CONVECTIVE FORECAST
VALID 06Z SAT 26/07 - 06Z SUN 27/07 2003
ISSUED: 25/07 14:51Z
FORECASTER: DAHL
General thunderstorms are forecast across southwest...central and east Europe.
SYNOPSIS
Main feature this period will be amplifying quasi-stationary upper longwave trough over the NE Atlantic ... with several vort maxima at its periphery expected to affect NW and central Europe on Saturday and Saturday night. Backing low-level winds ahead of the trough will advect moist subtropical air into W and central Europe. First vort max/associated occluded frontal system is progged to reach central Europe during the first half of the day ... with a developing frontal wave farther upstream ahead of the next vort max. This wave is expected to affect N France ... BeNeLux and W/N Germany Saturday night towards the end of the forecast period.
DISCUSSION
......W-central germany......
Thermodynamic properties of the theta-e plume/occluded front ahead of the first vort max ... and thermodynamic properties of the (same) airmass involved in the frontal wave is uncertain ... this airmass uses to be rather stable owing to weak lapse rates ... which is confirmed by GFS' CAPE forecasts.
Extensive cloud shield accompanying the occlusion ATTM over W Europe ... will most likely limit destabilization by diabatic surface heating. Moreover ... lightning data indicate that storms have failed to initiate in this airmass thus far ... though a few elevated TSTMS may be embedded. It appears that airmass will have to undergo substantial modifications to allow for TSTM development. Such a modification may occur ahead of the second vort max as elevated mixed layer originating from the Pyrenees/Iberia is advected northwards into France and W/SW Germany. Uncertainty exists with this scenario since it is solely based upon BOLAM output. Also, the suspected steep lapse rates would likely be east of the region of strongest UVV's ... and ATTM it appears that CAPE will be quite meager ... and likely elevated.
Current thinking is that a few weakening elevated TSTMS will be ongoing across extreme E France ... W Germany towards the beginning of the period ... travelling NEwrd. Depending on whether or not SFC-based CAPE can build up during the day ... storms may redevelop during the afternoon over E Germany ... but this appears to be rather unlikely ATTM. Given rather strong shear ... there is a slightly enhanced threat of severe evolution in case SFC based TSTMS form ... and region will have to be monitored for possibly upgrades.
Within the WAA regime of the developing frontal wave ... models generate deep (elevated) convection. Indications are that CAPE will be minimal and although shear in the cloud-bearing layer will be on the order of 40 to 50 knots ... severe evolution appears unlikely.
However ... substantial uncertainty exists with tomorrow's scenario primarily owing to lack of knowledge of the developing thermodynamic setup. Kinematic setup will be adequate for severe thunderstorms ... and slight changes in the thermodynamics may alter Saturday's convective scenario substantially ... and upgrades may be necessary tomorrow.
...SW Europe... ...Other regions... #
DCVA overspreading N Spain ... the Gulf of Biscay and W France will also be a potential focus for severe thunderstorms ... however ... above uncertainties exist across this region as well.
organized severe thunderstorm threat is quite low due to weak shear and weak CAPE. However ... small chance of briefly severe storms appears to exist if cells favorably interact with outflow boundaries/orography. Allover threat does not warrant a categorical risk though.