Storm Forecast
Valid: Thu 22 May 2014 06:00 to Fri 23 May 2014 06:00 UTC
Issued: Wed 21 May 2014 22:24
Forecaster: GATZEN

A level 1 was issued for southern Germany mainly for large hail.

A level 1 was issued for central and northern Germany and Denmark mainly for severe wind gusts and large hail.

A level 1 was issued for eastern France, Benelux, and south-eastern British Isles mainly for tornadoes and severe wind gusts.

A level 1 wa issued for western Germany mainly for severe wind gusts.

A level 1 was issued for Sardinia, Corsica and Tunisia mainly for large hail and excessive precipitation.

A level 1 was issued for eastern Ukraine, Romania, and Bulgaria mainly for excessive precipitation and large hail.

A level 1 was issued for north-western Russia mainly for large hail and severe wind gusts.

SYNOPSIS

A deep trough is quasi-stationary across western Europe. A strong south-southwesterly flow will persist from the west Mediterranean Sea to the Benelux countries and western Scandinavia. A ridge extends from the central Mediterranean towards the Baltic Sea and Belarus/Russia. It is flanked by a Black Sea trough and a short-wave trough moving north-eastwards across northern Russia.

DISCUSSION

Southern Germany

An ill-defined cold front will spread into southern Germany in the morning hours that advects some low-level moisture eastward, replacing the very dry air indicated by latest soundings. Diurnal heating and associated thermal low is expected to cause backing low-level winds in the noon and afternoon, with moisture pooling along a convergence line to the north of the Alps.

Above the boundary-layer, southerly winds will advect an elevated mixed layer from the northern Alps across southern Germany. Within the overlap, 0-30 hPa MLCAPE of more than 1500 J/kg is not ruled out.

The most important limiting factor will be a capping inversion that may suppress thunderstorms. However, when north-easterly upslope flow can evolve, initiation becomes likely in the afternoon at the conjunction of the convergence line and upslope flow along the Alps. Rapid storm organization can be expected due to 15 m/s 0-3 km bulk shear (200 m²/s² SRH), and supercells are forecast. Very large hail (5 cm) is expected to be the primary threat, together with isolated severe wind gusts. The storms will spread north-eastward building clusters. The severe threat is expected to decrease gradually in response to weaker vertical wind shear further north.

Central Germany to Denmark

The northern portions of the weak cold front will be the focus of new convective development on Thursday. Given backing winds to the north of the Alpine thermal low, moisture pooling can be expected along the convergence line. Although lapse rates are limited, 10 g/kg low-level mixing ratio and strong diurnal heating will likely result in CAPE up to 1000 J/kg.

QG forcing is quite limited, but weak DCVA can be expected due to the approaching trough. Additionally, warm air advection sets in to the north of the Alpine warm air plume. Initiation is forecast over the mountains in the noon hours and spreads out along the convergence zone in the afternoon and evening, merging to the storm clusters over southern Germany.

Deep layer vertical wind shear will be rather strong, mostly in the 15-20 m/s range, supporting storm organization. A few supercells may form capable of producing large hail and severe wind gusts. However, it seems that storms will rapidly cluster later on, limiting the threat. Storm clusters will likely move northwards and may go on until the end of the forecast period.

Eastern France into south-east British Isles, Benelux, and western Germany

A dynamic weather pattern is forecast for eastern France and western Germany. In the wake of the cold front, dry air advection is initially expected. Additionally, clouds and showers reaming from overnights convection may limit low-level heating. However, to the north of another mid-level jet streak that is located across southern France at noon, low-level backing winds are forecast, and a southerly flow will result in moisture increase. Given increasing lapse rates due to DCVA, some CAPE is possible. Storms are forecast to form along the convergence from eastern France to the south-eastern British Isles during the afternoon, spreading north-eastward and clustering.

Main focus of severe weather will be the nose of a low-level jet spreading north across eastern France. Latest GFS indicates 20 m/s winds at 850 hPa, resulting in strong low-level vertical wind shear and curved hodographs (0-1 km bulk shear 15 m/s and SRH near 200 m²/s²). Current thinking is that low-topped supercells can form that move north-eastwards. These storms may be capable of producing tornadoes, while the threat moves from eastern France to the Benelux countries and south-eastern British Isles in the evening. Large hail and severe wind gusts are also not ruled out.

Further east, storm clusters are forecast to move into western Germany, probably including linear segments and bowing lines. These storms may be capable of producing severe wind gusts and locally large hail. Increasing cold air advection is forecast in the evening hours and storms are expected to weaken from the west.

Corsica and Sardinia to Tunisia

The axis of warm low-level air will slowly move eastward over the west Mediterranean Sea. An elevated mixed layer originating from the Atlas mountains will be advected northward, overlapping with increasing low-level moisture. Due to the cold sea surface, CAPE will be elevated, but diurnal heating over Corsica and Sardinia as well as Tunisia may lead to initiation of surface-based storms. Strong vertical wind shear exceeding 15 m/s 0-3 km bulk shear and 20 m/s 0-6 km bulk shear together with 200 m²/s² SRH will support supercells, capable of producing very large (5 cm) hail. Excessive precipitation and severe wind gusts are not ruled out. Storms will weaken when moving across the cold sea later on.

Eastern Europe

Decreasing lapse rates and low-level moisture will result in decreasing thunderstorm activity along a convergence line from Russia across Belarus to the Balkans on Thursday. Main focus of storms is expected to be from the eastern Ukraine to Bulgaria. Vertical wind shear is weak over most places, but increases over western Bulgaria and Romania, where a few supercells are forecast. Excessive precipitation and large hail are forecast before the storms die in the evening hours.

Northern Russia

Ahead of the approaching short-wave trough, a tongue of rich low-level moisture will be located ahead of a cold front, and diurnal heating will result in CAPE. Low-level convergence will support initiation in a strongly-sheared environment, and large hail and severe wind gusts are forecast.