Extended Forecast
Valid: Sun 25 May 2008 06:00 to Mon 26 May 2008 06:00 UTC
Issued: Fri 23 May 2008 18:36
Forecaster: TUSCHY

SYNOPSIS

Intense upper - level trough nests over SW Europe as its core lurches around over the Bay of Biscay and Portugal / NW Spain. Model pool agrees quite well regarding strength and position as this feature reveals a neutral / slightly positive tilt. Numerous upper - level streaks are forecast to circle around this trough, one developing along its eastern side while building northwards and another 60 m/s streak rounding the base. This constellation should lead to no significant movement of the upper - level trough. In addition numerous short waves / vorticity maxima rotate around this feature as well resulting in a broad area of convective potential. Downstream of this system a strong ridge continues to strengthen while building north / northeastwards. Very warm to hot but mostly stable conditions continue under this ridge. The strong thermal gradient over far western Russia , already present during the past few days, shows sign of weakening as a weak depression slowly moves E / NE - wards along this boundary. Cold conditions prevail over far N - Europe.

A glance on the probabilistic maps reveals highest uncertainties along the UL trough over SW Europe and far N - Europe although the latter one is of little interest as no convective activity can be expected. Overall the major synoptic pattern should be handled quite well by global models.

DISCUSSION

... France, parts of Germany and the Low Countries ...

Thunderstorm development will be quite widespread as diffluent upper - level streamline pattern covers France, Switzerland and parts of Germany / Low Countries. Atmosphere is only weakly capped and some diabatic heating should help to break the cap during the late morning / midday hours. Convective debris from storms which occured the night before should limit insolation but a combination of moist boundary layer and steepening mid - level lapse rates should be adequate for widespread 200 - 600 J/kg MLCAPE. GFS has surface dewpoints locally near 20 °C which could be somewhat too optimistic but a combination of persistent southerly flow from the Mediterranean during the past few days ( readings there in the mid - / upper tens ) and evapotranspiration should help to push dewpoints in the 14 - 18 °C range. We do not want to go into too many details as small scale variations of short waves and placement of the jet streaks will lead to seesaw changes. Overall best shear and strongest instability are displaced but scattered thunderstorms should evolve during the day / evening hours over the area of interest, growing upscale into numerous clusters, which move northwards. There is a risk for more organized thunderstorms over central / eastern France and W / NW Germany and the Low Countries where DLS / LLS are enhanced. We decided not to introduce a level area due too all those uncertainties but parts of the highlighted area may see an upgrade.


... Central Mediterranean ...

Placed right under the strengthening jet streak, kinematics are impressive with DLS values of 30 - 40 m/s but the thermodynamics look meager. Tilt of the main upper - level trough precludes any cooling over the area of interest and hence forecast lapse rates are very weak east of Spain and around the Balearic Islands. Further towards the east, strong EML spreads northeastwards and despite very steep lapse rates due to strong WAA, airmass should stay capped. In addition there is no real vorticity maximum, which could support elevated thunderstorm development. The focus for numerous thunderstorms finally will be during the night hours between the Balearic Island and Sardinia and northwards. GFS develops a coupled jet structure with plenty of UL divergence, spreading over the area from the SW. No level area was issued due to missing instability but this area may also see an upgrade later on as shear would be more than adequate for organized thunderstorms.

Elsewhere diurnal driven thunderstorm activity is forecast with an isolated large hail risk over SE Europe, where shear could support a few multicell storms.

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