Storm Forecast
Valid: Sat 03 May 2025 06:00 to Sun 04 May 2025 06:00 UTC
Issued: Fri 02 May 2025 18:42
Forecaster: TUSCHY

A level 2 was issued across SW France for large to isolated very large hail, severe gusts and heavy rain.

A broad level 1 was issued for W- into E-CNTRL Europe for all kind of hazards with regionally enhanced magnitude.

A level 1 was issued for parts of Portugal and SW Spain mainly for heavy rain and an isolated tornado.

A level 1 was issued for E Spain mainly for large hail, severe gusts and heavy rain.

SYNOPSIS

Finally we see a shift in the long lasting blocking event although a complete break down of the standing wave pattern over S Europe won't occur even during this forecast. However, the strong ridge over the CNTRL Mediterranean weakens and so does the cut-off W of Portugal. Both features drift E.
The cut-off over SE Europe starts its re-entering phase into the westerlies but it keeps it tilt and amplitude during the forecast, which offers rather impressive negative height anomalies over Egypt before entering the Middle East from the W.

Exiting one blocking event and jumping right into the next one with rapidly rising heights near Iceland. This helps a progressive and amplifying wave to dive SE from Scotland to the Baltic Sea, where it interacts with a broad cyclonic gyre over Scandinavia. This change in geopotential geometry results in advection of a cool and more stable marine airmass from the NW.

An extensive and wavy airmass boundary exists from France all the way to the Ukraine and supports a broad overlap of enhanced shear along the frontal zone with a modest CAPE plume. We would talk about an extensive severe event if we would have better moisture and CAPE to work with but even with expected modest values we will see an extensive area with DMC probabilities.

DISCUSSION

... France, Germany, Austria, Slovakia, Poland to the Ukraine ...

Numerous low-amplitude waves circle the strong subtropical ridge and cross the broad and unstable warm sector from SW/W to the E. Some phasing with waves from the polarfront jet is possible and this could temporarily amplify any wave and subsequently its impact on forcing mechanisms on a more regional scale. Therefore CI remains a bit heterogeneous in NWP guidance rgearding the exact timing and location of CI. However enough background support should exist for temporal enhanced CI probabilities over most of the highlighted area. An additional driver however for many areas will be the orography or mesoscale convergence zones.

One more tangible QG lift mechanism however will be the strong and amplifying short wave, which crosses the North Sea during daytime hours SE. This feature approaches the wavy frontal boundary over Germany, where recent model runs now agree in the development of a weak frontal wave. This feature is well respresented by tightly clustered IFS-ENS members over CNTRL Germany, which exit towards Poland with a drop in MSLP by roughly 5 hPa in the median. These members remain zonally stretched, so the final placement of the wave remains still uncertain. However, enhanced frontogenesis and dynamics could cause a DMC hot-sport over Germany into Poland.

Expect a rapid uptick in CI from noon onwards with more discrete convection over CNTRL Poland due to more gentle lift ahead of the structuring wave. MUCAPE will be in the 300-800 J/kg range with DLS around 20 m/s and variable 0-3 km shear (5-15 m/s with an increase next to the front). A well mixed BL should support high based multicells and a few supercells with hail and strong to severe gusts. The severe risk diminishes beyond sunset with lowering CAPE and strengthening capping.
For CNTRL Germany, it becomes more messy. The first concern will be convection along the lifting part of the wave with enhanced frontogenesis by differetial diabatic heating and growing WAA. Beyond noon, CI is forecast and cells from E-CNTRL Germany into far W Poland will see a confined swath along the warm side of the surface front, where adequate CAPE and shear exist for a few supercells. Some hail and gusts are possible but the somewhat augmneted threat will be an isolated tornado risk with good LL veering and LCLs arund 400-600 m. Time for producing a tornado is limited as convection tends to shift on the cold side of the boundary with storms trending more elevated. A NEward facing mean storm motion supports this idea. Rapid weakening of the upper level winds also supports HP-ish storms, which bring heavy rain in a short amount of time with some fash flooding possible. Finally we could see more stratiform rain admit some isentropic lift along the boundary to impact this area, which would lower the overall severe risk. Nevertheless this area will be monitored closely in case a few storms ride the boundary E.
During the passage of the wave's inflection point and thereafter along the structuring cold front, scattered to widespread CI is forecast. Depending on the broadness of the evolving warm sector, a few longer tracked supercells S of the inflection point are possible. This activity moves into a deeper mixed airmass over S-Germany, so the risk of strong to severe gusts and some hail dominates. For sure an isolated late afternoon/evening tornado event is still possible as LCLs lower during this time period. In case we get a more compact wave going during the day, we could also face a cold front driven MCS event over CNTRL Germany into Bavaria. Some models increased thise signals in the past few runs with bowig segments crossing parts of N/CNTRL Bavaria E into far NW Czechia.
Finally the rainfall and flash flood risk should peak along the top of the wave, e.g. CNTRL Germany but also along the trailing cold front into SW-CNTRL Germany.

Otherwise the final extent of severe turns more nebulous for the rest of the areas.

France will see rich CAPE in excess of 1000 J/kg but DLS in the 10 m/s range so the main hazard will be clustering and slow moving convection with heavy rain, hail and strong to severe gusts. SW France could see numerous significant HP cells with large to isolated very large hail,before clustering into a temporarily more organized and cold pool driven MCS event. Constant weakening into the night hours is forecast as BL becomes more stable.
N France (including Paris) into Luxembourg reside along the front with a nice CAPE/shear space for multicells and a few supercells and/or bowing segments. Hail, strong to severe gusts and an isolated tornado are possible.

Parts of the German/Austrian border and the Main Alpine ridge could see numerous long-lived supercells with hail and strong to severe wind gusts. Isolated large hail is possible along the German/Austrian border and later-on also over SE Austria, as maturing cells enter a more unstable airmass. Same for any storm, which shifts E into far W Hungary and W Slovakia. Beyons sunset, the activity weakens. Towards the Carpathian Mountains numerous hail and severe gust events are also forecast during the afternoon, initiating along the orography with a movement to the SE.

Towards the W Ukraine, DLS improves during the afternoon and offers a time-frame of a few hours for numerous multicells and a few supercells to cross the area to the SE. Hail and strong to severe gusts will be the main hazard admit a mixed BL airmass.

No wonder we deal with a broad level 1 area. SW France was upgraded mainly for the dominant hail threat while Austria was kept in a level 1 due to still modest CAPE. However we could see a more focused swath of severe in this area. S-CNTRL Germany was also considered (e.g. N Bavaria), but too many uncertainties exist with the wave's amplitude, which has impact on the final severe risk. Same for the German/Austrian border, where swaths of hail and severe gusts are possible.

... Spain and Portugal ...

E into NE Spain still resides ahead of the dissolving cold front with moist onshore flow interacting with steeper lapse rates along the orography. A confined swath of 500-1000 J/kg MUCAPE evolves in a strong shear environment (DLS around 20 m/s). Passing low-amplitude waves combined with strong diabatic heating should spark numerous thunderstorms, which could produce large hail and strong to severe wind gusts. The main issue will be an increase of the cap towards the coast, which should lower the sever risk to the E.

Portugal into SW/W Spain will see a diurnal severe risk on an isolated scale due to modest CAPE (300-600 J/kg). 15 m/s DLS is enough for a few multicells with heavy rain and an isolated tornado risk next to heavy rain.