Storm Forecast
Valid: Fri 11 Jun 2010 06:00 to Sat 12 Jun 2010 06:00 UTC
Issued: Thu 10 Jun 2010 17:34
Forecaster: TUSCHY

A level 2 was issued for NE-Germany, N-Poland, Latvia, Lithuania, parts of Estonia and parts of W-Russia mainly for significant hail, damaging wind gusts and tornadoes, isolated strong events possible.

A level 1 surrounds the level 2 mainly for large hail, severe wind gusts and tornadoes.

A level 1 was issued for SE/E-Germany mainly for large hail and severe wind gusts.

A level 1 was issued for NW-Italy mainly for large hail.

SYNOPSIS

Quasi-stationary upper trough over Spain and France gets reinforced by another trough, which moves in from the north. This set-up assists in large-scale warm air advection ahead of this huge trough, which affects most parts of central Europe. Most parts of central/eastern Europe beneath the WAA regime will see hot/very unstable conditions, partially capped over E-Europe.
At lower-levels, a frontal boundary will be situated over the S-Baltic Sea, gradually transforming into an east/southeastward moving cold front.

DISCUSSION

... NE-Germany, N-Poland, Lithuania, Latvia, Estonia and extreme W-Russia ...

A concentrated outbreak of severe thunderstorms is forecast. Thunderstorms will produce large hail, significant events (5cm and more) possible, damaging wind gusts and tornadoes, strong ones possible.

Compared to the past days, upper-level support becomes stronger, as a potent trough over N-Europe digs southeastwards. At the surface, a west-east aligned frontal boundary is forecast to stall somewhere over the southern Baltic Sea/coast of NE-Germany and N-Poland, featuring a warm and very unstable warm sector to its south. Finally, a cold front over E-Germany gradually moves to the east and the quasi-stationary frontal boundary over N-Poland also transforms into a southeastward moving cold front (probably merging with the German one), so in respect of lift, enough mechanisms will be present.

This time, I absolutley have no reason to challenge GFS/WRF BL dewpoint forecasts of upper tens/lower twenties, which looks reasonable compared with latest synop reports. As WAA regime remains deep and strong, no significant increase in lapse rates at mid-levels is foreseen, but the amount of vertical temperature gradient is adequate for 1000 - 2000 J/kg MLCAPE with up to 3000 J/kg SBCAPE (the latter one also justifiable, given depth of moist BL air mass). Most significant CAPE may be released along and just to the south of the boundary, where moisture pooling effects will be in effect. The same for LL CAPE, which is bound to highest surface mixing ratios just south of the front.

With the approach of the Norway/Sweden trough, wind fields markedly increase throughout the atmosphere, resulting in impressive kinematic parameters. DLS of 20-30 m/s, 15-25 m/s 0-3km shear and 10-15m/s 0-1km shear overalp with aforementioned CAPE-rich air mass. Also, directional shear component is augmented along the boundary, but a more widespread increase is forecast during the evening hours over NE-Poland east/northeastwards.

The main uncertainty for this forecast package will be the final position of the boundary, which still reveals some discrepancies in latest model data. Also, convection from the night before may have an impact on when and where initiation will take place. However, initiation may occur over N-Germany (probably partially ongoing from the night before). Storms over NW Germany already find themselves in an environment, supportive for multicells/isolated supercells. Each more discrete thunderstorm may produce all kind of severe, although overall risk remains more limited. Conditions for supercells become way more supportive over NE Germay around noon onwards with large to very large hail and damaging wind gusts being the primary risk. Tornado risk does not yet look really eminent, although storms just have to have a slight deviant storm motion for ingesting an high amount of SRH.

Thereafter, conditions for large to very large hail, damaging wind gusts and tornadoes, significant events possible, become better with eastward progression and probably maximized somewhere over NE-Poland and to the north and east of that. This set-up features a good potential for a fairly significant and widespread outbreak of damaging supercells. An high-end level 2 was issued. Betimes, supercells start to cluster with the main severe risk changing to excessive rainfall, large hail and severe wind gusts. The latter one may become fairly widespread, if storms line-up. Thunderstorm activity gradually decreases from west to east during the day.

... Extreme W-Russia ...

As warm front boundary pushes eastwards, scattered thunderstorms are forecast to evolve. Shear and CAPE overlap will be more than adequate for long-lived supercells with all kind of severe and a level 2 was issued. Thunderstorms decrease after sunset.

...S/E-Germany ...

Forecast becomes more tricky over that part of Germany. A cold front drags south/southeastwards during the outlook and seems to stall somewhere over SE Germany. At upper-levels, weak ridging works in from the SW although right now, we see no real signal for any substantial thermal ridge, which could become established over that forecast area. However, models like GFS/WRF and EZMWF keep this cold front quite inactive with just sporadic initiation. Air mass ahead is very unstable with MLCAPE exceeding 1000 - 1500 J/kg and intense 0-3km/6km shear of 15-30 m/s, next to strong directional shear beneath 3km. Limited forcing and ridging aloft may indeed keep initiation very isolated, but any developing thunderstorm will be able to gain strength with a strong to severe wind gusts and large hail risk. In the past, this was a set-up for severe hailstorms over SE-Germany moving to the east just north of the Alps, finally entering N-Austria/SW Czech Republic as long-lived right movers, so this set-up definitively is worth watching.

We went ahead and issued a broad level 1 with low thunderstorm probabilities. However, an upgrade may become necessary when initiation becomes more widespread than currently anticipated.

...NW Italy and W-France ...

Isoalted large hail/strong wind gusts may accompany thunderstorm initiation over NW-Italy, but coverage remains quite limited. A marginal level 1 was introduced for that.

Scattered showers/thunderstorms evolve beneath the flat upper trough. Shear is too weak for anything long-lived and organized, but marginal hail may still occur beneath stronger updrafts.