Storm Forecast
Valid: Tue 09 Oct 2007 06:00 to Wed 10 Oct 2007 06:00 UTC
Issued: Tue 09 Oct 2007 08:55
Forecaster: VAN DER VELDE

SYNOPSIS

A low pressure area, descending from the British Isles into southwestern France, is the most dynamical system today. Some instability or strongly forced convection may be present at its cold front. Behind the cold front a weak comma cloud passes over Ireland and England, but lightning activity should remain rather limited. Convective actvity at the cold front during the night hours over western France is expected to increase a bit.

The western Mediterranean areas are fairly quiescent, with broad upper troughing creating conditions for continuous isolated thundery convection.

Slightly unstable Mediterranean airmass is found over and around the southeastern Balkan, an older low pressure system still capable of producing large precipitation sums in the western Greece area, some waterspouts are possible as well especially in the surface convergence zone with good GFS low level buoyancy indicated in the level 1. Vertical shear has decreased to values generally good for organisation but nothing special for severe weather... a sporadic large hail event may occur.


DISCUSSION

UK:

A rather marginal level 1. 00Z Castor Bay indicated slightly deep unstable profile with good 0-1 km shear. Thought to be present at the cold front during the morning, with decreasing GFS signals of instability, but strong convergence and wind shift, a tornado may not be ruled out if convective elements can be defined on radar. Deep layer shear seems to be lagging a bit behind the cold front though, and GFS computed SREH is also much less than the case two weeks ago.

France:

GFS suggests increasing instability along the cold front as it passes southward along the coast. Dep layer shear will still lag a bit behind the front, but 0-3 km SREH and 0-1 km shear vectors are 200-400 m2/s2 and around 12 m/s, respectively. This makes for a threat of shallow tornadic mesocyclones, probably embedded in a convective line oriented E-W along the cold front. Instability is augmented by advection of warm air in lower levels from the ocean. Hail (<2 cm) and isolated severe gusts are possible.

W-Italy:

GFS indicates good 0-3 km CAPE in the morning of Wednesday. The quiescent weather pattern seems supportive of waterspouts, with a band of low level convergence off the coast, and the center of the shallow low at 600 hPa is nearby, with PVA through the area. Wind profiles should look weak/erratic (supportive).