Storm Forecast
Valid: Wed 14 Jun 2006 06:00 to Thu 15 Jun 2006 06:00 UTC
Issued: Tue 13 Jun 2006 19:50
Forecaster: GATZEN

SYNOPSIS

Large cut-off low present over southeastern Europe ... that is filled with rather cool airmass affecting most of the Balkans, western Black Sea, and western Ukraine/Belarus. To the north and west ... upper ridge expands from western Mediterranean to Germany and further to northern Baltic States/Russia. Airmass in the range of this ridge is expected to be well-mixed on Wednesday ... and GFS indicates 900/700 hPa lapse rates of 7-9 K/km from Iberian Peninsula to France, Germany/Alpine region, Poland, and northern Russia. A frontal boundary is present at the northern edge of the warm airmass from Bay of Biscay to southern British Isles, southern Baltic Sea, and further to Finland. In the range of this boundary ... low level convergence is expected and boundary-layer moisture is forecast to increase gradually. Additionally ... small cut-off is expected over British Isles/the channel region ... associated with cold airmass spreading southeastward into France, Benelux, and northern Germany. CAA should be rather shallow underneath the ridge axis ... where latest model output indicates a surface low/convergence from northwestern Poland to central Germany and central France on Wednesday, 18 UTC. Another cut-off low is located SW of Iberian Peninsula ... where strong upper southerly flow is present ... while cool airmass spreads into Portugal behind a cold front.

DISCUSSION

Northern France, Benelux

Focus of expected potential for organized convection will be southeastern quadrant of developing cut-off low over British Isles. Increasing DLS is forecast at the southern edge of the cut-off low ... and DCVA is expected to spread southward. At lower levels ... well-developed frontal boundary should spread southward, too ... leading to low-level stabilization near the channel region ... while boundary-layer convergence should reach central France in the evening hours. North of the frontal boundary ... model outout shows rather warm/moist boundary-layer ... and CAPE should be present. Given QG forcing ... showers and thunderstorms are forecast to develop along the cold front and especially to the north of it. Convection may start elevated due to stable low layers ... but is expected to root to the boundary layer that is forecast to be moist/warm. Given easterly surface winds north of the frontal boundary ... and westerly jet aloft ... yielding DLS of 20 m/s and veering profiles ... thunderstorms will likely organize into multicells and mesocyclones ... capable of producing isolated large hail and severe wind gusts. An upgrade to level 2 may be warrant when thermodynamic profiles will evolve better than anticipated. Later in the day ... thunderstorms are expected to merge into an MCS moving eastwards into Benelux with severe wind gusts the most significant threat. While shear profiles weaken ... cluster of storms should also weaken during the night hours. Along the surface boundary over central France ... additional convection should form. Given weak DLS ... storms should be outflow-dominant ... forming a cluster. Given well-mixed low-levels ... severe wind gusts should be possible.

Iberian Peninsula

Warm airmass is present over Iberian Peninsula that is charcterized by relatively steep lapse rates above the boundary layer. Below an inversion ... low-level moisture in increasing over southern Iberian Peninsula. Additionally ... upper cut-off over eastern Atlantic evades eastward yielding to height falls. Models suggest that CAPE will build over the peninsula during the period. In the range of strong southerly jet at the eastern flank of the cut-off ... quite strong QG forcing should be possible ... and given low-level forcing along a cold front over Portugal moving eastwards as well as orographic lift ... rather strong capping inversion is expected to brake. Thunderstorms that form will likely organize into supercells given strong DLS ... and isolated large hail and severe wind gusts are expected. Thunderstorms should spread northeastward into western Mediterranean during the night ... while low-level cooling should lead to weakening of the storms.

Finland

Models show weak CAPE forming south of propagating cold front ... where latest sounding data show steep low-level lapse rates and quite rich moisture. Thunderstorms that form will have a potential to organize given enhanced vertical wind shear in the range of the upper jet at the northern flank of eastern European ridge. Although thermondynamic profiles are not expected to be very favorable ... and overlap of CAPE/shear is not clear ATTM ... a few severe hail/wind events are not ruled out. Thunderstorms should merge into a line leaving the sheared environment during the period ... while severe threat weakens. An upgrade to level 1 may be warrant later on.

Germany

In the range of propagating cold front/convergence ... thunderstorms should form. Weak vertical wind shear will be present ... and thunderstorms should be outflow-dominant. An isolated hail/wind event may occur.