Storm Forecast
Valid: Fri 08 Jun 2018 06:00 to Sat 09 Jun 2018 06:00 UTC
Issued: Thu 07 Jun 2018 23:26
Forecaster: PISTOTNIK

A level 3 is issued for Slovenia and N Croatia mainly for large hail and severe convective wind gusts.

A level 1 and level 2 are issued for N and E Italy, SE Austria, SW Hungary and Croatia mainly for large hail, severe convective wind gusts and excessive convective precipitation.

A level 2 is issued for the Adriatic Sea mainly for tornadoes and to a lesser degree large hail and severe convective wind gusts.

A level 1 and level 2 are issued for Bosnia-Herzegovina, Montenegro, parts of Serbia and Albania mainly for large hail and to a lesser degree severe convective wind gusts and excessive convective precipitation.

Level 1 and level 2 areas are issued for S Germany, N Austria, Slovakia, S-ernmost Poland, NE Hungary, Romania and the SW Ukraine as well as for NE Turkey, parts of Georgia, Armenia and the Russian Caucasus region mainly for large hail and excessive convective precipitation, and to a lesser degree severe convective wind gusts.

A level 1 and level 2 are issued from NW Germany into Switzerland mainly for excessive convective precipitation and to a lesser degree large hail.

SYNOPSIS

Another deepening polar trough and a surface cold front translate from Finland into W Russia. Otherwise, mostly weak pressure gradients remain in place across much of Europe. Cut-off lows at 500 hPa are located NW of Scotland, NW of Iberia and over central Italy.
Warm and moist air covers much of the continent south of England, S Norway and the Ukraine. It often allows moderate CAPE in reponse to daytime heating, mostly under weak vertical wind shear and weak dynamics. However, a confined window for an outbreak of organized storms opens up ahead of the Italian cut-off low, where particularly hot air wedges from SE Europe towards Austria and increasingly overlaps with enhanced vertical wind shear.

DISCUSSION

... Italian E coast, Adriatic Sea, Croatia, Slovenia, SE Austria, SW Hungary, Bosnia-Herzegovina, Serbia, Montenegro, Albania ...

Steep lapse rates, originally created over the Sahara desert and reinforced over the Appennines and the Dinaric mountains, spread over the area of interest. With 2m dewpoints of 16-20C over the Pannonian plains and up to 23C over the Adriatic Sea, around 2000 J/kg of CAPE are predicted to build on Friday, locally even more. Warm air advection will create a pronounced cap, but lift ahead of the cut-off low will start to increase from the SW as the day proceeds. At the same time, vertical wind shear rises to 15 m/s (20 m/s) across the lowest 3 (6) km under slightly veering profiles.

Scattered convection will likely still be left over from the previous night over N and E Italy or will form already before noon, aided by increasing synoptic lift and a mess of outflow boundaries and other disturbances of the low-level wind field. Competition and destructive interference may lower the severe weather risk somewhat, but at least scattered large hail, severe wind and heavy rain events are expected with this activity. Tail-end storms towards the southeast may organize best into supercells.
It is worth mentioning that the majority of the model pool, including convection-permitting models like WRF and MOLOCH, predict scattered activity across the northern and central Adriatic Sea throughout the forecast period. Scirocco winds would advect a very moist boundary layer with enhanced 0-1 km shear into these storms, which would very likely become supercellular. If such a high storm coverage over the Adriatic Sea indeed verifies, numerous tornadoes could form, most of them well off the coastlines, though.
In the evening and overnight, clustering storms will increasingly move onshore or form anew in coastal Croatia, Montenegro and Albania with a gradually decreasing but still enhanced risk of all kinds of severe weather.

Further north and east of the Adriatic Sea, it is most difficult to assess where and when the cap will be broken. Convective initiation will stay isolated at best until mid-afternoon. Possible sites for solitary early "cap breakers" are the southern Alps (if upvalley flow is strong enough) or outflow boundaries, laid out by several storm clusters from the previous night that are moving northward in E Austria and W Hungary near the nose of the warm air advection regime at the time of writing (Thu 22 UTC). Despite an expected struggle against the cap, these early storms, if they form, may already organize well and bring large hail.
In the course of the afternoon, the probability of convective initiation rapidly increases over the Dinaric mountains in W Slovenia and NW Croatia. While vertical mixing will keep both CAPE and wind shear lower in these initiating regions, storms will quickly intensify and organize as soon as they tap into the abundant CAPE reservoir and better shear further northeast. Convection-resolving models agree on quick upscale growth into a large MCS with a most likely track across much of Slovenia in the evening, including the bordering regions of Croatia and Austria. Supercells will likely bring large to very large hail in initiating stages or generally at the southern flank of such a system. In addition, a tornado is not ruled out, though this risk is mitigated by not too low cloud bases and rather inconspcuous low-level shear. Along the MCS's main body, severe straight-line winds will betimes become the main risk. They may become quite widespread and include some extreme events (>32 m/s).
The mature and finally slowly weakening MCS could persist well into the night or even into the next morning while it continues its track northeastward into E Austria, Hungary and possibly even Slovakia. It will likely bring a prolonged period of heavy rain, and flash floods become another dangerous hazard in the course of the night, especially in S and E Austria where soils are often already saturated. In addition, severe wind gusts or large hail are still possible at the leading edge or under more discrete updrafts, respectively.

Similarly abundant CAPE and enhanced vertical wind shear is expected over the eastern parts of Croatia and Bosnia-Herzegovina as well as in Serbia, but the capping inversion will be stronger towards the south (indicated by the Thu 12 UTC Trapani, Pratica di Mare and Brindisi soundings) and synoptic lift arrives later and stays weaker. It is uncertain whether the cap can be broken at all. However, every storm that forms over the Dinaric mountains and descends into the Pannonian plain can easily become supercellular with a risk of large to very large hail. Severe downbursts and flash floods become additional hazards in case convection manages to grow upscale. In that case, it may also persist into the evening and night.

... S Germany, N Austria, as well as NE Hungary, Slovakia, S-ernmost Poland, SW Ukraine, Romania ...

The northern fringes of the warm air ridge and the plentiful CAPE reservoir are rounded and grazed by slightly enhanced mid-level flow. This combination yields 0-6 km shear up to 15 m/s and patches of enhanced storm-relative helicity where CAPE may still reach 1000 J/kg. The overlap is best within a ~50 km wide belt downstream of the Alps (S Germany, N Austria) and N Carpathians (W-ernmost Ukraine into N Romania), where warm air advection is strongest and thermal upvalley/upslope circulations are opposed to the mid-level flow.
Scattered afternoon storms are expected over the mountains and will likely organize into multicells and a few supercells when they detach. Large hail is the main risk (5 cm size is not ruled out in case of supercells), while a few severe downbursts and flash floods may occur as well, especially in case one or two larger clusters form. Convection will weaken in the evening.
The limiting factor which precludes larger level 2 areas is morning fog or stratus, which may be quite tenacious under this warm air advection. Hence there are some doubts if the predicted CAPE values can indeed materialize.

... from Spain into Germany, as well as NE Turkey, Georgia, Armenia, and the Russian Caucasus region ...

Scattered thunderstorm activity in the other thunderstorm areas will be mostly tied to the daytime cycle. Flash floods are the main hazard. Moderately large hail is possible as well in initiating stages. Weak to moderate vertical wind shear will keep storms mostly disorganized and the severe weather risk low otherwise.