Storm Forecast
Valid: Thu 10 Jun 2010 06:00 to Fri 11 Jun 2010 06:00 UTC
Issued: Wed 09 Jun 2010 23:46
Forecaster: GATZEN

A level 2 was issued for portions of eastern France and western Alpine region mainly for severe wind gusts.

A level 2 was issued for south-western France mainly for very large hail.

A level 1 was issued for western France mainly for tornadoes and large hail.

A level 1 was issued for Benelux and western Germany mainly for severe wind gusts and large hail.

A level 1 was issued for northern Germany, northern Poland, southern Baltic States, Belarus, and western Russia mainly for large hail and tornadoes.

SYNOPSIS

A deep polar trough is present over western Europe, yielding a strong south-westerly mid-level flow across the west Mediterranean into France and Germany. While the trough shows some cut-off tendencies associated with rising geopotential over the North Sea, a 40 m/s mid-level jet streak ejects from its southern periphery into France. At lower levels, warm air advection will go on from France to the regions further east while the main frontal boundary is present along the southern North Sea and Baltic Sea.

DISCUSSION

France into Benelux, western Germany, and Switzerland

The first of two very strong mid-level jet streaks will affect France on Thursday. Winds will peak at 45 m/s at the 500 hPa level near the Pyrenees in the morning hours, when the jet axis will be directed towards southern Belgium. At lower levels, warm air advection will be present over most portions of France. Strong vertical wind shear is likely over most places, forcing is expected to be in place for most of the period, and low-level moisture will be around 10 g/kg over a broad area. The main uncertainty will be the low-level heating and release of CAPE given widespread showers and clouds.

GFS and ECMWF both indicate a strong low-level southerly jet from the west Mediterranean Sea directed towards central France. To the east of this jet, strong low-level warming due to daytime heating is expected over the western Alpine region, while clouds and showers will lead to a cold pool over southern central France. A surface low is expected to move north-eastward ahead of this cold pool during the afternoon hours.

Current thinking is that some moisture pooling due to easterly surface winds and low-level convergence underneath the warm air will increase the CAPE over the north-western Alpine region in the afternoon and evening hours. Initiation will be no problem given the strong forcing along the low-level front and increasing QG forcing due to the mid-level trough entering the area from the south. A line of thunderstorms seems to be most likely over the south-eastern portions entering the western Alps in the evening hours. The strong mid-level jet streak may support a MCS with several bowing lines due to the strong vertical wind shear, and widespread severe wind gusts may be the most significant threat.

Further north, more isolated storms are forecast in the evening hours to the north of the surface low. Given favourably veering profiles, supercells are expected to develop, capable of producing very large hail and severe wind gusts. Tornadoes may be most likely near the low-level jet across central France, where weak diurnal heating will likely be a limiting factor.

Increasing QG forcing is expected in the evening and night hours, and storms are forecast to cluster, forming one or two MCSs with bowing lines, spreading north-westward into Benelux and western Germany. Severe wind gusts and large hail will be the main threat, but excessive rain and tornadoes are not ruled out.

To the west of the main frontal boundary, a moist and rather warm air mass is expected across western France. While the low pressure centre will remain over the Bay of Biscay, southerly surface winds are likely over a broad region of western France and will lead to further warm air advection. In the afternoon hours, quite strong lift will spread northward ahead of the approaching mid-level trough axis, and models indicate that CAPE will increase, including rather strong low-level buoyancy. Best potential of severe weather is forecast to the north of the Pyrenees, where easterly surface winds are forecast and favourably veering profiles are expected. Supercells are forecast to produce very large hail and tornadoes. Further north, tornadoes and locally large hail are not ruled out as well, although the low-level vertical wind shear is expected to be rather weak. Storms are forecast to decay in the evening hours.

Northern Germany to western Russia

A warm front is present from northern Germany to western Russia, and intense moisture convergence is forecast at low levels. While a frontal wave will travel eastward from northern Germany to the Baltic States during the period, rather intense warm air advection is expected to spread eastward, while cooler air advects from the west in the wake of the boundary. Deep moist convection is forecast in the range of this frontal wave in the morning hours across northern Germany and will spread eastward during the day. Given increasing instability further east and moderate vertical wind shear, some cells are expected to organize, capable of producing large hail and severe wind gusts. Near the coasts, low-level vertical wind shear is forecast to be quite strong and a few tornadoes are not ruled out.

Central Mediterranean, Balkan region

Over the Mediterranean as well as over the Balkan region, rather moist low-level air and diurnal heating will likely lead to some CAPE. Weak forcing will be a limiting factor, and initiation is unlikely. Storms that manage to form along of sea breeze fronts or over mountains may become severe as moderate vertical wind shear is present over most places. Locally large hail will be the main threat.