Storm Forecast
Valid: Tue 12 Dec 2006 06:00 to Wed 13 Dec 2006 06:00 UTC
Issued: Mon 11 Dec 2006 23:08
Forecaster: TUSCHY

SYNOPSIS

A slow pattern change will be underway over the far NW Atlantic during the next 24 hours and onwards. High index pattern will transform into a more amplified upper-level high / trough constellation, when a pool of arctic air east of Canada will be advected towards the south.
As a result, an intense trough evolves south of Greenland and a strong upper-level jet will form on its downstream side with a broad 170kt streak.
This in turn will place a developing open wave northwest of Ireland in the favorable left-exit region, which is a good position for some strengthening.
This system will cross Ireland and Scotland during the daytime hours.
In the latter part of the forecast period, strong warm air advection will finally start to affect the extreme northwestern parts of Europe.
Meanwhile, very cold mid-levels will continue to cover most parts of Sweden and Finland, supporting pretty unsettled weather.
Another very interesting feature is a strong upper-level low over NW Africa, starting a slow ENE-ward movement. Very warm mid-level air from northern Africa is forecast to spread northward over the E-CNTRL Mediterranean, while a weak upper-level trough over Greece with cool mid-levels continues a slow eastward drift.
High pressure over E/NE Europe will bring stable weather conditions.

DISCUSSION

....SE Mediterranean...

Departing upper-level trough will support an area with scattered thunderstorm development over the area of interest.
Rapidly intensifying cap due to very warm mid-level temperatures (~ -7° at 500hPa ) will suppress activity from the west / southwest. Weak shear should limit severe thunderstorm potential.
Soundings from yesterday indicated a well mixed boundary layer and low-level lapse rates will stay fine due to SSTs at or above 17°C and cool low-levels ( even somewhat cooler than 24 hours ago ). There is a chance that some instability can be realized in the lower-levels and an isolated waterspout threat can't be ruled out.

... Ireland, Scotland and parts of United Kingdom...

Main focus for thunderstorm development will be an eastward shifting upper-level trough, affecting the area of interest during the daytime hours.
Attention turns to a surge of dry mid-level air, which will rapidly cross the Atlantic, affecting Ireland and N-UK / Scotland during the later morning - afternoon hours.
21Z WV images ( 11th Dec.) indeed show this surge of dry air south/southeast of Greenland.
Main concern for organized convection arises during the 11Z - 16Z time frame along the coastal areas of W-CNTRL / NW-UK and Scotland.
A tongue of higher boundary layer moisture content will cover those areas and GFS brings the dry mid-level air over CNTRL-UK at the same time.
Strong PVA and a not well defined but pretty strong UVV field will be present, too, so conditions will become favorable for thunderstorms to form.
There are signs that tornadic storms can evolve, due to low LCLs and extreme low-level shear ( SRH values of 0-1km ~ 200-300 J/kg) but there are still some uncertainties left.
E.g. instability will be on the low - end side but a bigger concern will be a pool of slightly warmer temperatures at 850hPa ( compared to yesterday ) over the area of interest, so weakening LL lapse rates can be expected and hence no concentrated instability maximum at lowest levels.
However, still warm SSTs could be enough for local steeper LL lapse rates along the coastal areas and will mention an enhanced tornado potential mainly along the coastal areas of W / N - UK and Scotland.
DLS of 20-25m/s will support an enhanced severe wind gust threat, too.

Also included Ireland in the level - 1 due to similar conditions !

... North Sea, Norwegian Sea and parts of Sweden and Finland...

Extensive area of steep mid-level lapse rates will be present and models like GFS indicate numerous features, which locally enhance the lift and isolated thunderstorm development will be possible over a broad area. Only highlighted the areas, where most concentrated thunderstorm coverage can be expected.
Shear / instability separation will limit possibility for organized storms to form.