Extended Forecast
Valid: Tue 29 Dec 2009 06:00 to Wed 30 Dec 2009 06:00 UTC
Issued: Mon 28 Dec 2009 00:14
Forecaster: TUSCHY
A level 1 was issued for W/NW France mainly for tornadoes, severe wind gusts and to a lesser extent for large hail.
A level 1 was issued for N-Portugal/NW-Spain mainly for tornadoes and severe wind gusts.
A level 1 was issued for extreme SW-Spain and the Strait of Gibraltar mainly for excessive rainfall.
SYNOPSIS
A potent cyclonic vortex over the NE-Atlantic steers numerous disturbances over NW-/W-Europe. Showers and thunderstorms are well possible in this environment. NE/E-Europe is covered by cold/stable continental air with warm and capped conditions over the Mediterranean.
DISCUSSION
... NW France ...
Unseasonably warm and moist air mass overspreads France during the forecast, as a warm front advances northeastwards. The air mass over the Bay of Biscay and western France becomes more unstable during the night hours, as mid-levels cool down, so widespread 200-500 J/kg SBCAPE are a reasonable forecast. A concern exists, as LL trajectories indicate the traverse over N-Spain and N-Portugal but also a gradual mixing and hence lowering of the BL moisture. This scenario has to be monitored in upcoming model outputs and we excluded the extreme SE Bay for now.
Otherwise the feature of interest will be a strong short wave, embedded in the SW-erly flow, affecting the Bay and NW-France during the latter part of the forecast. WRF/GFS but also EZMWF agree to a certain degree with this disturbance and the attendant surface wave. Main concern arises along and south of a warm front, which accompanies the wave. A favorable overlap of CAPE and shear (25m/s at 850hPa and intense speed/directional shear in the boundary layer) hint on a severe wind gust and tornado risk although the latter one depends on the final quality of the BL moisture. Model consistency of the path of this feature increased the confidence of this event and a level 1 was placed over parts of W/NW France. The severe and thunderstorm risk spreads eastwards during the morning hours and the level area may need a significant eastward adjustment in later outlooks.
... Extreme SE-France and SW-Switzerland ...
As the short-wave approaches from the west, some moisture influx from the Mediterranean is forecast. Beneath steepening mid-level lapse rates, isolated thunderstorms are possible in an environment with intense shear. The main uncertainty will be the magnitude of CAPE, but any deep updraft would be able to organize. Severe wind gusts and an isolated tornado are well possible.
... NW-Spain and N-Portugal ...
Best moisture influx is well displaced to the south, as high PWAT tongue again points to the Strait of Gibraltar. If thunderstorms are possible in this band of persistent lift, an upgrade may become necessary due to intense rainfall.
The main story however is the highlighted area over N-Portugal and NW-Spain, as steep mid-level lapse rates approach from the WNW. Some instability build-up is possible with intense shear along the coast, so severe wind gusts and tornadoes are well possible, especially during the night hours.
