Storm Forecast
Valid: Fri 03 Aug 2007 06:00 to Sat 04 Aug 2007 06:00 UTC
Issued: Fri 03 Aug 2007 10:29
Forecaster: VAN DER VELDE

SYNOPSIS

The east flank of a mid/upper trough and a weak cold front advance into central Europe, Italy and the Balkan. Steep mid-level lapse rates stretch northeastward from Africa over Italy into southeastern Poland. Instability on the order of 1000 J/kg MLCAPE is observed/forecast over Italy and the western Balkan.
Stable conditions prevail behind the front under a high pressure region over western Europe.
A low pressure system over western Russia moves slowly northward. The warm sector and strong lift should cause widespread thunderstorms.


DISCUSSION

...Italy, western Balkan...

00Z soundings hint at some 1000 J/kg MLCAPE and further destabilisation is likely to take place locally. Strong lift from the upper through guarantees widespread thunderstorms as a result, while moderate shear conditions aid storm organisation and severe weather chances. Deep layer shear is forecast to be around 15-20 m/s while SREH in places with low level backed flow (mainly forecast over Slovenia, and eastern Italy at 15Z) may reach 100-250 m2/s2, supportive of supercells. Low level shear is lacking as 925-850 hPa winds are only around 5 m/s, this limits tornado potential, while also LCLs are unfavourable (1800-2800 m). The latter may induce evaporative cooling needed for gusts, though. Large hail seems the main threat.
Low level CAPE is not forecast to be high over the Adriatic sea, but cannot rule out a waterspout as low level winds are modest and convergence strong.

... western Russia...

Strong instability around 1000-1500 J/kg is hinted at by 00Z soundings, weakly capped, while GFS model predicts this zone to be in 100-200 m2/s2 SREH and good low level shear due to backed winds ahead of a surface level convergence zone. Lack of strong deep layer shear (12 m/s)should limit the severe weather threat somewhat, but overall shear is sufficient for well-organized cells and mesoscale convective systems in the evening.
Expect large hail to be the main threat, but an isolated tornado or severe gust is not ruled out. Low LCLs ahead of the convergence zone and strong forecast 0-3 km CAPE are in favour of tornadoes, 0-1 km shear is marginal but increasing to more than 10 m/s around 18Z in the western part of the level 1 area.