Storm Forecast
Valid: Thu 17 Jun 2010 06:00 to Fri 18 Jun 2010 06:00 UTC
Issued: Thu 17 Jun 2010 04:01
Forecaster: GATZEN

A level 1 was issued for the central Balkans mainly for large hail.

A level 1 was issued for central Italy mainly for severe wind gusts and large hail.

A level 1 was issued for northern Italy mainly for large hail.

A level 1 was issued for southern Germany and surounding mainly for excessive rain and large hail.

A level 1 was issued for southern France mainly for tornadoes.

A level 1 was issued for portions of Romania and Bulgaria mainly for large hail and excessive rain.

A level 1 was issued for southern Sweden mainly for severe wind gusts and tornadoes.

SYNOPSIS

To the west of an amplified ridge over the Atlantic Ocean, Europe is dominated by rather low geopotential centred over southern France, where an intense cut-off low propagates north-eastward. This yields a strong mid-level south-westerly flow across the western Mediterranean region. To the north, a weak ridge stretches from the British Isles to eastern Europe, leading to a cool but calm situation, while a frontal boundary stretches from southern Germany to Romania separating the dry from moist air masses. From the north, another cold air outbreak starts at the end of the forecast period.

DISCUSSION

Central Italy to the Balkans

Ahead of the cut-off low, a strong south-westerly jet spreads across the western Mediterranean into central Italy and further into the northern Balkans. Winds will reach about 20 m/s at the 700 hPa level during the day. In the range of this jet, models agree about a frontal wave spreading eastwards, and warm subtropical air advects into southern Italy and the southern Balkans. The warm air advection regime is characterized by a rather cool boundary layer, but the low-level moisture increases to the north-east as ample moisture can be found to the south of a front over the northern Balkans.

Initiation seems to be most likely over the north-western Balkans. Outflow boundaries of Wednesday's thunderstorms and rich low-level moisture are expected, while some daytime heating will lead to upslope flow over the mountains. The cap will increase over the southern portions due to the warm air advection, limiting the chance of initiation, while upward vertical motion at the cyclonic flank of the jet will likely lead to steepening lapse rates further north, where about 1000 J/kg CAPE will be quite possible.

Main convective mode may be pulse storms initially, but increasing vertical wind shear and favorably veering profiles in the lowest 3 km will assist for mesocyclogenesis during the afternoon and evening hours. Large hail will be the main threat with every supercell that forms. Given the rather widespread convection, a high-end level 1 was issued. Tornadoes and severe wind gusts are not ruled out, but rather weak vertical wind shear in the lowest kilometre will limit the potential. Additionally, locally excessive rain and flash flooding is not ruled out. Storms may cluster into an MCS over the northern Balkans at the nose of the low-level jet given the QG forcing in the range of the approaching trough axis. Severe wind gusts and locally large hail are not ruled out, but the severe potential will gradually decrease during the night hours as the instability decreases.

Further west, low-level moisture is significantly weaker across the Mediterranean. Given the rather cool sea surface, only weak and strongly capped instability is expected, what limits the potential of thunderstorms. However, diurnal heating and associated sea-breeze convergence over central Italy may result in some storms that will have a high potential to organize given the strong vertical wind shear of about 20 m/s in the lowest 3 km. Large hail and severe wind gusts are forecast. Storms will likely weaken when they spread across the Adriatic Sea, so that severe potential will likely decrease.

Northern Italy to southern Germany and surroundings

A moist air mass is present over the Alpine region. On Thursday, QG forcing is likely at the cyclonic flank of the approaching jet streak. Given some diurnal heating, CAPE is likely over the region, as persisting forcing ahead of the trough axis and low-level convergence will result in widespread thunderstorms. Mostly unidirectional shear to the south of the Alps may be supportive for a few severe storms capable of producing large hail, but weak low-level convergence limits the chance of initiation. To the north of the Alps, strong veering is forecast while the wind speed is weak. This may result in some slow moving supercells that will likely merge into an MCS given the strong low-level convergence in the afternoon and evening hours. Excessive rain and local flash flooding will be main severe threat, but locally large hail is also possible. The intense rain may go on during the night hours over portions of southern Germany, while thunder becomes less likely due to decreasing instability.

Southern France region

In the range of the cut-off low, weak vertical wind shear, rather large low-level buoyancy and low-level convergence is expected to be favorable for non-mesocyclonic tornadoes, and a few events are forecast especially during the morning/noon hours. Later in the day, low-level convergence is forecast to decrease and severe potential will likely decrease.

Romania, Bulgaria

At the eastern flank of the central European ridge, rather dry air spreads southward into the Black Sea region. Rich low-level moisture is present to the south of the cold front and will be pushed further south during the day. In the range of this air mass, diurnal heating is expected and CAPE will likely develop. Although low-level convergence is rather weak, some storms are forecast during the day. Some moderate vertical wind shear is expected to the south of a weak mid-level trough that moves eastward over the Ukraine, and storms may organize posing a threat of large hail. Excessive rain is not ruled out as storms will move along the frontal boundary, where low-level convergence is best. In the evening hours, weakening forcing and decreasing instability will likely result in decaying storms.

Southern Sweden

An intense trough spreads southward at the end of the period, and strong cold air advection is expected over western Scandinavia. At the lee side of the Scandinavian mountains, models indicate a tongue of rather moist air that will remain until the end o the period, while strong forcing will lead to rapidly cooling mid-levels. As a consequence, instability may develop, and latest GFS shows an increasing parcel layer depth and decreasing equilibrium temperatures. Given the strong forcing, thunderstorms are forecast in the morning hours on Friday. While the low-level vertical wind shear is forecast to be rather weak, the deep layer vertical wind shear will likely increase to values of about 25 m/s, indicating at least a weak potential of severe thunderstorms. Severe wind gusts are not ruled out, and a tornado may also develop given the rather moist low-level air mass.