Storm Forecast
Valid: Sat 21 Aug 2010 06:00 to Sun 22 Aug 2010 06:00 UTC
Issued: Sat 21 Aug 2010 05:18
Forecaster: VAN DER VELDE

A level 1 was issued for southern UK (and to a lesser extent central Sweden) mainly for a chance of tornadoes and excessive convective rain.

SYNOPSIS

On the south side of a low pressure area between Scotland and Norway a strong WSWerly flow advects a warm and very humid airmass concentrated in a stationary frontal band. Over Scandinavia this curls back into a secondary low pressure center over central Norway and moves into Finland as warm front. In most of the region convection is barely surface-based, or capped today, and reaches EL at temperatures between a warm -20° and -30°C. Over central Scandinavia tops are colder.
With very low LCL, high moisture content and low CAPE, convection will be mostly rainy rather than thundery.
A trough in mid levels passes over central Italy.

DISCUSSION

...southern UK...

This area is not capped in GFS, shows a good deep convergence signal, and is in a strong shear regime, about 20 m/s DLS and more than 10 m/s LLS. GFS hints at a wave in the front. The main mode of convection is thought to be one or more training lines (streets), with an embedded possibly tornadic mini-supercell not ruled out. In the moist environment rain sums could locally become high, provided a decent amount of convection indeed forms. Rather strong but probably not severe gusts are likely.

...central Sweden and Norway...

An isolated tornado (or spout in Norway with good low-level CAPE) is not ruled out given low LCLs and low level shear and SREH being enhanced.