Storm Forecast
Valid: Mon 19 Mar 2007 06:00 to Tue 20 Mar 2007 06:00 UTC
Issued: Sun 18 Mar 2007 17:22
Forecaster: TUSCHY

SYNOPSIS

Dramatic changes are underway / have already occured over Europe.
Southward expanding polar vortex will reach the extreme northern part of the forecast area during the end of the forecast period .
At lower levels, a combination of a stout 1040hPa anticyclone over the Azores and a slowly filling low pressure area over Norway and Sweden will support an outbreak of arctic air over parts of western, southern , northern and central Europe. A broad area will be affected by this type of airmass and will see an active convective day.
Strong WAA on the downstream side of the impressive upper-level trough will bring warm conditions well towards the north, affecting eastern / northeastern Europe.

A developing lee-cyclogenesis over N-Italy will be another area to watch out for enhanced thunderstorm activity.

DISCUSSION

...Scotland, Ireland, United Kingdom, France, Belgium, the Netherlands, Germany, Switzerland and parts of Spain...

Temperatures of -35 to -40°C at 500hPa, -15°C at 700hPa and -5°C at 850hPa will cover a huge area, supporting mid-level lapse rates of 8-9K / km !
This kind of atmospheric stratification will admit SBCAPE release in the order of 100-300 J/kg.
Such a postfrontal convective event makes it difficult to pinpoint areas of enhanced (severe) thunderstorm activity that far out... especially over such an huge area.

GFS indicates numerous lines with enhanced moisture advection, various upper-level impulses and small UVV maxima, which will cross the area from the north. Topography and fluctuating boundary layer moisture will play a role as well as possible small-scale polar low development, which are hard to forecast by global models.
Summarized we can say that a broad area will see isolated to scattered thunderstorm development during the next 24 hours.

LL shear of 10-15m/s was calculated for Ireland, Scotland, UK and northern / western France with decreasing values further towards the east / southeast.
Boundary layer moisture is forecast to be very shallow and constant mixing will bring the LCLs to values of 750-1500m, which will lower the tornado possibility.
Storm relative helicity values stay pretty low , besides UK, Scotland and Ireland.

Decided to issue a level-1 for regions, where 850hPa flow is at least somewhat enhanced ( e.g. Ireland, Scotland and UK with 25m/s ), which poses a threat for an isolated severe wind gust. Shear / instability combination also points out the possibility for an isolated tornado in those areas.
Exactly the same for SW / W / NW / N France, where LL shear and instability favor the evolution of an isolated tornado/ severe wind gust event.
During maximum heating, storms could produce mostly sub-severe hail, although an isolated large hail event can't be ruled out, given still 10-20m/s DLS.

...Central Mediterranean...

Constantly deepening lee-cyclogenesis over N-Italy will be the focus for another area of enhanced thunderstorm possibility.

During the latter part of my forecast period, the atmosphere will start a rapid cool-down over the Balearic Islands, Sardinia and Corsica and 400-600 J/kg SBCAPE can be expected.
Despite the favorable environment, thunderstorms will have problems to evolve just south of France / east of Spain, because of expected long fetch, before dry continental airmass will be mixed out.
Further torwards the south, scattered thunderstorms can be expected in an environment with up to 50m/s DLS.
Low / mid-level flow won't be that strong, but don't want to exclude an isolated severe wind gust risk with the stronger storms.
LL shear of 10m/s, low LCLs and fine instability will also support an local tornado threat along / over Sardinia and Corsica, which was the reason for issuing a level-1.... otherwise confidence is low that storms will be able to gain severe thunderstorm status.

The same for an area between Sicily and CNTRL Italy.

Further towards the east, over Italy and the Adriatic Sea, conditions for isolated to scattered TSTMs will become favorable, too, especially during the latter part of the forecast.
DLS of 30-40m/s and locally up to 17m/s LL shear favor an isolated tornado / severe wind gust risk. Expected pretty strong forcing will compensate the warm temperatures at low / mid-levels.

...Aegean Sea...

A weakening upper-level trough will cross the Aegean Sea from the SW during the late hours and a few thunderstorms will manage to develop. Strongest shear will just be out of my forecast area ( south of Turkey ) and therefore no organized thunderstorms can be expected .