Storm Forecast
Valid: Thu 23 Jun 2016 06:00 to Fri 24 Jun 2016 06:00 UTC
Issued: Thu 23 Jun 2016 00:00
Forecaster: GATZEN

A level 2 was issued for northern central France into Belgium and north-west Germany mainly for severe wind gusts and large hail.

A level 1 was issued for western and central France mainly for large or very large hail.

A level 1 was issued from northern France, Netherlands, NW Germany and SE UK mainly for large hail, severe wind gusts, and tornadoes.

A level 1 was issued for southern Ukraine, Romania, Serbia, Bosnia and Herzegovina, Croatia and surroundings mainly for large hail, severe wind gusts, and excessive rain.

SYNOPSIS

A small and intense cut-off low is slowly digging over Sicily. Ridging strengthens from the west Mediterranean to central and eastern Europe in the wake of a weak upper trough that leaves the forecast region over the east Ukraine. Between the ridge and low geopotential over western and northern Europe, strong mid-level flow is expected from the Bay of Biscay to southern Scandinavia. At lower levels, rich moisture remains over large parts of eastern Europe south of a cold front across the central Ukraine. Another tongue of rich moisture is present along a frontal boundary stretching across west Europe.

DISCUSSION

France, south-east England, Benelux into western and northern Germany

Widespread thunderstorms will go on during the night hours along the frontal zone over western Europe and are expected to affect also Thursday morning. Ahead of the main trough axis, weak ridging can be expected in the late morning hours and decreasing activity may allow for some diurnal heating of the moist low-level air mass. Due to the overlap with an EML as indicated by latest Bordeaux ascent, high CAPE values of 1000-2000 J/kg are possible.

Initiation is most likely along the sea breeze convergence over northern France into the Netherlands. However, overcast conditions and rain of overnights storms may limit CAPE here. Further storms are expected along the southern edge of overnights outflow boundaries over France, Benelux, and northern Germany. Vertical wind shear will be strong with 15 to 20 m/s deep layer shear, and splitting storms or multicells / bow echoes are forecast. Main threat will be severe wind gusts along with large hail. Widespread hail / wind events are most likely in the corridor highlighted by the level 2. One or two MCSs can be expected to move into north-western Germany late in the period, with severe wind gusts the main threat.

Higher shear and more favourably curved hodographs are expected near the coasts including the south-east British Isles, and the sea-breeze could support more low-level convergence. If diurnal heating can lead to sufficient low-level buoyancy, supercells and bow echoes could form near the shores of France and the Netherlands, posing a threat of tornadoes, large hail and severe gusts.

Additional storms could form further south over western and central France along a weak cold front / convergence. Although vertical wind shear will be slightly weaker, isolated storms may nonetheless produce large or very large hail and severe gusts given the high CAPE values and around 10-15 m/s deep layer vertical wind shear.

Southern Ukraine, Romania, Serbia into northern Adriatic

An unstable air mass is present over this region. In response to diurnal heating, storms are expected to form especially over high terrain. Large hail, severe wind gusts and excessive rain are the main threats given high CAPE but rather limited vertical wind shear. Storms may cluster in the evening hours before decaying overnight.

Southern Italy

Rich moisture and steep lapse rates overlap over southern Italy at the north-eastern flank of the cut-off low. Storms are forecast during the day due to low-level convergence along the sea breeze front. Vertical wind shear around 10-15 m/s in the lowest 6 km will support supercells, capable of producing large hail and severe wind gusts. An isolated tornado event is not ruled out.

Alpine region

Isolated storms are forecast during the afternoon. Given high CAPE, some large hail and severe wind gusts are not ruled out.