Extended Forecast
Valid: Wed 28 May 2025 06:00 to Thu 29 May 2025 06:00 UTC
Issued: Mon 26 May 2025 17:22
Forecaster: TUSCHY
A level 1 was issued across most of Germany, Luxembourg, W Poland, Czechia into NE Austria mainly for strong to severe gusts, some hail and heavy rain. An augmented tornado risk exists for some parts and a strong event is possible.
A level 1 was issued for parts of N Italy into Slovenia mainly for large hail and severe gusts.
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. EZ-ENS is rather tightly clustered with the mentioned track but some NWP track spread is still noted in latest data for this wave. ICON is more along the southern fringe of the poor man's ensemble with GFS the fastest one. These issues still have lingering questions regarding final details about the convective setup 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 ...
From 06Z onwards until around noon, we already see an uptick in CI over Benelux into NW Germany. This activity starts elevated but turns surface based rapidly until noon and spreads E/SE thereafter. A decrease in activity occurs over far NW Germany thereafter.
From the Netherlands E into N Germany and NW Poland, 0-3 km and 0-6km shear will be more in the 10 m/s range with a rapid weakening trend of the winds at upper levels. Pulsating convection, probably organized into smaller line segments is forecast, although strongest lift and uncapped conditions probably support rather widespread CI especially from NE Germany into NW Poland. Gusts, some graupel/hail and heavy rain on a local scale will be the main hazard. However, deviating cells (e.g. by merging) will ingest enough SR inflow into low-based updrafts with LCL in the 400-600 m AGL range to support an isolated tornado risk. Latest forecast hodographs still show some backing above roughly 3km AGL, which would keep mesocyclone depth more restricted. LL CAPE is forecast to reside in the 100-250 J/kg range, which is more than enough for organized shallow mesocyclones. We therefore expanded the level 1 into this area.
Of most concern will be a belt from Luxembourg into CNTRL and SE Germany if we follow EZ/ICON. The cold front crosses this area during peak heating with constantly cooling mid-upper levels and despite mid-level CAA, positive Q vector convergence along passing short waves indicate enough background support for scattered CI (next to the cold front itself and diurnal heating). Next to the sloped frontal zone, 15 m/s 0-3 km shear and aoa 20 m/s DLS affect the unstable warm sector. Forecast hodographs are elongated and both models indicate spaciously enhanced helical values for 1 and 3km AGL with LCLs at or below 600 m. Any deviating cell would ingest lots of SR inflow which is rather streamwise at least for most regions. In addition, merging and potentially deviating cells will stay avoid of most of the FFD precipitation, so longer-tracked mesocyclones are possible.
The risks will be some hail (although rather mild mid-levels should lower that risk), strong to severe gusts and heavy rain (especially for convection, which passes repeatedly over the same area). The tornado risk is augmented and probably maximized over S-CNTRL into SE Germany, although more model consistency regarding exact MSLP/geopotential height configuration is needed for adding higher tornado probabilities on a regional scale (e.g. upgrade to a level 2). This includes the information of CAM data.
Further E over Czechia into NE Austria, shear remains borderline for multicells and transient supercells but strong LL inflow and modest 1/3km shear point to organized line segments with swaths of strong/severe gusts and hail next to an isolated tornado event.
The overall risk diminishes after sunset despite the activity over S Germany, which continues well into the overnight hours (including a local flash flood risk along the Alps).
... N Italy to Slovenia ...
A classic severe risk with deep/intense NW-erly winds evolve in the mentioned area as the supergeostrophic upper jet point towards the Alps during the day.
In the meantime, a moist BL airmass resides in the highlighted area ahead of the cold front, which gets partially blocked by the orography so a diurnal driven increase in CAPE is forecast with MUCAPE in the 1000-1500 J/kg range, whereas a rapid decrease of BL moisture and CAPE occurs towards Slovenia.
Forecast hodographs show elongated and rather straight signatures as DLS pushes into the 30 m/s range while 0-3 km shear resides more around 15 m/s.
CI is forecast during the afternoon as the upper part of the cold front crosses the Alps. Timing will be an issue as not much time is left for surface based convection but conditions are supportive of a few splitting supercells with severe gusts and large hail. Betimes, convection outruns best CAPE and turns more elevated with a decreasing severe risk. For now we just added a rather broad level 1, which probably needs some refinement in later updates.
... SE/E Europe ...
Numerous regions with enhanced thunderstorm probabilities exist but either shear or instability or both remain too marginal for anything severe. Spotty instances of hail, heavy rain and gusts are possible but this risk is driven mainly on the mesoscale.
Isolated large hail could become a concern for E Turkey into Azerbaijan and a regional upgrade could be performed in later updates.
