Storm Forecast
Valid: Fri 05 Jun 2020 06:00 to Sat 06 Jun 2020 06:00 UTC
Issued: Thu 04 Jun 2020 23:59
Forecaster: PISTOTNIK

Level 2 areas are issued for S to central Hungary and for S Italy mainly for excessive convective precipitation, severe convective wind gusts, large hail and to a lesser degree for tornadoes.

Level 1 areas are issued for Albania, Montenegro, parts of Bosnia-Herzegovina, Croatia, Slovenia, Hungary, Slovakia, parts of Poland, the Russian exclave of Kaliningrad, and parts of W and NW Russia mainly for excessive convective precipitation and to a lesser degree for large hail and severe convective wind gusts.

A level 1 is issued for W Germany for severe convective wind gusts and tornadoes.

A level 1 is issued for NE Turkey, Georgia, Armenia, Azerbaijan and the Russian Caucasus region mainly for large hail.

SYNOPSIS

A complex and dynamic weather pattern with heavy cyclonic activity influences most of Europe. A dominant cyclone, pronounced at all levels, has settled over the North Sea.
At its forward flank, an occluded frontal system initially stretches from S Finland towards the Ukrainian/Russian border and swings northeastward into Russia. A second, more pronounced frontal system emerges from the core of the North Sea low and affects a broad region from S Scandinavia to the central Mediterranean Sea. Over Poland, it opens up into a warm front that initially extends to the central Black Sea and crosses Lithuania, Belarus and the Ukraine in this forecast period, and into a cold front that initially stretches to the Tyrrhenian Sea and Tunisia and moves east, delayed by a subtle wave development behind the Alps. In its wake, a stream of maritime polar air from the North Atlantic is advected across the British Isles into central Europe. An embedded comma feature gradually organizes into a third frontal system, helped by diurnal heating over land, while it crosses BeNeLux and Germany.
Each of these three features is accompanied by convective activity, which can partly organize thanks to an overlap with enhanced shear beneath mid-level jet-streks and/or areas of strong synoptic and mesoscale lift. In order to tame this otherwise very chaotic setup, they are discussed in chronological order below.
More quiescent conditions under warm air masses are confined to the SE (Turkey and Near East) and SW of our forecast domain (Iberia).

DISCUSSION

... first frontal system: W Russia ...

An environment of low to moderate CAPE (several hundred, regionally perhaps up to 1000 J/kg) allows scattered thunderstorms. Most of the activity is daytime-driven, though a few storms are not ruled out round the clock, especially in case of upscape growth into MCSs and/or secondary initiation along outflow boundaries.
In general, the decaying stage of the occlusion and an accompanying mid-level trough keep the amount of vertical wind shear and synoptic lift on the low side. The only exception is a smaller and more vivid vorticity maximum in the left exit of a mid-level jet, which is located near the border triangle between Belarus, the Ukraine and Russia at Fri 06 UTC and moves to the east afterwards. Some CAPE will likely be available under its forward flank with synoptic lift and moderately enhanced vertical wind shear. Better organized storms are therefore possible and could produce large hail or severe wind gusts.
Otherwise, weak vertical wind shear, moist air overworked by daily convection, and almost front-parallel storm motion provide a prototypic environment for some excessive rain events, plus the odd other risks in case "mesoscale accidents" open transient time windows for discrete, more explosive storms, e.g. at colliding outflow boundaries.

... warm front of the second frontal system: E Poland, SW Belarus, much of the Ukraine ...

Excellent wind profiles are present near the warm front with strong vertical wind shear (0-3 km shear: ~15 m/s) and veering winds (0-3 km storm-relative helicity: 100-300 m^2/s^2). Moisture accumulates near the warm front, but forecast models are discordant if, and how much, CAPE shall evolve: GFS and ICON-EU provide agressive scenarios with up to 1500 J/kg, whereas ECMWF and UKMO keep it confined to a few hundred J/kg. Arguably, the amount of insolation and daytime heating will play a decisive role.
Since the warm air advection regime creates a rather strong cap and its synoptic lift does not seem sufficiently backed up by positive vorticity advection, a late and rather sporadic storm initiation is a plausible scenario even in the case indeed robust CAPE should evolve. This forecast therefore leans to a more conservative scenario and issues only a low probability lighting area, mostly for sporadic elevated convection along the warm front plus a limited chance for surface-based initiation towards its warm side. Nonetheless, a careful monitoring is recommended, as any surface-based storm can turn supercellular with all kinds of severe weather, including an enhanced tornado risk.

... cold front of the second frontal system: from N Poland to S Italy ...

Rather moist air under modest lapse rates contributes to CAPE on the order of 500 J/kg ahead of the slowly advancing cold front. An overlap with a pronounced southerly low-level jet creates quite remarkable 0-1 km shear, reaching around 10 m/s across wide areas and even up to 15 m/s across the Pannonian Plain in Hungary and around. Vertical wind shear does not increase much across deeper layers anymore, but appears nonetheless sufficient for storm organization.
The main trough axis lags several hundred kilometers behind the cold front, hence little synoptic-scale lift support exists. However, the cold front itself, various outflow boundaries surging ahead, smaller-scale vorticity maxima and orography provide plenty of mesoscale sources of lift. Scattered thunderstorms are therefore expected mostly between noon and late evening.
Almost front-parallel storm motion will likely result in destructive interference and undercutting cold air, hence excessive rain seems to be the primary risk, especially in case of backbuilding or training activity. However, leading storms which manage to propagate towards the warmer air can organize well and pose an increasing risk of large hail, severe wind gusts and isolated tornadoes. While most areas are covered with a level 1, a belt across S and central Hungary and an area of S Italy are upgraded to a level 2, mostly due to steeper lapse rates and higher CAPE up to 1500 J/kg. It is difficult to give any details about the expected storm mode, but there are indications for a maximized wind risk in Hungary (possible bowing segments under strongest 0-1 km shear) and a maximized rain risk in S Italy (strong warm air advection ahead of a Saharan elevated mixed layer plus orographic lift of the southerly flow).
Towards the far south, i.e., Sicily, Malta and around, the capping will likely be too strong to be eroded by a combination of the mentioned sources of lift. The level 1 is therefore extended across Sicily, where the orography is still helpful, but not beyond it towards Malta and the open sea.

... comma / emerging third frontal system: BeNeLux, N France, Germany ...

Daytime heating creates marginal CAPE, especially at the forward flank of a strong vorticity maximum in the left exit of a 40 m/s mid-level jet. Scattered, low-topped convection is predicted. Synoptic lift, and hence the chances for showers to become deep enough for limited lightning, are maximized across BeNeLux and NW Germany, whereas vertical wind shear rapidly increases towards the south. Depending on the amount of daytime heating, a limited overlap of sufficient lift and enhanced shear is possible from NE to W Germany. A level 1 is issued for low-topped multicells, supercells or bowing lines with a risk of isolated severe wind gusts or tornadoes. The main time frame is from 12 to 18 UTC.

... NE Turkey, Georgia, Armenia, Causcasus mountains ...

Scattered afternoon convection is expected over the mountains under moderate CAPE and mostly weak vertical wnd shear, but a possibility of local shear enhancement in thermal upslope flows. The main risk is moderate (2-4 cm sized) hail. Storms will likely fail to propagate to the lowlands, where the capping is strong, especially towards the Black Sea.