CONVECTIVE FORECAST
VALID 06Z SAT 16/10 - 06Z SUN 17/10 2004
ISSUED: 15/10 19:16Z
FORECASTER: DAHL

There is a slight risk of severe thunderstorms forecast across the central Mediterranean and the central Balkans.

General thunderstorms are forecast across central Europe ... the Mediterranean ... SE Europe.

SYNOPSIS

Main feature this period will be extensive quasi-stationary upper low ... centered over the S North Sea and extending as far southwards as to the central Mediterranean ... exhibiting several perturbations at its periphery. Vertically aligned with the upper low ... SFC low is covering most of Europe with wave cyclones expected to exist ahead of the peripheral upper vort maxima. W of the large-scale low ... plume of moist subtropical air is advected into SE/E portions of Europe.

DISCUSSION

...Central Mediterranean Regions...
One of the foci for convective development will be vigorous vort max expected over the central Mediterranean at the beginning of the FCST period.

Thermodynamic setup within the theta-e plume ahead of this feature appears to be somewhat weak based on Friday's 12Z launches. However ... indications are that low-level moisture depth over the central and E Mediterranean is somewhat variable ... and shallow SFC-based moist layer apparent in Friday's ascents may locally be substantially deeper ... and support SFC-based CAPE of several 100 J/kg. As mixed and relatively deep boundary layer generated over Spain is advected eastwards ... profiles are gradually destabilized as the lowest levels moisten over the water ... sufficiently to support TSTMS over the W Mediterranean as of Friday evening.

Shear will be more than sufficient for severe TSTMS with up to 20 m/s at 850 hPa and 25 to 35 m/s at 500 hPa.

Current thinking is TSTMS will be ongoing ahead of lead vort max located over the central Mediterranean by Saturday 06Z. These TSTMS may develop into short lines and bow echoes with severe-straight line winds ... also supercells are anticipated with an additional threat for large hail and also tornadoes given low LCL's and strong low-level shear.

Upstream vort max is progged to affect the NW Mediterranean late in the day ... and strong forcing for UVVs along with weak instability could support scattered TSTMS ... some of which may briefly attain severe levels.

...SEE TEXT areas...
Depth of convective mixing beneath the upper thermal low is somewhat uncertain as weak low-level CAA/DAVA appear to be quite effective in shrinking the convective boundary layer. Given mid-October insolation though ... and Friday's development ... confidence is high that mixing will be sufficiently deep for scattered TSTMS ... which may be augmented my small-scale vort maxima revolving around the inner periphery of the mean upper low. It looks that most of this activity will not develop in strongly sheared environment ... and allover severe threat looks to be quite low. But ... any deep convection that manages to form close enough to the upper jet at the flanks of the upper low ... with chances of this to happen being slightly higher in the SWRN SEE TEXT area than in the ERN SEE TEXT area ... will have increased potential for becoming mesocyclonic with an attendant threat for marginally severe hail ... strong/severe wind gusts and maybe even a brief tornado or two.

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