Storm Forecast
Valid: Tue 21 Nov 2006 06:00 to Wed 22 Nov 2006 06:00 UTC
Issued: Tue 21 Nov 2006 05:21
Forecaster: VAN DER VELDE

SYNOPSIS

A large low pressure system with its core pressure below 965 hPa north/northeast of Scotland affects a large part of western Europe today. In a northwesterly upper flow with a 65 m/s jet max at 300 hPa, a sharp shortwave causing enhanced convection reaches Ireland in the early morning, southwestern UK late morning/afternoon, northern France in the evening and finally even into Corsica and Italy by early morning. GFS predicts that in the lee of the Alps the surface trough will deepen to a 990 hPa low.

DISCUSSION

...Ireland, southwestern UK, central France...

This is a zone where instability appears to be present along with strong deep layer shear, north of the jet axis. Low-level shear would be enhanced by friction over land to moderate to strong values around 10 m/s. LCL heights are low, between 400-800 m. There appears to be some chance that storms at the passage of the upper trough could develop mesocyclones, and therefore bear a chance of an isolated tornadic event.
For central France the same could be possible, though instability may be less, e.g. GFS has small positive lifted index values but no 0-3 km CAPE, for what it is worth - the very strong synoptic/mesoscale forcing at the nose of the trough and left jet exit may create an organized storm system nonetheless... MM5 lets precip peter out a bit over land while NMM suggests more intense convection over land.
However, wind fields are forecast to be not strong enough near instability to think of a gusts threat, and forcing perhaps too linear/widespread to think of a tornado threat, with also weak storm-relative helicity and low-level CAPE.

...Corsica, Italy...

A more significant pool of instability will be present when the cold airmass within the upper trough reaches the relatively warm Mediterranean Sea. The upper divergence at left exit of the jet and cyclonic vorticity advection in front of the upper trough meet an area of low-level warm air advection, a recipe to cyclogenesis. Under these conditions a well-organized MCS can be expected to develop with ease, maintained by moderate deep layer shear (15 m/s 0-6 km), but the ingredients appear too marginal all together for a severe weather threat. An isolated waterspout or flash flood events may occur.