Storm Forecast
Valid: Tue 28 Nov 2006 06:00 to Wed 29 Nov 2006 06:00 UTC
Issued: Tue 28 Nov 2006 04:33
Forecaster: VAN DER VELDE

SYNOPSIS

A large old low pressure area is present south of Iceland. Below an upper trough a reservoir of cold mid level air (steep lapse rates) is slding from the Atlantic into the British Isles Tuesday morning and afternoon. A new cold front has developed at the interface of a tongue of slightly enhanced theta-e between Portugal and the southern UK.
Other regions of interest for deep convection are the cold pool south of Sicily and the zone near the Balearic islands where a weak old cold front enters the Mediterranean Sea from Spain.

DISCUSSION

...southern and western UK...

A jet overlaying the frontal zone creates 35 m/s deep layer shear (0-6 km), while unstable air marches up into the country mostly behind the frontal zone. Low level shear is computed to be moderate, 7-10 m/s over 0-1 km. Not being convinced that favourable shear and and instability overlap sufficiently, and not seeing much in the way of low-level veering winds with height, and no strong sources of lift, I have decided to go with just a normal thunderstorm threat category.

...Sicily, Tunesia, Libya area...

Not very steep mid level lapse rates, no strong convergence signals, no LFC heights as low as LCL heights in GFS, but synoptically a classic situation that may produce a few waterspouts. Because of above factors, a level 1 is not issued.

...Balearic islands...

After days of GFS anticipating convection in this area, it may finally happen at the end of the forecast period. GFS and NMM are in favour, with small convective precipitation signals, strong low level convergent winds and weak mid level rising motion in an area of enhanced CAPE. Also MM5 gives some precipitation. It will not be much special in terms of organisation, activity and lack of severe weather.