Storm Forecast
Valid: Mon 02 Jun 2008 06:00 to Tue 03 Jun 2008 06:00 UTC
Issued: Sun 01 Jun 2008 21:31
Forecaster: TUSCHY

SYNOPSIS

The upper -level trough over SW Europe shifts slowly towards the north. Attendant forcing and disturbances support a broad area of thunderstorm activity over most parts of central Europe. The advection of a cool and more stable airmass continues over E - Europe.

DISCUSSION

... Belgium, the Netherlands, extreme E - France / W - Germany ...

As described in the latest extended forecast the main discrepancy within the model pool is the number and intensity of disturbances, coming out of the base of the strongly negative tilted upper - level trough, centered over S / central France during the forecast period. GFS still has a compact short wave, crossing the area from the SE during the afternoon and evening hours. At the surface no front is available and no real foci for initiation can be detected. GFS develops a channel of low surface pressure somewhere over Belgium and the Netherlands, running southeastwards which is characterized by a convergent flow but this feature is quite diffuse and broad so this is no real help to pinpoint potential initiation. However, upper - level disturbances and a weakly capped environment should all support scattered to widespread thunderstorm initiation already during the midday hours.

Thermodynamics are favorable for intense updraft evolution. A modified EML approaches from the S / SE resulting in steepening lapse rates. Although not as steep as the past few days over Germany it should be enough as boundary layer remains very humid and warm. Yesterday, readings south of the area had dewpoints between 14°C and 17 °C so GFS with 16°C to 19°C does not look too bad. 1000 - 1500 J/kg MLCAPE can be expected with the most robust release over the Netherlands.

Deep layer shear is not that strong with 10 - 20 m/s, increasing from east to west but strong instability release should offset the light shear. Of interest is an area of strong SRH values, running from extreme E - France to Belgium and S - Netherlands. In addition, GFS develops some LL shear in the order of 10m/s and slightly enhanced SRH 1 values so we can't exclude an isolated tornado report mainly over Belgium and S / Netherlands where LCLs are somewhat lower.

Thunderstorm initiation should be quite early already starting during the late morning and midday hours with the strongest and most widespread development over E - France and W - Germany. Those storms should rapidly develop into a cluster of thunderstorms which moves NW - wards, crossing Belgium during the afternoon and evening hours and affecting SE - UK during the night hours. Environment is favorable for numerous large hail reports mainly during the initiation when storms stay more discrete. Shear and thermodynamics also point to the chance for a few significant hail events with hail being larger than 5cm. A few strong to severe wind gusts and an isolated tornado also can't be ruled out. As thunderstorms mature / cluster the risk for extremely severe events diminishes but the risk of large hail / an isolated tornado remains. Torrential rain and flash flooding will likely accompany this cluster as slow storm motion and rich BL moisture will be present. This risk extends all the way to SE - UK where flash flooding will be a potential threat. Further towards the north over the Netherlands, the coverage of thunderstorms is somewhat less but environment also supports large hail and strong to severe wind gusts.

... Most parts of Germany ...

The ingredients come together over W - / central / SE - Germany for scattered thunderstorms with an enhanced flash flood risk. Shear is very weak as DLS decreases from 10 - 15 m/s to around 5 m/s from west to east. Instability release is strong with steepening mid - level lapse rates and a rich BL but we think that wet downdrafts will undercut strong updrafts too fast to support longer lived thunderstorms. Nevertheless, pulsating thunderstorms and maybe a few multicells will be possible with a large hail and strong wind gust risk. GFS develops some low - end SRH values over W / S - central Germany and this could also foster numerous large hail reports. Although not completely excluded we think that the environment is not supportive for significant hail events so we decided to stick with a level - 1 area. Thunderstorms should evolve into a loosely organized and north / northeastward moving line of storms which should undergo weakening after sunset. Dependant on the internal structure of this line, a few strong wind gusts will accompany the system which will likely affect S / SW and S - central Germany. Slow storm motion, abundant BL moisture and persistent signals of deep convergence in the lowest 2km point to an excessive rainfall risk especially when thunderstorms cluster betimes. Flash flooding will be a distinct possibility !

North of the level - 1 area, non - existing shear and moderate instability release will preclude organized thunderstorms. An isolated large hail event can't be excluded but otherwise the main risk will be marginal hail and strong wind gusts.

... Rest of Europe ...

The European streamline pattern supports an extensive area of mainly diurnal driven thunderstorm activity. Weak shear and modest instability release should support pulsating thunderstorms and multicell storms with an isolated large hail and strong wind gust risk. We upgraded parts to the broad general thunderstorm area where environment looks most promising for large hail reports. Although thunderstorms will decrease in intensity and coverage after sunset, there will likely be numerous thunderstorms during the night hours.

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