Storm Forecast
Valid: Tue 06 Nov 2007 06:00 to Wed 07 Nov 2007 06:00 UTC
Issued: Tue 06 Nov 2007 00:09
Forecaster: TUSCHY

SYNOPSIS

A potent mid-/upper-level jet curves around a leisurely east/southeastward moving high pressure area, centered southwest of Ireland.
As this jet enters a trough over the North Sea a constant increase of the trough's amplitude will be the result while this feature rapidly crosses Denmark, Germany and Poland from the northwest.
This constellation leads into a very dynamic pattern as the jet only gets stronger north of Ireland, reaching up to 170kt during the end of the day at 300hPa and which will be the focus for rapid cyclogenesis during the next days.
Cold conditions prevail over eastern Europe, as ridging builds northward and a combination of diminishing cold air advection and nocturnal radiation helps to create a broad pool of around / sub-zero temperatures at the surface.
The Mediterranean is characterized by stable conditions in the west ( with a strong Mistral-event going on )and a more unstable central and eastern Mediterranean, where lower geopotential heights support some thunderstorms.

DISCUSSION

...Germany and extreme NW Poland...

The focus for some thunderstorms arises as upper-level trough races southeastwards. The most difficult task with this feature is the fact that this trough exhibits a strong tilt downstream with height. The result will be that the time-frame for any development will be very limited, and as CAA at lower levels approaches from the north, substantial warming at mid-levels already starts to suppress any deeper convection from the NW.
Latest IR-images show a postfrontal airmass, looking not very impressive with cloud tops staying quite warm and strongest activity being defined to the narrow trough.
So we decided to show the best potential for a few thunderstorms mainly between 06 and 15Z, shifting from the coastal areas of NW Germany rapidly eastwards. We extended the highlighted area well inland, as latest AVHRR indicates SSTs over the southern North Sea of 12-14°C . Therefore expect a well mixed BL to advect inland, increasing the chance for some low-end instability release before mixing out well onshore.
After 12Z, temperatures at 500hPa increase and should suppress the chance for deep convection.

The risk for a few thunderstorms was extended eastwards, including NW Poland.

A few thunderstorms should also develop over western Norway .

Thunderstorms could produce gusty winds and marginal hail.

As CAA also gets active at the lower levels the focus then turns to more shallow-type convection. Latest thinking is that only traces of instability will be available over central Germany which should limit the chances for updraft electrification although an isolated SFLOC report over central and eastern Germany can't be ruled out completely.

An interesting area will be SW Germany during the later afternoon and early evening hours as atmosphere at and below 700hPa cools down. GFS is quite consistent in enhacing 0-3km CAPE as BL stays comparatively moist. EL temperatures stay cold enough for shallow convection and we highlighted the area beside many negative points ( like no clear UVV signal, bad timing although quite dynamic situation could offset / cause a delay in the development of the nocturnal stable boundary layer and diminishing QPF signals in the models ) because modified soundings indeed indicate the risk for such a scenario.
Even in the absence of any electrification more intense convection cells could pose a threat for an isolated funnel / short - lived tornado report as instability dispersal and kinematics are favorable.
Extreme eastern France and Belgium should be too close to the high pressure area to see any noteworthy convection.


...Skagerrat and Kattegat...

The main risk for a few thunderstorms should arise between 00 and 06Z as surface pressure falls and a tongue of somewhat higher moisture gets advected towards the area of interest. Some low-end instability release can be expected mainly offshore. LL shear will be enhanced mainly along the coastline and it can't be excluded that storms exhibit LL rotation, but overall risk looks like to be too low for increasing the probabilities.
Strong wind gusts will be the main threat.

Otherwise areas with isolated to scattered thunderstorms over the Mediterranean were highlighted.
No thunderstorm organisation is expected.