In navolging van Alwin's thread: Estofex voor woensdag

.... 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.