Storm Forecast
Valid: Wed 02 Jul 2025 06:00 to Thu 03 Jul 2025 06:00 UTC
Issued: Wed 02 Jul 2025 02:03
Forecaster: TUSCHY

A level 3 was issued for the far E Netherlands into NW Germany mainly for swaths of damaging wind gusts.

A level 2 was issued for NE France to SE Denmark for severe wind gusts. Large hail is possible mainly in the western part of this level area.

A level 1 surrounds both other levels for a hail, strong to severe downburst and heavy rain risk.

A level 1 was issued for parts of the Alps to SE France and Italy mainly for hail, strong to isolated severe gusts and heavy rain.

A level 1 was issued for parts of N Algeria mainly for large hail and severe downburst gusts.


SYNOPSIS

A stubborn 587 dam ridge over CNTRL Europe shifts E a bit as the European wave pattern adjusts somewhat to the east. This takes the constantly intensifying ridge gradually towards E Europe, also highlighted by an ongoing easterly ageostrophic flux regime, which betimes turns more W. So next to the gradual shift of the ridge axis, we also see some height increases towards the subtropical and NE Atlantic, which keeps the brisk zonal flow regime alive.

Meanwhile, a progressive trough (embedded in this zonal flow over the NE Atlantic) lifts NE along the NW fringe of the ridge and affects S Norway/Sweden during the night. A constant de-amplification trend occurs but this wave still offers an impressive positive PV anomaly/core. Hence its radial influence exceeds the main core, which has impacts on today's thunderstorm forecast as its wind field and lift both affect parts of the unstable airmass to the S. The most compact Q vector convergence spreads from the English Channel/Benelux to the NE into Denmark.

A cut-off over SW Russia still increases its amplitude towards the Middle East and brings unsettled and thundery conditions to this area.

The European heat dome gets framed by a wavy front, which extends from the Bay of Biscay to the German Bay further NE into Sweden and the Baltic Sea/Finland. There is no clear signal of a focused LL depression, but EZ-ENS trends to constantly lowering MSLP over SE Norway/S-Sweden, as the progressive upper wave approaches from the SW. Convective-wise of most interest will be a prefrontal LL confluence zone/surface pressure trough well ahead of the gradually eastward sliding cold front. This area of LL mass convergence probably extends from NE France to Benelux into NW Germany. Embedded weak LL vortices may enhance the severe risk on a regional scale like one affecting NW Germany during this forecast.

Despite some increase of dust load compared to yesterday, the setup won't support dust-induced convection on a regional scale, which would limit diabatic heating, as weak LL/mid-level flow was present over the place of entry (NW Africa) during the past 24-48h.

DISCUSSION

... France to Benelux into NW Germany and parts of Denmark/S-Sweden ...

A regional outbreak of organized thunderstorms is forecast for parts of the highlighted area.

The first and earliest round of convection (probably ongoing from the previous night) evolves along the cold front itself, which is placed mostly offshore W of Benelux. These storms will ride the frontal zone in an impressive kinematic environment but they stay elevated for most of the time. NWP guidance diverges a bit regarding the depth of the cap offshore and along the coasts, but effective DLS in the 15-20 m/s range will be enough for numerous elevated supercells, which will be accompanied by hail and heavy rain. This activity extends from N-CNTRL France to W Belgium/Netherlands into the Germany Bay and probably also into N-Denmark during the evening hours. Nowcasting is very important as any increase in forward speed to the E (e.g. growing cold pools) might take the updrafts into weaker CIN with an increase of the overall severe risk. Right now only a few ensemble members support that idea, as most of them keep storms along the sloped frontal zone, where they get steered to the NE. We covered this activity in a level 1, which resides rather close to the level 2 however, as some models now support healthy UH tracks near the coast for those supercells.

The main concern for organized convection however evolves displaced from the cold front along the mentioned zone of LL mass convergence.

Intense diabatic heating beneath the African EML plume mixes the BL up to 3 km AGL and we probably see isolated to scattered CI around noon onwards mainly along the orography of CNTRL France. This high-based activity evolves in a weak shear environment with MUCAPE in the 500-1000 J/kg range (probably lowered by entrainment). CAMs produce excessive gusts, which might be overdone a bit (as was the case yesterday) but still a few severe downburst events are well possible next to some hail. Betimes, erratic storm motions tend to cause temporal clustering, which increases the local heavy rainfall risk. We also see an increase of mid-level and LL moisture betimes from the SW and S respectively, which increases the local heavy rain threat a bit towards the evening.

