Storm Forecast
Valid: Thu 04 Sep 2025 06:00 to Fri 05 Sep 2025 06:00 UTC
Issued: Wed 03 Sep 2025 15:44
Forecaster: TUSCHY

A level 2 was issued for parts of SE France into far S Germany mainly for severe gusts and regionally also for excessive rain.

A level 1 surrounds the level 2 for similar hazards but with a lowered risk level. In addition, heavy rain, hail (isolated large) and a few tornado events are possible.

A level 1 was issued for parts of S UK mainly for an isolated tornado, gust and heavy rain event.

A level 1 was issued for parts of N Algeria mainly for large hail and severe gusts. A significant event is possible.

A level 1 was issued for parts of E-Europe mainly for isolated hail, gusts and heavy rain.

SYNOPSIS

Initial slightly positive/near neutral NAO pattern flips over to a blocking pattern, as geopotential heights over E Europe/Russia continue to rise. A formely prominent trough over NW Europe fills and starts to break apart, as it interacts with the blocking ridge to its E. This trough breaks up into a mobile and NE-ward lifting low-amplitude wave with another vorticity maximum working its way from E-CNTRL France to the Alps/S Germany.

A zonally elongated upper cut-off exits the Ionian Sea E and impacts Greece/the Aegean Sea during the forecast period. Elsewhere, positive height anomalies suppress DMC activity for most parts of Europe.

The fragmenting and slowing long-wave trough over W/CNTRL Europe gets framed by an elongated/wavy frontal zone, which runs from Spain to Switzerland into Sweden/Norway. The ongoing trough-baroclinic zone interaction results in the most focused organized DMC activity for this forecast period.

DISCUSSION

... S/E France ...

Initial upper trough configuration is not supportive of any focused DMC activity. Instead, persistent and constantly increasing background lift on the synoptic-scale supports an active WCB regime with a broad shield of convectively enhanced precpitation, affecting SW to E-CNTRL France.
This activity already takes profit of an initiating return flow event from the W-CNTRL Mediterranean, where positive SST anomalies persist. This moist/unstable inflow (MUCAPE pushing up to 1000 J/kg already) combined with 15 m/s 0-3 km and 20 m/s DLS favor a few rotating updrafts already before noon (focused along the Massif Central) with excessive rain/some hail and strong gusts. Curved lower parts of the hodographs with very low LCLs also point to a growing tornado risk with this activity.
From here on we expect two corridors of severe. The first one affects the area Massif Central to the S.

CAMs/LAMs differ compared to coarser IFS/ICON solutions. The latter ones show a rapid flare up of DMC activity along/S of the Massif Central down to the coasts near Marseille, whereas others stay way more reluctant. We stick to the active solutions given approaching high-ampitude trough interaction with a plume of 1-1.5 kJ/kg MUCAPE. Discrete storm mode is suppressed but speed/directional shear, modest LLCAPE and lowered LCLs favor an ongoing tornado risk next to excessive rain, some strong susts and isolated hail.

The second area of interest extends from the Massif Central E, where organizing storms cross a well sheared airmass with augmented SRH and sufficient DLS/LL for long-lived mesocyclones. Concerns exist that these storms leave the thinning moist return flow and suffer from lowered BL moisture, which is especially true for far SE France. However, a moisture tongue could expand in time into Switzerland to take these storms matured/organized into this area - either more discrete of lined-up as an (partially PS) MCS.

If we can get clustering storms down to the coast, less moisture advects into the growing convection over E France, wich could increase discrete storm mode a bit (including a N-ward expansion of severe). If we don't see CI that far S, a favorable setup for a PS MCS event along the S/SE Massif Central is expected with excessive rainfall amounts in a short amount of time.
Anyhow, this activity weakens during the night as it crosses SE France into NW Italy, where CAPE diminishes. Enhanced DLS still favors a few organized event, that's why a level 1 was expanded far E.

For now a rather broad level 2 was issued to encompass all issues/uncertainties mainly for a combined isolated hail/severe gust and excessive rain risk. In addition a tornado risk exits.

We expanded the level 1 far W towards CNTRL France to catch a spotty severe wind gust and isolated tornado risk with the passing cold front. An adequate CAPE/shear space for isolated severe exists.

... Switzerland into S Germany ...

