Storm Forecast
Valid: Thu 10 May 2012 06:00 to Fri 11 May 2012 06:00 UTC
Issued: Thu 10 May 2012 06:20
Forecaster: GATZEN

A level 1 was issued for northern and central France, Benelux, northern and central Germany, Denmark and the western Baltic Sea mainly for severe wind gusts, large hail and tornadoes.

A level 1 was issued for northern adn eastern England mainly for tornadoes.

A level 1 was issued for south-east Europe mainly for large hail and excessive precipitation.

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

SYNOPSIS

An Atlantic short-wave trough will move eastward during the period, reaching Ireland at Friday morning. A strong jet streak from the northern Bay of Biscay to the North Sea will expand into the Baltic Sea. To the south-east of this jet streak, an unstable air mass characterized by steep lapse rates is present from Iberia to central France and further into the Baltic States and south-east Europe. Best lapse rates have developed over Spain, but rather steep low-level lapse rates can also be found north of the Alps. The best low-level moisture is situated along a broad frontal boundary stretching from the Bay of Biscay over The Channel region into northern Germany. Rich boundary-layer moisture is also present over France and Germany, where 0-1 km mixing ratio exceeds 8 g/kg.

While the jet streak will only slowly progress eastward, strong quasi-geostrophic forcing is not expected over most places. Low-level convergence near the frontal boundary will be likely, though.

DISCUSSION

Northern France to north-western Germany

Rich boundary-layer moisture of 10 g/kg in the lowest km will spread north-eastward during the day. The moist air will overlap with well-mixed air masses across western, central, and northern France into northern Germany, where CAPE of 500-1000 J/kg is likely (1000 J/kg across central France). Initiation is likely due to daytime heating over central and northern France in the late morning hours. Thunderstorms are forecast to move to the north-east where many clouds are likely near the frontal boundary, and low-level buoyancy will be rather limited.

Strong vertical wind shear is forecast especially from northern France to northern Germany (15-20 m/s 0-3 km bulk shear). Well-organized storms capable of producing large hail, severe wind gusts and tornadoes are therefore not ruled out. However, current scenario with embedded storms and weak low-level buoyancy limits the severe potential over most places. Large hail seems to be most likely over central France into western Germany, where best instability will be located and storms may be isolated.

British Isles

Strong QG forcing can be expected at the cyclonically-sheared flank of the mid-level jet streak. The affected air mass indicates nearly moist-adiabatic lapse rates and may become unstable due to diurnal heating. Low-level moisture will further improve from the south-west during the day. CAPE may develop with the best potential across northern/eastern England in the noon and afternoon hours.

Given the strong forcing ahead of a convergence moving eastward during the day, showers and thunderstorms are forecast. These storms have a good potential to organize given the very strong vertical wind shear of 20 m/s in the lowest 3 km. Mesocyclones may develop that can produce tornadoes given strong low-level vertical wind shear of 15 m/s. Large hail and severe wind gusts may be possible as well. Limiting factor is the broad cloud coverage and associated weak low-level heating.

South-eastern Europe

A moist and unstable air mass is located over a broad region and will result in about 1000 J/kg MLCAPE in the noon and afternoon hours. Thunderstorms are expected to develop during the day. Given rather weak mid-level flow, a focus of organized convection is not given. However, stronger pulse storms are possible and locally large hail, excessive precipitation strong to severe gusts are not ruled out. There is also a weak risk of mesocyclones especially near sea-breeze fronts and a tornado is also not ruled out completely.

Central Italy

Rather weak lapse rates across central Italy are expected to improve during the day and may result in weak instability given increasing low-level moisture along the western coasts that spreads eastward with the sea-breeze front. Upslope flow may result in some thunderstorms. A mid-level jet streak that moves south-westward at the flank of low geopotential over south-east Europe leads to increasing deep-layer vertical wind shear. Well-organized multicells are forecast capable of producing locally large hail.