Storm Forecast
Valid: Sun 06 May 2007 07:00 to Mon 07 May 2007 06:00 UTC
Issued: Sun 06 May 2007 07:10
Forecaster: DAHL

SYNOPSIS

Synoptic setup has not changed much since yesterday ... the main difference relevant for convective evolution is that a well-pronounced upper vort max at the E periphery of the central/south European long-wave trough will overspread the Ionian Sea and the S Balkans in the evening and night hours. This feature will induce weak cyclogenesis mainy over Greece. Progressive upper flow pattern is establishing over northern portions of Europe, with lead vort max reaching southern Scandinavia towards the end of the forecast period. Intense and comparatively large SFC low-pressure complex is accompanying this feature, affecting much of the N Atlantic and Scandinavia.

DISCUSSION

... Balkan States ...

Main issue today seems to be a lack of moisture as revealed by the 00Z ascents from S Italy and the S Balkans. Though low to mid-level lapse rates are rather steep, it seems that air mass will remain strongly capped with only minimal CAPE, especially over the southern parts of the warm sector. However, it seems likely that the LL moisture is deeper locally than what has been revealed by the ascents, and thus confidence exists that the thermodynamic environment will be, or become, favorable for deep convection again.

Low-level and deep-layer shear will remain well adequate for severe storms, given 10 m/s 0-1 km shear and again 25 to 30 m/s deep-layer shear.

Large-scale focus for convective initiation will be the vort max that affects the Ionian Sea and the south Balkans in the late evening hours. Models seem to converge at not initiating convection where upper-level Q-vector convergence will be maximized, but rather somewhat downstream over the S Balkans, with a focus in the QPF field over southern and central Romania. However, simulated CAPE is very weak over this area in the evening hours, and it seems rather unlikely that e.g. upscale growth into one or more MCSs will occur.

It seems that any isolated storm developing along and ahead of the cold front over the Ionian Sea and Greece will have the potential of becoming supercellular, with a threat for large hail and severe wind gusts. Rather strong capping and somewhat high-based nature of the storms should limit the tornado threat. However, development of CAPE over these regions is questionable and currently not supported by the models. A level one will thus not be issued.

Farther downstream over the Balkans ... extensive cloudiness, partly associated with convective debris is currently present, but leave the area during the next couple of hours per latest satellite imagery. Though lapse rates have partly weakened compared to yesterday, 500 to 1000 J/kg CAPE should develop, though GFS is reluctant to come up with any CAPE over the central Balkans. However, limited confidence exists with this solution given what appears to be small Cb's that recently developed in a region void of CAPE in the GFS over N Greece. TSTM activity may become rather widespread, and mostly of multicellular nature, though isolated mesocyclones may also occur. Main threats should be hail and wind gusts, and possibly an isolated tornado or two given strong low-level shear. Peak activity should occur in the late evening hours.