Storm Forecast
Valid: Fri 10 Apr 2009 06:00 to Sat 11 Apr 2009 06:00 UTC
Issued: Thu 09 Apr 2009 14:32
Forecaster: TUSCHY

SYNOPSIS

The trough over western Europe spreads eastsoutheastwards, splitting into a discrete cyclonic vortex over Spain, whereas the rest stalls just west of UK. Another upper low exits Tunisia during the morning hours northeastwards, entering the Tyrrhenian Sea during the night hours with a constant weakening trend. A small but active cold-core low at higher levels affects east/southeast Poland. The global models agree well regarding track and strength of this feature.

At the surface, a broad surface high causes quiescent conditions over E/NE-Europe as cool and dry conditions still affect those areas. The idea of a developing surface depression east of Spain during the night hours is well accepted in the model pool with GFS showing the strongest development. Otherwise, weak pressure gradients and warm/humid conditions assist in another day with scattered thunderstorm activity over central Europe.

DISCUSSION

... E-France, Belgium, the Netherlands, Luxembourg and the S-North Sea ...

Despite weak signals of northward traveling short-waves along the eastern fringe of the trough, no synoptic scale forcing mechanisms are expected. A departing wind maximum at 500hPa over the western North Sea places the area under the right entrance region although divergence values remain slim during the day. Therefore thunderstorm development will be tied to orographic support, surface convergence zones and boundary layer moisture dispersal mainly over E-France and extreme W-Switzerland. Furthermore GFS backed off the eastward progress of the trough during the past few runs, which shifted focus for initiation to W-Switzerland and N-Belgium/Netherlands.

The surface features dewpoints of 9-12°C which is already enough for daytime driven initiation despite weak lapse rates throughout the atmosphere. WRF and GFS have most robust MLCAPE release over Benelux with up to 500J/kg, decreasing rapidly towards the south/east.

Although thunderstorm initiation can be anywhere in this broad area, extreme W/NW Switzerland and Benelux northwards have the most promising conditions for initiation. Shear at all levels is weak at best, so strong wind gusts and marginal hail are possible. A very isolated large hail risk over Benelux can't be ruled out if CAPE release indeed approaches the optimistic GFS output, but confidence in that scenario is too low for a level-1. Thunderstorms move northwards over the North Sea, decreasing in coverage/intensity after sunset.

... Central / E-central Europe ...

Various regions over central Europe will see short-lived thunderstorms, daytime driven and mostly confined to mountainous areas. Marginal hail and gusty winds are the main hazard with that activity. Thunderstorm coverage and intensity decrease after sunset.

... Portugal, Spain and the western Mediterranean ...

Thunderstorm chances increase from west to east during the day as an evolving vortex at higher levels swings in from the northwest and as surface depression develops over SE-Spain during the evening/night hours. Cold mid-levels combined with some diabatic heating ought to be enough for scattered showers/thunderstorms. Surface moisture behind a SE-ward moving, decaying frontal boundary is scarce and mixing during the day should keep dewpoints quite low throughout the day, so instability release will be marginal at best. A lowering WBZ favors hail with stronger storms, especially if updrafts penetrate high enough to take profit of 10-20m/s DLS.

Another focus for initiation will be the eastward moving cold front, placed somewhere over E/NE Spain during the afternoon hours. The front becomes stationary over NE-Spain, while the southern part over SE Spain re-strengthens during the evening hours, as pressure falls over SE-Spain. Better moisture ahead and some diabatic heating should lead to an increase in convection along the front with increasing thunderstorm chances but no severe weather event is expected.

Finally, thunderstorms also grow around the Balearic Islands after sunset as the surface depression evolves and drifts northeastwards. Both shear and instability are too slim for any level area, so nothing severe is forecast.