Forecast Update
Valid: Fri 11 Jan 2008 23:00 to Sat 12 Jan 2008 06:00 UTC
Issued: Fri 11 Jan 2008 22:52
Forecaster: TUSCHY

SYNOPSIS

Please refer to the outlook issued at Thu 10 Jan 2008 20:51.

DISCUSSION

...Extreme SW-France...

An update became necessary to reflect a more favorable environment for thunderstorms than anticipated / older model runs indicated.
Latest IR images show an area of enhanced convective activity over the southern / eastern Bay of Biscay, spreading towards the east / southeast as main upper-level trough slides eastwards.
Latest VWPs indicate a quite diffuse wind field in the lower troposphere, which should become better structured during the next few hours, as a 25m/s streak at 700hPa moves onshore over N-Spain between 01Z and 03Z.
Current surface observations indicate quite weak and variable winds ahead of the Cu-field. Marginal BL moisture and steep mid-level lapse rates at or above 7K/km create modest instability values along the coast, rapidly easing onshore.
In addition to the favorable placement of the 700hPa streak, a weakening PVA trail and attendant modest forcing will cross the area from the west during the next few hours.
Low LCLs, enhanced helicity values at lowest 1000m ( around 200J/kg) and traces of instability well inland favor a sporadic thunderstorm report over the highlighted area and an isolated short-lived tornado can't be excluded. In addition, marginal hail can accompany stronger storms.
Overall risk seems to be too low for warranting a level area.

No significant changes were done to the rest of the forecast as other regions seem to be on track.
We extended the TSTM area over SE France a little bit to the north as numerous thunderstorms evolved along the eastward shifting cold front. Time-frame for additional thunderstorms is limited and activity should decrease during the next 2-3hours.
Gusty winds and mainly marginal hail will be the main risk.
Wind field at LL level stays backed just ahead of this convective line with intense LL shear. An isolated tornado can't be ruled out but threat will be too low for issuing any level-area.