Storm Forecast
Valid: Fri 27 Nov 2009 06:00 to Sat 28 Nov 2009 06:00 UTC
Issued: Thu 26 Nov 2009 18:01
Forecaster: TUSCHY
A level 1 was issued for severe wind gusts and to a lesser extent for tornadoes.
SYNOPSIS
Extensive cyclonic flow affects most parts of Europe. Embedded in this steering flow are numerous disturbances, which affect NW/central and N-Europe during the period. Geopotential height falls also start over the central Mediterranean, resulting in a growing thunderstorm risk during the late night hours.
DISCUSSION
... NE-France, Luxembourg, Belgium, the Netherlands and extreme W-Germany ...
... Until roughly 12 UTC...
Numerous waves of enhanced thunderstorm chances expected for today. First one is associated with the NE-ward moving short-wave during the early morning hours. This wave is about to exit NE-France during the start of the forecast and is over far NE Germany roughly 6h later. At the surface, a weak cold front affects Belgium and Luxembourg during the morning hours and NW-Germany later-on. A rapidly decrease in strength of this front is likely as prefrontal moisture tongue decays. Hence, the main chance for isolated thunderstorm activity will be over Belgium, Luxembourg and extreme W-Germany between 06-12 UTC. Despite marginal CAPE, a gradually diminishing quality of the BL and bad timing, the strength of the attendant vorticity lobe with a nice UVV maximum, dry mid-/upper levels, steep mid-level lapse rates and the strength of the shear, especially at lowers levels,require a level 1 for isolated severe wind gusts ( H850 wind up to 25m/s) and tornadoes (SRH1 augmented along the cold front). Marginal hail is already possible beneath strongest storms. Generally, the severe risk and coverage of storms will diminish thereafter over N-Germany due to very limited surface moisture access. Forced nature of this convection however could result in isolated, electrified convection as far east as N-central Germany.
... 09-18 UTC ...
Aforementioned short-wave rides along the SE fringe of a broad mid-level trough, which affects NE-France, Belgium and the Netherlands after 09 UTC. Cooling mid-levels and persistent influx of some offshore moisture ought to yield a good environment for another round of showers/thunderstorms. This convection will be bound to the diurnal diabatic heating cycle but will also be fostered by the approach of a weak vorticity maximum, as the mid-level through axis passes by - perfect timing implied. There is a broad overlap of marginal CAPE and enhanced LL speed/directional shear, probably maximized over Belgium and the Netherlands, where moisture is still adequate. A level 1 for isolated tornadoes, isolated severe wind gusts and marginal hail was issued and even large hail can't be ruled out, where more persistent updrafts evolve. ECMWF and GFS still differ in the amount of instability and how far inland convection will move, so we decided to stick to best BL moisture advection and coldest mid-levels for a level 1. However, areas further inland may see an upgrade, if new data indicates a better chance for inland convection. Strong wind gusts accompany convection as far east as NW-Germany, before overall activity decays after sunset (not offshore).
... Thunderstorm areas along the coast of Ireland, Scotland, UK and NW/N-France ...
Showers and thunderstorms are forecast during the outlook with strong gusts and marginal hail being the main risk. The activity decreases from west after sunset, as WAA of another disturbance works its way towards the east.
... Parts of the central Mediterranean ...
Thunderstorm chances increase during the night as upper trough draws near. Shear at all levels is quite strong, but expected instability does not yet justify to go with a 1. Bulk of activity remains offshore, but decaying storms already affect the coastal areas with some better rain amounts during the early morning hours (28th Nov.)
