Storm Forecast
Valid: Sat 02 Sep 2006 06:00 to Sun 03 Sep 2006 06:00 UTC
Issued: Fri 01 Sep 2006 18:22
Forecaster: DAHL

SYNOPSIS

Strong Atlantic upper jet is propagating eastwards ... with lead vort max reaching the weastern Baltic Sea/S Scandinavia by Sunday 06Z. This feature will be accompanied by intense frontal wave moving from the British Isles across the North Sea
along the south periphery of large-scale surface low ... reaching Denmark towards the end of the forecast period. Narrow upper longwave trough over E Europe is progged to close off into an upper cut-off low ... and associated DCVA-related ascent should promote weak cyclogenesis over the E Black Sea ... advecting very moist and unstable air into the extreme eastern portions of the forecast area.

DISCUSSION

central Ukraine ... W Russia

Tuapse 12Z launch from Friday 12Z reveals about 3000 J/kg MLCAPE ... this air mass should return into the extreme eastern forecast area in response to weak cyclogenesis over the E Black sea. Deep-layer shear is progged to be on the order of 25 m/s, LLS
ranging from roughly 5 m/s to 15 m/s ... the higher values probably being outside the region where bulk of SFC-based convection may occur. In the WAA regime N of the SFC low ... SRH should reach 400 J/kg (0-1 km) and 600 J/kg (0-3 km).

GFS 12Z places bulk of precip amidst WAA regime over the central Ukraine ... suggesting that storms may be embedded in stratiform precip ... with
limited chances of becoming surface based. However ... storms may also develop farther E ... and should rapidly become supercellular. Main threat will likely be large hail and damaging winds ... though a few tornadoes may occur as well despite weak large-scale LL shear.

If BL air mass W of the warm front happens to be at least weakly buoyant ... elevated convection may root down into the BL ... ingesting 600 J/kg 0-3 km SRH ... with a concomitant threat for tornadic supercells.

Threat is quite conditional ... but pattern may favor isolated extreme severe weather ... unfortunately mainly E of the froecast area. Have included region of possibly elevated convection from which SFC-based convection may develop in the level one area.

UK ... North Sea ... N-central Europe

Very large hodograohs will be associated with the intense frontal wave over north-central Europe ... but it seems that free convection will be suppressed given absence of positive CAPE. Forced convection may become an issue ... but given present inability to forecast this type of convection ... will not base a categorical risk on it. Situation should be monitored though and an upgrade may be warranted should convection indeed develop. In the wake of the frontal wave ... GFS hints at possibility of some convection over the UK. But convection should not be too widespread ... and will thus not issue a categorical risk ATTM. If indeed widespread TSTMS develop ... an upgrade may be considered given very strong shear profiles.

Iberian Peninsula

Isolated high-based late-afternoon TSTMS may develop over the Iberian Peninsula. These will pose a threat for isolated/short-lived downbursts. Models do not suggest any convective development ... and activity should indeed remain quite isolated. Will thus not include it in a TSTM/risk area.