Storm Forecast
Valid: Sat 01 Sep 2007 06:00 to Sun 02 Sep 2007 06:00 UTC
Issued: Fri 31 Aug 2007 21:00
Forecaster: DAHL

SYNOPSIS

More of a propgressive upper flow pattern is establishing over Europe. A southern-stream trough located over the northern Mediterranean as of Friday evening will initially phase with a northen-stream trough and eventually close off into a cut-off low over the Balkans towards the end of the period. Another, slowly weakening and quasi-stationary upper low is present over the eastern Iberian Peninsula. According to the upper-flow configuration ... strong low-level bearoclinic zone will extend from central Russia across the Balkans into the central Mediterranean on Saturday, making slow eastward progress. The air mass southeast of this boundary is characterized by a well-pronounced/deep EML and is quite unstable with peak CAPEs of around 2000 J/kg.

DISCUSSION

... southern Italy ... central and eastern Balkan States ...

It seems that the air mass over southern Italy and the Ionian Sea will remain quite unstable with MLCAPEs locally in excess of 2000 J/kg. However, the EML is expected to effectively cap the BL air, and given somewhat limited large-scale upward motion, storms may remain rather isolated.

Better forcing for upward motion should be present farther downstream over the Balkans. Also, somewhat warmer/drier boundary layer may reduce the capping somewhat. Shear should be strongest over the Balkans, probably around 20 m/s, decreasing towards the west.

The GFS simulates upscale growth of the convection into a large MCS over N Bulgaria/S Romania late Saturday evening/night, which appears quite likely given ample CAPE and strong shear. Falling pressure ahead of the system indicative of increasing mesoscale ascent should also aid in sustained convective development.

Present thinking is that scattered thunderstorms will initiate over Italy and the central Balkans during the day. The south Italian storms may, despite somewhat limited shear, rapidly become severe given quite strong thermodynamic fields. Expect wind gusts and hail both reaching severe limits locally. Further northeast over the Balkans, a more serious severe weather threat seems to exist. Very large hail, damaging wind gusts and possibly also a few tornadoes may occur, especially late in the evening. The latter is due mainly to enhanced low-level shear in response to weak cyclogenesis over the Aegean.

... southern Spain ...

Isolated storms could also form over southern Spain in weakly unstable air mass. It appears that moist LL air near the Mediterranean coast will be strongly capped, so that it may be more likely that storms sevelop farther inland, having a rather high-based nature. This suggests that a few strong outflow wind gusts may accompany the storms. However, anticipated degree of severity and coverage do not seem to necessitate a categorical risk at the moment.