Storm Forecast
Valid: Wed 21 May 2008 10:00 to Thu 22 May 2008 06:00 UTC
Issued: Wed 21 May 2008 10:05
Forecaster: GATZEN

SYNOPSIS

A rather broad upper trough is present over Europe, with numerous centers over Scandinavia, west of British Isls, Germany, France, and Italy. The main trough axis extends from Germany to southern Adriatic and turns north-eastward into Balkans during the period. A strong upper jet streak curving around the southern edge of the associated short-wave trough will enter the western Black Sea region late in the period. At low levels, very warm subtropical air mass has been advected into Turkey and Black Sea region ahead of a frontal boundary from southern Aegean to Slovakia. Cool and mostly dry air mass has spread across most of Europe in the range of the upper trough.

DISCUSSION

Bulgaria, Romania, Moldova, south-western Ukraine

Main focus of expected severe threat is the western Black Sea region. A tongue of very warm subtropical air expands from western Turkey to western Ukraine at the eastern flank of the upper trough. Latest soundings indicate that this air mass is characterized by step mid-level lapse rates especially from western Turkey to eastern Romania at this time. Below the inversion, low-level moisture is more than adequate for diurnal instability as indicated by latest Bucharest ascend with around 10 g/kg mixing ratio. Latest surface observations show that low-level dewpoints reach more than 16°C along the Bulgarian/Romanian Black Sea coast, indicating that low-level moisture may increase during the day over most regions given easterly winds. Visible satellite image also shows that strong diabatic heating will be possible leading to sufficient instability in the range of 1000 J/kg.

During the next hours QG forcing is expected especially over northern Bulgaria ahead of the approaching jet streak and associated short-wave trough due to DCVA and WAA to the north-east of the frontal boundary/outflow boundary now present over western/central Bulgaria. Latest models indicate that a surface low will develop over western Bulgaria along this frontal boundary during the day. To the east of this low, strengthening southerly winds are forecast just above the boundary layer, reaching up to 20 m/s at the 850 hPa level as indicated by latest BOLAM model run. Given south-easterly winds in the boundary layer, strong low-level vertical wind shear may develop in the afternoon and evening hours, and latest GFS model run suggests 20 m/s 0-3 km vertical wind shear over southern/central Romania in the evening hours.

Expect initiation during the next hours over Bulgaria initially along the frontal boundary, storms are forecast to spread north-eastwards while additional storms are forecast to develop over southern/central Romania in the afternoon/evening hours. Limiting factor may be moderate capping inversion, but strong diurnal heating and mid-level height falls will likely weaken this cap. Some uncertainties exist with regard to low-level vertical wind shear and veering profiles, but given easterly winds over most of Romania at this time, quite high low-level SRH seems to be likely, and storms will rapidly organize. Supercells are forecast capable of producing large hail and severe wind gusts. Tornadoes are also forecast given veering profiles and locally strong low-level buoyancy especially along old outfow-boundaries from overnights convection. Some tornadoes may be strong especially in the evening hours when southerly low-level jet intensifies over Romania. Given persistent QG forcing during the night hours, convection will likely merge into on or two MCS moving northward into western Ukraine and Moldova, where severe wind gusts and isolated tornadoes will be the main severe threat.

Belarus, western Russia

At the northern edge of the east European warm air regime, a frontal boundary is present form north-western Ukraine to Belarus and western Russia. Along the frontal boundary, additional thunderstorms will likely develop given some instability due to rich low-level moisture and diurnal heating s well as rather weak convective inhibition. Storm that form will likely organize given moderate vertical wind shear and veering profiles, and large hail as well as isolated tornadoes are forecast.

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