CONVECTIVE FORECAST
VALID 06Z THU 13/11 - 06Z FRI 14/11 2003
ISSUED: 12/11 22:48Z
FORECASTER: DAHL

General thunderstorms are forecast across the central Mediterranean.

General thunderstorms are forecast across North Sea and the British Isles.

SYNOPSIS

Though split-flow pattern will remain in place across eastern portions of Europe ... upper high ATTM over NE central Europe is expected to weaken some and to move east ... allowing Atlantic upper troughs to gradually penetrate farther into the western portions of Europe. Weakly meandering southern frontal zone is stretching across the Mediterranean. Main feature this period will be intense baroclinic wave ATTM over the central North Atlantic ... which is progged to induce quite strong cyclogenesis at the SFC ... which will affect the British Isles Thursday evening/night. Weak southern-stream trough will cross the central Mediterranean regions during the FCST period.

DISCUSSION

...Central Mediterranean...
Plume of weakly unstable airmass is residing over the south-central Mediterranean Sea as indicated by GFS ad BOLAM theta-e 850 hPa analyses and 12Z/18Z radiosonde data. A few TSTMS are detected by lightning network and satellite data at the nose of this plume where low-level WAA is maximized. This activity should continue throughout the period and cells may conglomerate into one or more larger clusters especially early Thursday morning. Also ... as DCVA associated with the upper trough overspreads this airmass towards the afternoon ... TSTM activity should increase. Soundings and numerical analyses suggest that only a few 100 J/kg CAPE are available ... and given quite meager shear profiles ... threat for organized severe appears to be quite low though a few waterspouts could occur over the warm sea waters.

Farther west ... polar air will likely deepen as the trough slowly digs SE ... Lightning network currently indicates CG's with quite shallow convection over the west-central Mediterranean Sea. As this convection deepens ... TSTM activity is expected to increase. There appears to be a slight threat of a few waterspouts but allover severe potential will be minimal given the poor thermodynamic and kinematic fields.

...British Isles...
Potential focus for convective development will be the wake of the cold front and the cold front itself associated with the developing cyclone over the Atlantic.

Depth of convective mixing in the polar airmass in the post-frontal environment will strongly depend the on opposing effects of low-level CAA ... and DCVA ... the latter of which which could provide a region of relatively deep convective mixing. Current forecasts do not show substantial CVA in the postfrontal environment but strong CAA ... and indications are that convection will be too shallow for TSTMs.

Along the cold front ... strongly forced line convection could form in weakly unstable/stable warm-sector airmass. Strong gradient flow and isallobaric winds are progged to generate 0-3 km SRH up to 500 J/kg ... but supercell threat appears to be rather low given expected narrow forced lines of convection ... with chance of isolated updrafts being rather low. More likely convective mode will be bowing line segments which will primarily pose the threat of severe wind gusts. Even if convection turns out to be too shallow for lightning ... vertical momentum transfer along the front will likely augment the wind gusts. Small risk exists that sufficient vertical vorticity is created along the front as it locally bows out ... and based on past observations ... a few brief tornadoes could occur.

However ... this scenario is quite uncertain since the thermodynamic structure of the warm-sector airmass is not known yet as is the small-scale structure of the front ... which will both determine the character of the convection along the front. Later obs have to be awaited to specify severe threat. An upgrade to SLGT will likely be necessary if convection indeed forms along the cold front.

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