Storm Forecast
Valid: Fri 01 Aug 2008 06:00 to Sat 02 Aug 2008 06:00 UTC
Issued: Thu 31 Jul 2008 17:14
Forecaster: TUSCHY
SYNOPSIS
A strong low pressure system over the British Isles tilts over in a more zonal direction. Attendant surface cold front and upper trough axis cross NW and parts of central Europe with scattered thunderstorm development. A strong cold front pushes southward over NE Europe while hot and stable conditions persist over the Mediterranean.
DISCUSSION
... North Sea, parts of the Netherlands and Germany ( aside from S - Germany ) ...
An extensive, arc shaped cold front will be the main player as it crosses those areas from the W / SW during the day. This frontal boundary is best structured over the North Sea due to a more cross-frontal background flow, pushing the cold front rapidly to the north / northeast. Forcing and strong upper divergence provide persistent convective activity over the North Sea, which decreases during the day as instability vanishes / tongue of highly modified EML clears away. A few elevated storms are still not out of the question and hence we went with a quite extensive thunderstorm area.
Further to the SE, aforementioned cold front is already present over the Netherlands and W - Germany and we think that this boundary is still active regarding shower / thunderstorm activity due to a favorable pool of BL moisture just ahead of this front and strong upper divergence. This front runs in a slowly weakening and eastward retrenching thermal ridge over E - Germany and forward speed slows down as mid - / upper level wind field backs ahead of the main upper trough axis over N - France. Current thinking is that a more or less N - S aligned band of showers and thunderstorms re - / develops during the day, moving eastwards over central Germany and ENE over N / NE Germany. Despite a moist BL, quite warm mid - levels and barely 10 - 15 m/s DLS should keep the severe risk limited. Strong LL CAPE release could support a few more organized storms with hail approaching our criterion, but that's too marginal for a level - 1.
The area, which could get a few severe thunderstorms is E / NE - Germany, where better diabatic heating, constant surface pressure fall during the day and a convergence zone ahead of the main surface front could result in a few thunderstorms, breaking the cap. As models fail / have seesaw changes with this possible development we want to wait for more informations and an update / upgrade may become necessary later - on.
... S - Germany, western Alpine region, SE France and N - Italy ...
Slowly eastward moving surface front nestles up against the Alpes during the late afternoon and evening hours, providing good BL moisture. Combined with better mid - level lapse rates, MLCAPE of 700 - 1200 J/kg are likely to evolve during the day. Both, frontal forcing and diabatic heating should help to erode the cap until the early afternoon hours with scattered thunderstorm development expected. Wind field at all levels is slightly better with DLS around 15m/s and multicells could produce a few large hail / strong wind gust reports.
During the evening hours, upper trough axis finally approaches from the WSW, resulting in another burst of widespread thunderstorms mainly over Switzerland and W - Austria. Model pool is in general agreement in developing a large cluster of storms somewhere over central Switzerland, moving to the northeast. Marginal hail and strong wind gusts can occur but the main risk will be a flash flood risk, running from Switzerland over W - Austria to SE Germany during the night hours.
We issued a broad level - 1 area also for SE France and N - Italy as combination of modest DLS / moderate instability release could result in a few large hail reports over a broad area.
... S - central Sweden, S - Finland, Estonia and eastwards ...
A strong cold front plows southward during the day. At the same time, a strong upper trough axis crosses the highlighted area from the WNW so plenty of forcing will be available for scattered thunderstorm initiation. Latest model runs continue to place Estonia and extreme S / SE Finland in the most favorable thermodynamic environment with MLCAPE values approaching 1000 J/kg at the afternoon / evening hours. The wind field is more or less parallel aligned to the surface front, resulting in its slow - down and this should limit the risk for an organized squalline with widespread severe wind gusts ( in addition, the wind field in the lowest 2 km is not very strong ). Despite that, a solid line of showers and thunderstorms should re - / develop along / just ahead of the front and strength of DLS ( 15 - 20 m/s ) and instability release call for an isolated large hail / strong wind gust risk and hence a level - 1 was issued.
LL speed shear but not directional shear becomes stronger over extreme W - Russia and and isolated tornado can't be ruled out, although instability release will be quite limited with 200 - 500 J/kg MLCAPE. We may have to expand the level area to the east if more instability can be released than currently anticipated.
