Storm Forecast Valid: Tue 06 May 2014 06:00 to Wed 07 May 2014 06:00 UTC Issued: Mon 05 May 2014 18:14 Forecaster: PISTOTNIK
A level 1 was issued for parts of France, Belgium, the Netherlands, Luxemburg and W Germany mainly for tornadoes and severe wind gusts.
A meandering zonal flow has established over Europe. A progressive upper-level trough moves across the British Isles into BeNeLux and N France. Another one exits into Russia. Fairly low geopotential is also present over SE Europe and the E Mediterranean region, while transient ridging stretches from the W Mediterranean into Central Europe.
... BeNeLux, France, Germany ...
The frontal system of a surface low over the North Sea provides the main focus for deep convection. Its warm front moves over Germany, the Alpine region, the Czech Republic and Poland until the evening, while the cold front forms up along the continental coast of the Channel. In-between, a tongue of rich low-level moisture (8-9 g/kg) enters the warm sector with a 15 m/s low level jet from the SW. A prominent feature on the forecast maps, and confirmed by latest satellite imagery, is a dry intrusion which is about to overrun the moist warm sector at higher levels. The cold front will then likely attain a well-marked "split front" character. First convection my already emerge near the warm front in the afternoon hours, when it is crossed by one or two forerunning vorticity maxia over the N half of Germany. However, instability will likely be elevated and too weak for electrification. Confidence in the build-up of surface-based and more robust CAPE up to 400 J/kg increases further west, where stronger lift ahead of the main trough axis and cooling upper-levels enter the scene. Near the surface, weak warm air advection will likely persist till the evening, and the clearing of the upper-level clouds should allow a few hours of insolation. Showers and thunderstorms are forecast to initiate near the French North coast in the afternoon and to spread eastward into BeNeLux and W Germany in the evening. Linear forcing and strong wind shear (0-6 km shear around 20 m/s, 0-3 km storm-relative helicity of 100-200 m^2/s^2, and 0-1 km shear in excess of 10 m/s) favour the development of a convective line. Isolated severe wind gusts are possible, and the combination of strong directional low-level shear and cloud bases around or below 1000m points to an enhanced tornado threat with any more discrete updraft that manages to form. Further South, convective initiation will be more isolated over Central France, and an organization into multicells or even one or two supercells is possible. Severe wind gusts and an isolated tornado are possible with this activity as well, and localized hail approaching or exceeding 2 cm in size is not ruled out. Convection will move deeper into Germany overnight, but will become elevated and will gradually lose its severe weather threat.
... British Isles ...
Scattered to widespread, weakly electrified showers will form in response to diurnal heating of the postfrontal air mass. Frictional convergence of the Southwesterly background flow and sea breezes might promote training storms along the Irish South coast and over Cornwall with a slightly enhanced risk of heavy precipitation. Otherwise, limited instability and weak vertical wind shear will keep the severe weather risk low.