Storm Forecast
Valid: Fri 30 Nov 2007 06:00 to Sat 01 Dec 2007 06:00 UTC
Issued: Thu 29 Nov 2007 21:48
Forecaster: TUSCHY

SYNOPSIS

A glance on the 100hPa forecast maps indicates major changes in the upper atmosphere over the northern Pacific Ocean and despite the fact that this evolution has no imminent influence on the European weather ( at least not yet ) it should be included into this synopsis as a short side-note.

The main player will be an intense depression SE of Greenland ( with an official pressure of 952hPa yesterday at 12Z ). This feature is forecast to move towards the east while weakening. Despite the overall weakening trend, this trough is surrounded by an intense upper-level jet and hence the system is still quite dynamic.
Models are quite consistent in developing a wave SW of Ireland during the morning hours although the final strength of this system is still somewhat uncertain. The wave will cross UK / Scotland from the SW while intensifying into a strong depression.

Weak pressure gradients prevail over the Mediterranean supporting numerous areas with thunderstorms.

The pool of cold air over N/NE Europe is still in place and hence convective activity will be unlikely in those areas.

DISCUSSION

... United Kingdom ...

A difficult forecast is in store for most parts of the UK.
A developing wave west / southwest of Ireland will cross UK during the late morning and afternoon hours while gradually intensifying.
As this wave is in its developing stage it is characterized by a broad warm sector with temperatures at 850hPa increasing to 5-6°C, overspreading the UK from the SW during the morning hours. Forecast soundings show a strengthening cap and despite a well mixed BL and surface temperatures well above 10°C no convective activity is anticipated to occur in the warm sector.

The attention then turns to an active cold front, reaching western UK sometime between 14 and 16Z, crossing UK rapidly towards the east and affecting the eastern part of UK between 21 and 00Z.
There are persistent signals during the past few model runs of GFS that a dry slot will intersect the eastward racing cold front over western UK in an environment with strong lift. In addition, BL is quite moist as tongue of up to 8g/kg mixing ratio advects northeastwards just ahead of the cold front. A compact and intense DPVA field should also affect SW / W UK during the evening hours. Hence there are indeed a lot of signs that the cold front could become thundery or that a small line of enhanced convective activity on its backside could develop.
In any case, LL shear of 15m/s and DLS of 40m/s would be available and 0-1km helicity values along the front would also point to an enhanced tornado and signficantly augmented severe wind gust threat. Right now, confidence is by far not that high to increase the probabilities to a level-2 as instability is very shallow / mainly confined to the postfrontal airmass but development of the environment just ahead and along the cold front has to be monitored closely.

Another area will be the postfrontal airmass and hence a time frame between 18Z and 00Z over SW / S-UK.
The influx of moist air from the S/SW just ahead of the eastward racing cold front goes on, supporting low LCLs. Aforementioned dry slot continues to overrun the surface front and higher resolution models reveal strong vorticity signals in an environment with up to 200 J/kg MLCAPE. Even GFS has a line of strong UVV, crossing S-UK during the evening hours and this could be a hint on a potential mesoscale feature ( e.g. a comma cloud ).
LL shear in the postfrontal airmass is still significantly enhanced although directional shear eases and so does the helicity values. We should not forget that those are outputs of a global model and if indeed such a mesoscale feature evolves, directional shear could increase considerably .
Right now we will stick with a strong wording which is not yet represented in the level scheme, but the area has to be monitored.

Intense shear and at least low-end instability would be supportive for tornadic cells and the degree of the shear would also support an isolated strong tornado event, if parameters indeed turn out to be as good as currently anticipated.
Another risk not only along the cold front but also in the postfrontal airmass will be severe wind gusts as impulse of intense low-/mid-level wind could easily get mixed down. Any organized line could be accompanied by bowing segments with an enhanced wind gust risk!

Around 00Z, the risk of a local tornado event increases again over SW UK and along the whole west coast of UK as mid-level cold pool finally arrives. Enhanced convective activity can be expected and as LL shear along the coast overlaps with increasing instability fields, the risk of a few tornado reports will increase. This threat should exist during the rest of the forecast period.

The same for Ireland mainly between 21Z and 00 Z, as LL shear and instability both increase and overlap.

...Ionian and Aegean Sea...

An eastward shifting upper-level trough will be the focus for scattered thunderstorms in an overall weakly sheared environment. The main risk will be an isolated spout report west of Greece during the morning hours, as the 0-3km instability is enhanced.
Despite the weak shear, strong instability release could support a few strong updrafts and hence gusty winds and mostly marginal hail could accompany those storms.