Storm Forecast
Valid: Thu 05 Apr 2018 06:00 to Fri 06 Apr 2018 06:00 UTC
Issued: Thu 05 Apr 2018 00:24
Forecaster: GATZEN

A level 1 was issued across southern Italy, Slowenia, Croatia, and Bosnia and Herzegovina mainly for severe wind gusts and large hail.

SYNOPSIS

A deep and intense trough is situated over western Europe. Its axis will cross Germany and Poland on Thursday. The southern portions of the trough move over the central Mediterranean and start to form a closed low over south-east Europe at the end of the period. Warm air masses are present to the east of this trough, whereas polar air affects western Europe and the Mediterranean in its wake. New warm air advection sets in over the Iberian Peninsula ahead of an approaching Atlantic trough.

DISCUSSION

Baltic States, Belarus, western Russia

Steep lapse rates situated between 650 and 800 hPa as indicated by latest soundings over Germany will spread north-east ahead of the trough axis. Low-level moisture is limited but mixing ratio exceeds 7 g/kg along a band of enhanced moisture that overlaps with the steep lapse rates. In response to some diurnal heating, weak CAPE is expected. A cold front moving north-east will provide lift to initiate storms in the noon and afternoon across the Baltic States. Storms will likely decay offshore so that they may not reach southern Finland to the north, but some isolated storms may form there as well. Further south, initiation is less likely over Poland and northern Belarus due to weaker low-level convergence along the front, however, isolated storms are not ruled out.

Storms form in strong deep layer shear around 20 m/s in the lowest 6 km, allowing for well-organized multicells and supercells. Main limiting factor for severe storms is the weak CAPE. Although isolated large hail and severe wind gusts are not ruled out, overall potential is expected to be less compared to the day before farther south-west. A level 1 is not issued for this reason. Storms may merge to a linear system when entering western Russia in the evening where stable low levels are expected to reduce the severe threat further.

Italy, Adriatic Sea, western and central Balkans

Weak CAPE will form ahead of the southern portions of the approaching trough due to steep lapse rates spreading east and relatively rich moisture exceeding 7 g/kg. Thunderstorms are forecast along and ahead of a cold front that moves east during the day. Storms that form will profit from strong deep layer vertical wind shear exceeding 20 m/s in the lowest 6 km.

Multicells and supercells are not ruled out, capable of producing large hail and severe wind gusts. However, weak CAPE limits the severe potential and mid-level winds will also weaken during the period so that storms will gradually weaken. A level 1 highlights the area across the northern Balkans and Italy with the main potential where vertical wind shear is highest in the noon hours.

To the west of the cold front, showers and thunderstorms near the base of the trough are forecast to be non-severe given the weak vertical wind shear.