Storm Forecast
Valid: Mon 06 Aug 2007 06:00 to Tue 07 Aug 2007 06:00 UTC
Issued: Sun 05 Aug 2007 23:13
Forecaster: GROENEMEIJER

SYNOPSIS

A closed mid-/upper-low centered over north-central Greece moves slowly northeastward and reaches SE Bulgaria on Tuesday morning. A weak mid-/upper- low is located over the Central Russian Upland and is quasi-stationary. Upstream, an elongated ridge stretches from northern Algeria to the western Alps to the Baltic Sea and the Gulf of Finland. Further west, a trough stretches from east of Iceland over Ireland, where an embedded closed low is located, to the western Iberian Peninsula.

DISCUSSION

Eastern Balkans...

Ahead of the mid-/upper-low over northern Greece an west-to-east oriented surface trough is initially located over Northern Greece and European Turkey. Advection of warm air containing abundant moisture is advected westward off the Black Sea, leading to 1000-2000 J/kg MLCAPE according to the GFS model. Strong forcing for upward vertical motion should have led to widespread convective storms by the beginning of the forecast period. Although the strongest winds in the mid- troposphere should remain out over the Black Sea and to the south of the highest CAPE, about 20 m/s 0-6 km shear is expected with a relatively high amount of that confined in the lowest few kilometers. North of the aformentioned surface trough -effectively a surface warm front- around 200 m2/s2 storm relative helicity is forecast. This all suggests that if storms can develop more or less in isolation, tornadic supercells are a possibility. The magnitude of the forcing suggests however, that most convection will be clustering rapidly. The resulting large MCSs should have a risk of producing large hail and some damaging winds. Overall, however, we don't want to exclude the chance of tornadoes with storms that are initially isolated, nor the smaller chance that they will occur with the more clustered activity later. The severe threat of the storms should diminish during the late evening as their focal point moves towards Moldova.

Northwestern Spain...

East of cold front stretching from the Benelux over France to central Spain, a zone of relatively weakly capped-moist air is situated.
Influx of moist air from the western Mediterranean into Spain should even lead to around 1000 J/kg MLCAPE. Storm activity will increase during the day. Isolated to scattered large hail and gusty winds can be expected.

Benelux, Northeastern France, Western Germany...

Convergence east of the cold front discussed above will almost certainly lead to the development of convective storms. Over the Benelux, 15-20 m/s deep-layer shear is expected, which should be sufficient for storm organization into multicell clusters. Given that about 1000 J/kg MLCAPE should be able to form, storms will be capable of producing large hail and gusty, locally severe winds.
Further south, in the range of the Vosges, Jura, orographic influences are likely to enhance the longevity of the otherwise poorly-organized storms, so that some large hail is possible there as well, marginally warranting a level 1.