CONVECTIVE FORECAST
VALID 06Z SAT 01/11 - 06Z SUN 02/11 2003
ISSUED: 31/10 22:51Z
FORECASTER: DAHL

There is a moderate risk of severe thunderstorms forecast across southern portions of Italy and the central Adriatic Sea.

There is a slight risk of severe thunderstorms forecast across south and central Italy ... the Adriatic and the NW Balkan States.

General thunderstorms are forecast across the western and central Mediterranean ... and south/southeast central Europe.

SYNOPSIS

Main feature is a deep large-scale upper trough which is expected to extend from north-central Europe across the W Mediterranean and the Iberian Peninsula into Algeria by the beginning of the forecast period. Though associated extensive SFC low ATTM centered over France ... is progged to begin to fill during the period ... ample southerly flow regime is expected to be maintained across the central and southern Mediteranean and across E-central Europe into Scandinavia. Main cold-frontal boundary will stretch from NE Algeria across the central Mediterranean into N Germany by Saturday 12Z ... and make only slow eastward progress.

DISCUSSION

...Central Mediterranean ... Italy...
Southerly low-level flow is extending deeply into N Africa ... pushing steep lapse rates from central Algeria northeastwards. These will overspread the south-central Mediterranean Sea early on Saturday ... and later S and central Italy and the Adriatic Sea ... which is expected to give rise to MLCAPE in the 1000 to 1500 J/kg range. Indications are that deep low-level moisture with SFC dewpoints at or above 15°C will be in place beneath the steep lapse rates.

Kinematic profiles appear to be quite favorable for severe evolution ... MM5 initial analyses appear to be about 20 knots below what has been revealed at 850 hPa and at 500 hPa by rawinsonde ascents. About 85 knots at 500 hPa and 50+ knots at 850 hPa are simulated ... which could turn out to be an underestimation. GFS's initial analyses appear to match the observations much better ... and it starts out with about 40 knots on Saturday morning across the central Mediterranean ... and increases 0-6 m shear to 70+ kts late Saturday evening. 0-1 km shear is assumed by GFS to be on the order of 30 knots. SELY SFC winds will promote 0-3 km SRH in excess of 300 J/kg per GFS. The SELY SFC winds are also supported by the meso ETA solution. Though exact shear magnitudes remain somewhat uncertain ... it looks that kinematic fields will likely be more than sufficient for organized severe thunderstorms.

Intense vort max at the periphery of the mean trough is progged to cross the central mediterranean during Saturday afternoon/evening. Current model fields suggest that associated UVM regime will remain somewhat west of where the best kinematic/thermodynamic are expected. However ... isentropic lift at the nose of the EML plume may provide focus for elevated convection which may root down into the boundary layer over the warm sea waters. SFC-based convection should develop along the cold front ... and primarily affect S and central Italy on Saturday afternoon/evening ... and the Adriatic/NW Balkan States late Saturday night/Sunday early morning. If about 1500 J/kg CAPE are indeed realized ... along with 300 J/kg 0-3 km SRH and 70+ knots deep-layer shear ... substantial threat for supercells and bow echoes would exist ... which will be capable of producing the entire range of severe convective weather ... large hail, damaging wind gusts and tornadoes. Primary mode will likely evolve into short lines and bow echoes later on ... with the main threat shifting towards damaging wind gusts. Tornado threat could be especially enhanced if orographically-induced low-level flow perturbations increase storm-relative veering over the mountainous terrain of S Italy.

Potential problem with this scenario may be extensive mid-level cloudiness/stratiform precip associated with the pre-frontal WAA regime ... which could limit diabatic SFC heating over S Italy ... minimizing CAPE there. Also ... lack of focused UVV's along the cold front could turn out to limit convective development. However ... present 850 hPa theta-e analyses and SAT imagery suggest that much of the cloudiness is dissipating in association with the dry EML air ... and confidence that CAPEs in excess of 1000 J/kg will be available is sufficiently high to warrant a moderate risk.

...Western Mediterranean Sea...
In the post-frontal environment over the Mediterranean Sea ... rather deep cellular convection will likely be maintained along/ahead of the thermal-trough axis. Neither shear nor thermodynamic profiles appear to be alarming over this area ... and severe TSTM threat looks to be rather low. Nonetheless ... strong wind gusts ... a few waterspouts and a few small-hail events may well occur with the stronger cells.

It appears that an isolated CG lightning strike or two will also occur farther NW with rather shallow Cb's ... Indications are that this activity will be too isolated to warrant a large GEN THUNDER area across western Europe.

#