Storm Forecast
Valid: Wed 28 May 2025 06:00 to Thu 29 May 2025 06:00 UTC
Issued: Wed 28 May 2025 01:54
Forecaster: TUSCHY

A level 1 was issued across most of Germany, Luxembourg, E-France, W-Czechia into N Austria mainly for strong gusts, (isolated large) hail and heavy rain. A few tornado events are possible and a strong event is not ruled out.

A level 1 was issued for parts of N Italy into Slovenia/far S Austria mainly for isolated large hail and strong gusts.

A level 1 was issued for parts of the Adriatic Sea mainly for heavy rain, some hail and gusts.

A level 1 was issued for parts of the Lesser Caucasus Mountains mainly for large hail, severe gusts and heavy rain.

SYNOPSIS

This forecast period is characterized by positive geopotential height anomalies over SW Europe into the far E subtropical North Atlantic and negative heights over Scandinavia and SE Europe. Further E into W Russia, a long-lived blocking ridge persists.

A shift of the zonal flux is noted as f.ex. ageostrophic geopotential flux forecasts shift from a NE-erly direction into NW Russia to a more S-erly direction, which adds more energy into the quasi-stationary trough over SE Europe. This feature starts an amplification trend deep into N Africa during the following days.
Also, the more zonal aligned background flow over NW into CNTRL Europe starts an amplification trend in response to a deepening NE Atlantic trough.

The height configuration results in an anomalous intense mid/upper jet over NW Europe, which points towards CNTRL Europe while further intensification occurs in response to increasing supergeostrophic flow along the anticyclonic side of the building W-European ridge.

Convective-wise of main interest is a progressive open wave over the far S German Bay, which shifts E/NE atop the Baltic Sea into Finland. A temporal intensification trend occurs before it leaves the jet dynamics behind. It is interesting to see an homogenization trend with the deterministic runs as ICON, GFS, UK and EZ converged with both timing and placement of the wave and the upper trough, whereas EZ-ENS spread increased a bit compared to yesterday. The main reason most likely is that ENS has problems to capture the short/weak intensification trend of the wave as deterministic runs show multiple rounds of re-structuring vortices within the wave. Overall NWP guidance now seems stable for most of the important features for this convective event over CNTRL Europe.

Of interest is the wave's history, encorparating a tropical airmass into its structuring warm sector. Layered PW data indicates rich moisture up to the 500-300 hPa layer, so a lot of moisture to work with (highlighted by a broad fetch of positive EFI and SOT values over NW into CNTRL Europe).

The wave stretches a warm sector over most of Germany into W Poland, before a leisurely E/SE ward shifting cold front approaches later this day. The cold front remains best structured over N Germany with an attendant pressure couplet but turns more diffuse further S.

More thunderstorms are forecast next to the upper trough over SE Europe but the worked-over airmass and rather weak shear lowers the general severe risk.


DISCUSSION

... Belgium, Luxembourg into Germany and Czechia/W-Poland/parts of Austria ...

A rather complex structure of the upper low with numerous smaller but well structured IPV maxima impacts today's convective event.
A lead impulse during the morning affects CNTRL Germany and is placed in the left exit of a powerful 120 kt upper jet with focused upper divergence. Plenty of deep moisture ahead of that impulse results in an extensive area of more stratiform rain with point source soundings showing warm mid-levels and PW values up to 30mm. This area of rain pushes E and affects E-CNTRL Germany around noon. This probably delays CI only a bit for CNTRL Germany and in fact latest NWP runs increased the forward speed of this wave.

Parts of the Netherlands E to N Germany into NW Poland will see early and widespread CI, before a more stable postfrontal airmass behind the cold front filters in with a gradual decrease in activity. Forecast hodographs show a rather destructive depth of the backed wind field 2-3 km AGL and widespread CI hampers more discrete cells. Hence pulsating convection should stay mostly sub-severe with gusts, spotty heavy rain and graupel/small hail. Deviating cells by merging could temporarily increase SR inflow into the 15 kt range, so transient shallow mesocyclones with an isolated tornado risk are still possible. As latest NWP data comes in a tad weaker regarding shear, we now opted to downgrade parts of yesterday's level 1.

