Storm Forecast
Valid: Thu 20 Jun 2013 06:00 to Fri 21 Jun 2013 06:00 UTC
Issued: Thu 20 Jun 2013 00:19
Forecaster: GATZEN

A level 2 was issued for south-western and central Germany, the Netherlands, western Alps and surroundings mainly for severe wind gusts and to a lesser extend large or very large hail, excessive precipitation, and tornadoes.

A level 2 was issued for north-western Italy and south-eastern France mainly for large or very large hail.

A level 1 was issued fo eastern France and Belgium mainly for large hail and severe wind gusts.

A level 1 was issued for southern Sweden and western Poland to northern Balkans mainly for large hail and excessive precipitation.

SYNOPSIS

A long-wave trough approaching over the north-eastern Atlantic merges with the west-European cut-off low that will accelerate northwards as negatively-tilted short-wave trough. The broad ridge from the Mediterranean to Poland will extend into the Baltic Sea region. A plume of very warm air is tied to this ridge, and latest soundings indicate an elevated mixed layer over a broad region. Rich boundary-layer moisture will allow for high CAPE values especially across central Europe ahead of the approaching short-wave trough on Thursday.

DISCUSSION

Eastern France, Germany, Benelux countries, northern Italy

The axis of the short-wave trough will cross western central Europe on Thursday. A mid-level jet streak ahead of the trough will spread eastward. Strong differential cyclonic vorticity advection will therefore affect eastern France and northern Italy as well as western/central Germany during the day and evening hours.

At lower levels, a plume of very warm air is centred over Germany and an elevated mixed layer between 850 and 550 hPa is indicated by latest soundings (Meiningen). It will likely remain in place given backing winds ahead of the approaching trough axis. Rich boundary-layer moisture is present in the same region allowing for MLCAPE in the order of 2000-3000 J/kg. Severe thunderstorms affected parts of a region from France to the North and Baltic Sea, leaving a complex low-level set-up on Thursday.

Current thinking is that remaining convection will affect the western regions on Thursday morning, and convective outflow may spread well into central Germany. Clouds will gradually disappear and strong diurnal heating is expected especially across central and eastern Germany. Within the unstructured surface low over Germany, two convergence zones are supposed to form: One between the hot air mass across central Germany and a humid and slightly cooler air mass across northern Germany and another due to the cold front that spreads into southern Germany later on. Rapidly increasing convective activity is expected along these convergence zones that may merge to one or two squall-lines moving eastward.

Despite high CAPE in the order of 2000-3000 J/kg, vertical wind shear will be only moderate as indicated by latest GFS model run, with about 15 m/s deep layer bulk shear and 10 m/s 0-3 km bulk shear. At the initiation point, vertical wind shear may be stronger due to the approaching jet streak, and an overlap of deep layer shear > 20 m/s with the high CAPE values is possible. Therefore, very large hail is not ruled out. Due to the linear upscale growth, the main threat will likely be high winds further east. Given the strong mid-level forcing and high instability, widespread convective wind storms are not ruled out, although models indicate rather weak low- to mid-level winds. Additionally, large hail and excessive precipitation is forecast as well as tornadoes.

To the west of the surface low, diurnal heating is expected to be weaker given clouds and increasing convective activity. However, adequate CAPE is possible as well. Given the increasing forcing, widespread convective activity is expected. Strong vertical wind shear reaching 25 m/s 0-6 km and 20 m/s 0-3 km bulk shear will be favourable for storm organization. Spitting storms may form capable of producing large or very large hail, high winds, and tornadoes. This is expected especially from southern France to northern Italy, where storms may be rather isolated for a while. Further north, the high coverage of storms will likely result in MCSs moving north-eastward with high winds and excessive precipitation the main threat. Dependent on the low-level set-up, strong heating and high CAPE values are possible, and a potentially derecho is not ruled out as well. If confidence increases during the morning hours, an update to level 3 may be required.

Southern Sweden to Balkans

Near the axis of warm air, rich low-level moisture and an EML will overlap. Rather high CAPE will build during the day. Initiation is expected along outflow boundaries spreading into the Baltic Sea region and Poland from the west. Additionally, storms will likely initiate further south over hilly terrain. Vertical wind shear will be weak and organized storms are not likely. However, given the high CAPE values, isolated large hail and excessive precipitation are forecast. Best potential exists in the northern portions, where some forcing and increasing vertical shear will be possible in the afternoon hours. Storms will weaken after sunset, before more convection will spread into Poland and Sweden from the south-west in the morning hours.