Storm Forecast
Valid: Fri 22 Aug 2008 06:00 to Sat 23 Aug 2008 06:00 UTC
Issued: Thu 21 Aug 2008 19:19
Forecaster: TUSCHY

SYNOPSIS

A situation, we saw so often this summer. Ahead of a slowly eastward progressing upper trough, an extensive frontal boundary forms from Spain all the way to Poland, separating a warm/humid airmass to its south/east from much cooler and drier air to its west. This boundary becomes active as scattered thunderstorms evolve, although environment is not favorable everywhere for organized thunderstorms. Hot and dry conditions continue over SE / E-Europe.

DISCUSSION

... NE-Spain and S-/SE France ...

A cold front arrives over NE Spain during the late morning hours from the NW, moving slowly southeastwards, before stalling somewhere over extreme NE-Spain. Combined with a weakening, eastward progressing vorticity lobe, enough lift will be present for scattered initiation, advancing from the NW to the SE already during the midday/early afternoon hours.

Ahead of the cold front, daytime driven onshore flow, fostered by falling surface pressure over SE Spain should advect a very moist BL airmass well inland, maximizing mixing ratio ahead/just along the front. In addition, the Spanish pulme overspreads the area of interest with steep mid-level lapse rates, which will be present during the afternoon hours. This will help to boost MLCAPE values to near 1500 J/kg with even higher MCUAPE values over a small area over extreme NE Spain. Warmer LL and hence capping should not be a problem as diabatic heating and strengthening convergence should break the cap easily.

Along the positive tilted upper trough, a broad belt of strong wind maxima covers E-Spain, including our area of interest and 25m/s 0-6km bulk shear and up to 200 m^2/s^2 SRH3 overlap, resulting in an enhanced large hail/ severe wind gust threat with the highest severe wind gust risk more inland, where LCLs are well above 1.5km (strong downbursts). A few significant hail events are likely and hence a small level-2 was issued. LCLs drop rapidly during the late afternoon hours/next to the coastline and as GFS hints on strong LL CAPE release, an isolated tornado event will be also possible. Thunderstorms, although diminishing in organisation, go on during the night hours along the coast.

The same environment and foci for development will be present over S/SE France although DLS is somewhat weaker. The main risk will be large hail and a few severe wind gusts. The threat also includes extreme NW Italy.

... SE-Germany and the Czech Republic...

Latest reality vs. model output check at 12Z indicated that GFS was running a few degrees to high with surface dewpoints over SE Germany, catching the airmass better over Austria/E-France. The surface dewpoints in the Munich soundings (00/12Z) changed not a lot and despite good diurnal heating/mixing, a constant increase of dewpoints over SE Bavaria was noticed. Advection and evapotranspiration could push dewpoints to above 15°C tomorrow afternoon but still some doubts exist that near 20°C can be realized as GFS output wants to. Steepening mid-level lapse rates and moist BL should yield widespread 500 to 800 J/kg MLCAPE, with even higher values if GFS is on track regarding dewpoint forecast. DLS of 20m/s, strong veering with height and potential robust instability release support organized thunderstorm-multicells and isolated supercells- with a large hail and isolated severe wind gust threat.

Thunderstorms will start over SW-Germany/N-Switzerland and the N-range of the Alpes during the midday hours and storms will move NE-ward in an increasingly more favorable environment. A speed max at 700hPa moves in from the west during the afternoon hours and combined with a pronounced UVV max. thunderstorms will line up. In general the severe wind gust risk will be higher over the N-risk level area as 0-3km shear increases to at or above 20m/s. In contrast to that, the hail risk will be enhanced with more discrete storms over S/SE Bavaria and discrete storms ahead of the potential MCS and if instability release matches model output even a significant hail event can't be ruled out, given strong veering with height. Later outlooks have to evaluate the final risk but an upgrade looks unlikely right now. A cluster of storms moves NE-wards and while the risk for severe events diminishes as instability vanishes/storms become more elevated, the risk for torrential rain continues with a localized flash flood risk.

As the main trough axis approaches during the morning hours, another cluster of stratiform rain/embedded thunderstorms from Switzerland will cross S/SE Germany although environment for thunderstorms becomes more hostile and the main thunderstorm activity will stay confined south of the western Alps.

... N-Europe..

A wide area over N-Europe will see diurnal driven thunderstorm development in a weakly sheared/moderately unstable airmass. Marginal hail and gusty winds will accompany stronger storms and the activity will diminish after sunset.