Storm Forecast
Valid: Thu 21 Jun 2007 06:00 to Fri 22 Jun 2007 06:00 UTC
Issued: Wed 20 Jun 2007 23:42
Forecaster: GATZEN

SYNOPSIS

Strong subtropical high ridges into central Mediterranean and east Europe. Warm desert air mass with impressive EML is present in the range of this ridge as indicated by latest soundings (DTTA Tunis, LIRE Pratica di Mare, LDDD Zagreb). Below the capping inversion, high theta-e and rich low-level moisture has developed over northern Mediterranean, Balkans, and most of Alpine region. Further west, mid-level trough over western British Isles is expected to weaken. A strong southwesterly jet will affect south-western, and central Europe at its eastern flank. An embedded short-wave trough/jet streak is expected to migrate northeastwards over western Mediterranean, Alpine region, and Germany during the period. Latest model guidance indicates 300 hPa winds of 40..45 m/s over France. While the jet moves across the frontal boundary present from western Alps to northeastern Germany on THU morning, cyclogenesis and strong UVV is forecast. Cool maritime air mass remains over west Europe and spreads into Iberian Peninsula, southern France, western Alps, and Germany during the period. Extremely eastern and northern Europe will be affected by cool/dry air mass in the range of another trough.

DISCUSSION

Eastern Germany, Czech Rep, western Poland

Focus on THU will be the strong frontal wave forecast to build along the frontal boundary over central Europe. Although models do not agree well about the exact position of the low, it seems to be likely that it will move across eastern Germany into northwestern Poland during the day. Models do consistently predict intense precip over parts of Germany at the cold side of the boundary, where cool temperatures will be a consequence. North and east of the low, warm air mass will likely spread as far west as the Poland/German border. Latest GFS indicates well low-level mixing with strong lapse rates and high surface temperatures/rather weak boundary-layer moisture over Poland, and Austria. It seems to be likely that the same air mass will be present over Czech Rep. In the range and just W of the surface low, weak low level winds (mostly from the north) are expected across northeastern Germany, where boundary-layer moisture is forecast to increase during the day.

Strongest convective development is expected to be associated with the upper vort-max/jet streak spreading into southern/eastern Germany during the day. and associated UVV. Convective clusters will likely intensify over southern/central Germany in the morning hours. While the surface low and associated convection moves northward during the day, cold front over southern Germany will likely accelerate eastwards. A linear MCS is quite realistic to develop over southeastern Germany moving into Austria in the afternoon hours. Given the strong pressure gradient and rapidly moving cold front, severe wind gusts are not excluded along the leading edge of the gust front. Brief gust front tornadoes may also occur.

Farther north, initial focus of expected severe convection will be pre-frontal boundaries ahead of approaching cold front. Current scenario is that sinking and inversions will inhibit initiation until the afternoon. Due to strong diabatic heating, storms may develop during the day, given locally rich boundary-layer moisture and steep lapse rates aloft. Storms that form may organize as DLS in the order of 20 m/s will be available in the warm sector as well as 0-3 km SRH of 100..300 J/kg. Supercells that form will have the potential of producing tornadoes given 100..150 J/kg 0-1 km SRH, 10 m/s LLS, quite low LCL heights and locally strong low-level buoyancy. Very large hail and severe wind gusts are not excluded.

Later in the period, cold front convection will likely spread into northeastern Germany, western Poland, and Czech Rep. Latest model guidance seems to indicate a strongly-forced convective line. The strong cold pool expected over central Germany as well as moderate DLS, LLS, and QG forcing may be sufficient for severe wind gusts along the leading edge of the front. Although later observations will be necessary for an upgrade, want to mention the possibility for a high wind event over the affected region, with pre-frontal boundaries as a focus of severe winds. Embedded supercells may also produce tornadoes and large hail. Convection is expected to weaken over Poland due to dry low-level air mass in the evening/night hours.

Southern France, Alpine region, northern Balkans

Although low-level air mass will be mostly capped, QG forcing at the cyclonic flank of upper jet stream may be sufficient for a few thunderstorms. Models predict moderate instability and shear, and a few supercells are not ruled out, capable of producing large hail and severe wind gusts.

Southern Norway/Sweden

At the northern edge of the frontal boundary/convergence zone over central Europe, warm and moist air mass is expected over southern Scandinavia. Weak CIN and quite some low-level forcing may be sufficient for a few thunderstorms during the day. In the range of strong northwesterly jet over Scandinavia, moderate DLS will be present, and some thunderstorms may organize into multicells. Large hail is forecast to be the main threat.