Storm Forecast
Valid: Thu 03 Sep 2015 06:00 to Fri 04 Sep 2015 06:00 UTC
Issued: Wed 02 Sep 2015 23:02
Forecaster: GATZEN

A level 2 was issued for the south-east Alpine region and the northern Adriatic mainly for large hail and severe wind gusts and to a lesser extent tornadoes and excessive rain.

A level 1 was issued for the southern and eastern Alps and northern Balkans mainly for severe wind gusts and to a lesser extent large hail, excessive rain, and tornadoes.

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

A level 1 was issued for the northern Ukraine and south-western Russia mainly for severe wind gusts and to a lesser extent large hail.

A level 1 was issued for western Turkey mainly for large hail.

A level 1 was issued for the Atlas mountains mainly for large hail.

A level 1 was issued for parts of the North Sea mainly for waterspouts.

SYNOPSIS

A deep trough has formed across western Europe with its main vort-max centered across the North Sea. A moderate to strong south-westerly mid-level flow is expected across large portions of the forecast region. A weak mid-level trough slowly moves east across the Aegean and Turkey. Cold air has spread into the north-western half of Europe. The frontal boundary has become parallel to the flow from the west Mediterranean to the northern Balkans and south-west Russia, and frontal waves can amplify over the eastern Alps and the Ukraine. These waves can allow some overlap with moisture along the frontal boundary with steep lapse rates conserved across the Balkans and the Ukraine. Given strong vertical wind shear due to strong south-westerly mid-level winds, severe thunderstorms are expected.

DISCUSSION

North-eastern Spain

Clusters of embedded storms are expected in the morning hours ahead of a short-wave trough passing by. CAPE will be limited to about 500 J/kg and storms may be elevated. However, given about 25 m/s deep layer vertical wind shear, multicells are expected, capable of producing large hail and severe wind gusts. Storms may spread across the west Mediterranean ahead of the trough during the day.

In the wake of the trough, cold air advection will limit storm coverage especially in the north, but isolated storms may still be possible as CAPE remains close to the Mediterranean. These storms may become supercells given the strong vertical wind shear, and the risk of large hail and severe wind gusts goes on until the night hours. Late in the period, main activity will shift to the Mediterranean.

Northern Italy into southern Alps

A rather moist and unstable air mass has been sampled by latest soundings across northern Italy and will be in place on Thursday as well. Storms are expected during the day that increase in coverage ahead of an approaching trough. With strong vertical wind shear of about 25 m/s in the lowest 6 km and 8 m/s in the lowest km, well-developed storms, including supercells, are expected. Large hail, severe wind gusts, and tornadoes are forecast. In the northern portions, storms will likely cluster or form mesoscale convective systems that move eastward into the northern Balkan region. Severe wind gusts will be the main threat, but excessive rain, large hail, and tornadoes are not ruled out. Convective activity shifts eastward later on, but isolated storms are likely in the wake of the trough given remaining CAPE.

Ukraine into south-west Russia

Moist and unstable air is expected in the vicinity of the frontal boundary. A frontal wave will travel across the region, providing lift. Clusters of storms are expected to develop. With moderate to strong vertical wind shear exceeding 10 m/s in the lowest 3 km, supercells and multicells are the dominant storm mode, although some storms in the northern and western parts may be elevated. Severe wind gusts are expected to be the main threat given well-mixed air masses at the warm side of the front. Large hail is not ruled out.

Western Turkey

Ahead of the Aegean trough, a cool air mass becomes unstable due to diurnal heating. Additionally, rather rich low-level moisture has spread into western Turkey. Initiation is expected in the noon hours. Storms are forecast to become severe as 0-3 km vertical wind shear is about 10 m/s, and large hail is not ruled out. Overall threat is limited due to weak deep layer shear and a tendency of short-living storms.

Across the Atlas, upslope flow of moist air overlaps with well-mixed air masses, and diurnally heating will allow for storms over the mountains. Although deep layer vertical wind shear is limited, 0-3 km 10 m/s bulk shear is present, and supercells are not ruled out. Large hail will be the main threat.

Southern Germany

Ahead of a cold front, rather moist air mass is expected across southern Germany in the morning hours. As a mid-level trough approaches, frontogenesis leads to a cold front that enters the region in the late morning hours from the west. Given some CAPE due to diurnal heating, thunderstorms are expected to form that travel eastward into northern Austria. Vertical wind shear is about 7 m/s in the lowest 3 km, and some storm organization is expected like linear segments at the leading gust front. Locally large hail and severe wind gusts are forecast, but overall threat seems to be to marginal for a level 1 given the weak instability.

Southern North Sea

Near the trough centre, weak vertical wind shear and good low-level buoyancy create a favourable environment for waterspouts.