Storm Forecast
Valid: Sat 28 Jun 2008 06:00 to Sun 29 Jun 2008 06:00 UTC
Issued: Sat 28 Jun 2008 01:57
Forecaster: DAHL

SYNOPSIS

Progressive upper flow pattern has established over Europe, with a meandering westerly current extending across the continent. One of the imbedded troughs is expected to dig across the NE Balkans late on Saturday. A deep-layer frontal zone is stretching roughly from France across the southern Alps towards the Ukraine, with very moist and unstable air south of the boundary. Except for a SFC low over the NE Atlantic off the British Isles, which affects parts of the British Isles and NW central Europe, quiescent synoptic conditions prevail over the forecast area.

DISCUSSION

... Balkans ...

Very moist and unstable air is present over the Balkans as well as over Italy, locally exhibiting CAPEs on the order of 2500 J/kg. It seems that large-scale forcing for ascent will overspread the NE Balkans ahead of an upper trough in the evening hours. Thunderstorms should develop along and ahead of the cold front associated with this trough in the early afternoon hours. This front should stretch from the NW Balkans into the Black-Sea region early on Saturday evening. However, some uncertainty exists on the thermodynamic fields immediately ahead of the front. Indications are that moisture depth varies substantially over the Balkans, and the majority of the storms may develop in weakly instable, inverted-V type environments, posing mainly a threat for severe wind gusts and large hail. There will only be quite a narrow strip of 10 to 15 m/s DLS and positive CAPE over the N Balkans. This, coupled with rather unimpressive QPF fields of large- and mesoscale models, suggests that the threat for widespread severe weather is somewhat reduced compared to previous days. Still, pulse-type storms may develop in essentially unsheared environment farther south towards Bulgaria during peak-heating hours with a brief threat for damaging winds and severe hailfall.

There exists some chance that the strong thermodynamic fields may coincide with the moderately strong over the N Balkans shear for a while so that well-organized multicells could occur, some of which may well contain mesocyclones. In this case, expect (very) large hail along with destructive wind gusts. Local moisture pooling as well as locally-augmented shear profiles, e.g. near outfow boundaries, may also support strong low-level rotation with some threat for tornadoes. However, this scenario is not supported by the models at the moment, and will thus only introduce a level one threat at this time. However, an upgrade may be required if storms are to become more widespread than currently anticipated.

... Italy ...

Isolated storms are also expected over Italy, where shear should be minimal, but instability will likely be quite impressive. However, considerable variability of low-level moisture depth has been revealed by Saturday's 00Z ascents, so that MLCAPE should range from close to zero to about 2500 J/kg. Large-scale shear should be rather weak, but a variety of mesoscale opportunities for shear to be augmented (orography, mesoscale boundaries) will likely exist. If a cell in a 2500 J/kg thermodynamic environment interacts with these regions of enhanced shear, a threat for very large hail and severe wind gusts will exist. However, this threat is somewhat conditional ... and given a lack of large-scale kinematic support, a level two threat will not be introduced at the moment.

... NE Germany ... NW Poland ...

Weak CAPE should develop within weak occluded frontal system over NE Germany and NW Poland in the evening hours. Deep shear in the 15 to 20 m/s range, low-level shear of about 10 m/s along with minimal capping and a moist boundary layer, suggest that a few mesocyclones may occur, with a threat for marginally severe hail as well as for a brief tornado or two. Severe events should remain too isolated for a categorical risk, however.

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