Storm Forecast
Valid: Mon 31 Aug 2009 06:00 to Tue 01 Sep 2009 06:00 UTC
Issued: Sun 30 Aug 2009 20:53
Forecaster: TUSCHY

A level 1 was issued for parts of United Kingdom and southern Scotland mainly for severe wind gusts.

A level 1 was issued for parts of extreme northern Spain and SW-France mainly for large hail.

SYNOPSIS

A strong Atlantic trough approaches Europe during the forecast. The sector ahead of the trough becomes increasingly unstable and susceptible to thunderstorm development over W-Europe. Hot conditions continue over the Mediterranean with afternoon thunderstorms over the central parts. Another trough over eastern Europe drives a cold front eastwards with cool and stable conditions in its wake.

DISCUSSION

... Spain and France ...

Global models are all over the place regarding initiation but at least they converge in respect of timing, showing the most active stage of thunderstorm activity during the night hours.

Warm and increasingly unstable air for surface based parcels covers parts of northeastern Spain and southwestern France during the evening hours with widespread MUCAPE release over Spain and France due to a strengthening EML tongue. Main focus for initiation is the area, where surface based convection during the evening hours is possible (e.g. NE-Spain), with increasing uncertainty thereafter for isolated, elevated activity. Forcing only glances the areas of interest in form of eastward traveling vorticity lobes and dependant on their strength, isolated thunderstorms are possible from central Spain to northern France.

Speed shear between 0-6 km of 15-20 m/s and augmented SRH3 values of 150-300 m^2/s^2 support a large hail risk with those thunderstorms but only those regions were highlighted with a level 1, where confidence for initiation is the highest. A larger level may be needed in later updates, if ECMWF (W-France) or GEM(Spain) are on track. An area from central Spain all the way up to N-central France was included into low thunderstorm probabilites (not yet made public).

... Central Mediterranean and parts of SE-Europe ...

Mainly diurnal driven thunderstorms over the Mediterranean and an augmented thunderstorm risk along an eastward moving cold front over E-Romania, Moldova and central Ukraine are forecast. Instability of 1000 J/kg MLCAPE over the Mediterranean but weak shear and stronger shear but limited instability over SE-Europe preclude an organized thunderstorm risk. Isolated large hail is possible, but too sparse for a level area.

... Northern United-Kingdom ...

GFS and ECMWF indicate some low-end instability with GFS even showing some MLCAPE between 12UTC-18UTC. Mid-level lapse rates steepen somewhat after 15 UTC and a very limited time-frame for an isolated thunderstorm is possible. 20m/s speed shear in the lowest 3km points to an augmented severe wind gust risk and due to the risk of showers and isolated thunderstorms, a level 1 was issued, with low probability lightning contours (not yet made public). The risk ought to diminish rapidly after 18 UTC.