Storm Forecast
Valid: Wed 13 Jun 2007 06:00 to Thu 14 Jun 2007 06:00 UTC
Issued: Wed 13 Jun 2007 06:25
Forecaster: GATZEN

SYNOPSIS

Low geopotential over Scandinavia, British Isles and W of Bay of Biscay yields a south-westerly flow over north-western Europe. Strong mid-troposphere jet streaks are expected to move across southern Scandinavia and north-western Iberian Peninsula, while rather weak winds are forecast south-east of the main upper jet. At lower levels, a wavy frontal boundary stretches from western Iberian Peninsula to northern France, and Baltic States. Warm air advection is expected from northern France to southern British Isles as well as over northern Poland, while cooler air mass spreads into Bay of Biscay/northern Iberian Peninsula and into Germany. While the cool air mass will be rather stable, warm air mass is characterized by inverted-v-profiles up to around 750 hPa near the frontal zone. To the south, rich low-level moisture has developed over Mediterranean, where diurnal heating will likely build CAPE.

DISCUSSION

Southern British Isles, the Channel region, northern France

A cut-off low present W of Bay of Biscay will merge to the long-wave trough present over Scandinavia. Associated upper vort-max/short-wave trough is forecast to accelerate north-eastward into north-western France in the evening hours. This leads to rather strong south-westerly winds over western Europe, and quite strong warm air advection is expected over northern France, and southern British Isles. Air mass now situated over southern France will likely spread into northern France. This air mass is characterized by steep low-level lapse rates as indiacted by latest soundings, and diurnal heating will likely lead to moderate instability during the day. To the north, low-level air mass over the Channel region and southern British Isles will likely be capped, while instability is expected to increase during the day due to the advection of steep mid-level steep lapse rates. Mid-level height falls expected in the range of the short-wave trough will also lead to further destabilization. Convective initiation is forecast along a cold front that moves eastward over western France during the day. Given south-easterly low-level winds over France and weak to moderate mid-level winds ahead of the trough, some deep layer vertical wind shear is expected to lead to organized thunderstorms along the cold front. Low-level veering profiles will also be marginally favorable for supercells, posing a threat of severe hail. Given inverted-v-profiles, severe wind gusts are forecast with the stronger cells. During the day, thunderstorms may merge/organize ahead of cold pools moving ENE-ward over France. Bowing lines may develop, capable of producing severe wind gusts. Especially over northern France, low-level winds are backed to the east, while low-level moisture is also expected to be quite rich along the frontal boundary, and a slight chance for tornadoes is forecast. Further north, low-level air mass is forecast to be rather cool over the Channel and southern British Isles. Given strong QG forcing due to WAA and DCVA, as well as steep mid-level lapse rates advecting northward, convection is forecast to develop that may be elevated over most places. However, every storm that roots to the boundary layer will have the potential to be severe given quite favorable veering profiles, and a tornado is not ruled out over the affected region is the afternoon/evening hours. During the night, thunderstorms will likely go on, clustering into mesoscale convective systems. A moderate inflow of warm/unstable air from the south will likely feed the systems that will spread across eastern France, western Germany, and Benelux until the morning. Main threat will be severe wind gusts along the leading edge in the range of bowing lines, but decreasing vertical wind shear and QG forcing are forecast to lead to weakening of the storms.

Poland, Baltic States, southern Finland

East of the main long-wave trough over Scandinavia, a cold front moves eastward over Poland, Finland, and Baltic States. Ahead of the frontal boundary, warm and unstable air mass characterized by steep lapse rates is present. Given low-level forcing in the range of the cold front, expect that thunderstorms will develop initially along old outflow-boundaries, where low-level moisture is rather high. Some deep layer vertical wind shear may be sufficient for multicells and brief mesocyclones, capable of producing large hail and severe wind gusts. Over southern Finland, amount of instability is questionable at this time due to some clouds and rather dry boundary-layer. However, when instability will realize indeed, storms will likely organize given strong deep layer vertical wind shear. Bowing lines and some supercells are not ruled out, with severe wind gusts the main threat. An isolated hail or tornado event is not ruled out. Thunderstorms are expected to cluster along the cold front during the day moving eastward into western Russia/Belarus, where vertical wind shear is low, but quite well-developed inverted-v-profiles may be sufficient for isolated severe wind gusts, though.