Plume of very warm and dry air at 850mb, evident on 12z Bordeaux and Trappes (Paris) radiosonde ascents above hot but moist surface airmass advecting north is providing a strong cap (lid) to convection this afternoon across France, though 850mb plume is creating steep lapse rates which combined with high temps approaching the mid 30s degrees C, is likely to yield MLCAPE of 1500-2000 j/kg. Cooling of mid-upper level temps with approach of upper trough to the west and convergence over N and W France is forecast to erode this cap Friday evening.
Model forecasts indicate thunderstorms breaking out this evening across N/W France, already there are storms across W France, which will then track north towards S England tonight. A strengthening southerly jet stream moving up from France and low-to mid level flow backing more SE'erly, as shown on Bordeaux and Trappes ascents, will increase vertical shear - allowing storms to organise upscale into an MCS, which may exhibit bowing line segments on its forward side, bringing the risk of damaging wind gusts, and also one or two supercells could be embedded. Large amounts of MLCAPE and strong directional shear shown by GFS suggests threat of large hail. The storm system is also likely to produce frequent cloud-to-ground lightning. PWAT (precipitable water) values of 39mm indicated by GFS are very high- so excessive rainfall and flash flooding are a real threat too. Given the likely organisation of an MCS bringing widespread threat of wind damage, large hail and flash flooding - have issued a MODERATE risk of severe storms.
The situation will be monitored, given storms have still yet to form, with updates possible to the categorical risk of severe weather.
