Storm Forecast
Valid: Sun 05 Jun 2011 06:00 to Mon 06 Jun 2011 06:00 UTC
Issued: Sat 04 Jun 2011 16:39
Forecaster: TUSCHY
A level 1 was issued for parts of NE-France, Belgium, the Netherlands and parts of NW-Germany mainly for excessive rainfall, isolated large hail and a low-end tornado risk.
A level 1 was issued for NW-Italy mainly for excessive rainfall amounts.
A level 1 was issued for parts of Hungary, the Czech Republic and Slovakia mainly for a few severe downburst events.
SYNOPSIS
As an upper trough over the far E-Atlantic starts to amplify to the south, long term quasi-stationary upper low over the W-Mediterranean starts to move towards the east/northeast, approaching Italy during the night. Also, high pressure area over the North Sea weakens and a broad/flat low pressure channel evolves over W/central Europe. This configuration allows widespread thunderstorm development during the forecast period.
DISCUSSION
... Belgium, the Netherlands, NW-Germany and parts of NE-France....
During the night hours, a warm front continues to lift to the north, placing those areas in a very moist warm sector. Models agree well in a persistent convergent wind field as warm front stalls somewhere along the coast or just offshore. Postfrontal air mass gets characterized by deep and moist profiles with 925 hPa dewpoints in the mid tens, elongated moisture transport vectors/enhanced moisture flux, PWAT at or above 35 mm and nearly non-existent storm motion vectors. In addition, a modest right entrance jet at 300 hPa keeps SR-anvil layer wind field enhanced. Bound to neutral lapse rates profiles, about 1 kJ/kg MLCAPE seems reasonable... immediately along the convergence zone, even higher values are possible.
Scattered thunderstorms evolve during the day in a weakly capped environment and move in general towards the NW or N. Storms tend to reveal marginal net-movement over NE-France, Belgium and the Netherlands but could become more progressive over NW-Germany, as shear increases somewhat. Also, some weak directional shear is forecast, which may even assist in some updraft rotation/better organized thunderstorms. Regarding the threat, large hail and strong downdraft winds are well possible during the initiation stage, transforming more into a strong to isolated severe wind gust risk, as thunderstorms cluster/temporarily cold pool organization occurs. However, the main hazard will be excessive rain. During the night, thunderstorm activity onshore gradually decays but keeps going offshore, as large thunderstorm clusters persist.
Just as a side-note: Augmented LL-CAPE is forecast with low LCL-environment. Can't completely rule out an isolated funnel/tornado event, especially if outflow boundaries locally modify PBL wind field.
... CNTRL/SW-Germany ...
Overall, not many changes compared to areas further north. However, missing moisture pooling, not as moist low/mid troposphere and weaker anvil-layer winds limit the overall risk somewhat. Nevertheless, scattered to widespread thunderstorms develop early in the day, spreading leisurely towards the northeast. There is a chance that a loosely organized MCS evolves....however, weak forcing/shear may only assist in a few strong and isolated large hail events. Again, excessive rainfall events are possible, especially when thunderstorms cluster. Activity gradually decays after sunset.
... NW-Italy ...
Finally, quasi-stationary upper low gradually starts its journey to the E/NE, approaching the area of interest during the night hours. Already during the morning hours, scattered thunderstorms develop in an environment characterized by persistent moisture advection from the south and 10-locally up to 15 m/s DLS. Not sure that a lot of insolation will occur with rapidly clustering thunderstorms over SE-France, Sardinia and Corsica around noon onwards, which may limit overall risk fo severe events. Excessive rainfall and isolated large hail will be the main risk. This is also the case further east over N/N-CNTRL Italy.
Concerns increase that a potential bigger flash flood risk could evolve during the evening and night hours over NW-Italy, especially from Turin to Milan and northwards. As upper low draws near from the west the wind field at low/mid-levels constantly backs and hence assists in the advection of a very moist airmass towards NW-Italy. This, combined with a modest LLJ may allow back-building thunderstorms with rapid clustering forecast. Excessive rainfall will be the main hazard. Main uncertainty remains how daytime thunderstorm activity modifies BL air mass, but for not we think that main clustering occurs further west until sunset.
... Hungary and parts of the Czech Republic and Slovakia ...
Scattered thunderstorms evolve during the day in an environment characterized by modest CAPE/weak shear. The main reason for the level 1 was the advection of a dry mid-level air mass atop of the moist LL one, creating a set-up favorable for strong to severe downbursts. Next to that, isolated large hail may accompany stronger thunderstorms.
Otherwise, weak kinematics preclude a more widespread severe risk. As instability outside of the level 1 still locally approaches 500 - 1000 J/kg, isolated large hail/strong wind gusts are possible within the 50-% thunderstorm areas, however this risk will be dictated by mesoscale/storm-scale effects.
