Storm Forecast
Valid: Thu 26 Apr 2012 06:00 to Fri 27 Apr 2012 06:00 UTC
Issued: Wed 25 Apr 2012 22:16
Forecaster: GATZEN

A level 1 was issued for northern France, Benelux, north-western Germany, Denmark, and southern Sweden mainly for severe wind gusts.

A level 1 was issued for eastern Romania mainly for large hail.

A level 1 was issued for northern Turkey mainly for large hail.

SYNOPSIS

The east European trough cuts off over the west Black Sea region and looses intensity. To the north and west, a ridge builds from the central Mediterranean to the Ukraine. Together with a deepening polar trough over the north-eastern Atlantic, a very strong mid-level jet will develop stretching from southern Portugal to southern Finland at the end of the period.

DISCUSSION

... Northern France, Benelux, north-western Germany, Denmark ...

Embedded in the strong mid-level jet stream, a vort-max will travel north-eastward at Thursday. In the morning, this vort-max is expected over north-western France. At low levels, a wave is forecast along the frontal boundary. Warm air advection will result in rather strong QG forcing that expands north-eastward during the afternoon and evening hours.

Main uncertainty is the amount of CAPE available to the north of the surface front. Latest models indicate rather cold low-level air masses up to 700 hPa, while warmer air may be present at 500 hPa. This would lead to rather low cloud tops as indicated by latest GFS. Additionally, low-level moisture will be quite weak with only 5 g/kg 0-1 km mixing ratio. This moisture may increase due to low-level convergence along the sea breeze front and some moisture advection from the south-west.

Current thinking is that the strong forcing will likely create clouds and rain along the frontal boundary. At the cold side, the forcing could result in weak instability and showers and thunderstorms are expected after noon, when low-level heating is strongest. The activity will rapidly spread north-eastward during the afternoon and evening. Given the strong vertical wind shear in the lowest 6 km, fast moving showers and thunderstorms may evolve bowing lines. Given the strong background flow and the fast propagation, some severe wind gusts may occur. The strong low-level vertical wind shear may increase the potential of tornadoes as well. The weak buoyancy will limit this potential, but an isolated event is not ruled out, though.

... Western Romania ...

Near the base of the cut-off low, thunderstorms are forecast in the afternoon hours. Given rather weak CAPE in the cold air mass and weak vertical wind shear, chance of strong updrafts are limited. Isolated large hail may be possible with stronger storms.

... Northern Turkey ...

Ahead of the cut-off low, a westerly low-level flow will lead to some cold air advection into the southern Black Sea. This cool air will be capped by an elevated mixed layer that develops over central Turkey. Daytime heating will result in weak MLCAPE around 500 J/kg. With weak vertical wind shear, only isolated large hail is forecast.