Mesoscale Discussion Valid: Sat 09 Aug 2014 16:00 to Sat 09 Aug 2014 19:00 UTC Issued: Sat 09 Aug 2014 16:58 Forecaster: VAN DER VELDE
...central France and northern Spain...
Recovery of lapse rates is taking place by lift provided by a broad upper trough and low level warm air advection. The 12Z soundings do not capture conditions inside the surface convergence zone running from Biscay to Switzerland. Dewpoints of 19-20°C (13-15 g/kg mixing ratio) are observed within this zone, with falling pressure, and initiation of storms has started before 15Z in central France. CAPE should be 500-1000 J/kg (GFS). Over Spain the low levels are drier and warmer. The region has to deal with a stronger CIN outside the convergence zone. In northern Spain it is questionable if CIN will be overcome. A second chance comes after sunset when a weak trough passes.
Storms that form have good chances of becoming supercells with large hail and perhaps isolated severe wind gust threat, thanks to more than 20 m/s 0-6 km shear and 200-300 m²/s² of SREH (0-3 km). The Lannemezan profiler shows increasing SWerly winds from 20 kts to 40 kts at 6 km which combined with backed surface winds (10 kts ENE) confirm 25 m/s deep layer shear. Along the central France convergenze zone LCLs are low. This combined with local 10 m/s 0-1 km shear in eastern France supports additionally a chance of an isolated tornado.