CONVECTIVE FORECAST
VALID 06Z SUN 24/10 - 06Z MON 25/10 2004
ISSUED: 23/10 19:06Z
FORECASTER: DAHL

General thunderstorms are forecast across the Iberian Peninsula and S France ... across the S UK and over Greece.

SYNOPSIS

Large-scale Atlantic upper trough will slowly move east ... and expand as amplifying vort maxima orbit around its periphery. One such vort max is progged to affect portions of W Europe on Sunday afternoon/evening. Weak upper cut-off low is expected to developp over the Aegean Sea during the period. At low levels ... SWLY flow is stretching from deep SW Europe into NW Russia ... with rather quiescent conditions over the Mediterranean regions. Main cold front is expected to roughly parallel the continental-European W coast at the beginning of the period ... and move east ... stretching from central Iberia across central France into central Germany by Monday 06Z.

DISCUSSION

...Iberian Peninsula ... central France...
Question of the day will once more be the degree ... and the existence of CAPE. Models coherently prog weak CAPE in narrow theta-e plume preceding the cold front ... which is supported by latest satellite and SFLOC data which indicate scattered TSTMS along the front. Large scale UVV's should increase some ahead of vort max ... and scattered TSTMS should be possible throughout the period within this theta-e plume. Shear profiles will likely be sufficient for an isolated severe TSTM event ... main threat being severe wind gusts and maybe a large-hail/tornado event. TSTM coverage and expected weakness of thermodynamic fields precluding SLGT ATTM.

...S France ...
SLY boundary-layer winds are expected to increase over the W Mediterranean and S France ... which will advect Mediterranean moisture beneath steep mid-level lapse rates. Uncertainty exists about the depth of this moisture ... but it looks that it is rather non-uniform and locally sufficiently deep for TSTMS. MLCAPE may rise as high as 1000 J/kg ... but will likely be fairly strongly capped and convection may fail to initiate. If initiation occurs ... severe evolution is likely given 20 to 25 m/s deep shear and 3 km SRH in excess of 200 m2/s2. Main threats expected to be damaging winds and marginally severe hail though low LCLs and 0-1 km shear of about 10 m/s may promote a tornado or two. Confidence in initiation remains uncertain ATTM and a SLGT does not appear to be warranted.

...S UK...
In the evening hours ... postfrontal convection may form over the British Isles ahead of intense vort max associated with the center of the upper low. Strong shear may support formation of small mesocyclones with an attendant threat for large hail ... severe wind gusts and a tornado or two. Timing is somewhat unfavorable though ... and threat should be too small for a SLGT.

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