CONVECTIVE FORECAST
VALID 06Z FRI 26/12 - 06Z SAT 27/12 2003
ISSUED: 25/12 15:06Z
FORECASTER: DAHL

General thunderstorms are forecast across Central Mediterranean

General thunderstorms are forecast across The British Isles ... France ... BeNeLux.

SYNOPSIS

Main feature on the upper-air analyses/forecasts is the digging Atlantic upper trough which will reach the western parts of Europe late in the forecast period. Extensive Mediterraean upper low is expected to continue to weaken while slowly moving east. At low levels ... cyclogenesis is progged in advance of the Atlantic upper trough ... with the cyclone expected to be centered over N Great Britain by Friday 18Z. However ... model runs are not quite consistent with respect to the strength/extent/position of this feature. Mediterranean SFC low will eventually fill and give way to EWD shifting central European high-pressure SFC ridge.

DISCUSSION

...Ionian Sea ... Aegean Sea ... south-central Mediterranean Sea...
A few TSTMS will likely continue beneath upper cold pool ahead of rather small-scale vort maxima ... primarily over the Aegean and Ionian Sea ... and along the coasts of Greece and the S/W coasts of Turkey. Soundings reveal nearly saturated neutral profiles ... and given weak shear ... severe TSTM threat is pretty low. However ... the stronger cells could produce small hail given rather low WBZ heights. Also ... a waterspout or two cannot be discounted.

...British Isles ... west and central France ... BeNeLux...
More difficult scenario could evolve along the cold front of the vigorous Atlantic low ... pre-frontal air mass will likely be quite stable. However ... strong/deep upward vertical motion along/ahead of the front ... and strong low-level forcing along the edge of the cold front could promote lines of narrow and shallow convection with a few lightning strikes. Given large hodographs ... bowing segments and severe wind gusts would likely accompany the passage of the cold front. Small tornado threat would also exist ... possibly owing to shearing instability which could promote small-scale vortices along the front. This would affect the British Isles in the late evening hours ... and later on W and central France and the BeNeLux states.

This scenario is somewhat speculative at the moment ... but it appears that it is sufficiently likely to be discussed. So ... will include those regions in a GEN thunder where there is a slightly enhanced threat for severe TSTM events. If line convection indeed forms and confidence in this scenario increases ... an upgrade to SLGT could be necessary.

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