CONVECTIVE FORECAST - UPDATE
VALID 20Z SAT 17/01 - 06Z SUN 18/01 2004
ISSUED: 17/01 20:26Z
FORECASTER: DAHL
General thunderstorms are forecast across the N Iberian Peninsula ... S and W France ... N Italy and the N Balkan States.
SYNOPSIS
As strong vort max ATTM over the Biscay continues to dig south ... cyclogenesis is in full progress over France ... with the SFC low expected to be centered over the N Gulf of Genoa at the end of this forecast period. Associated warm front will stretch across N Italy ... the N Adriatic Sea into the N Balkan States ... with the cold front curving across the W Mediterranean Sea into Gibraltar. Vort max over the Adriatic Sea will shift northeastwards and reach the Ukraine late in the period.
DISCUSSION
...Biscay and adjacent coastal regions...
The Gulf of Biscay is currently covered with relatively deep cellular convection producing occasional lightning ... this activity appears to be linked to mesoscale DCVA maxima at the periphery of the mean trough which will affect the region through the night. Expect scattered TSTMS to persist over the night over the Gulf and coastal regions of W France and N Spain. TSTMS should decay as they move inland ... low-level shear on the order of 40 knots ... and 70+ knots deep-layer shear are supporting a slight severe straight-line winds threat. Also ... the stronger cells may well produce small hail. The strong ... largely unidirectional shear may promote isolated splitting storms with rotating updrafts which may briefly produce marginally severe hail. Lack of large-scale low-level veering and limited chances of low-level shear modification suggest that tornado threat is not particularly high.
...Spain ... S France ... W Mediterranean Sea ... N Italy...
Possibly elevated TSTMS are forming over N Spain in strong frontogenetic regime at the moment. Bordeaux, France, 12Z sounding may be a sample of the air mass currently affecting N Spain ... as supported by BOLAM 850 hPa theta-e fields. CAPE is minimal in this air mass ... and given the elevated nature of the convection ... severe threat appears to be quite small. TSTMS should expand south- and northwards late in the period as frontogenesis will increase during the second half of the night. Additional elevated TSTMS should develop late in the night E/NE of the low center over N Italy along and N of the strengthening warm front.
...N Balkan...
It appears that orographic forcing at the Dinaric Mountain range is promoting current TSTM activity at the Adriatic E coast along and N of thermal boundary ... this activity may initially weaken ... but rejuvenate and expand east- and westwatds late in the night as frontogenetic forcing associated with the deepening low farther upstream affects the region. Magnitude of shear in the cloud-bearing layer ... and meager instability suggest that organized severe TSTM threat will be rather small.
#