Forecast Update
Valid: Thu 15 May 2008 10:00 to Fri 16 May 2008 06:00 UTC
Issued: Thu 15 May 2008 10:14
Forecaster: TUSCHY

SYNOPSIS

Diffuse omega - like pattern is present over Europe with a major trough just west of Europe and another, smaller one over SE Europe. In - between, weak ridging has established but the ridge is too flat and faint to suppress convection. Weak pressure gradients at all levels prevail from Spain to Belarus and widespread thunderstorms are forecast.

DISCUSSION

... Highlighted areas ...

A few more hours needed for initiation in most places so expect initiation to occur mainly between 11Z ( western parts of the outlook )and 13Z ( for the rest of the highlighted area ) .

Weak shear and low - end / modest instability release won't support organized thunderstorms with a severe weather risk but strong pulsating thunderstorms could locally bring strong wind gusts and hail, approaching our criterion. Dewpoints at the surface changed not a lot to yesterday and in conjunction with very steep LL lapse rates and modest mid-level lapse rates, 100 - 500 J/kg MLCAPE are possible again. Enhanced SRH values and / or better DLS and / or stronger instability release resulted in a level - 1 for a few regions as hailstones could reach 2 - 3 cm in diameter. Evaporational cooling in dry column below high based storms should enhance downdraft strength with strong wind gusts possible but it would be a gamble to pinpoint those spots. Thunderstorms will decrease in intensity and coverage after sunset but locally enhanced forcing mechanisms could support thunderstorms until 06Z ( e.g. NW / W - France / Belgium and the Netherlands / SE Germany to W - Ukraine ). Slovakia, N - Romania and W - Ukraine will have the best chance to see an isolated tornado report as LCL comes down during the night and LL shear / SRH increases somewhat. Threat is too conditional to upgrade.

Another risk is a flash flood risk as background flow is very weak. Areas, where thunderstorm coverage will be enhanced ( e.g. S / SW Germany, Belgium and the Netherlands and S/SE Switzerland ) could see an upscale growth of thunderstorm activity into a few loosely organized thunderstorm clusters. Please keep in mind that this risk is not implemented in our risk scheme.


... S - Germany , Belgium and the Netherlands ...

We decided to spend a few more lines for those areas as there could exist a hail threat, equaling a level - 1. Latest synop reports and modified soundings reveal potential modest instability release in an overall weakly sheared environment but wind field and hence DLS strengthened somewhat in the mid-levels compared to yesterday with some veering present. A PVA maximum crosses those areas during the evening / night hours from the SW and should cause enhanced thunderstorm development. At the moment we decided against issuing a level - 1 as shear and instability are too weak but a level - 1 may be issued later on mainly due to an isolated large hail risk.




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