Storm Forecast
Valid: Sun 25 May 2025 06:00 to Mon 26 May 2025 06:00 UTC
Issued: Sat 24 May 2025 17:24
Forecaster: TUSCHY

A level 1 was issued for parts of S Greece and the Aegean Sea mainly for hail and heavy rain. An isolated tornado is possible next to strong gusts.

A level 1 was issued for parts of NW Russia and far E Finland mainly for hail, heavy rain, gusts and a few tornadoes.

SYNOPSIS

Over E-CNTRL and E Europe, the longwave trough breaks up into numerous more or less defined lower amplitude waves. One shifts N/NE over the Baltic Sea and moves into Finland during the afternoon onwards. Over SE Europe, two waves start to phase into one dominant trough, which approaches the Aegean Sea during the end of the forecast. To the E, a blocking ridge remains stationary which causes the wavy and N-S aligned surface front to stall somewehere from NW Russia south towards the W Black Sea. EZ-ENS has diffuse member clusters for weak/temporal cyclonic vortices in the lower troposphere with a somewhat better focused area over the Aegean Sea and far NW Turkey. Otherwise we could see weak depressions over Moldova and also over E/NE Finland, which translate N along the synoptic-scale front. These vortices could impact the final degree of thunderstorm chances on a regional scale.

In the meantime a high-index pattern becomes established over NW into CNTL Europe as a broad cyclonic vortex shifts E towards Scotland while facing positive height anomalies over the far E subtropical Atlantic and into far SW Europe. The net result will be an anomalous mid/upper jet, which evolves W of Ireland and spreads E betimes. Numerous weak low-tropospheric waves circle that vortex and spread unsettled conditions well into CNTRL Europe.


DISCUSSION

... Germany ...

Most of Germany resides in a broad warm sector during the day before a cold front approaches from the W/NW during the late afternoon onwards. The cold front is not well structured with the more focused post-frontal pressure rise affecting N Germany, whereas weak signals exist further S. This causes the cold front to lag behind over CNTRL/SW Germany compared to the frontal passage to the N. In addition, a cyclonic SW-erly flow overspreads the warm sector and numerous weak signals of Q vector convergence point to passing waves. In addition, MSLP forecast also points to weak/progressive pressure troughs although this may be more a signal added by the orography. Nevertheless, all this information points to various LL convergence zones (partially on the mesoscale), which serve as foci for more enhanced thunderstorm activity.

A lingering uncertainty is how fast the WAA clouds shift E during the day with better diabatic heating thereafter spreading into CNTRL Germany during the afternoon/evening hours. ID2 f.ex. keeps the more strafiorm rain and thick clouds over CNTRL Germany well into the afternoon hours, which would limit CAPE build-up that far E.

Surface dewpoints remain in the low to locally mid tens with slightly higher readings for the temperature. A 0-2 km AGL veered wind profile beneath weak backing aloft (here: differential temperature advection) results in a more focused 0-3 km CAPE build up with local peaks approaching 200 J/kg. Forecast SR hodographs indicate rather strong 0-1 km storm relative inflow of 20 kt with LCLs more in the 400 to 700 m AGL region.
In case we get enough clearing, the stage is set for numerous small and eastward moving line segments with strong gusts and graupel/small hail. Shallow mesocyclones are possible and a somewhat (but not focused) tornado risk exists. A better overlap of ingredients exist roughly from Saarland E/NE into NW Bavaria and Thuringia/W-Saxony, which could become a focus for a spin-up or two.

A level 1 was not added as (despite the tornado risk) no other hazard approaches severe criterion.

... NW Russia into far E Finland...

Another round of early and widespread CI is forecast along the synoptic-scale front. Initiating cells evolve in 400-800 J/kg MUCAPE and 15 m/s shear at 0-3km and 0-6 km levels (near the most sloped part of the front itself). Cells initating near the front will acquire temporal rotation with hail, gusts and a few tornado reports possible. Betimes the more dominant risk becomes heavy rain with clustering convection.
A somewhat more enhanced tornado risk could evolve over Karelia (W of the White sea), where storm relative inflow further increases. Another broad level 1 should cover that event with clustered reports certainly possible.

... S Greece into the Aegean Sea ...

Ahead of the structuring trough, strong WAA advects a dry BL airmass beneath a pronounced EML layer into the area of interest, which results in elevated showers and thunderstorms before noon into the early afternoon hours. Thereafter, cooling mid-levels along the periphery of the upper trough with steadly increasing lift atop improving BL moisture creates a setup, which supports CI during the afternoon into the overnight hours.

MUCAPE of 500-1000 J/kg overlaps with up to 25 m/s 0-3 km and 0-6 km shear which is more than enough for organized DMC activity. However a rather warm layer below 800 hPa questions how much of the shear gets incorporated into growing updrafts as they stay more elevated. Hence the main risk will be hail, isolated large, and heavy rain. However any cell, which ingests more surface based CAPE/shear could be accompanied by severe gusts and an isolated tornado event. EZ points to that risk near Athens and surrounding coastal areas. A broad level 1 should cover this event for now.

... N Ireland to N UK and S Scotland ...

An active day convective-wise is forecast with low-end CAPE and modest shear. Gusty winds and a few funnels are possible with some LL CAPE, but nothing severe is anticipated.