Further N, from NE France into N Germany, a constantly strengthening and gradually eastward moving LL convergence zone turns very unstable during the afternoon with up to 1500 J/kg MUCAPE for most areas ahead of the convergence zone. CAPE increases to 2500 J/kg along the periphery of the convergence zone itself.
Forecast soundings show broadening mid-level CAPE profiles atop a deeply mixed BL with inverted-V signatures. The gross DCAPE paramter has local peaks up to 1500 J/kg, so a mix of heavy precipitation load with effective evaporation drives any downdraft/growing cold pool. With BL wet bulb temperatures 10-15 K below the 2m peaks, effective deep cold pools are forecast.
Strong kinematics near the frontal zone with 15-20 m/s 0-3 and 0-6 km shear weaken further E. A confined belt of favorable CAPE/shear space evolves from parts of Benelux into N Germany and probably also into SE Denmark.

Initiating cells over E-Belgium/E-Netherlands and far W/NW Germany during the afternoon pose an hail risk with a few large events near the convergence line not ruled out (3 to 4 cm in diameter). Heavy rain is also a concern with PW values of 30-40 mm and a surge of rich mid/upper tropospheric moisture from the SW into the area of interest. The main story however will be strong to severe downburst events with a few damaging events not ruled out (gusts in the Bft 10-12 range). This initial events are more localized ones.

The main story however will be the IPV maximum passage to the N, which adds more background dynamic and increases shear also near the convergence zone. The NE ward passing wave with initiating cells over Benelux ensures an elongated corridor of adequate shear and CAPE for long-lived thunderstorm activity well into N Germany. Growing cold pools will not only get support by internal storm-scale dynamics (RIJ dynamics), but also by improving shear in the 0-3 km layer with up to 15-20 m/s forecast. This amount of shear atop a well mixed BL is concerning regarding a long-tracked severe bow echo event, which rides along the LL convergence zone over NW into N Germany. Swaths of damaging gusts and some hail are possible. This activity also affects parts of Denmark any may spread into S Sweden during the night but uncertainties increase how unstable the airmass becomes that far N next to increasing capping issues. In case EZ verifies, a mix of multicells/ a few supercells and bowing segments may also affect parts of Denmark.

Some uncertainties remain regarding timing of CI over NW Germany and Belgium/the Netherlands, which could impact the regional outcome, where to expect the most severe gusts. If we see less CI over NW Germany during the afternoon and more focused CI further W over E Benelux, we could see a long-tracked severe bow echo over NW into N Germany, which is favored by ICON D2 f.ex.. Otherwise we could see more destructive interference of numerous clusters with not one dominant wind maker.

Despite those lingering uncertainties, enough confidence for a more widespread severe wind event exist to add a confined level 3 upgrade. This area was added just along the highest CAPE axis, where the wave passes by with persistent and focused severe gust signals of ID2 and embedded in a positive EFI CAPE/shear space (with positive SOT), pointing to this unusual setup.

The level 2 and 1 areas were broadened to the E as storm-dynamics could take convection further E than currently forcast by the NWP pool.

During the night, decreasing CAPE, strengthening CIN and a more stable nocturnal BL should induce a gradual weakening trend of this activity but we would not be surprised to see a partial non-thundery storm-forced pressure wave to affect parts of NE Germany well into the night.
Further S over CNTRL Germany, a weaker background support and less thunderstorm coverage probably causes shorter lived cold pool events. However a gusty pressure wave may affect the area and hence the level 1 was expanded S.

Over S Germany, once again CI occurs along the orography during the day with pulsating conection posing a local severe downburst, hail and heavy rain risk.

... Alps to Italy and S France into Spain ...

Lowering thickness and strong diabatic heating support an active day for mountain convection. Scattered to widespread slow moving thunderstorms bring isolated heavy rain, hail and strong to severe downburst events. Betimes, convection tends to move off the orography but a stronger cap should then induce a constant weakening trend.

... E Scotland and parts of E England ...

Far SE England could see some morning activity along the cold front, but these thunderstorms push rapidly E/offshore.

Thereafter we expect another round of thunderstorms during the afternoon along a converging zone, which extends along the NE coast of England into SE Scotland. MUCAPE up to roughly 500 J/kg overlaps with decent DLS but it remains unclear how far inland CAPE develops. Latest idea is to expect numerous thunderstorms near the coast, which move E towards the North Sea. Some small hail and strong gusts are forecast, which is not enough for an upgrade.

... N Algeria ...

Onshore advection of a marine airmass from the 4-6 K too warm W-Mediterranean sneaks beneath steeper mid-level lapse rates along the orography. This results in a belt of up to 2000 J/kg MUCAPE. Despite weak shear, a risk of a few damaging gusts and some hail exists and a level 1 was added.

... Lightning areas ...

Over SW Russia, expect another day with a worked-over airmass, featuring modest MUCAPE and weak to moderate shear. Isolated instances of hail, gust and heavy rain are possible, but no focused severe risk exist to upgrade any area.