Storm Forecast
Valid: Thu 21 May 2009 06:00 to Fri 22 May 2009 06:00 UTC
Issued: Wed 20 May 2009 22:30
Forecaster: GATZEN

A level 2 was issued for eastern France and Germany mainly for severe hail and tornadoes.

A level 1 was issued for aouthern and eastern France, Alpine region, Czech Republic, and Poland mainly for tornadoes, severe wind gusts, and large hail.

SYNOPSIS

A south-westerly flow affects western and central Europe at the edge of low geopotential over northern and western Europe, and a rather strong mid-level jet streak is expected to travel from France to Belarus during the period. To the south, a subtropical high ridges into the central Mediterranean and the Balkans, while a cut-off low is centered over southern Turkey. A nearly flow-parallel, wavy frontal boundary from northern Iberia to central Poland will be the focus of convective activity on Thursday.

DISCUSSION

Eastern France and Alps to Germany and Poland

A frontal boundary is forecast from eastern France across central Germany to western Poland on Thursday. Steep mid-level lapse rates are forecast to spread north-eastward from southern France into Germany, while quite rich low-level moisture is expected along the frontal boundary from central France to central Germany. Latest GFS indicates that a strong 30 m/s mid-level jet streak will travel eastward north of the Alps during the day, and it seems to be likely that steep lapse rates will spread north-eastward across the frontal boundary, resulting in CAPE in the order of 1000 J/kg. Latest ECMWF and UKMO do not show this strong jet streak over southern Germany, resulting in a weaker thermodynamic profiles what seems to be more likely to me.

Ahead of the main frontal boundary that is expected from north-western Germany to north-western France in the afternoon hours, south-westerly to southerly winds are quite likely in the warm sector, turning to more south-easterly directions over southern and eastern Germany, and strong vertical wind shear around 20-25 m/s is expected over a broad area from France to Poland. Favourable veering profiles are most likely ahead of a weak surface low moving north-eastward across central Germany during the afternoon hours as well as north of the Alps east of the wind shift line.

Main limiting factor for deep moist convection will be limited low-level heating due to many showers and clouds as warm air advection will likely result in quasigeostrophic forcing in the night and morning hours. Forcing will likely increase ahead of the jet streak during the day, and coverage of showers and thunderstorms is forecast to increase. Given favourable shear profiles, well-organized multicells and supercells are expected to form over central Germany, moving eastwards. These storms will have a potential of producing tornadoes. Limiting factor may be rather weak low-level buoyancy as dewpoints are expected to be well below 20°C s well as limited low-level vertical wind shear below 10 m/s in the lowest kilometre. However, the situation has to be monitored closely as latest GFS model run indicates stronger shear as well as strong daytime heating over eastern Germany, and the tornado threat may turn out to be more robust. Large hail and severe wind gusts are also forecast with the stronger storms.

Further south, stronger heating will likely result in CAPE of more than 1000 J/kg from central France to southern Germany, but weak forcing may limit chance of initiation north of the Alps initially. In the range of the approaching mid-level jet streak, intense convection is expected to form over central France and will likely spread eastward in the afternoon and evening hours. Given strong vertical wind shear and favourable veering profiles, supercells are forecast that will move eastwards and may merge into mesoscale convective systems, capable of producing very large hail and severe wind gusts over southern Germany. The threat of tornadoes is expected to be rather low north of the Alps given rather dry low-level air mass. The best potential seems to exist from central France to south-western Germany in the early afternoon hours.

Benelux to Denmark

Strong QG forcing is forecast at the cyclonic flank of the mid-level jet streak from the southern North Sea to the Baltic Sea, and strong vertical wind shear is also expected by latest models. Uncertainty exists about latent instability north of the frontal boundary. Current thinking is that low-level air mass will be rather cool, while moist adiabatic profiles are likely above the boundary-layer. Elevated thunderstorms are forecast to move north-eastwards into Denmark during the period. Storms that root to the boundary-layer may become severe given strong veering profiles, but chances seem to be too low to issue a level 1.