Storm Forecast
Valid: Fri 01 Feb 2008 06:00 to Sat 02 Feb 2008 06:00 UTC
Issued: Thu 31 Jan 2008 17:33
Forecaster: TUSCHY

SYNOPSIS

A powerful cyclonic vortex over the northern North Sea ( central pressure around 960hPa but gradually rising ) moves slowly towards the east / northeast, as cold air on its upstream side overspreads most parts of NW Europe.
An intense boundary sets-up over W-central Europe, separating a cool and dry airmass to its north from a much warmer and more humid airmass to its south.
Otherwise stable conditions prevail over the Mediterranean and eastern Europe.

DISCUSSION

... Parts of west-central France, mainly an area NW of the Massif Central ...

There will be a time-frame for about 6 hours, where environment becomes supportive for an isolated thunderstorm chance.
As the active cold front slides southwards, prefrontal onshore flow strenghtens gradually with attendant robust moisture advection. Mixing ratio of 7g/kg averaged over the lowest 1000m and dewpoints at or above 9°C are present well inland. CAA at 500hPa is underway and the - 24°C isotherm overspreading the area of interest during the afternoon hours ( at least this is what GFS showed during the past few runs ).As a consequence of a strengthening WAA regime just ahead of the front, lapse rates at mid-levels steepen quite signifcantly to 7K/km.
A limiting factor could be the trough axis and attendant compact UVV field, which arrives after sunset and hence too late but placement under the right entrance region of a potent upper-level streak should be supportive for some lifting mechanism.
By now ( 31.Jan at 17Z ) persistent and strong onshore flow is already present, which should replace current meager dewpoints with aforementioned values, keeping 11-12°C SSTs over the eastern Atlantic in mind. Including this information in model forecast soundings, at least low-end instability looks increasingly likely during the early afternoon hours. Main instability release looks like to be confined to the 900-700hPa region. Veering in this level is not that strong, but strength of the wind field will be quite intense ( up to 30m/s at 850hPa ), resulting in enlarged and mostly straight hodographs.
Any thunderstorm, which manages to develop will be low-topped with an isolated severe wind gust and marginal hail risk, the latter one mainly with storms, which develop in the prefrontal airmass, where mid-level lapse rates are steeper and helicity values at lowest 3km are the strongest .
A general thunderstorm area was issued but only a very isolated thunderstorm is expected.

Thermodynamics become too hostile further towards the east / northeast and should suppress electrified updrafts.

...NW Spain and NW Portugal ...

The same event like mentioned above, beside somewhat warmer mid-levels. This confines the instability release to the lowest 200hPa, which should be too weak for strong updrafts and electrification sustained by EL analysis.
Rate of convective activity will be on the increase during the afternoon / early evening hours, but overall weak thermodynamics / kinematics preclude any severe threat.

...Denmark...

Numerous more or less well developed convergence zones cross Denmark during the forecast period, with one of them having the main attention. This boundary is forecast to arrive just about 13-14 Z from the NW.
A modest structured theta-E ridge helps to bring better LL moisture to NW and W-Denmark, as mid-levels stay cold. This already is enough for some instability release as GFS has 100-200 kJ/m^2 ICAPE over NW Denmark.
Moisture plum fans out rapidly and overall environment is not supportive for more than an isolated SFLOC report .
Beside a strong to possibly isolated severe wind gust risk, marginal hail will be possible, too as the WBZ stays low.
In addition, onshore moving storms ingest notable helicity ( e.g. SRH-1 at or above 200 J/kg ) and with low LCLs, an isolated funnel / short-lived tornado report is possible.
Forecast soundings over N-Denmark have quite weak lapse rates, but strong veering at lower levels, which is at least marginal supportive for an isolated tornado report.
Thunderstorms / convective segments should undergo rapid weakening while moving onshore and hence the overall risk seems to be too low for any risk-level.

...Wales...

Direction of the incoming flow ( N/NNW ) only favors one area where convection can move well inland and this is SW-UK, otherwise,shadow effects and a long fetch over Scotland and UK prohibits any enhanced convective activity, despite the very cold-mid-levels. Parameters of models do not indicate anything severe .
The same for the coastal areas of N-Ireland and Scotland.

...Coastal areas of eastern UK and SE-UK....

A combination of a southward moving trough axis and a fading back-bent occlusion create an environment, assisting a sporadic thunderstorm event over the area of interest, mainly between 15Z-18Z.
A well defined and rapidly southward moving convergence zone is evident in surface streamline maps with better LL moisture but also warmer temperatures at mid-levels.
The latter one should be the main negative point, as only traces of instability onshore and up to 200 J/kg offshore can be realized.
The main concern right now is a short spin-up if convection indeed develops along the convergence zone as helicity values are enhanced.
Threat and coverage is too low for issuing any threat/level area, but the region has to be monitored for better instability release during the next few model runs.