CONVECTIVE FORECAST
VALID 06Z SAT 17/01 - 06Z SUN 18/01 2004
ISSUED: 16/01 19:53Z
FORECASTER: DAHL

General thunderstorms are forecast across the Biscay ... N Spain and W France.

SYNOPSIS

Main feature will be intense vort max ATTM S of Iceland ... which will rapidly dig southeastwards and reach the western Mediterranean Sea by early Sunday morning. Ahead of this vort max ... cyclogenesis is progged over S France at the SW periphery of the weakening central European low-pressure system late on Saturday. At Sunday 06Z ... SFC low is expected to be centered over the NW Mediterranean Sea ... with the trailing cold front curving from E of the Balearics southwestwards into Gibraltar. Another vort max is expected to cross N Italy and the N Adriatic Sea during the day.

DISCUSSION

...Biscay ... N Spain ... W France...
Field of enhanced Cu's with the chance of comma-cloud formation will likely be present ahead of the Atlantic vort max. Mesoscale structure of the vort max/CVA fields appears to evolve rather complicated ... with GFS indicating development of two rather small successive vorticity/CVA maxima. Fairly deep field of Cu's/Cb's is already accompanying the vort max ... and present thinking is that it will continue to strengthen ... advect into east France early Saturday morning ... and decay as it moves farther inland. Deep-layer shear will be supportive for a few strong outflow winds ... possibly isolated exceeding severe levels. Second vort max may be accompanied by deep cellular convection as well ... but will also likely decay after landfall. Allover severe threat appears to be limited due to weak thermodynamic support and little large-scale low-level veering.

...NW Mediterranean Sea ... Adriatic Sea...
Considerable uncertainty exists with respect to the degree of destabilization of the air mass over the west and north Mediterranean. Modified subtropical/Atlantic air mass beneath the DCVA-regime ahead of the Atlantic vort max will likely not undergo significant low-level moistening over the Mediterranean owing to short residence times and only minor SST differences between the east Atlantic and the W Mediterranean Sea. Though no representative sounding of the pre-frontal air mass is available at the moment ... indications are that lapse rates will be quite weak and that development of CAPE is rather uncertain ATTM. Hence ... have little confidence in convective development along the NW Mediterranean cold front late Saturday night/early Sunday morning. In the wake of the front ... air mass should initially be too dry to promote significant TSTM threat.

Vort max farther downstream could promote a few Cb's/TSTMS over the Adriatic ... but coverage is expected to be too small for a gen thunder area ATTM.

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