Storm Forecast
Valid: Thu 11 Jun 2009 06:00 to Fri 12 Jun 2009 06:00 UTC
Issued: Wed 10 Jun 2009 23:51
Forecaster: GATZEN

A level 2 was issued for western Ukraine, Belarus, and western Russia mainly for large hail and tornadoes.

A level 2 was issued for north-eastern Italy region mainly for large hail and tornadoes.

A level 1 was issued for Central Balkans to western Romania, and western Ukraine mainly for large hail.

A level 1 was issued for northern Italy to northern Balkans mainly for large hail and tornadoes.

A level 1 was issued for Eastern Germany into Austria and Czech Republic mainly for severe wind gusts and tornadoes.

SYNOPSIS

A subtropic high ridges into the Iberian Peninsula. A mid-level jet over western Europe intensifies as a trough will move across the North Sea. The trough axis becomes negatively tilted over central Europe and provides strong QG forcing. As a consequence, a low pressure system that moves into southern Scandinavia will intensify. While rather cool air masses flood into most of western Europe, unstable air masses remain from northern Italy to eastern Europe.

DISCUSSION

Central Balkans to western Romania, western Ukraine, Belarus, and western Russia

An unstable air mass is present over a broad region of eastern Europe, characterized by an EML from Greece to western Ukraine and western Russia and rich boundary-layer moisture. High CAPE in the order of 2000 J/kg will likely build during the day. On Thursday morning, a cold front is forecast to spread into this region and will become quasi-stationary in the southern portions, while a frontal wave will lead to warm air advection from Belarus to western Russia.

Models indicate that a jet streak will evolve in the range of the frontal boundary that will be strongest from the western Ukraine and Belarus to western Russia, where strong vertical wind shear is forecast. Especially in the afternoon and evening hours, 15 to 20 m/s 0-3 km and 10 to 15 m/s 0-1 km vertical wind shear are quite likely. This will be sufficient for supercells that are forecast to produce large or very large hail. Tornadoes are forecast especially over Belarus and extremely western Russia where low-level vertical wind shear and veering profiles are strongest. Excessive rain is expected to be most likely over western and central Belarus, where low-level convergence and moisture are forecast to reach the highest values.

In the southern portions of the risk area, vertical wind shear will be weaker, but 10 to 15 m/s winds at the 700 hPa level are possible. Thunderstorms are likely along the low-level convergence, and some supercells are forecast to develop, capable of producing very large hail. Isolated severe wind gusts and local flash flooding are not ruled out with stronger cells, while the tornado threat is forecast to be rather low. Convective activity is forecast to spread further east in the night hours as the trough axis of the central European trough moves across the region. The potential of severe weather will only decrease slightly from the Ukraine to Russia as strong low-level winds will continue ahead of an old front that develops to the west.

Northern Italy to northern Balkans

Most regions of western and central Europe are affected by cold air advection in the wake of the low pressure system moving across the Benelux and northern Germany during the day. In the wake of the Alps, rather moist and warm air masses will likely remain during the day. As the mid-level jet streak spreads across the Alps, a lee cyclone is forecast to develop over northern Italy that provides weak low-level winds and maybe even south-easterly winds at its eastern flank. Low-level moisture will therefore increase during the day. In the wake of the Alps, mixed mid-level are likely, and CAPE is forecast to build especially over north-eastern Italy, where moisture pooling is strongest.

QG forcing will increase during the day and low-level convergence is quite likely in the range of the low pressure system, and thunderstorms are forecast especially near the Alps initially, spreading across north-eastern Italy, southern Austria, and most of the northern Balkans during the period. Strong vertical wind shear is forecast reaching 15 m/s in the lowest 3 km and 30 m/s in the lowest 6 km. The best veering profiles may evolve over north-eastern Italy, where supercells are most likely. Large or very large hail is forecast with these storms. Low-level vertical wind shear is quite weak in the latest model fields, but it seems to be plausible that stronger westerly winds at the 850 hPa level will spread over parts of northern Italy. Given rather strong low-level buoyancy especially over the eastern portions of Italy and the northern Balkans, tornadoes are not excluded. Cold air advection will likely lead to stabilization in the evening hours, and thunderstorms are forecast to weaken rapidly.

Eastern Germany into Austria and Czech Republic

Ahead of the cold front that crosses western central Europe during the day, a tongue of rather moist air mass originating from the Bay of Biscay spreads eastward north of the Alps. Strong forcing at the cyclonic flank of the mid-level jet streak is expected to destabilize this moist air mass due to thermal indirect circulation. Ahead of the main cold front, the unstable air mass is expected in the morning hours from south-eastern Germany and northern Austria to northern Germany. Strong linear forcing along the cold front will likely cause a convective line that moves across southern Germany and northern Austria until noon.

Given strong low-level vertical wind shear and around 25 m/s winds in the 850 hPa level, severe wind gusts are not ruled out along this convective line. Tornadoes may also evolve along the leading line given 10 m/s low-level vertical wind shear, but rather cool low-levels will likely limit the potential in the morning hours. This potential may slightly increase over northern Austria and the Czech Republic when diabatic heating will increase low-level buoyancy. The convective line will weaken further east as the warm sector occludes and the trough axis turns northward. Cold air advection will likely limit convective activity in the wake of the cold front rapidly.

The Benelux and western/central Germany

In the wake of the low, rather cool air mass spreads into western central Europe. From the British Isles, a second trough axis moves eastward, providing QG forcing. Models suggest that some low-level moisture increase will be present from the Benelux to western and central Germany, and cool mid-level air mass will likely favour some instability. Showers and thunderstorms are not ruled out in the afternoon and evening hours, and given strong vertical wind shear, an isolated severe wind gust or tornado is not ruled out completely. However, threat seems to be too low for a level 1.

Central Finland

A strong southerly mid-level jet will evolve across Finland during the period. At lower levels, some warm air advection is likely, and moisture pooling is forecast over the central portions. Showers and thunderstorms are most likely in the range of a mid-level trough moving northward during the afternoon. As mid-level winds increase, deep layer vertical wind shear will be sufficient for organized storms, capable of producing isolated large hail, but indications are too weak for a level 1.

Eastern Spain, northern Africa, Sardinia, Corse, Italy

In the range of the building ridge, convective activity is limited. Along sea breeze fronts, isolated thunderstorms may form, though. These storms will have a potential of large hail and severe wind gusts over the western portions due to well-mixed air mass and steep lapse rates, while the storms in the eastern portions may profit from increasing deep layer vertical wind shear during the period, and large hail is not ruled out. Storm coverage is forecast to be too limited to highlight a level 1 area.