Storm Forecast
Valid: Thu 06 Sep 2018 06:00 to Fri 07 Sep 2018 06:00 UTC
Issued: Thu 06 Sep 2018 04:43
Forecaster: GATZEN

A level 2 was issued across the Ligurian and northern Tyrrhenian Sea, central Italy, and northern Adriatic mainly for wind gusts and extreme rainfall.

A level 2 was issued across the eastern Balck Sea region for large hail, wind gusts and extreme rainfall.

A level 1 was issued across southern France, Alps, western Germany mainly for excessive rain.

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

DISCUSSION

SYNOPSIS

A strong westerly mid-to-upper-level jet is situated across the Mediterranean that forms a weak ridge between troughs west of Iberia and across Turkey. A vort-max embedded in this flow travels from the Iberian Peninsula to Italy on Thursday, affecting the west Mediterranean during the afternoon and evening. Farther north, weak flow dominates Europe except for an intense cut-off low that moves from Scotland to the North Sea. Some enhanced southerly mid-level winds are also expected at the eastern flank of the southeast-European trough. At lower levels, warm and relatively moist air masses are situated over most of Europe. Colder and drier air spreads from the British Isles towards the Bay of Biscay and into the Benelux countries. Relatively cool air is also present across Scandinavia. A zone with rich humidity extends along the frontal boundary from the Bay of Biscay towards the North Sea and farther into the Baltic States.

West Mediterranean

Rich low-level moisture and modest lapse rates overlap over the west Mediterranean. Capping is large in the southern parts, whereas CIN is less to the north, where some large-scale lift is ongoing downstream of the south-west-European trough and associated cold front. A pre-frontal tongue of warm air with steep lapse rates has spread across the west-Mediterranean where it overlaps with rich moisture, and MLCAPE may exceed 2500 J/kg according to latest GFS model (00 UTC soundings are missing). Storms that formed in the evening along the Spanish coast have merged to an MCS currently ongoing between the Balearic Islands and the French coast.

Current thinking is that the MCS will move on during the next hours and will affect the west Mediterranean in the morning hours. Heavy rain and gusty winds that may exceed severe limits locally will be the main threats. During the day, QG forcing will spread east with the mid-level short-wave trough. It will affect the Ligurian and Tyrrhenian Sea in the afternoon and evening. Initiation is expected over Corsica and Sardinia, and storms are expected to merge to another MCS that moves towards central Italy during the night hours. Additional storms may form over the Apennines, spreading east into the Adriatic Sea region. Vertical wind shear is limited with about 15 m/s deep layer and 10 m/s 0-3 km bulk shear. However, given large CAPE and local modification of the low-level wind field, storms may still organize. Main threats will be severe wind gusts and excessive rain, along with locally large hail.

In the wake of the mid-level trough, a convergence zone is expected along the east coast of Spain. More storms may form here, and long hodographs support storm organization and supercells are not ruled out, capable of producing large hail and severe wind gusts.

Southern Alpine region

In a weakly sheared environment, diurnally driven storms are forecast. With rich moisture and slow storm motion, excessive precipitation is the main threat. Large hail and severe wind gusts are not ruled out.

Eastern France into Alps and western Germany

A warm and moist air mass is present ahead of the frontal zone from southern France towards to western Alps and farther into western Germany. Some warm air advection is expected while diurnal heating will lead to modest CAPE in the order of 1000 J/kg. Current thinking is that storms form along a convergence line and merge to clusters. Given weak vertical wind shear, severe threat is limited, however, excessive rain is forecast given the slow storm motion and rich moisture. Over north-western Germany and surroundings, vertical wind shear increases in the afternoon. Although low-level cold air advection may already cause decaying storms, there is a weak chance of some better organized updrafts, capable of producing large hail and severe wind gusts. Interaction with the sea-breeze convergence close to the North Sea may even support a tornado given 7 m/s 0-1 km vertical wind shear.

Western Russia

The tongue of moist air that extends from the southern Baltic Sea towards western Russia overlaps with steep lapse rates that have been advected northward at the flank of a Russian high pressure system. Daytime heating will result in CAPE around 1000 to 1500 J/kg. A frontal boundary will be present to the north-east near the Finnish border. During the day, a mid-level short-wave trough passes to the north, causing some large-scale ascent. Together with low-level warm air advection and frontogenesis, capping will likely break and storms will form or re-develop. Vertical wind shear will be modest in the lowest levels, supporting some multicells. Deep layer shear is weak, though, and storms may be rather disorganized. However, a few severe wind gusts and large hail reports are not ruled out with the stronger storms. Additionally, a tornado may be possible due to slightly enhanced low-level vertical wind shear in the northern parts of the level 1 region.

Black Sea region

Numerous storms have formed in the Black Sea region that is influenced by an upper trough. On Thursday, storms will continue across this region. Excessive precipitation is the main threat in the western parts. Farther east, stronger deep layer vertical wind shear will support multicells and supercells, capable of producing large or very large hail and severe wind gusts.