Storm Forecast
Valid: Sat 20 Dec 2014 06:00 to Sun 21 Dec 2014 06:00 UTC
Issued: Sat 20 Dec 2014 03:04
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for the North Sea and large parts of North-central Europe for severe convective wind gusts and to a lesser degree for tornadoes.

A level 1 was issued for a small part of the Norwegian Sea for severe convective wind gusts.

SYNOPSIS

A strong zonal flow is present across Europe. To the south of a powerful cyclone centered over Finland, a long-wave trough starts to amplify southeastward. In its wake, temporary ridging overspreads the British Isles along with beginning warm air advection ahead of the next strong cyclone.
Southwestern Europe and the Mediterranean region are under anticyclonic influence.

DISCUSSION

... N-central Europe ...

In the left exit of a mid-level jet streak, an intense vorticity maximum ejects southeastward at the back side of the main trough. Rising motions ahead of it will cause rapid upper-level cooling and create an area of neutral to slightly unstable stratification over the Netherlands, Germany, Poland, the Czech Republic, northern Austria and Slovakia in the course of the day, whose southern part will overlap with strong 0-3 km shear (~20 m/s) and storm-relative helicity (200-400 m^2/s^2). Showers and low-topped thunderstorms will evolve, and short bowing lines with widespread severe wind gusts between 25 and 30 m/s can be expected. More discrete updrafts could evolve into supercells and bring one or two tornadoes as well.
The strongest lift is expected in the level 2 area, where convection has the best chances to become thundery. The highest risk is moving southeast in the 12 to 21 UTC time frame. Surrounding the level 2, still quite widespread severe wind gusts are possible, but the more doubtful presence of lightning and thunder is the limiting reason for staying with a level 1.
Convection will spread further into Poland, western Belarus and Ukraine, Slovakia and Hungary overnight while it gradually weakens.

... Norwegian and North Sea ...

Showers in the stream of deeply mixed polar air may still grow deep enough to produce a few lightning strikes. Due to a relaxing wind field, the risk of severe wind gusts decreases.
The only exception is a small region to the southwest of the Lofoten Islands, where a polar low may move onshore between 18 and 24 UTC. Next to a transient risk of wind gusts around 25 m/s, heavy showers of a rain-and-snow-mix may occur.