Storm Forecast
Valid: Sun 31 May 2009 06:00 to Mon 01 Jun 2009 06:00 UTC
Issued: Sun 31 May 2009 07:00
Forecaster: GATZEN

A level 1 was issued for Sardinia to central Italy and central Adriatic mainly for excessive rain and large hail.

A level 1 was issued for eastern Iberian Peninsula mainly for large hail.

A level 1 was issued for north-western Ukraine, south-western Belarus, and eastern Poland mainly for large hail and tornadoes.

A level 1 was issued for northern Morocco and Algeria mainly for large hail and severe wind gusts.

A level 1 was issued for northern Finland and north-western Russia mainly for tornadoes and severe wind gusts.

SYNOPSIS

A broad high stretches from British Isles across southern Scandinavia to western Russia. A cut-off low centered over Slovakia extends westward over the Alps into southern France. A mid-level jet streak is forecast to move across the central Mediterranean at the southern flank of this low that is filled with cool air masses. A rather warm air mass is present from the Black Sea across eastern Europe and the southern Scandinavian region as well as over the Iberian Peninsula.

DISCUSSION

Sardinia to central Italy and central Adriatic

Latest ascends across Italy indicate relatively steep lapse rates and rich boundary-layer moisture around 10 g/kg mixing ratio. Daytime heating will likely result in moderate CAPE values of 500-1000 J/kg today. Increasing QG forcing is likely over central Italy today at the cyclonic flank of a 30 m/s mid-level jet streak reaching southern Italy in the evening hours, while some warm air advection is forecast at low levels. Showers and thunderstorms are forecast to go on over central Italy. Given southerly low-level winds ahead of low surface pressure across the western Mediterranean Sea and strong mid-level westerly winds, 20 m/s deep layer vertical wind shear is forecast. Multicells seem to be the dominant storm mode, as favorable veering is not indicated over most places.

Forecast weak winds at the 700 hPa-level across central Italy indicate slow moving storms and back-building multicells will have a potential to produce excessive precipitation. Large hail is also forecast especially when supercells will develop. Severe wind gusts and tornadoes seem not to be likely a low-level vertical wind shear is forecast to be weak. Storms will likely merge into mesoscale convective systems given increasing QG forcing during the afternoon and evening hours, and the threat of excessive rain will continue from Sardinia to central Italy through-out the night hours and may also spread into the central Adriatic until the morning. A slight risk of large hail and local severe wind gusts is forecast especially at the southern flank of the convection, where deep layer vertical wind shear is forecast to increase in the range of the mid-level jet.

Northern Italy, southern France, eastern Iberian Peninsula

Underneath the trough axis of the European cut-off low, showers and thunderstorms will likely develop in response to diurnal heating and low-level convergence. Given weak deep layer vertical wind shear around 10 to 15 m/s, single cells and storms clusters are forecast. Best potential for organized storms exists over the south-eastern Iberian Peninsula, where 0-3 km vertical wind shear is forecast to increase in the evening hours in the range of moderate north-westerly winds at the 700 hPa level, and large hail is forecast as well as isolated severe wind gusts given steep lapse rates. Further north-east, only isolated severe weather is forecast including local flash flooding given slow moving storms.

North-western Ukraine and south-western Belarus, eastern Poland, eastern Germany

Rather steep lapse rates and rich low-level moisture are present to the north of the cut-off low and have spread into northern Germany. Diurnal heating will help to destabilize this weakly-capped air mass, and thunderstorms are forecast. Increasing QG forcing and vertical wind shear are forecast to affect the northern Poland region during the afternoon, and large hail is forecast. Additionally, low-level vertical wind shear will increase ahead of a cold front moving northward during the day. A slight chance of tornadoes is expected near the frontal boundary in regions with good low-level moisture and buoyancy. Further west, isolated large hail is not ruled out completely. Excessive rain is also not completely ruled out, but threat seems to be too weak for a level 1 at this time.

Northern Morocco and Algeria

A rather strong mid-level jet is forecast across northern Africa at the southern flank of the European trough. A trough axis will move eastward across northern Morocco during the day providing some QG forcing. At lower levels, moist air masses with mixing ratios of 10-13 g/kg are present near the Mediterranean Sea, and northerly winds will advect this air mass into the Atlas mountains, where upslope flow will evolve. Thunderstorms are forecast to develop over the mountains. Quite strong vertical wind shear and favorable veering profiles are indicated by latest models and supercells may be capable of producing very large hail and severe wind gusts.

Northern Finland

At the northern flank of the Scandinavian high, a short-wave trough travels eastward with the strong mid-level westerly flow, providing strong QG forcing in the range of a surface low advecting warm air mass northward. At the northern flank of the warm air mass, low-level moisture pooling is forecast. Models expect destabilization especially from northern Finland to north-western Russia in the afternoon and evening hours, when the mid-level trough approaches. Limiting factor is actual cloud coverage over portions of this region and associated weak diabatic heating.

Showers and thunderstorms are forecast to develop during the afternoon and evening hours spreading eastwards. Given strong low-level winds exceeding 20 m/s at the 850 hPa level, strong vertical wind shear is forecast. Favorable veering and SRH of 100-200 m²/s² in the lowest kilometer as well as moist low-level air mass indicate a potential of tornadoes, especially when mesocyclones will develop. Isolated large hail and severe wind gusts are not ruled out.