Storm Forecast
Valid: Wed 23 May 2007 12:00 to Thu 24 May 2007 06:00 UTC
Issued: Wed 23 May 2007 12:15
Forecaster: GATZEN

SYNOPSIS

A ridge axis extends from northern Atlantic to northern France, Poland, and further to south-western Russia. To the north, a strong upper jet stream is present over northern British Isles/southern Scandinavia at the southern flank of intense upper long-wave trough. At the anticyclonic exit of the upper jet stream, a weak short-wave trough migrates over Belarus region today. Further south, two cut-off lows affect the Iberian Peninsula and southern Balkans/Aegean/Black Sea region. At lower levels, a frontal boundary stretches from Bay of Biscay to central France, southern Germany, south-eastern Poland, northern Belarus, and western Russia. While stable air mass is present to the north of this boundary, unstable air mass is present especially in the range of a quite narrow band just to the south. This air mass is characterized by steep low- and mid-level lapse rates and quite rich boundary-layer moisture, yielding CAPE values up to 1000 J/kg. Every storm that forms within this air mass will have a low chance to produce large hail given steep mid-level lapse rates and some reports from eastern Germany on TUE. But expected coverage/intensity is too low for a categorical risk level. Weak instability due to poor boundary-layer moisture is present over most of the Mediterranean.

DISCUSSION

Belarus/western Russia region

Latest soundings indicate quite steep lapse rates both at low levels and mid-levels as well as quite rich deep moisture with mixing ratios around 10..12 g/kg over Poland/northern Belarus. Latest observations indicate that strong diurnal heating and dewpoints around 17°C do likely yield rather high CAPE around 1000 J/kg, and latest satellite images show that storms have already initiated over central Belarus. Current thinking is that convection will go on over most of the area south of the frontal boundary during the next hours, while likeliest convective mode will be single cells given rather weak vertical wind shear. However, a weak upper trough migrating over Belarus during the day may be responsible for QG forcing as well as deep layer vertical wind shear of around 10..15 m/s locally. This leads to the suggestion that the chance for isolated mesocyclones is higher than over the surrounding areas. Given steep lapse rates and rather high CAPE, storms may produce large hail. Local flash flood is also possible, while severe wind gusts are not expected to be likely given rather weak cold pool potential and expected organization of convective clusters. Storms will likely merge into clusters during the day, and threat for severe convection is expected to decrease in the evening/night hours, as upper trough weakens and low-level buoyancy decreases.

Iberian Peninsula

Warm and moist air mass spreads westward into south-eastern Iberian Peninsula as indicated by latest observations. Although boundary layer was quite cold at noon, strong diurnal heating is expected during the next hours. In the late evening, weak upper jet streak/vort-max will spread into south-eastern Iberian Peninsula, and initiation may be possible. Latest model output suggests deep layer vertical wind shear around 25 m/s in the afternoon/evening hours. Additionally, a weak surface low pressure system present over northern Algeria is expected to provide backing low-level winds, yielding favorable hodographs and 0-3 km SRH values of 100..200 J/kg. Storms that form will rapidly become supercells, capable of producing large hail and severe wind gusts. Limiting factor is a strong capping inversion, but upslope flow and diurnal heating as well as outflow boundaries of overnight's convection may be strong enough to break the cap especially over the mountains. In the late evening hours, QG forcing and vertical wind shear will likely increase as indicated by latest model output, while low-level air mass will become quite stable. Current thinking is that a few isolated storms are possible, but coverage will likely be small. A few large or very large hail reports are not ruled out as well as severe wind gusts. Overall threat seems to be quite low, though, and decide to issue a level 1 that may require an upgrade when strong storms will develop over south-eastern Spain.

Eastern Balkans

A weak frontal boundary is present over Bulgaria in the range of the cut-off low over Aegean. To the north of this boundary, moist and slightly unstable air mass is present over north-eastern Bulgaria and Romania. Thunderstorms have already initiated over Romania. Given moderate deep layer vertical wind shear of around 15 m/s at the north-eastern flank of the cut-off low, a few organized storms are forecast. Limiting factors are quite weak forcing as well as weak instability and SRH values. However, isolated supercells are not ruled out capable of producing large hail. Tornadoes are not expected given weak low-level vertical wind shear.