Storm Forecast
Valid: Sat 17 Mar 2012 06:00 to Sun 18 Mar 2012 06:00 UTC
Issued: Sat 17 Mar 2012 04:36
Forecaster: VAN DER VELDE

SYNOPSIS

Relatively warm springtime temperatures prevail in most of Europe, advected by southwesterly flow. However, an Atlantic depression installs itself over the British Isles, sending a cold front into France, Belgium and the Netherlands during this forecast term. A secondary system is an upper cut-off low / PV anomaly which traversed the Iberian Peninsula the last two days, now moving from southern France to Germany.
Unstable air is found in the cold airmass over the ocean, including parts of Ireland and UK, possibly also in the prefrontal warm airmass over southwestern France, and very local perhaps over Germany (in GFS).

DISCUSSION

... southwestern France...

All major models (ECMWF, GFS, HiRLAM, WRF) predict a surge in precipitation at/just ahead of the cold front around 18Z starting north of the Pyrenees moving off to the NNE and persisting for quite long. This strongly suggests clustered thunderstorms or an MCS will form, much like in summer, but in a less unstable environment, only <200 J/kg MLCAPE. Bulk shear 0-6 km reaches over 20 m/s, in principle supportive of supercells, however the poor instability may not stand up to the shear and isn't likely to be supportive of strong persisting cells, the energy also drops rapidly after the late development, and therefore level 1 seems already too optimistic. An isolated marginally large hail event cannot be ruled out though. Mixing ratios should be monitored (6-7 g/kg average over 0-1000m in HiRLAM and GFS). With the lasting drought in Spain and France, soil moisture and boundary layer absolute humidity in the models might be inflated if based on climatology.