Storm Forecast
Valid: Wed 04 Jun 2025 06:00 to Thu 05 Jun 2025 06:00 UTC
Issued: Tue 03 Jun 2025 17:44
Forecaster: TUSCHY

A level 3 was issued for parts of NE Switzerland into S Germany and Czechia for widespread severe to damaging wind gusts and damaging hail with swaths of extrmely severe possible.

A level 2 surrounds that level 3 with similar hazards but somewhat lowered probabilities. A regional tornado threat (including a strong event) is possible f.ex. for parts of Switzerland. A rainfall-driven level 2 was added for parts of SE Switzerland.

A level 1 surrounds the level 2 and 3 with similar hazards but lowered probabilities. Heavy rain and an isolated tornado are also forecast.

A level 1 was issued for parts of Romania mainly for large hail and a few severe gusts.

A level 1 was issued for parts of CNTRL Sweden mainly for an isolated tornado event and gusts/hail.

A level 1 was issued for parts of SE Ukraine into Russia for hail, strong to severe gusts and heavy rain. An isolated tornado event towards Russia is possible.

SYNOPSIS

An unseasonable deep cyclonic vortex (with nearly unsharted MSLP minima of 970 hPa or less for this time of year) still sits atop the Faroer. This extensive gyre encompasses most of NW Europe and faces a steadily intensifying and quasi-stationary deep warm-core ridge over SE Europe. Beside that positive height anomaly, another one remains anchored over the far E subtropical (NE) Atlantic. This height configuration over Europe results in one intense/anomalous upper jet approaching far W/NW Europe during this forcast period with another one building over CNTRL Europe into Scandinavia in response to supergeostrophic response of the northward building SE European ridge.

Numerous in this progressive flow regime embedded more or less pronounced low-amplitude waves / IPV maxima cross most of NW into CNTRL Europe. DMC-wise the most interesting one moves into France during the day and approaches Germany during the end of the forecast. This wave helps to push a cut-off over Spain rapidly NE while opening up into an open-wave. This lead-impulse crosses Switzerland around noon and Germany until the evening hours before racing NE into Czechia/Poland during the night (the nocturnal intensification trend of the wave is probably the reflection of an intense convective cluster, which then accompanies the wave). This lead-impulse becomes the main player regarding today's severe weather setup for CNTRL Europe.

Over far SE Europe into Russia, two progressive and merging upper troughs shift E. Interaction with better moisture and CAPE results in another area of enhanced thunderstorm probabilities.

Starting from the deep vortex over the Faroer, an elongated occlusion bends SE and then back to Sweden before opening up into a broad warm sector, which covers parts of Germany into Poland, Czechia and the Alps. This warm sector gets framed by a wavy and only slowy eastward shifting cold/warm front, which bisects Germany from NE to SW and a structuring and NE ward moving warm front over Poland.

Another depression over SW Russia directs a cold front SE which also affects the eastern parts of the Ukraine.

DISCUSSION

An outbreak of severe thunderstorms with extremely severe hazards is forecast over parts of CNTRL Europe!


... SE France to NE Italy, Switzerland to parts of Germany into Czechia and parts of Poland ...

Generally speaking today's setup offers an extensive overlap of moderate to high CAPE as a N-ward advecting African EML (enhanced by regional orography) overspreads an unseasonable moist fetch with PWATs partially in the 90(+)% percentile ranks of the NWP climatology (also highlighted by a broad fetch of positive EFI/SOT water vapor flux forecasts from EZ-EFI). This plume of unstable air interacts with constantly strengthening winds at mid/upper levels and pushes EFI CAPE/shear regionally in the SOT +1 region. This extensive overlap of ingredients offers a broad area with well organized and long-lived DMC activity. Hereinafter we split the broad area into certain sub-regions for a more focused discussion.

... S Germany into Czechia and far SW Poland ...

Probably the most dire overlap of highest CAPE and shear is forecast to evolve in this area.
A firm SW-erly mid-level flow interacts with the Alpine crest. Consequently sinking/adiabatically warming air warms up the lower troposphere over SE Germany into Czechia, which serves as a NE-ward spreading cap. BL response in form of lowering MSLP over SE Bavaria pulls rich BL moisture from N Austria W beneath the cap and the EML (enhanced by the Alps). Resulting loaded gun soundings are forecast with CAMs indicating MUCAPE from 2000 to 3500 J/kg from SE Germany into N Austria. These peak values subside gradually into Czechia and far SW Poland.

DLS steadily increases from SW with peak values near 30 m/s and 0-3 km shear values up to 20 m/s. Forecast hodographs show long signatures and WAA driven a modest curvature mainly for the lower 3-4 km AGL. Initiating cells probably first move into the own (vented) precipitation but as soon as they move off the hodogaphs with rapdily organizing mesocyclones, the udraft's path should stay away from the FFD's precipitation. This casts some uncertainties in how long any more discrete cell stays (semi) discrete.

