Storm Forecast
Valid: Sat 25 Nov 2006 06:00 to Sun 26 Nov 2006 06:00 UTC
Issued: Fri 24 Nov 2006 21:28
Forecaster: TUSCHY

SYNOPSIS

An intense upper-level trough just west of Europe will continue to advect an unusual warm airmass over most parts of Europe.
Intense cyclogenesis will occur on both sides of this trough, one affecting areas N/NE of Scotland and another one over the N-CNTRL Atlantic. An interesting side not is the intense pressure drop of the latter one with the Ocean Prediction Center forecasting an estimated pressure drop down to 948 hPa till the end of the forecast period.

Although cold temperatures are present over parts of NW Russia, no significant southward advection of this airmass can be expected due to the current synoptic constellation.
Most parts of the Mediterranean will stay dry and warm.

DISCUSSION

... United Kingdom...

First time frame and area of interest will be S / CNTRL - UK at 06 - 11Z . Models like GFS indicate a dragging and slowly NNE-ward shifting upper-level trough, crossing most parts of UK till 12Z.

18Z IR images (24th November ) showed an already well defined cyclonic swirl just west of NW Spain, but cloud tops were still pretty warm and no thunderstorm activity had been seen till 21Z. This system will rapdily shift towards the NNE.
No significant strengthening of this system will be expected, because it is well displaced of the best upper-level divergence.
A pronounced warm-up at 850 hPa should help to cap the warm sector covering all of S/SE UK at 06Z, although mid-levels will feel no significant change ( also indicated by still fine 2-4 km lapse rates ).
Main concern for a few thunderstorms to evolve will be a slowly eastward shifting cold front, where best UVV values and low-end instability are forecast to overlap.
A broad level-1 was issued because only small changes in the set-up could cause a signficant eastward shift and don't want to exclude an isolated storm east of the cold front ( over SE UK ) due to the general thermodynamic set-up ( regarding the lapse rates ).
Low-end instability is forecast and will be coupled with outstanding shear values ( DLS of 30-40m/s and about 15m/s LLS ) so each convective development ( not necessarily electrified ) can cause severe wind gusts and marginal hail.
The tornado threat will be enhanced along the cold front, where LCLs will be very low.

The forecast gets only more complicated after 12Z, when first upper-level trough will finally depart UK towards the NE, while another one will approach Ireland and Scotland during the early evening hours. At the same time, another upper-level trough is forecast to slowly come closer to UK from the SW.
All those systems will bring a prolonged period of locally enhanced lift over a pretty broad area and this also makes it difficult to pinpoint special areas of enhanced severe weather risk that far out due to frequently switching vorticity advection fields.

A pool of significantly colder mid-level air will arrive during the early afternoon hours over UK and should support a rapid steepening of the lapse rates. Hence strong convection can be expected with a risk for isolated large hail and severe wind gusts.
Negative LI values and low-end instability release mainly along the W/SW coast of UK and Scotland, coupled with low LCLs and strongly enhanced SRH values indicate the risk for a couple of tornadoes over the broad level-1 area.
The main inhibiting factor for a more significant thunderstorm event looks like to be the failure of a compact UVV field combined with pretty dry mid-level air over CNTRL UK.

NW France was also included into a level-1 due to similar conditions, discussed above.

...NW Morocco...

Approaching trough and steepening lapse rates should support widespread 200-400 J/kg SBCAPE in an environment with 20 - 30 m/s DLS, so an enhanced severe wind gust threat can be expected.

... Coastal areas of France ...

Strong onshore flow with adjacent advection of higher dewpoints far inland and slowly cooling mid-levels should support a few thunderstorms. Strong shear and an impressive veering wind field will be present . Kinematic parameters will be more than adequate for storm rotation and hence a tornado can't be ruled out. Main threat will be a severe wind gust risk.