Storm Forecast
Valid: Thu 08 Jun 2017 06:00 to Fri 09 Jun 2017 06:00 UTC
Issued: Wed 07 Jun 2017 22:14
Forecaster: GATZEN

A level 2 was issued for western Turkey mainly for large or very large hail and to a lesser extent severe wind gusts and tornadoes.

A level 1 was issued for Greece, Bulagria, Moldova, southern and eastern Romania, central Ukraine mainly for excessive precipitation and to a lesser extent large hail and tornadoes.

A level 1was issued for northern Ukraine and western Russia mainly for excessive precipitation and tornadoes and to a lesser exrent large hail.

A level 1 was issued for northern Spain to France, Benelux countries mainly for large or very large hail and severe wind gusts.


SYNOPSIS

An amplified ridge over eastern Europe connected to a blocking high over Finland will remain quasi-stationary. Upstream, the European trough splits, with its northern part lifting into Scandinavia and its southern vort-max forming a cut-off low that interacts with another closed low currently over the Ionian Sea. Both features will form a broader trough from Romania to the Adriatic region late in the period. Further upstream, the west-European ridge is pushed east ahead of a new trough entering western Europe. The evolving south-westerly flow causes strong warm air advection from Iberia towards France and south-western Germany. Cold air advection affects the Balkans and central and southern Mediterranean behind a cold front that enters the Black Sea region and western Russia.

DISCUSSION
Western Turkey
Ahead of the approaching cut-off low from the Ionian Sea, a strong mid-level jet streak is directed towards western Turkey. Latest models show height falls during the day and mid-level flow reaching 20 m/s. At lower levels, advection of an elevated mixed layer is expected to overlap with a moist low-level air mass with a mixed layer mixing ratio around 10 g/kg. This will result in CAPE but also quite strong capping that can be reduced by diurnal heating and some lift as the main vort-max approaches in the afternoon and evening.

Current thinking is that some storms can develop. Vertical wind shear of about 20 m/s in the lowest 0-6 km and favorably veering profiles with 150-200 J/kg 0-3 km helicity overlaps with moderate CAPE of about 1000 J/kg. Most prominent convective mode will be therefore supercells, capable of producing very large hail and severe wind gusts. In the northern parts, shear is weaker, but multicells and supercells can nontheless produce large hail. Given ongoing dynamic forcing, storms are expected to cluster in the evening and night hours, posing a threat of severe wind gusts. Increasing low-level vertical wind shear in the evening and night hours indicates some tornado risk as well, focussed in the north-western portions. Additionally, locally excessive precipitation and flash floods are not ruled out.

Greece to Bulagria, Moldova, southern and eastern Romania, central Ukraine

In the center and north of the Mediterranean cut-off low, pretty deep moisture is expected, partly due to the frontal boundary approaching from the west and partly due to Mediterranean moisture advected from the south. Cool mid levels and low-level convergence will support initiation with storms spreading east during the period.

While vertical wind shear is weak, most storms will be weakly organized. However, given rather slow moving storms, deep moisture and moist profiles, there is the risk of flash floods with these storms. Additionally, large hail is possible with the stronger storms. Non-mesocyclonic tornadoes are also not ruled out given quite good low-level buoyancy and low cloud bases.

Northern Ukraine into western Russia

Along the occlusion point of the east European frontal system, slight cyclogenesis is expected. Rich low-level moisture is expected to develop and CAPE may reach more than 1000 J/kg. Weak vertical wind shear is indicated by latest models. Storms that form may produce excessive rain and flash floods. Additionally, large hail is not ruled out. Tornadoes may also form in the moist and buoyant air mass with low LCL heights.

Northern Spain to France, Benelux countries

Ahead of the approaching upper-level trough, quite strong warm air advection is expected, and large-scale ascent can be suggested. Given strong south-westerly upper-level winds, moderate to strong deep-layer vertical wind shear will evolve. An elevated mixed-layer advects north-east and will provide steep lapse rates. Main uncertainty is low-level moisture so that initiation of storms is somewhat questionable. However, models indicate increasing moisture during the day and CAPE could develop.

Capping inversion is expected to weaken due to lift ahead of the approaching trough and associated cold front. Current thinking is that a couple of isolated storms form that evolve into supercells given favorable vertical wind shear. Large or very large hail is possible with these storms. Quite large cold pool potential can result in outflow dominant storms that form clusters capable of producing severe wind gusts while spreading north-east into northern France and Benelux ahead and along the cold front.

Ireland

In the centre of the west-European trough, low-topped storms are expected in a weakly-sheared environment. Some large hail may be possible and also a weak tornado due to the low cloud base and low-level buoyancy. Threat seem to be marginal and a level 1 was not issued.