Storm Forecast
Valid: Sat 08 Dec 2007 06:00 to Sun 09 Dec 2007 06:00 UTC
Issued: Fri 07 Dec 2007 22:31
Forecaster: DAHL

SYNOPSIS

Upper trough moving into the N Mediterranean on Friday evening is simulated to close off into a cut-off low on Saturday. Attendant SFC low that has formed over the Gulf of Genoa will move into the south Balkans by Sunday morning, while undergoing some weakening. Quite intense SFC low will move from the eastern Atlantic into Ireland during the period. Trailing cold front will affect the British Isles, and later France, Benelux and Germany.

DISCUSSION

... western Europe ...

Yet again, convective development along the cold front of the SFC low moving towards Ireland is quite uncertain. It seems that deep CAPE will not develop in the pre-frontal air mass (with the exception of southern/eastern France early Monday morning in the GFS 12Z run). Most likely scenario seems to be the development of intermittent shallow/narrow and strongly forced linear
convective segments along and ahead of the front. This convection may produce sporadic lightning, apparently strongly depending upon the local meso-beta/gamma-scale upward forcing along/ahead of the cold front. Abundant horizontal vorticity will be in place, and some updrafts may attain rotation, hinting at a slightly enhanced threat for a brief tornado or two. However, main effect of these convective lines will likely be a slight augmentation of the already (nearly) severe background gradient flow. It seems that best large-scale forcing for vertical motion will exist over France after Sunday 00Z, when GFS 12Z also simulates reasonably deep CAPE.

... Ireland ...

Postfrontal convection will likely affect Ireland late in the period ... when the largest SFC pressure gradients will move across the region. Abundancy of low-level vorticity suggests that chance for rotating updrafts is large, but sustenance of the updraft is uncertain given immense shear. Main severe weather threat should be tied to the background flow, with the contribution from the convection probably being too low to warrant a level-one threat. Lightning should also be quite sporadic with this activity.

... central Mediterranean ...

Though CAPE will likely rermain quite unimpressive ... moderately strong shear profiles should maintain some threat for severe thunderstorms. Expect isolated marginally severe wind gusts and possibly also some hail, but threat is somewhat marginal. Beneath the upper cold pool, some waterspouts may occur, though LL lapse rates are also not particularly strong. Still, a level-one is marginally warranted.