Storm Forecast
Valid: Thu 09 Jun 2011 06:00 to Fri 10 Jun 2011 06:00 UTC
Issued: Wed 08 Jun 2011 22:54
Forecaster: GATZEN

A level 2 was issued for eastern Bulgaria and surroundings mainly for large hail.

A level 1 is forecast for Romania, Bulgaria and surroundings mainly for large hail.

A level 1 is forecast for western Urkaine, north-eastern Poland, and southern Sweden mainly for excessive rain.

SYNOPSIS

A high over northern Scandinavia is flanked by polar troughs centred over the northern British Isles and south-western Russia. A convergence zone is located along the axis of warm air from southern Sweden and eastern Poland to the western Black Sea and the Aegean Sea. The Mediterranean is affected by a westerly mid-level flow and embedded vort-maxima.

DISCUSSION

Western Black Sea

The convergence zone near the warm air axis will be located over the western Black Sea region. Rich boundary-layer moisture is forecast (11 g/kg) combined with steep mid-level lapse rates (7K/km between 2000-4000m). In response to diurnal heating, CAPE in the order of 2000 J/kg is forecast by latest GFS model. On Thursday, a mid-level short-wave trough will approach the area, leading to lift. Models expect falling surface pressure along the convergence line associated with increasing south-easterly surface winds near the Black Sea. Above the boundary layer, south-westerly winds are forecast to strengthen in the afternoon hours as a mid-level jet streak approaches over the Aegean Sea region.

Storms are forecast that will likely spread eastward during the day. Supercells are forecast especially over eastern Bulgaria and surroundings where favourably veering hodographs are possible. Large or very large hail is forecast, and supercells that form may be also capable of producing tornadoes and severe wind gusts. Storms are forecast to cluster in the afternoon and evening hours as the mid-level trough leads to further lift, and the main threat is forecast to turn to severe wind gusts and excessive precipitation gradually.

Turkey

Along the southern parts of the convergence and warm air axis, weaker low-level forcing is expected. Storms that form will be more isolated. The relatively strong vertical wind shear that reaches 15 m/s in the lowest 6km is expected to support some supercells or multicells capable of producing large or very large hail and locally severe wind gusts. The convective activity is forecast to spread gradually eastward during the period.

The Balkans to Poland

Near the warm air axis and to the east of a surface cold front that moves across Poland, western Slovakia and the Alps during the period, a warm and moist air mass is present on Thursday. Diurnal heating will result in instability even though the mid-level lapse rates are relatively weak over most places. Lift is expected along the cold-front as well as due to the approach of the mid-level trough center. Storms that form are forecast to cluster given rather weak vertical wind shear over most places. Excessive rain is forecast to be the main threat, especially over eastern Poland and western Ukraine, while large hail is not ruled out with the stronger storms especially over Romania and Bulgaria. Storms will likely decay from the north-west as dry and cool low-level air approaches during the day.

Southern Sweden

A tongue of warm and moist air spreads north-westward ahead of the cold front and a south-easterly low-level jet develops. Models suggest rich boundary-layer moisture and also rather steep mid-level lapse rates, and CAPE of around 500 J/kg is forecast. Strong lift can be expected due to the warm air advection and low-level convergence. Thunderstorms are forecast that will likely go on during the day. The main threat seems to be excessive rain along the surface front. Additionally, tornadoes are not ruled out given increasing low-level vertical wind shear reaching 10 m/s in the lowest km. Storms are forecast to become gradually elevated during the night and will spread into Norway, where excessive rain is also possible.

Eastern Spain

The axis of the western rough will spread into eastern Spain in the evening and night hours. The associated jet streak will lead to increasing vertical wind shear that may reach 20 m/s in the lowest 6 km. Main uncertainty will be the instability as the low-level moisture is poor and the timing of the trough does not overlap with the daytime heating. Current thinking is that isolated storms may develop in the evening hours, and isolated large hail and severe wind gusts are not ruled out. A level 1 threat may become necessary later on when the instability becomes more evident.