Storm Forecast
Valid: Thu 26 Jun 2025 06:00 to Fri 27 Jun 2025 06:00 UTC
Issued: Wed 25 Jun 2025 22:47
Forecaster: PISTOTNIK

A level 3 is issued for NE Italy and S Austria for severe convective wind gusts and large hail.

A level 2 is issued for the rest of the Alpine region, SE Germany, the Czech Republic, W Slovakia and SW Poland for large hail, severe convective wind gusts and excessive convective rainfall.

A level 2 and level 1 are issued for Hungary, N Serbia, Slovenia, NE Croatia and the N Adriatic Sea mainly for severe convective wind gusts.

A level 1 is issued for larger parts of Germany mainly for large hail and severe convective wind gusts.

A level 2 and level 1 are issued for the E Ukraine and SW Russia mainly for severe convective wind gusts and large hail, and to a lesser degree for tornadoes.

SYNOPSIS

An unseasonably strong zonal flow remains in place with an axis around 50N, supported by two pronounced cyclones over NW Russia and over Iceland. The cold front of the former cyclone crosses the Ukraine, the cold front of the latter one the British Isles. In-between, a gentle mid-level ridge with warm air advection moves from central Europe gradually eastward.
Of particular importance is a rather small cut-off low from SW Europe, whose forward flank set the stage for a severe thunderstorm outbreak over France on Wednesday. This cut-off low re-unites with the frontal zone over France and travels across Germany into Poland on Thursday. The severe storm hazard at its forward flank consequently shifts eastward into central Europe.
Hot and quiescent conditions dominate the entire Mediterranean region.

DISCUSSION

... central Europe ...

Along with the warm air advection, the French low (or short-wave trough to be) brings a pretty impressive elevated mixed layer (EML) into central Europe, which originated in NW Africa and was/is reinvigorated over the Spanish Plateau, the Pyrenees and the Alps. Moisture pooling beneath its capping inversion results in the buildup of robust CAPE on the order of 500 to 1500 J/kg north of the Alps and 2000 to 3000, locally up to 5000 J/kg south of the Alps (where capping is even stronger, though). Over the Alps itself, CAPE magnitude is confined to some hundred J/kg due to stronger vertical mixing, but still sufficient for thunderstorm initiation even over the highest Alpine terrain. The EML is predicted to overspread SE Germany and the Czech Republic as well and to fan into E Germany and SW Poland, which roughly outlines the northernmost extent of the remarkable CAPE reservoir. With the gradual approach of the mid-level jet, 0-6 km shear shall rise from initially 10-15 m/s to nearly 20 m/s by evening.
Lift processes are manifold, from synoptic lift ahead of the impressive short-wave trough and in the warm air advection regime to frontal cross-circulations to thermal upvalley and upslope wind systems. Therefore scattered to widespread thunderstorms are expected to erupt from noon onwards mainly along the frontal boundary across Switzerland, SE Germany and the Czech Republic, as well as considerably ahead of it over the Austrian and - a little later - parts of the Italian Alps. Storm development will be explosive as soon as the capping inversion is broken. While long-lived supercells seem unlikely under just a tad too weak vertical wind shear and the expected high storm coverage, numerous storms should acquire at least temporary rotation especially in their initial stages, in the probably brief period when they manage to stay inflow-dominant. Large hail is likely with any of these storms, and a handful of very large hail events (5+ cm) are foreseen. As the convection matures and taps into increasingly hot air with high T/Td spreads towards the east (e.g., expected 2m temperatures in the mid-30s and 2m dewpoints in the 15-18C range anywhere east and south of the Austrian Alps), strong to extreme downbursts betimes turn into the main hazard. High-resolution models indicate quick and organized outflow fronts that surge south- and eastward as soon as the storms detach from the mountains and enter the Alpine forelands in the late afternoon or early evening. Concerns for a possible longer swath of hurricane-force wind gusts are highest in SE Austria and adjacent areas of Hungary and NE Slovenia. Southward into the rest of Slovenia and Italy, synoptic lift from the short-wave trough is much weaker or even absent, rendering it less likely that secondary storms can initiate and be sustained outside of the orography. Nonetheless, scattered storms can continue to fire up well into the first half of the night, and at least isolated to scattered large hail, severe wind and (later and to a lesser degree) excessive rain events are still possible until the big CAPE reservoir is finally depleted. Even in the absence of further storms, the outflow winds can also still be strong to severe deeply into Slovakia, Hungary, Serbia, Croatia, Slovenia, the N Adriatic Sea and N Italy.
Meanwhile, storms closer to the front line in Switzerland, SE Germany and the Czech Republic will continue to travel NE-ward and stretch into Poland in the evening. While extreme events are much less likely, the overlap of moderate CAPE and moderate deep-layer shear can still suffice for scattered large hail and severe wind events, along with excessive rain in case of training convection and/or a few large clusters that could form. These hazards gradually decrease in the evening and first half of the night.

Behind the cold front, lift from the short-wave trough is simulated to overcompensate subsidence from cold air advection, and create CAPE still on the order of 500 to 1000 J/kg in conjunction with daytime heating across the rest of Germany. Vertical wind shear of 15-20 m/s across the 0-6 km layer, and descending into the 0-3 km layer, are sufficient for multicells, transient supercells and bowing line segments with a few (marginally) severe hail and wind events. This activity can start quite early, from late morning onwards, but will likewise taper off already around sunset.

... E Ukraine into SW Russia ...

The cold front of the eastern cyclone over NW Russia makes a quick progress across the E Ukraine and SW Russia. A band of augmented moisture ahead of the front line results in moderate CAPE up to 1000 J/kg, overspread by quite strong vertical wind shear across all levels (0-6 km shear around or above 20 m/s and even 0-1 km shear up to 15 m/s). An outbreak of severe thunderstorms is expected in this setup as well, and a linear storm mode with scattered to widespread severe gusts appears likely. South of the cold front, discrete storms can easily turn supercellular and produce large hail and, owing to the strong low-level shear, perhaps one or two tornadoes. Limiting factors that speak against more widespread extreme hail and wind events are the rather moderate lapse rates aloft and a fairly strong cap that could suppress convective initiation towards the south, where synoptic lift is weaker.