CONVECTIVE FORECAST - UPDATE
VALID 11Z WED 15/10 - 06Z THU 16/10 2003
ISSUED: 15/10 10:52Z
FORECASTER: DAHL

There is a slight risk of severe thunderstorms forecast across south-central Mediterranean regions.

General thunderstorms are forecast across the southern and central Mediterranean and S/W Iberia.

SYNOPSIS

Omega-type block pattern is present over central and northern Europe ... with the upper high being centered over the north Sea. Vort max at the SW periphery of extensive upper low over E Europe adjacent to the upper ridge ... is crossing the Balkan States ATTM and will move into the Black Sea later this period. Weak zonal frontal zone is stretching across the Mediterranean ... with weak eastward-progressing vort max being present over the west Mediterranean ... which will accelerate east as it is ingested into the east-European upper low late Wednesday night. At low levels ... weak SFC low associated with the southern-stream vort max is present over the W Mediterranean.

DISCUSSION

...Mediterranean...
Rapid convective development has occurred over the western Mediterranean Sea ahead of the southern-stream vort max amidst weak theta-e plume ... which is advecting northeastwards ahead of the weak low over the W Mdeiterranean. Thermodynamic quality of this airmass is rather poor with nearly neutral lapse rates ... however ... continued advection of Sahara-desert airmass atop the BL could gradually improve the lapse rates though the EML is extending only up to about 700 hPa over the N Sahara ATTM. So ... CAPE is not expected to improve drastically during the period ... and should generally be in the 500 to 1000 J/kg range. However ... deep low-level moisture is in place ... with SFC dewpoints around 19°C ... yielding very low LCL heights with temperatures around 23°C.

Soundings show substantial low-level veering and about 30 knots deep-layer shear ... with the strongest increase in wind speed below 700 hPa ... suggesting that the storm inflow may contain quite some helicity. GFS suggests sheets 0-3 km SRH in excess of 300 J/kg ... Meso-ETA and 10 Z SFC obs support this by indicating SLY to ELY SFC winds.

Current thinking is that storms are likely to contain mesocyclones. The hail and damaging wind threat appears to be rather low owing to the weak thermodynamic support ... though a few isolated large-hail events cannot be discounted. In the presence of the favorable low-level shear profiles ... and the low dewpoint depressions in the boundary layer ... any supercells that forms appears to have fair chances of producing a tornado or two.

As storms progress east ... deep shear should gradually increase ... as low-level thermodynamic and kinematic setup will likely remain essentially unchanged ... expect the tornado threat to continue throughout the period ... especially as the storms cross the Italian Peninsula where orographically-induced flow perturbations may contribute to augmented low-level SRH. Also ... a few large-hail and damaging wind events should be expected. Uncertainty exists as to whether or not storms will remain SFC based throughout the perdiod ... tornado threat would be greatly diminished if storms happened to become elevated.

Elevated Convection N of the frontal boundary stretching across the NE Mediterranean ... will likely remain sub-severe. However ... if updrafts manage to root down into the boundary layer ... ELY surface winds would provide favorable shear in the inflow for supercells ... and a few hail events and possibly a brief tornado could occur. However ... threat appears to be too conditional for a SLGT ATTM.

#