Valid: Mon 12 Dec 2011 06:00 to Tue 13 Dec 2011 06:00 UTC
Issued: Sun 11 Dec 2011 17:19
SYNOPSIS and DISCUSSION
Before another major branch of the polar vortex becomes established over far NW Europe, numerous weaker upper troughs/waves affect major parts of Europe.
The most active one will be a diffuse wave, crossing the central Mediterranean during the forecast from west to east while weakening. At the surface, pressure fall overspreads Italy and the Adritatic Sea with a broad LL depression in progress....again weakening during the night hours as another upper low drops to the south over the central Mediterranean and as CAA over SE-France weakens (a more zonal flow becomes established to the north/over France).
This set-up assists in scattered thunderstorms with heavy rain mainly along the westward facing coasts of Italy, Croatia, (Bosnia and Herzegovina), Montenegro, Albania and Greece. Shear remains too weak for substantial organization of the convection, although 2 more or less conditional severe risks exist:
- an isolated waterspout risk along any convergence zone along the coast with enhanced LL CAPE
- an augmented training shower/thunderstorm risk with messy alignement of mesoscale convergence zones and up to 15 m/s LL flow (locally aligned parallel to convergence zones).
No level 1 was issued for that event due to the conditional nature of coverage and intensity of any DMC.
Isolated thunderstorms are also forecast beneath numerous upper troughs over the N-North Sea/S-Norwegian Sea, far NE Atlantic, parts of Ireland and Scotland. Of note is a potential warm seclusion process with significant core temperature anomalies forecast by globel model data NW of Ireland (00Z onwards). Latest data indicates a successive slower approach to Ireland, so no impact by damaging wind gusts is expected during this forecast period. However the cold front passage has to be monitored for better postfrontal CAPE availability mainly over S/SE-UK. Right now, forecast soundings only indicate shallow convection and there is no reason to go with a lightning area. However, strong to severe wind gusts may mix down even withouth deeper updrafts with briskly 25 m/s at 850 hPa.