Storm Forecast
Valid: Fri 22 May 2009 06:00 to Sat 23 May 2009 06:00 UTC
Issued: Fri 22 May 2009 09:51
Forecaster: GATZEN
A level 2 was issued for north-eastern Italy and south-eastern Austria region mainly for large hail.
SYNOPSIS
An intense trough moves across central Europe today, causing intense QG forcing. The axis of warm and unstable air mass ahead of the trough is situated from the Alps to south-eastern Poland and into western Ukraine in the afternoon hours. Neutral to unstable lapse rates in the range of the trough axis will spread across southern Scandinavia and northern Germany. High pressure over the Mediterranean will ridge into France, the Bay of Biscay, and also into the Black Sea region. A cut-off low west of the Iberian Peninsula is associated with a strong mid-level jet ejecting into Iberia.
DISCUSSION
North-eastern Italy, south-eastern Austria, Slovakia, Hungary, Slovenia, south-eastern Poland
Cool low-level outflow has spread into southern Germany and northern Austria, while warm and moist air mass is present form southern Austria to Slovenia. Ahead of the approaching trough axis, models suggest that the warm air mass will spread north-eastwards, and a rather strong increase in low-level moisture is likely. This may result in CAPE in the order of 500-1000 J/kg during the afternoon from eastern Austria to south-eastern Poland, as rather steep mid-level lapse rates are present. Best forcing is expected north of a mid-level jet streak moving across the northern Balkans in the next hours.
Limiting factor is rather cool low-level air mass, and most storms may be elevated initially. Many clouds will limit diurnal insolation to the west, but latest satellite images show dissipating clouds over south-eastern Austria and Slovenia. In the next hours, initiation of thunderstorms is expected along the leading frontal boundary, and thunderstorms will likely spread eastwards during the period. With deep layer vertical wind shear of 25 m/s, 0-3 km vertical wind shear of 15-20 m/s, and favourable veering profiles, ingredients seem to be very supportive for supercells from north-eastern Italy to south-eastern Austria, and very large hail and damaging wind gusts are forecast. Tornadoes are not ruled out, but weaker low-level vertical wind shear will be a limiting factor. Storms will move eastwards and north-eastwards into Slovenia, Slovakia, Hungary, and south-eastern Poland, where severe threat is expected to be lower due to weaker instability and forcing.
South-eastern Germany to northern Austria, Czech Republic, and central Poland
Along the trough axis, thunderstorms have formed over south-eastern Germany that move rapidly eastward in the range of the mid-level jet. Although low-level air mass is rather cool, chance for surface-based storms increases with daytime heating. Storms that root to the boundary-layer will encounter strong low-level vertical wind shear, and tornadoes are not ruled out in regions with some diabatic heating and increased low-level buoyancy. Locally large hail is also forecast with the stronger storms.
Belarus and western Ukraine region
Along the northern portions of the cold front, cool air mass is expected to spread eastwards rather quickly. Chance for severe thunderstorms seems to be lower as instability ahead of the front decreases. Best potential for supercells seems to exist over the western Ukraine region. Large hail and local wind gusts are not ruled out over most of this region, though.
Iberia
Ahead of the cut-off low, a cold front moves into Iberia during the period. To the north-east, strong daytime heating is expected to create inverted-v-profiles. Although low-level moisture is limited, CAPE will likely develop especially in the eastern and northern portions. Initiation is most likely along the cold front over central Iberia in the afternoon hours. Strong vertical wind shear will be favourable for supercells capable of producing large hail and also severe downbursts. Thunderstorms may merge into mesoscale convective systems while moving north-eastwards given strong forcing during the night hours.
