Storm Forecast
Valid: Thu 28 May 2009 06:00 to Fri 29 May 2009 06:00 UTC
Issued: Wed 27 May 2009 18:32
Forecaster: GATZEN

A level 1 was issued for Central Mediterranean and southern Balkans mainly for large hail.

A level 1 was issued for Poland, eastern Germany, Czech Republic, and northern Austria region mainly for severe wind gusts and tornadoes.

A level 1 was issued for Northern Adriatic mainly for severe wind gusts and large hail.

A level 1 was issued for Bulgaria, Romania, and central Ukraine mainly for large hail, extensive rain, and severe wind gusts.

SYNOPSIS

As a high builds over western Europe, a polar trough amplifies into central Europe. The trough surrounded by a strong mid-level jet reaching around 55 m/s over northern Germany will start to cut off during the period. Downstream, another low is expected across the central Mediterranean. At lower levels, warm air masses left over eastern Europe will spread north-westwards ahead of the approaching trough. Strong cold air advection is expected over the northern Balkans.

Central Mediterranean and southern Balkans

Steep mid-level lapse rates and rich boundary-layer moisture will likely result in CAPE values around 1000 J/kg from southern Adriatic to southern Italy, and the Tyrrhenian Sea. Some QG forcing will be likely in the range of the weak mid-level trough as a mid-level jet streak curves around its southern flank. At lower levels, low-level convergence is likely especially along sea-breeze fronts and outflow-boundaries, and thunderstorms are forecast to develop. Given around 10 m/s 0-3km vertical wind shear due to westerly wind at the 700 hPa level and weak winds at the surface, some multicells and supercells are not ruled out. Limiting factor is relatively weak deep layer vertical wind shear, but models expect at least 15 m/s in the evening hours that may be sufficient for storm organization. Best potential for severe thunderstorms seem to exist over southern Italy and Sicily, where diurnal heating will result in strong instability while low-level convergence is forecast. Large hail and severe wind gusts are expected to be the main severe threat. Tornadoes are not completely ruled out along of outflow-boundaries, where low-level buoyancy and wind shear as well as convergence may be favorable.

Poland, eastern Germany, Czech Republic, and northern Austria region

Intense QG forcing is forecast as the strong mid-level jet streak moves across Germany. A cold front is forecast to travel southwards over Germany and Poland during the day. Ahead of this front, models indicate a narrow tongue of rather humid low-level air mass and profiles will likely be moist adiabatic in the morning hours due to the strong lift. Instability will likely develop in response to daytime heating. Given the strong low-level convergence along the cold front, a strongly-forced narrow convective line is expected to develop, moving from the southern Baltic Sea to the northern Balkans during the period.

Given strong low-level vertical wind shear of 10 m/s and favorably veering profiles especially over eastern Germany and western Poland, mesocyclones are not ruled out ahead of the convective line, capable of producing large hail. Tornadoes are not ruled out, while rather cool boundary-layer may limit the potential. Supposed strong diabatic heating head of the cold front in the morning hours, the tornado threat may be more robust, and a few events would be possible. Severe wind gusts are also forecast along the cold front.

In the wake of the cold front, strong cold air advection is expected, but diurnal heating may be strong enough and lift seems to be likely due to strong differential cyclonic vorticity advection. Given negative temperatures at the 850 hPa level and low-level heating, instability is likely. Thunderstorms are forecast given cold equilibrium temperatures that will likely organize near the German border, where vertical wind shear will be strongest. Severe wind gusts are expected to be the main severe threat given dry low-level air mass and steep lapse rates as well as strong low-level vertical wind shear. Marginally severe hail is not ruled out with the stronger cells. Convective activity is expected to weaken after sunset while spreading into Czech Republic, Slovakia, and northern Austria.

Northern Adriatic

Low-level moist air mass will likely remain over the northern Adriatic region until the night hours, when the mid-level trough will approach and leads to strong lift. Steepening lapse rates will likely result in instability, and showers and thunderstorms may develop late in the period. Given very strong deep layer vertical wind shear up to 40 m/s, severe storms may evolve, capable of producing large hail and severe wind gusts. Tornadoes are not expected to be likely given the cool boundary-layer in the morning hours.

Bulgaria, Romania, and central Ukraine

Models indicate some instability ahead of the European trough during the day, and thunderstorms are likely to develop in the warm air mass in the wake of the low-level frontal boundary, where best boundary-layer moisture is forecast. Although deep layer vertical wind shear will be rather weak, a few severe events are not ruled out. Large hail seems to be the main severe threat with the stronger storms that may organize along outflow-boundaries. Extensive rain may also occur due to the slow propagation speed of the convective cells. Severe wind gusts are also not ruled out along the leading frontal boundary given dry air mass to the east and strong downdraft CAPE.