Storm Forecast
Valid: Tue 24 Aug 2010 06:00 to Wed 25 Aug 2010 06:00 UTC
Issued: Mon 23 Aug 2010 21:19
Forecaster: KOROSEC

A level 1 was isssued for extreme southern Germany, Austria, NW Hungary, Slovenia and N Italy mainly for large hail, excessive convective rainfall and strong wind gusts.

A level 1 was issued for Denmark, southern Scandinavia and parts of Baltic States mainly for strong winds and brief funnel/tornado threat.

A level 1 was issued for eastern Poland, Belarus and parts of western Russia mainly for large hail, strong to severe wind gusts, intense rainfalls and tornadoes.

SYNOPSYS

A broad upper trough amplifies over north-central Europe. Embedded in this trough, a robust vort-max is located over southern Scandinavia. One short-wave trough crosses the alps, while another wave moves from Belarus into western Russia. The main surface cold front extends from the surface low centered over southern Scandinavia SW-wards across Poland towards the Alps while slowly moving SE-wards towards northern Balkans by Wednesday. A large ridge with stable conditions persists over southern Europe.

DISCUSSION

... southern Germany, Austria, NW Hungary, Slovenia and N Italy ...

Ahead of the SE-wards moving weakening front, rather strong insolation will create unstable airmass and around 500-1000 J/kg or even higher (northern Italy) should become available. Moderate deep-layer shear of 15-20 m/s will be sufficient for organized storms. Moderate forcing should lead into numerous organized multicells and a couple of isolated supercells, capable of producing locally large hail and strong winds. Those should also bring the intense rainfall threat given their slow moving progress and locally, flash floods could occur. More favorable curvature/hodographs can be expected especially over parts of N Italy, which should support better rotating storms and higher large hail threat. Several models suggest that clustering into an early evening MCS is possible along the southern Alpine flank, moving across NE Italy and Slovenia overnight. However, it seems that the surface front will arrive quite early in the afternoon across the eastern part of level 1, where surface based storms become questionable given the CAA/NE flow in the lowest levels. Still, a level 1 was extended into these areas mainly due to a possible rainfall threat with the possible evening MCS.

... Denmark, southern Scandinavia and parts of Baltic States ...

An unseasonably strong upper trough/surface low slowly moves across the southern Scandinavia. South of this low, strong westerlies support quite strong shear, only partly overlaping with the shallow instability. Models suggest that across the warm sector a widespread activity with multicells and a few low-topped supercells should occur, given the EL tops up to -30°C only and moderate speed shear in place. A few of funnels/brief tornadoes could also occur given the quite strong (10-15 m/s) of LL shear and low LCL heights. A convective initiation is also expected along the NE-wards advecting warm front over southern Finland. With moderate shear/SREH available, more organized cells could indeed have supercellular characteristics and bring some marginally large hail threat, as well as strong wind gusts.

... eastern Poland, Belarus and parts of western Russia ...

A disturbance/wave which affected Poland yesterday, will be rapidly moving eastwards. Enough ascent should be available for storms to fire along the surface front. With moist BL ahead of the front, its reasonable to expect that 1000-1500 J/kg of MLCAPE should become available. As the wave will be embedded in the westerlies, at least 20 m/s of deep-layer shear will be available and rapid organization is expected when storms manage to initiate. Backing flow and favorable veering profiles suggests that well organized multicells and supercells will be possible, capable of producing large hail, damaging winds and a few tornadoes. Especially where SREH will be maximized (e.g. interaction of old boundaries with the front itself). Later in the afternoon, storms might merge into a larger system while moving into western Russia. However, parts of Belarus and western Russia might require an upgrade into level 2 if the severity of the storms will be more widespread than currently expected.