Storm Forecast
Valid: Fri 05 Dec 2008 06:00 to Sat 06 Dec 2008 06:00 UTC
Issued: Thu 04 Dec 2008 17:32
Forecaster: TUSCHY
SYNOPSIS
Extensive cyclonic vortex covers NW Europe steering the large-scale flow over almost the entire forecast area. Various more or less pronounced disturbances are embedded in this cyclonic flow, resulting in thundery conditions for numerous regions. A combination of forecast boundary layer moisture quality, degree of forcing, agreement of model pool and expected degree of destabilisation were all considered while drawing the general thunderstorm areas. Marginal level-1 areas were included over the north-central Mediterranean, where shear/instability overlap fosters an low-end tornado/isolated large hail and strong to severe wind gust threat.
DISCUSSION
... A concentrated swath, including north/north-central and eastern France ...
A strong vorticity lobe traverses the level area from NW to SE at 09Z onwards. Moisture at low-levels remains marginal but rapid cool-down of the mid troposphere should still induce neutral/marginal unstable conditions over the forecast area. The streamlines near the surface become more cyclonic betimes with some moderate convergence and weak moisture advection present as the disturbance passes by. The timing is good with some diabatic heating possible and EL temperatures hint on potential deep convection with very thin vertical CAPE profilers. Enhanced convection is possible with the passage of the disturbance and degree of forcing also supports the idea of a forced line of storms, probably divided into numerous smaller line segments. Sporadic lightning could accompany activity and hence the level-1 is embedded in a general thunderstorm area. 25m/s at 850hPa is enough for strong to isolated severe wind gusts, especially when convection lines up. Next to marginal hail, this will be the main risk. However the airmass is well mixed and directional shear is very strong, so a tornado can certainly occur. The risk continues all the way to south-Germany, however, as strongest forcing passes south of the Alps, S-Germany was not included.
