CONVECTIVE FORECAST
VALID 06Z SAT 25/10 - 06Z SUN 26/10 2003
ISSUED: 24/10 19:14Z
FORECASTER: DAHL

General thunderstorms are forecast across the N Mediterranean and the Balkan States.

General thunderstorms are forecast across the British Isles and N France.

General thunderstorms are forecast across SW/W Iberia.

SYNOPSIS

Rather unusual upper flow pattern in place ... mainly featuring impressively intense WSW-ENE oriented upper frontal zone extending from NW Africa across the Mediterranean and across the Black Sea into Russia. Deep extensive trough is covering NRN and central Europe ... and a quasi-stationary closed upper low is anchored just off the Iberian W coast. At low levels ... main baroclinic zone is stretching across the west Mediterranean into Turkey. Airmass S of this boundary is slightly unstable per latest radiosonde data ... but unsampled plume of steep lapse rates is advected off the Sahara desert per various 850 hPa theta-e analyses. Unseasonably cold polar/arctic airmasses are dominating the parts of Europe located N of the frontal zone.

DISCUSSION

...Mediterranean...
Airmass S of the frontal zone is only marginally unstable at best since SFC-based moisture is quite shallow as revealed by radiosonde data across the S-central and SE Mediterranean. EML off the Sahara desert will likely move into the S-central Mediterranean Sea during Saturday/Saturday night which should enhance CAPE somewhat ... as also suggested by the 00Z GFS solution which assumes CAPE in excess of 1000 J/kg beneath this plume. Though quite an impressive mid/upper jet is in place ... models continue to show dearth of perturbations along the frontal zone ... suggesting minimal dynamic upward forcing in this nearly equivalent barotropic environment. Another challange exists as to which of the CVA sheets displayed in the model fields are noise and which are true signals. It appears that lobe of CVA associated with elongated vort max expected to be over N Italy on Saturday morning ... will affect N Italy ... the N Adriatic and the N and central Balkan States during Saturday afternoon/evening. Farther S along the low-level boundary ... no significant signal is present in the vorticity and vorticity advection fields of the current model runs. So ... expecting a few shallow TSTMs in association with the N Italian vort max ... and low probabilities of deep convection farther S along the front. Instability in the polar airmass is expected to be really meager ... but in the presence of 70+ kts deep-layer shear a few brief severe TSTM events could occur. Most likely mode appears to be rather short-lived and small cells capable of producing some hail ... possibly approaching severe levels and an isolated severe wind gust or two. A tornado or two cannot be excluded but the probability does not appear to be significantly enhanced.

...Iberia...
Low-level WAA ahead of the Iberian upper low ... and DCVA-related ascent will likely create an environment conductive to scattered thunderstorms ... which should form along and SW of warm-frontal boundary stretching across the SW part of Iberia as suggested by numerical 850 hPa theta-e forecasts. Indications are that storms will have too little thermodynamic support to pose substantial threat of becoming severe. However ... if storms happen to become surface-based ... inflow should contain more than 100 J/kg 0-3 km SRH. Also ... deep-layer shear may be marginally sufficient for severe evolution for deeper SFC based storms. However ... uncertainty with this scenario precludes a SLGT ATTM.

...British Isles...N France...
Shallow convection may develop downstream from intense vort max crossing the N Sea and the British Isles during the day. Lapse rates will be nearly neutral ... but strong shear (60+ kts in the lowest 6 km) may promote a few marginally severe wind gusts and some hail. Also, a brief tornado or two cannot be ruled out but this will depend on local flow modifications either caused by orography or by interaction of updrafts with outflow boundaries/gust fronts. Allover severe threat appears to be rather low though.

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