Forecast Update
Valid: Mon 23 Nov 2009 18:00 to Tue 24 Nov 2009 06:00 UTC
Issued: Mon 23 Nov 2009 18:28
Forecaster: TUSCHY

A level 1 was issued for Belgium, the Netherlands, Luxembourg and most parts of Germany mainly for severe wind gusts and tornadoes.

SYNOPSIS

Please refer to the outlook issued at Sun 22 Nov 2009 21:15 UTC.


DISCUSSION

... Level 1 ....

An update was issued to highlight the enhanced tornado risk over N/NE-Germany. The rest of the outlook seems to be on track.

The 10°C isodrosotherm finally reached the southern North Sea and 06/12 UTC sounding data reveals an impressive modification of the lowest 2-3km AGL. Larkhill (12 UTC) is the most representative one, sampling the immediate airmass and wind field just south of the upper wave. Not much to say in respect of speed and directional shear in the lowest levels, so rotating updrafts are a distinct possibility. Cold front is still hard to detect with the front now crossing W-Belgium/the Netherlands. Prefrontal activity (short, SW-NE aligned line segments) evolves in a moist and sheared environment.

Latest IR loop indicates a significant increase of deep convection, now occurring over Belgium and the Netherlands, spreading rapidly towards the east. Overall environment remains favorable for deep convection, as high IPV airmass overspreads the very moist lower/mid-levels. Due to the robust moisture advection and current widespread initiation, the level 1 was expanded northwards/northeastwards, now inlcuding all of N-Germany. Latest GFS run supports this idea, keeping convection going all night long all the way to NE-Germany until 06 UTC. Conditions for shallow or even deep mesocyclone evolutions remain favorable and therefore, the wording for an augmented tornado risk was increased over all of N-Germany. The main concern arises over N-central/NE-Germany after 21 UTC, when SRH-1 values also exceed 200 m^2/s^2 along the eastward racing cold front with a sharp gradient over extreme N-Germany, where the tip of the wave advances eastwards. Any thunderstorm, riding along those boundaries (cold front or at the occlusion point of this wave) poses an enhanced risk for tornadic activity and an isolated strong event is possible, given adequate LL CAPE and shear overlap. Next to the tornado risk, severe wind gusts (along bowing line segments) and marginal hail are all possible.

Further south, over extreme NE-France, central and southern Germany, no major changes were performed. Still enough LL CAPE is forecast along the cold front for a forced line of shallow convection, mixing down up to 30m/s from the lowest 1-3km, so severe wind gusts during the cold front passage are likely.

An upgrade was discussed due to the enhanced tornado risk, but no potential concentrated swath of severe/extremely severe events is seen in the forecast data, which would justify that.