Storm Forecast
Valid: Wed 25 Jun 2025 06:00 to Thu 26 Jun 2025 06:00 UTC
Issued: Wed 25 Jun 2025 00:03
Forecaster: TUSCHY
A level 2 was issued across most of France mainly for large to very large hail and severe to damaging gusts. Numerous extreme events (partially clustered in swaths) are possible.
A level 1 surrounds the level 2 with a regionally enhanced hail and/or gust and/or rain and/or tornado threat.
A level 1 was issued for far E Poland into the NW Ukraine mainly for a gust risk.
SYNOPSIS
Once again we're in the progress of another shift of the dominant weather regime towards increased blocking. Until then however we still deal with the zonal flow pattern over CNTRL into E Europe between anomalous low geopotential heights over Scandinavia and a pronounced ridge over the CNTRL Mediterranean into the Balkans.
The long-lived cut-off W of and now over the Iberian Peninsula lifts N/NE during this forecast as it integrates into the westerlies to the N. During this process the cut-off opens up into a wave, which continues to weaken. Nevertheless, using PV cross sections through this wave, it still reveals a robust upper-level PV anomaly until the end of the forecast, which reflects itself in a maintained augmented mid/upper-level wind field. In addition, latest deterministic runs but also IFS-ENS remain optimistic in showing a weak LL response of the upper PV anomaly in the form of a weak/diffuse LL depression, which crosses NW France during the day before passing by Benelux.
Regarding synoptic-scale fronts, we see an extensive warm sector over S into CNTRL Europe, which gets framed by a wavy front. Of main interest could be a structuring cold front over W France, which gets drawn E/NE by the evolving LL depression. In the meantime, the attendant warm front lifts N/NE and affects parts of CNTRL Europe (e.g. advects a surge of hot air towards the Alps and Germany). The cold front becomes the main attractor for DMC activity.
DISCUSSION
... France ...
A widespread and potential high-impact severe thunderstorm event unfolds for most of France during this forecast period. The NE-ward lifting wave crosses a broad warm sector, which is characterized by modest CAPE and strong shear. Hence an outbreak of organized thunderstorms is anticipated.
Regarding lift, the initial compact IPV maximum (cut-off) elongates into a NW-SE aligned IPV streamer (wave), so there is abundant background lift, which affects the warm sector from the SW to NE. Scattered to widespread CI is anticipated despite lingering cap issues in the warm sector.
Along the BL, we see a very humid BL airmass over SW France, which extends up to 1 km AGL with 2m dewpoint readings in the lower twenties and BL mixed mixing ratios aoa 14 g/kg. This moisture advects to SW France both from the 3 to 4 K too warm SE Bay of Biscay and the 4 to 5 K too warm W-Mediterranean. In addition, latest layered PW data still shows a plume of rich mid/upper level (modified tropical) moisture within the WCB, which overspreads W France during the day. At the same time, the gradually eastward pushing surface cold front advects better moisture from the Bay of Biscay into all of W France. So during the afternoon, W into SW France will see deep and rather moist profiles with limited entrainment. Present steeper mid-level lapse rates assist in widespread 1000-2000 J/kg MUCAPE with up to 3 kJ/kg over SW France.
Further E towards CNTRL and E France, onshore advecting moisture from the W Mediterranean gets mixed out rapidly due to strong diabatic heating and forecast soundings show impressive inverted-V profiles with BL dewpoints in the lower tens and temperature in the mid thirties. Wet-bulb temperatures at the surface above 20C already indicate how effective evaporative cooling becomes for any storm, which enters this airmass from the W/SW later-on. With the African EML atop, partially capped 500-1000 J/kg MUCAPE can still be realized.
The wave's broadening wind field (in response to the reforming IPV) overspreads most of the warm sector with 0-3 km shear in the 10-15 m/s range and DLS up to 20 m/s. Both shear values increase either along the frontal zone over W France (especially DLS) but also later on over E-France in response to orographic support (0-3 km shear regionally in excess of 20 m/s).
Forecast hodographs show very long and mostly straight signatures but with regionally enhanced curvature (e.g. along the cold and warm front but also over SW France). Any deviating storm stays away from its own rain cooled imprint and long-tracked and organized thunderstorms are forecast - a mix of severe multicells and mature/long-lived supercells.
