Storm Forecast
Valid: Fri 12 Jun 2020 06:00 to Sat 13 Jun 2020 06:00 UTC
Issued: Fri 12 Jun 2020 01:45
Forecaster: PISTOTNIK

A level 1 and level 2 area issued for a large belt from SW Russia, the Ukraine, N Romania, Hungary, Slovakia, the Czech Republic, E Germany, Poland, Lithuania and Belarus for large hail, excessive convective precipitation, severe convective wind gusts and to a lesser degree for tornadoes.

A level 2 is issued for south-central France mainly for excessive convective precipitation and to a lesser degree for large hail, severe convective wind gusts and tornadoes.

A level 1 is issued for S and E France mainly for excessive convective precipitation, large hail and to a kesser degree for severe convective wind gusts.

A level 1 is issued for SE Romania, Bulgaria, N and central Turkey mainly for excessive convective precipitation and large hail.

Level 1 areas are issued for NE Algeria, NW Tunisia and S Turkey for large hail and severe convective wind gusts.

SYNOPSIS

A blocking pattern is in place. A large anticyclone over Scandinavia is opposed by an active cyclone over the Bay of Biscay and a filling mid-level low from the S Balkans to Turkey. Very warm air from Russia is advected westward into E and central Europe at the northern flank of the cut-off lows.

DISCUSSION

... from the northern Black Sea region to S Poland and NE Germany ...

Similar to Thursday, the highest severe weather potential arises in the belt of maximized warm air advection, where low-level moisture accumulates to robust CAPE and synoptic lift works on the erosion of the (initially pronounced) capping inversion. With expected dewpoints in the upper tens, CAPE between 500 to 1500 J/kg should be available again after some hours of daytime heating. Vertical wind shear is rather weak with values around or below 10 m/s even across deep layers, but warm air advection creates veering low-level hodographs with moderately enhanced 0-3 km storm-relative helicity between 50 and 150 m^2/s^2. All in all, this environment seems prime for some well-organized storms with a strong tendency to grow upscale and produce all kinds of severe weather, which was already confirmed by numerous severe weather events on Thursday (including hailstones up to 7 cm in size).
Downstream effects of the ongoing evening and nighttime convection will dictate mesoscale details about the storm distribution on Friday. Latest observations and the model pool give some hints that the outflows of Thursday's various large storm systems could merge into one dominant convergence line during the night, whose most likely position on Friday morning is somewhere near a line Warsaw (Poland) - Khmelnytskyy (SW Ukraine) - Chisinau (Moldova), where it could phase with a sea breeze front pushing inland from the Black Sea coastlines into the S Ukraine and SW Russia. To the NE of this boundary, low-level moisture is expected to be shallow and vulnerable to thermal mixing processes, which should create dry and deeply mixed vertical profiles and reduce CAPE to a few hundred J/kg as soon as daytime heating takes over. Hence this boundary will attain the characteristics of a dryline and will not only be the prime site of convective initiation, but also the northeastern margin of the area at the highest severe weather risk.
Under a prnounced cap and limited synoptic lift (aside from the ongoing warm air advection), convective initiation will happen rather late, possibly not before early to mid-afternoon, and may even completely fail across some areas. However, once it happens, it will likely set off a cascade of secondary initiations at outflow boundaries. Best chances for the "convective nucleus" to form exist over and around Moldova, from where storms would likely move into N Romania and the SW Ukraine and possibly enter Hungary, Slovakia and Poland if they survive into the night. In general, storms can contain embedded supercells and will quickly grow upscale into large clusters. The main risk at early stages is large to very large hail, followed by cold-pool-driven severe wind gusts and very heavy precipitation later on. One or two brief tornadoes are not ruled out in case of favourably colliding gust fronts or outflow boundaries.
Signals for convective initiation are less convincing further eastward along the sea breeze front in the Ukraine and SW Russia and further nortwestward towards central Europe. Scattered storms can form over mountains in Romania, Slovakia, the Czech Republic and SW Poland, but may struggle against the cap once they detach from them. A low-probability scenario, shown only by WRF, is a surviving overnight MCS over Hungary whose outflows could improve the chances for new storms on Friday also over flat terrain and further westward. However, at the time of writing (Thu 23 UTC), there are no indications for this outlier scenario to materialize.
In E and N Germany, stratus clouds will probably break up too late to allow surface-based storms already in the afternoon. Chances for scattered storms to form or to move in from the Czech Republic and Poland increase in the evening and overnight, but they will likely be elevated and their severe weather risk is limited.

... N Poland, Kaliningrad, Lithuania, Belarus, N Ukraine into W Russia ...

To the north, the body of very warm air is framed by an almost stationary frontal boundary which starts moving southward on Friday. Particularly abundant low-level moisture has accumulated on its warm side. 2m dewpoints peaked at 20-22C around the bordering areas of Belarus, Lithuania, Latvia and Russia on Thursday, and 2500 J/kg CAPE were sampled by the 12 UTC Velike Luki sounding. (The Moscow and Smolensk soundings indicated even higher CAPE values, but were discarded due to their obviously low data quality.)
Under strong daytime heating, CAPE is expected to reach 1000-2000 J/kg across wide areas on Friday and even 2000-3000 J/kg in the blob of the highest moisture which will move towards central Poland. A lack of synoptic lift, or even weak subsidence as the cooler air starts trickling south, limits the confidence in convective initiation. However, similar to the southern area, the first storm that forms may quickly set of a cascade of secondary initiations, which could spread erratically outward under weak vertical wind shear und pose a high conditional risk of large (isolated very large) hail, severe downbursts and excessive precipitation. A level 2 is issued for central and NE Poland, where the model pool agrees best in initiation in the late afternoon to evening. Otherwise, a level 1 and a low probability thunder area cover this "low-risk high-impact" scenario.

... France, BeNeLux, W Germany, Switzerland ...

Ahead of the western cut-off low, a few hundred J/kg CAPE in moist and moderately warm air overlap with strong vertical wind shear (15-20 m/s across the 0-3 km layer). Synoptic lift from a short-wave trough and frontal lift of a slowly advancing cold front across France will likely create scattered storms in the course of the day.
The highest severe weather risk evolves over south-central France, where constantly backbuilding storms originating from the Massif Central could create more than 100 mm rain within a few hours and produce a very dangerous flash flood situation (probably maximized in the creeks discharging northwards). With higher CAPE up to 1000 J/kg and enhanced 0-1 km shear under a southerly low-level jet, tail-end storms can contain mesocyclones with a risk of large hail, severe wind gusts or tornadoes.
Further north and east, the lower CAPE magnitude and the almost front-parallel storm motion limit the severe weather risk and shift it betimes to heavy rainfall. Convection will gradually become elevated and weaken in the evening and overnight while it spreads into BeNeLux, Germany and Switzerland.

... S Balkans, most of Turkey ...

Beneath the eastern cut-off low, scattered daytime-driven storms form in an environment of low to moderate CAPE and weak vertical wind shear. The main risks are heavy rain and localized marginally large hail.

... NE Algeria, NW Tunisia, S Turkey ...

Daytime heating of moist onshore and upslope circulations create some hundred J/kg CAPE, which overlap with enhanced vertical wind shear beneath the subtropical jet (up to 15 m/s in S Turkey, up to 25 m/s in Algeria and Tunisia). Afternoon storms will stay isolated to scattered, but pose a main risk of large hail (especially with good access to coastal moisture) and severe downbursts (especially in the drier, more deeply mixed air further inland).