Storm Forecast
Valid: Thu 06 Jun 2019 06:00 to Fri 07 Jun 2019 06:00 UTC
Issued: Wed 05 Jun 2019 22:08
Forecaster: GATZEN

A level 2 was issued for northern Serbia, southern Hungary, and surroundings mainly for large hail and to a lesser extent severe wind gusts, excessive rain, and tornadoes.

A level 2 was issued for southern Czech Republic and north-eastern Austria mainly for excessive rain and to a lesser extent large hail and severe wind gusts.

A level 1 was issued across much of eastern Europe mainly for excessive rain and large hail and to a lesser extent severe wind gusts.

A level 1 was issued for north-eastern Germany into the Check Republic mainly for excessive rain and large hail and to a lesser extent severe wind gusts.

A level 1 was issued for northern Italy mainly for large hail and excessive rain and to a lesser extent severe wind gusts.

A level 1 was issued for central Scandinavia, central Finland and north-western Russia mainly for large hail and excessive precipitation and to a lesser extent severe wind gusts and tornadoes.

A level 2 was issued across southern Russia/Ukraine mainly for excessive rain and large hail and to a lesser extent severe wind gusts.

A level 1 was issued for northern Turkey mainly for large or very large hail and to a lesser extent severe wind gusts.

SYNOPSIS

Several vorticity maxima eject from the base of an intense north-eastern Atlantic trough and affect western Europe. A negatively tilted trough will move across the British Isles and the Benelux countries on Thursday. Another short-wave trough follows over the Bay of Biscay, associated with intense cyclogenesis and a severe storm system across the Bay of Biscay. Downstream, a weakening mid-level trough migrates slowly over central Europe, spreading into Poland during the period. A deep warm anticyclone extends across northern Europe. This large-scale situation is associated with southerly mid-level flow across most of Europe. The main frontal boundary between warm and moist air masses over central and eastern Europe and colder polar air across western Europe will progress very slowly over east Germany.

DISCUSSION

Balkans towards the Ukraine/Belarus and surroundings

Rich low-level moisture is present across most of the central Balkans, overlapping with a plume of moderate to steep lapse rates. CAPE is expected in the order of 1000 J/kg, with the highest values of 2000 J/kg over northern Serbia and surrounding. During the noon and afternoon, CIN weakens and storms are forecast to initiate over mountain ranges and spread into the lowlands later on. Vertical wind shear is weak near the trough center but increases to 15 m/s south-easterly deep layer shear from towards eastern Poland ahead of the trough and 15 m/s north-westerly shear in the wake of the trough across Slovakia, Hungary, and northern Serbia. Here, we expect multicells and some supercells to form. With high CAPE, large of very large hail is possible. Additionally, severe wind gusts are forecast. In the evening, storm-relative helicity increases across southern Hungary and northern Serbia along with increasing low-level shear. Storms forming in the evening can become tornadic in that area. In the remaining region, in particular close to the trough center, weak shear is present and storms will be capable of producing excessive rain and locally large hail.

NE Germany

A cold front moves east during the day. Warm air east of the cold front is unstable with rich low-level moisture and skinny CAPE profiles. Frontal lift will support storms to re-develop at late morning and go on until the night over the same area. Initially, 10-15 m/s south-easterly deep layer vertical wind shear can support multicells moving along the cold front with a tendency of backbuilding. Later in the period, weak vertical wind shear will lead to stationary storms within the frontal boundary. Excessive rain will be the main threat with 40-80 mm in short time. With the stronger shear in the morning and noon, intense storms can also produce large hail. Severe wind gusts may occur at the eastern flank of the thunderstorm area where deeper boundary layers are expected. The overall severe threat is high-end level 1 and could even justify a level 2 at this time.

Southern Germany into Austria and southern Czech Republic

A convergence line moves east in the morning and becomes stationary near Vienna. During the day, a zone of rich moisture extends from northern and eastern Austria to the Czech Republic, overlapping with moderate lapse rates. Rather skinny CAPE profiles along the weak CIN and shear are expected and excessive rain is forecast. Farther south, the threat decreases due to smaller storm coverage. Over the southern Czech Republic and north-eastern Austria, stronger south-easterly low-level flow ahead of the convergence line will lead to stronger deep-layer vertical wind shear around 15 m/s. Here, multicells and a few supercells are forecast, capable of producing large hail and also excessive rain due to backbuilding. Severe wind gusts are not ruled out.

Over southern Germany, diurnal heating will lead to some CAPE building in the noon. Storms are expected ahead of the cold front that enters Bavaria at noon from the west. Given increasing deep-layer shear in the afternoon (up to 20 m/s), storms can organize including the chance of one or two supercells. Large hail is the main threat. Additionally, severe wind gusts are expected.

Northern Italy

Rich low-level moisture overlaps with moderate lapse rates over northern Italy, and models indicate CAPE in the order of 1000 J/kg. With south-westerly winds, upslope flow is expected and storms are forecast to initiate over the southern Alps. Propagation is expected to the south-east and storms will be anchored to the terrain for some time, increasing the potential of flash floods. Furthermore, with easterly low-level winds, deep layer shear increases to 20 m/s, and low-level hodographs are curved, supporting supercells. These are forecast to produce large or very large hail. Right moving cells can propagate into the Po valley in the afternoon and evening, and locally, large hail and severe wind gusts are forecast.

Southern and central Scandinavia

A warm and moist air mass with moderate lapse rates is present on Thursday. In a weakly sheared environment with moderate CAPE (500-1500 J/kg) and weak CIN, local storms are forecast. These can produce large hail, severe wind gusts and excessive rain locally. Stronger deep-layer shear affects southern Norway and Sweden as well as central Finland into north-western Russia. Multicells and brief mesocyclones will pose an increased threat of severe wind gusts and large hail. Additionally, increased low-level shear up to 10 m/s point to a tornado risk with isolated supercells during the afternoon and evening.


Northern Turkey

With south-westerly mid-level flow, mixed air masses are advected towards the Black Sea. During the day, low-level onshore flow will advect low-level moisture into Turkey, resulting in CAPE up to more than 2000 J/kg. Vertical wind shear around 15 to 20 m/s will support multicells and supercells, capable of producing large or very large hail. Severe wind gusts are not ruled out.

Southern Ukraine/Russia

Rich low-level moisture, CAPE around 2000 J/kg, and weak flow are forecast. Along the sea breeze convergence, quasi-stationary storms are forecast to produce flash floods. Additionally, large hail and severe wind gusts are expected due to the high CAPE and the potential of pulse storms.