Valid: Sat 28 Jun 2014 14:00 to Sat 28 Jun 2014 21:00 UTC
Issued: Sat 28 Jun 2014 14:39
Forecaster: VAN DER VELDE
...France to southwestern Germany...
A surface low pressure center lies currently (14Z) at the west coast of France (Bordeaux). A large Atlantic upper trough moves into the region and will provide destabilization of the mid and upper levels, which is needed as 12Z soundings still show inversions at 900 and 450 hPa. A stationary frontal zone is well-defined by thermal gradients and a sharp wind shift (NE/S) running from some 100km north of Bordeaux to Stuttgart approximately.
Observed surface mixing ratios in France vary at 13Z between 10.5 and 13 g/kg with potential temperatures ranging between 26 and 32°C, the higher values over the Massif Central.
Inserting these values into Bordeaux and Nimes 12Z soundings, the cap can be bypassed in some locations and some 1000 J/kg SBCAPE could be obtained, but MLCAPE will be lower and mixed parcels may have some trouble with the cap.
In fact, satellite shows initiation over northern Massif Central oriented SW-NE and along the front in the west (WSW-ENE). The storms will grow in a strong kinematic environment with large but relatively straight hodographs. Observed Bordeaux 600 hPa wind is 60 kts with 40 kts at 900 hPa. This directly translates into a primary threat of severe convective wind gusts, especially as cold pools join into an MCS. Deep layer shear values of 15-20 m/s and SREH of 150 m²/s² initially and over 250 m²/s² predicted by GFS at 18Z (SW France) support supercells. 0-1 km shear ranges from 10 m/s in the afternoon to over 15 m/s during the evening (GFS). Together with relatively low LCL (descending to <1000 m in the evening) this underlines potential for tornadoes, and supports bow echoes. Large hail will be possible as well, but the upper level inversion represents a mixed phase region without much buoyancy which could be a limiting factor.
After 18Z the mid level cold front comes in from the west and will likely trigger storms in SW France moving upscale to a somewhat parallel-stratiform MCS over the southeastern quarter of France (this will happen beyond the validity time of this MD).