First round of storms is also the more questionable one as EPS data constantly indicates numerous pseudo-members for discrete CI in the open warm sector over SE Bavaria. Main area of concern aligns with a constantly N/NE ward mixing dryline with foehn induced drying to sub 10C 2m dewpoints behind the dryline and peaks up to 20C near the dryline itself. Enhanced BL convergence and a constantly weakening cap due to ongoing diabatic heating and BL moisture pooling could indeed support a few discrete cells during the afternoon hours over S/SE Bavaria. CAPE/shear space could support giant hail (probably in the 5 to 10 cm range), damaging downbursts (driven by internal storm dynamics) and heavy rain. The same is valid for N Czechia, where cap is not such an issue.

Cap probably holds firm into most of N Austria but we expanded the level areas a bit into N Upper Austria/far N lower Austria to account for the approaching wave's background support during peak heating with 3000-4000 J/kg weakly capped MUCAPE.

More confidence in widespread severe/extremely severe evolves beyond noon along the approaching wave's periphery, which sparks scattered thunderstorms f.ex. over Switzerland. As this activity taps into the unstable warm sector airmass, a rapidly organizing MCS/derecho event is forecast to spread rapdily NE into Bavaria during the evening and late evening hours. Very impressive SR 0-2km inflow not only supports a balanced inflow/RIJ but also embedded supercells along the line. Widespread severe/damaging gusts (peaks in excess of 120 km/h are well possible) mixed with large to very large hail (hail diameter up to 5 cm or higher) create a dangerous setup with a risk of swaths with extreme damage.

Upscale growth into an MCV event over Czechia is possible as book-end vortices could increase gusts even more on a regional scale.
Betimes (beyond sunset) this activity starts a gradual weakening trend as inflow air becomes more stable over Poland. Nevertheless, this very dynamic convective event could persist well into SW/CNTRL Poland.

Heavy rain is possible especially near merging cells and later along the northern fringe of the structuring (potential) MCV event.

We upgraded parts of the mentioned area to a hail/wind driven level 3!

... CNTRL into NE Germany and NW Poland ...

700 hPa winds ramp up in this area, which pushes 0-3 km shear up to 25 m/s on a regional scale and with 0-2 km SR inflow into the 20-25 kt range. Questionable insolation and a broad area of isentropic driven more stratiform rain affects the mentioned region during the forenoon hours, but forecast soundings show not much modification/diabatic heating is needed for 400-800 J/kg MUCAPE with at least temporal breaks in the thick cloud shield. Latest CAMs constantly pointed to isolated CI over CNTRL into NE Germany. Despite modest CAPE, impessive kinematics especially for the lowest 2-3 km point to the risk of a few better organized and longer-lived mesocyclones. Very low LCLs and mentioned shear indicate a growing concern for a tornado or two. In case a region with better diabatic heating indeed materializes, the severe risk would ramp up rapidly and CAPE/shear space would support even longer tracked supercells and bowing segment with a growing severe wind gust threat, some hail and an isolated tornado. Heavy rain is also forecast. For now we keep most of this area in a broad level 1 but embedded storm-scale swaths of level 2 conditions are not ruled out. However we nudged the level 2 N into NE Germany to account for an increasig risk of organized convection moving through the warm sector.


... SE France to NW Italy and into Switzerland ...

The level 2 was expanded that far S to account for numerous longer tracked supercells in up to 1500 J/kg MUCAPE with 20 m/s DLS. WAA driven (and modified by the local orography) curved hodographs support the mentioned risk with large to very large hail, severe gusts, heavy rain and an isolated tornado. The level 2 was expanded into SE Switzerland as environment becomes steadily supportive for training multicells and supercells, which move into the orography. 6h rainfall amounts in excess of 200 l/qm on a regional scale are possible during the evening into the overnight hours.

Parts of NE Switzerland were added to the level 3. Initiating cells move through a very favorable CAPE shear space for long-lived supercells, which will be accompanied by large hail, severe to damaging gusts and heavy rain. A favorable low-tropospheric stratification with intense SR inflow supports strong/long-lived mesocyclones with a risk of a strong tornado event. We added the level 3, where we expect mature supercells to exit into Germany. We explicitly want to point out that the strong tornado risk extends into the level 2 area over W/N Switzerland.

... CNTRL Romania ...

CI along the Carpathians is forecast as cap wanes with up to 2 kJ/kg MUCAPE and roughly 10 m/s DLS. Initiating cells pose a large hail threat with isolated severe downbursts and heavy rain. As thunderstorms leave the orography, increasing cap should induce a general downtrend of activity (next to rising geopotential heights aloft).

... Far E Ukraine into Russia ...

Along the SE ward moving cold front, a line of strong to isolated severe thunderstorms brings a mix of hail, strong gusts and heavy rain. The severe risk increases towards Russia, where some large hail, heavy rain, severe gusts and a tornado or two are possible.

... Parts of S/CNTRL Sweden ...

Scattered thunderstorm erupt during the daytime hours with weak CAPE but strong 0-3 km and 0-6 km shear. Numerous shallow mesocyclones with some hail, strong gusts and an isolated tornado event are possible.