Storm Forecast
Valid: Fri 01 Jul 2022 06:00 to Sat 02 Jul 2022 06:00 UTC
Issued: Fri 01 Jul 2022 00:50
Forecaster: PISTOTNIK

A level 3 is issued for N Poland for large hail, severe convective wind gusts and to a lesser degree for tornadoes.

A level 1 and level 2 are issued for NE Germany and NW Poland for large hail, severe convective wind gusts, excessive convective precipitation and tornadoes.

A level 1 is issued for the Czech Republic, Austria and N Italy for excessive convective precipitation, large hail and to a lesser degree for severe convective wind gusts.

A level 1 is issued for E Slovakia, E Hungary, the W Ukraine and Romania mainly for severe convective wind gusts.

A level 1 is issued for NE Turkey and Georgia mainly for excessive convective precipitation and large hail.

SYNOPSIS

A blocking mid-level ridge with hot air stretches from the central Mediterranean region all the way to Scandinavia. It is flanked by pronounced cut-off lows next to the British Isles and over W Russia. In the wake of the eastern one, cold air advection overspreads the Ukraine, the Black Sea region and Turkey. At the forward flank of the western cut-off low, a negatively tilted mid-level trough ejects NE-ward and accelerates a formerly lingering cold front into central Europe. The tail of the cold front, lacking synoptic lift support, becomes inactive and fades over the W Mediterranean region and Iberia.

DISCUSSION

... E Germany, W half of Poland, Czech Republic, Austria, N Italy ...

Between the blocking ridge and the western cut-off low, a plume of hot subtropic air is advected into east-central Europe, bounded by the lingering frontal boundary to the west. In the center of this plume, maximum daytime heating resulted in temperatures up to 38C in Serbia, Hungary, Slovakia and W Romania on Thursday, but also in deep vertical mixing up to 700 hPa that made surface dewpoints drop to 7-13C in the course of the day. In contrast, low-level moisture was bottled up and robust CAPE was built along the western fringes of the "heat dome", where the hot airmass overspread somewhat cooler near-surface air (either the cold front or outflows from previous thunderstorms) or was undercut by it, depending on mesoscale oscillations of the frontal boundary. These differing environments were well sampled by the Thu 12 UTC soundings: hot and dry with little CAPE in Budapest (HU) and Poprad (SK), much moister with CAPE values between 500 and 2500 J/kg in Zagreb (HR), Vienna (AT) and Prostejov (CZ), and extremely moist with 3500 J/kg CAPE in Tarnow and Warsaw (PL).
This setup is replicated on Friday, when the richest low-level moisture will be advected further north- to northwestward across Poland and drawn closer to the frontal boundary, which roughly follows the German border to Poland, the Czech Republic and Austria until noon before it accelerates eastward with the impetus of the approaching mid-level trough. CAPE magnitude will likely be similar to Thursday, with an agreement on an arc-shaped belt of maximized CAPE around 2000 J/kg and possibly again up to 4000 J/kg (per ICON-EU and ICON-D2) across N and W Poland. Southerly mid-level winds also pick up, and forecast models agree that the outer parts of this belt will increasingly overlap with up to 20 m/s vertical wind shear across the 0-3 and 0-6 km layers, creating a confined but impressive window for organized and severe storms.
Scattered, partly embedded and initially elevated storms may already be active in the morning in S and E Germany. They may become surface-based or new storms become increasingly likely towards afternoon. Initially, a time frame with discrete, multi- or supercellular convection is anticipated for some hours, and large to very large hail is the main risk in this stage. Storms will move NNE-ward but will propagate eastward, pushed ahead by outflow boundaries and the cold front. As they encounter hotter air and the mid-level jet descends in the course of the afternoon, a transition into some bowing line segments is a distinct option, the more the sharper the dryline gradient between well-mixed air to the SE and accumulated low-level moisture to the NW becomes. Overall, convection-resolving models show reasonable agreement on one or two swaths of severe to extreme wind gusts. The combined hazard of some extreme wind and hail events appears robust enough to warrant a confined level 3, in particular thanks to the extraordinary CAPE magnitude.
A few tornadoes might become an issue NW of the axis of highest severe weather risk, i.e. in NE Germany and NW Poland, in case pockets of particularly rich low-level moisture and/or strongly backed surface winds should emerge. Excellent wind profiles for tornadic supercells would be in place in the sea breeze regime, but it is rather unlikely that the sea breeze will move far enough inland and be exposed to enough daytime heating to sustain surface-based convection.
By evening, storms will likely become pretty widespread. Several large storm clusters may emerge and pose an additional hazard of excessive rain. However, all hazards will quickly diminish as soon as convection moves offshore and decouples from the surface, though it may move far enough northward to reach some Swedish and Finnish coastlines.

Further south in the Czech Republic, Austria and N Italy, CAPE is a little lower (around 1000 J/kg, regionally up to 2000 J/kg) and cold-air advection largely compensates positive vorticity advection ahead of the trough, leaving little lift support beyond the cold front itself and the orography. At least scattered afternoon and evening storms are foreseen, but the hail and wind risk is lower than in Poland and conditional upon surface-based convection, which may be confined to mountainous areas, whereas storms moving NE-ward into N Austria and the Czech Republic could all to soon become undercut by the cold front and decouple from the surface.
South of the Alps, a strong cap and the lack of synoptic lift become an increasing issue. It appears therefore unlikely that the storms from the Austrian and Italian Alps will propagate into lower terrain, despite still plentiful CAPE.

... from SE Poland to Greece ...

Scattered afternoon storms are expected in the hot, deeply mixed airmass, in particular over mountains. Some severe, perhaps even dry downbursts are possible. In addition, lightning strikes may spark forest fires (though this hazard is not explicitly covered by our threat level scheme). The probabilities for large hail and heavy rain are limited by low CAPE, weak vertical wind shear and by the very dry air, respectively.

... NE Turkey into Georgia ...

Some hundred J/kg CAPE are present. A background of subtle synoptic lift plus onshore and upslope circulations are expected to trigger scattered afternoon storms. Vertical wind shear is weak across the 0-3 km layer but increases to ~20 m/s across the 0-6 km layer thanks to a mid-level jet streak. Next to heavy rain, some better organized storms therefore also pose a hail risk.