Over CNTRL Germany (North Rhine Westphalia, Hessen to Saxony), conditions improve rapidly for CI behind the departing stratiform rain. CAMs indicate a gradual decrease in 2m dewpoints during the day with clearing/diabatic heating and mixing but 2m dewpoints in the low to mid tens remain adequate for 400-700 J/kg MUCAPE. DLS increases into the 15-20 m/s range with 0-3 km shear just a tad weaker. A deeper LL WAA regime with backing aloft supports the idea of stronger/deeper mesocyclones with gusts, isolated large hail, heavy rain and an isolated tornado event. Multiple rounds of strong to isolated severe thunderstorms are possible.

The sweetspot for the highest severe risk extends from E-France, Saarland/Rhineland Palatinate E towards S Thuringia/SW Saxony and into S-Germany. All models show a BL response to a second IPV maximum, which crosses W-CNTRL Germany during the afternoon, resulting in lowering MSLP over SW Germany. More backed near BL flow strengthens during the peak time for heating and results in a broad area of rather impressive helicity values (e.g. 3km SRH up to 300 m^2/s^2), which overlaps with 0-3 km CAPE up to 300 J/kg on a regional scale. Also, mid-level backing weakens and hodographs offer more long and curved signatures towards SW Germany with DLS aoa 20 m/s.
Lingering uncertainties remain how fast the airmass recovers from the first lead impulse during the forenoon hours and how much background lift overspreads the mentioned area. Confidence for at least scattered thunderstorms is rather high, however. Finally, vented precipiation remains near the updrafts and covers the mean storm motion vector, which increases odds of a more rapid line-up of storms.

Initiating cells will pose a hail threat as MUCAPE up to 1000 J/kg overlaps with favorable kinematics and any more discrete cell could produce large hail. Robust mesocyclones move through enhanced LLCAPE with a steadily improving strong streamwise inflow in the 30 kt range. At the same time, LCLs hover at or below 600 m AGL. Any more discrete cell or merging cells pose a tornado threat, which increases into the evening hours and this parameter space also supports a strong tornado event. Heavy rain is another issue with repeated thunderstorm crossing the same area.
From E-France into SW Germany, CAPE/shear space supports long-tracked supercells with all kind of hazards including a strong tornado event. CI is more isolated but any dominant cell could become long-lived and severe.

Bands of convection interact with the complex orography of the Alps and heavy rain is possible on a local scale.

A level 2 upgrade was considered from far E France to CNTRL Germany but mentioned uncertainties, modest CAPE and potential destructive interaction of cells precluded an upgrade. However a few regions with clustered severe reports are possible.

... N Italy to Slovenia ...

Not much change to the discussion of the extended outlook. The main issue will be a rapid decrease of MUCAPE towards Slovenia and strong synoptic-scale subsidence towards NW Italy, where mostly capped MUCAPE exceeds 1000 J/kg. The shape of the level 1 was reduced but still a few splitting supercells with isolated large hail and strong gusts are forecast with 20 m/s DLS and 15 m/s 0-3 km shear (long and straight hodographs). The severe risk ends around sunset at storms outrun the CAPE plume and turn more elevated.

We issued an offshore level 1 for parts of the Adriatic Sea for a growing nocturnal thunderstorm risk. Despite staying more elevated, strong to isolated severe gusts, heavy rain and hail will be the hazard with those cells.

... The Lesser Caucasus Mountains ....

An upgrade for large hail and severe downburst gusts was performed as up to 1500 J/kg MUCAPE overlaps with 15 m/s DLS and well mixed lower parts of the troposphere. Interaction of the convection with the orography also supports a few flash flood events with clustering and temporarily slower moving convection.

...Other lightning areas ...

Numerous areas see augmented probabilities for thunderstorm activity.
Moldova is placed beneath a slow moving upper low. Near zero shear with up to 500 J/kg MUCAPE points to slow moving convection with heavy rain and small hail. An isolated tornado is possible with augmented LL CAPE and low LCLs. Modest CAPE and rapid clustering precluded an upgrade.