hodographs will be mostly unidirectional and well-organized storms are not forecast. An isolated marginal hail event is not ruled out.

Spain

A frontal boundary is present across northern Spain. It will slowly move southward until Thursday, leading to the advection of cool but rather moist air into the Iberian Peninsula. During the day, a mid-level jet streak will enter central and southern Iberia. Ageostrophic flow will lead to weak warm air advection along the frontal boundary, resulting in some storms. Although there will be strong deep layer vertical wind shear of 25 m/s between 0-6 km, lower level vertical wind shear is expected to be rather weak. Although an isolated large hail event is not ruled out, mostly non-severe convection is forecast.

Western Turkey

Near the south-east European cut-off, thunderstorms are expected in a weakly-sheared environment with rather weak CAPE. Larger CAPE may develop across western Turkey as well-mixed air may overlap the moist maritime air along a dryline/sea-breeze front in the afternoon hours. This overlap may result in large CAPE values, and an isolated large hail event is not ruled out.