Storm Forecast
Valid: Fri 30 May 2008 06:00 to Sat 31 May 2008 06:00 UTC
Issued: Thu 29 May 2008 23:08
Forecaster: TUSCHY

SYNOPSIS

The dominant feature during the next day is the huge upper - level trough, placed over the Bay of Biscay and SW Europe. The so far dominant trough weakens while moving northward over the central Mediterranean and Alpine regions and another trough builds in from the NW, resulting again in a disturbed weather pattern over most parts of Europe. Strong ridging, characterized by an impressive EML and unseasonably hot conditions prevails over parts of central Europe although weakening as influx of hot and dry air from the south preliminarily vanishes. Downstream of this ridge, cold air overspreads most parts of E - Europe, suppressing deep moist convection.

DISCUSSION

... W - Alpine region, Switzerland, E - France, the Low Countries, SW / W and NW - Germany ...

The broad level - 2 area was issued due to the possibility of a few significant hail events with numerous severe hail reports.

What was missing during the past few days ( beside strong capping ) finally seems to kick in from the south as a compact upper - level vorticity maximum approaches from the central Mediterranean during the early morning hours. A chain reaction, being made up of a weakening ridge over Germany / E - Autria, hence a weakening geopotential height gradient resulting in a weakening UL wind field and hence a constantly decelerating and weakening trough, makes this forecast quite uncertain. This uncertainty is also reflected in the model pool with quite substantial QPF differences as to intensity, placement and coverage of strongest precipitation. The main reason should be the different handling of the weakening and opening upper level trough. At lower levels, numerous more or less pronounced convergence zones are forecast by GFS which could serve as foci for initiation although weak convergence signals at lowest 2km indicate that those zones are pretty transient and not well structured.

Thermodynamics over the area of interest are quite impressive with expected MLCAPE of 1 to 2 kJ / kg and even higher SBCAPES. Although mid - level lapse rates undergo a temporarily weakening trend during the midday hours, they will support this robust instability release. Yesterday, 20Z, dewpoints between 16 and 19 °C were reported over the area of interest and this rich BL moisture will be present again so we will stick with instability forecasts of the otherwise too optimistic GFS output.

The day will start with numerous showers and constantly weakening thunderstorms mainly over western Germany / eastern France and the Low Countries. At the moment we think that the most likely scenario will be that one or two small clusters of thunderstorms will cross those regions from the south. Decoupled from the BL, they could pose an isolated large hail and strong wind gust threat as DLS stays strong but thunderstorms should weaken during the morning hours. After this time - frame, different solutions are possible. We think that the most likely one will be that numerous thunderstorms ( re ) develop over SW / W Germany in a weakly capped environment during the midday and early afternoon hours. DLS between 15 and 20m/s, SRH around 200 J/kg, increasing rapidly over W / NW Germany, quite steep mid - level lapse rates and favorable instability release in the hail growth zone between - 10 and - 30°C all point to a risk for isolated supercells and severe hail with each more discrete thunderstorm and significant hail larger than 5cm will be possible. It is a bit more uncertain, how far northwards thunderstorm evolve but GFS had persistent convective signals as far north as the Netherlands, covering W / NW Germany. Any thunderstorm which manages to develop will also pose a significant hail threat as 20m/s DLS and intense storm relative helicity overlap. GFS also points to an isolated tornado risk over W / NW Germany , E - Belgium and the Netherlands as LL shear increases during the afternoon and evening hours, but this depends on the exact evolution of the LL wind field wich is very uncertain that far out. Degree of thunderstorm coverage will depend on insolation and local convergence zones / outflow boundaries from tonight's storms as any forcing at mid - / upper - levels is missing. During the evening and early night hours , most models seem to agree that a small disturbances will come out of the decaying UL trough over Switzerland and the western Alpine region, crossing Switzerland / SW Germany and E - France from the SE during the evening and night hours. This is also the time - frame when LL / mid - level cool down from the south. A cluster of thunderstorms will likely evolve over Switzerland / SW Germany, moving NW / NNW - wards with a large hail and strong wind gust threat, reaching E - France at around 21Z and Belgium a few hours later. Finally, BOLAM also develops thunderstorm along the N - Alpine region during the afternoon and evening hours, which move slowly northward before weakening. Conditions for large hail are not quite as supportive as further towards the west but still a few large hail reports could be expected. BL is well mixed and a dry subcloud layer could result in locally strong to severe downdraft wind gust reports.For now we will go with the optimistic BOLAM output as some forcing overspreads the area from the south and hence initiation looks quite likely.

Such as the past days, there is a risk for excessive rainfall with those thunderstorms as background flow is not that strong and GFS points to a strong moisture inflow into those storms. Local flash flooding could result from this .

... N - Italy ...

Placed under the base of the decaying upper - level trough, widespread thunderstorm activity is forecast. Shear weakens betimes but is strong enough for an isolated large hail report mainly over NE Italy. Slow storm motion and moist airmass should support excessive rainfall which could be threatening in those regions which already were affected by training thunderstorm activity during the past 48 h. Thunderstorm activity will decrease after sunset.

... Parts of SE Europe ...

Strong instability release but only moderate shear in the range of 10 - 15m/s DLS will result in numerous strong thunderstorms. A mixture of pulsating thunderstorms and multicells will evolve with an isolated large hail and strong wind gust threat. Thunderstorms will continue during the night hours as mid - / upper levels stay supportive for further development.

... SW Europe ...

Modest instability release and weak shear will preclude thunderstorm organisation. Scattered thunderstorms will occur with strong wind gusts and marginal hail. While thunderstorms over S / SW France are mainly diurnal driven, storms will keep going over Spain during the night hours as mid - level cold pool moves in from the west.

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