Storm Forecast
Valid: Tue 15 Jan 2008 06:00 to Wed 16 Jan 2008 06:00 UTC
Issued: Tue 15 Jan 2008 02:56
Forecaster: VAN DER VELDE

SYNOPSIS

A large low pressure area passes today with its core over the British Isles with MSL pressure predicted by GFS below 976 hPa. At the time of writing a large warm-frontal shield is present followed by a mesoscale convective line or weak comma cloud near the apex of the cloud mass, likely becoming an instant occlusion. Strong warm air advection ahead of the cold front creates highly veering winds with height and SREH > 400 m2/s2 in a long broad zone.

Behind the cold front increasing amounts of CAPE are predicted as result of increasing cold air advection. By 15Z, southern UK and Bretagne are first exposed to the unstable airmass. GFS 18Z suggests, by theta-e fields of BL and 700 hPa, that the cold front has a split-level structure, with the 700 hPa cold front well inland and a band of higher BL mixing ratios (or theta-e) following. While deep layer shear is moderate behind the midlevel front, SREH is practically absent.

GFS proceeds to forecast a wave structure in the surface cold front over the southern UK. By 18Z-21Z, if the model is still on track, strong and deep convergence over the SE-UK is associated with this feature, and patches of enhanced 0-1 km shear > 20 m/s appear as well. A small bubble of somewhat enhanced SREH now follows behind the midlevel front and before the surface occlusion over SE-UK. CAPE seems also enhanced off-shore, advected inland.


DISCUSSION

W-France:

Showers with isolated thunder are possible behind the mid-level cold front, likely in the form of a line. Given the relative absence of low level shear and helicity, the situation seems mostly conducive to gusts, some severe, with a minor chance of a tornado.
More thundery showers can be expected towards the end of the night over the Bay of Biscay.

S-UK:

Advection of CAPE into the wave passing during the evening along with shear enhancements described above could be sufficient for a tornado event, most likely over the eastern part of the area included in level 1, save any model inaccuracies at this term. However, the absence of an intense PV anomaly and 0-3 km CAPE and not huge shear suggest the threat is fairly low.

NW-Spain:

This is an area where GFS does forecast CAPE during the afternoon, but perhaps elevated (most unstable parcels yield better results here today). This is in the same split-level front between surface and mid level cold front as in France. Hence, refer to that discussion.