Extended Forecast
Valid: Wed 06 Aug 2008 06:00 to Thu 07 Aug 2008 06:00 UTC
Issued: Mon 04 Aug 2008 23:08
Forecaster: TUSCHY

SYNOPSIS

A strong upper trough just west of Europe shifts eastwards with intense WAA on its downstream side, overspreading most parts of central Europe. The ridge is not too strong and numerous disturbances circle around the ridge during the forecast period with various areas having thunderstorm chances. Strong CAA downstream of a broad cyclonic vortex over W-Russia prevents deep convection to form over most parts of NE and E-Europe.

DISCUSSION

.... Parts of France, Belgium and the Netherlands ....

Ahead of a strong upper trough just west of France, a hot and humid airmass spreads NE-wards during the forecast period with dewpoint calculations in the upper tens over France and Belgium / Netherlands. On the 4th of August, dewpoint readings over S-France were between 16°C and 19 °C, so those values are not too unrealistic. Atop of this favorable BL airmass, a Spanish Plume overspreads the area from the SW with very steep mid - level lapse rates so we can expect very unstable profilers over France, although mostly capped during the day due to very warm lower levels ( e.g. 850hPa temperatures between 20°C and 25°C over E / S-France ). MLCAPE values should reach widespread 1000-2000 J/kg and SBCAPE/MUCAPE values well above 3000 J/kg. Digging into details that far out is not useful as models like GFS try to develop numerous more or less potent short-waves, which will serve as foci for thunderstorm initiation, but model consistency regarding timing / exact track and strength of those lifting mechanisms is still considerably. It looks like storms will start over NW-France and later in the night also over SW-France, moving east-/northeastwards just west of the capped airmass over central / eastern France.

We are concerned about the extent of the 20-25 m/s 0-6km bulk shear and very high instability overlap, but also about a broad area with strong veering throughout the atmosphere, resulting in widespread strong helicity values (e.g. SRH 3 between 200 and 400 m^2/s^2). Each thunderstorm will have a good chance to rotate, producing large and possibly significant hail, but also strong to severe wind gusts. LL shear increases during the night hours mainly over E/NE France and a tornado threat could evolve for storms, moving into this environment but later outlooks will include new model informations to evaluate the final tornado risk.

Strong DLS and 500-1000 J/kg MLCAPE also overlap over Belgium and the Netherlands during the evening and night hours and storms will pose a large hail / strong to severe wind gust risk. We did not include those areas in the level-2 area as overall risk for significant events looks less likely.

... N - Italy ...

10-15 m/s DLS and strong instability release with up to 1000 J/kg MLCAPE will be present as a short-wave crosses the area from the west during the midday hours. Large and isolated very large hail, but also severe wind gusts due to strong downdrafts will be the main threat from those storms. For now, an high-end level-1 is enough, but if confidence in a more robust hail risk increases, an upgrade could become necessary.