Storm Forecast
Valid: Thu 12 Apr 2018 06:00 to Fri 13 Apr 2018 06:00 UTC
Issued: Wed 11 Apr 2018 23:26
Forecaster: GATZEN

A level 1 was issued across south-eastern and central Germany for large hail and severe wind gusts.

SYNOPSIS

North European blocking will continue on Thursday. Low geopotential across south-western Europe flanks the ridge. Ahead of its negatively tilted axis, a moderately strong south-easterly mid-level flow affects central Europe. This corresponds to a tongue of warm air masses advected from south-eastern Europe towards to North Sea. This warm air mass is characterized by steep lapse rates and relatively rich low-level moisture as indicated by latest Kuemmersbruck sounding.

DISCUSSION

South-eastern Germany towards the Netherlands

Main convective activity is expected along the nose of the warm air masses across parts of Germany. Diurnal heating of the moist air mass will result in CAPE up to 1000 J/kg in the afternoon hours along and east of a convergence line from south-eastern to central Germany. Weak synoptic-scale forcing will likely limit chances of storms first. In the afternoon, QG forcing increases ahead of a negatively tilted trough axis approaching over the Alpine region. Convection initiation is expected over south-eastern Germany and may spread north-east along the convergence line, where CAPE is best. Storms may eventually spread towards the North Sea and to the Benelux countries where low-level warm air advection is present. Some models indicate that these storms will reach the North Sea or parts of Belgium in the evening and night hours.

Deep-layer vertical wind shear of about 15 m/s can be expected along the convergence line thanks to moderate mid-level south-easterly flow and weak winds on the cold side of the convergence line. Multicells are forecast that may merge to MCSs as indicated by latest high-res models. Large hail and severe wind gusts are not ruled out and require a level 1 in the region with best CAPE / shear overlap over south-eastern Germany into central Germany. Further north, some uncertainty exists since colder low-level air masses may limit low-level buoyancy and storms may become elevated. To the east of the convergence line, weaker forcing will only allow for a few storms capable of producing large hail locally.

Northern Balkans to southern Poland and Ukraine

Within the warm air mass across south-eastern Europe, steep lapse rates overlap with rich low-level moisture and CAPE around 500 J/kg is expected in the afternoon. Storm initiation is forecast over hills especially in the evening when the trough axis of the southwest-European trough approaches. Weak vertical wind shear will limit storm organization, however, a few large hail reports are likely. Over the northern Balkans, multicells are more likely due to stronger deep layer vertical wind shear but weakly veering profiles. As CAPE will be weaker as well, there is no significant higher severe weather threat in this area.

Storms will weaken after sunset over Poland and the Ukraine. Farther south, synoptic-scale forcing is stronger and may support thunderstorms through-out the night.

Sicily and southern Italy

A well-developed trough axis and associated PV intrusion crosses southern Italy in the evening hours. Although lapse rates will increase and overlap with rather rich low-level moisture, low levels may remain stable due to the cool maritime boundary layer. Current thinking is that only small areas of substantial surface-based CAPE will form. Strong deep layer vertical wind shear supports multicells and supercells, capable of producing large hail. Storms will quickly move across the sea where they will become elevated.