Issues regarding the final storm mode persist for this area as DMC activity approaches from the W during the afternoon. A more S-erly cross-Alpine flow increases lapse rates a bit especially towards SE Germany, where adequate BL moisture pushes MUCAPE in excess of 1000 J/kg. Some issues persist in NWP data regarding low-tropospheric mass response, as some data hints at more E/NE-erly BL flow up to the northern Swiss border with others keeping the E-erly BL flow component confined to SE Germany. With the mentioned flow above, confidence is high enough to expect a mixed (synoptic-scale) mass reponse/Alpine pumping event to keep BL flow out from the E/NE until the DMC activity approaches from the W during the afternoon/evening.

Over Switzerland, IFS/ICON both agree in a favorable CAPE/shear space ahead of the afternoon convection with 15 m/s at 3/6 km AGL with GFS running a bit weaker in the lower troposphere. In reality, shear should be a tad stronger along the complex orography.
We either see a structuring bowing segment ahead of an intensifying pressure wave/storm-internal RIJ, which affects W/N Switzerland with swaths of severe/damaging gusts, some hail and heavy rain. Or we see evolving/rapidly organizing convection along the orography, which shifts into NE Switzerland and afterwards into S Germany with swaths of damaging gusts. ID2 EPS data (probably over-confident) has a strong favor to the second solution, with EZ driven models (at least partially) to the first scenario. In addition, with the more discrete CI possibility (also expanding further N), we could also see more discrete storms affecting SW into CNTRL Germany with sufficient CAPE/shear for severe next to limited/spotty discrete storms over S/SE Germany ahead of the main show during late afternoon/evening.

For now, we expanded the French level 2 into S-Germany to account for numerous solutions (and despite mixed signals for the general setup). EPS and deterministic data however create a confined/favorable synoptic and mesoscale setup for a potential long-lived bowing system to cross parts of the Swiss/German level 2 area NE with widespread strong to severe gusts and some hail. In addition, if we can get the 25kt 0-2 km SR inflow pointing into a growing MCS, embedded temporal tornadic circulations cannot be ruled out.

... S UK, NE France to Belgium/the Netherlands ...

A progressive vorticity maximum crosses far S-UK E/NE before approaching the North Sea during late afternoon/evening in a weakening stage. This feature is accopmanied by a plume of somewhat better low tropospheric moisture with 200-500 J/kg MUCAPE spreading NE from the coastal areas of S-UK with peak values of 500-900 J/kg over SE UK during noon.
Forecast hodographs are rather straight, so with effective modest shear in place, a few better organized thunderstorms could affect the S/SE coast of SW and SE England, then the London area and East of England from the morning into the early afternoon hours. Some graupel/hail, gusty winds and a low-end tornado risk is present next to bursts of heavy rain. The rain risk increases along the tip of the wave , which roughly affects Wales and areas E towards the Midlands, where heavy rain becomes an issue with clustering convection. A level 1 was issued to cover that risk, which ends during the afternoon hours.

Further SE, over NE France to the Netherlands and far W/NW Germany, some modest CAPE builds up ahead of the approaching cold front from the W and some models show a better CAPE/shear space on the regional scale (like ID2 over Belgium, Luxembourg and far W Germany). A few transient rotating updrafts with a low-end tornado risk are possible with some gusts and graupel.

... E/SE-Europe ...

An extensive moisture tongue runs from SE Europe N/NE into Belarus with mainly modest CAPE and weak shear. A confined level 1 was added for isolated large hail and strong gusts, where MUCAPE exceeds 1500 J/kg and overlaps with 10 m/s DLS. A few severe multicells are possible. CI is also rather uncertain with weak background support and lingering CIN. Hence only a confined upgrade area was added. More active elevated convection occurs NE of the level 1 but this remains mostly sub-severe.

A few hail/rain and gust events are possible over the E Dinaric Alps into Greece, but no focus for maximized severe exist and hence no upgrade.

... N Algeria ...

A few afternoon thunderstorms evolve with weak/modest shear but MUCAPE in the 1-2 kJ/kg range. A few large hail and severe downburst events are possible and a significant event is not ruled out. Limited CI precludes an upgrade for now.

... S-CNTRL Sweden ...

Especially ICON provides a nice setup for a few temporarily rotating thunderstorms beyond noon, but weak synoptic-scale forcing (temporal subsidence) keeps CI uncertain and more isolated to scattered. We could see a few gust/small hail issues with the more robust updrafts and would not rule out an isolated spin-up with lowered LCLs. However, uncertain forcing and diverging NWP guidance (with less aggressive EZ data) precludes an upgrade for now.