The first round of isolated CI remains very uncertain as an eastward pushing morning outflow from the thunderstorms during the previous night moves into the capped airmass over W/SW France. NWP data agrees in capped conditions but we cannot exclude an isolated cell along the far W Pyrenees and over NW France, where cap weakens a bit. In addition, we also cannot rule out a weak MCV, evolving out of tonight's activity, which then shifts towards NW France before noon.
However the main show begins during the afternoon, as the cold front starts to move inland over W France. Scattered to widespread CI is forecast and mentioned CAPE/shear space supports all kind of hazards. During initiation and during the (semi-) discrete phase, large to very large hail is forecast with numerous instances of hail in excess of 5 cm. Even an isolated giant hail event up to 10 cm in diameter is possible e.g. over SW France, where MUCAPE exceeds 3000 J/kg with 25 m/s DLS. An isolated heavy rain risk is possible, before thunderstorms speed-up and despite augmented LCLs, an isolated tornado event can't be ruled out with expected intense mid-level mesocyclones atop rich BL moisture.
Otherwise the gust risk becomes an imminent and dominant risk. Initially driven by intense precipitation loading, robust downdrafts also evolve as convection moves into the well mixed airmass further E. Rapid cold pool growth is forecast and evolving RIJs get fed both by CAPE-driven storm scale pressure perturbations and also by 30-45 kt 0-3km shear pointing into growing cold pools. Swaths of damaging and regionally extreme gusts are possible with peak gust values in excess of 30 m/s likely. It remains still unclear, which clusters become the dominant ones betimes (with subsiding/CI suppressing conditions along its peripery) but the CAPE shear space supports the complete cold front from NW France into SW France for such intense events.
During the evening into the overnight hours, the risk of severe to extreme events continues and numerous long-tracked and severe bow echoes / MCS events cross into CNTRL/N-CNTRL France. The combined gust and hail risk could cause swaths of serious damage to infrastructure and vegetation.
N-France needs to be monitored closely regarding CAPE build-up. ICON places 25 m/s 0-3 km shear in 1000-1500 J/kg MUCAPE. The shear vector is aligned near parallel to a coast-parallel baroclinic zone (sustained by the inland moving sea breeze front). If this setup verifies, a severe bow echo could plow NE with level 3 worth gust conditions!
It remains unclear how fast decreasing CAPE and stronger CIN induce a weakening trend to this activity, which is also driven by internal dynamics. Also, CI along the Massif Central could influence the cold front convection as this activity becomes more dominant betimes. Overall a gradual decrease in severity is anticiapted over far NE France towards Belgium and Luxembourg.
The acticvity, which moves towards E-France could see a slower weakening trend due to better connection to the Mediterranean moisture, which gets funneled NE by 25 m/s 0-3 km shear. This could bring the gust risk near the German border. Nowcast dictates that risk and uncertainties increase over this area.
Beyond midnight, isolated thunderstorms again spread towards NW France near the upper wave but this activity should stay mostly sub-severe.
A potential level 3 upgrade consideration took some time. It remains unclear, where the most extreme gust events will evolve along the cold front with ongoing substantial NWP spread. Also the upper wave weakens with less compact lift for a focused event along the cold front intersection and finally more robust CAPE remains over W/SW France with decreasing CAPE/increasing CIN to the E.
However, comparing severe MCS/derecho events from 2018 to 2022, the CAPE/shear space over W/SW France reveals enough similarities to expect numerous gust/hail swaths, wich would match our level 3 (e.g. N France as mentioned in the text and SW into CNTRL France). Late timing of mature activity and mentioned uncertainties preclude an upgrade for now.
... E-Poland, Baltic States into the Ukraine ...
A progressive and strengthening upper trough races SE through the mentioned areas. An increasing low/mid-tropospheric wind field accompanies this wave and overspreads an airmass with weak to moderate MUCAPE (peak values in the 200-400 J/kg range). Focused 0-3 km CAPE supports a few low-topped rotating updrafts with 20 m/s 0-3 km shear (DLS is not effective as updrafts should struggle to grow that deep). Strong to severe gusts and a tornado or two are possible with low LCLs and rather straight hodographs. It seems like deepening CAA lowers effective SRH with WAA enhanced LL hodograph curvature staying to the E of the forecast convective precipitation. Some small hail is possible as well. A broad level 1 was issued, mainly driven by the gust threat.
