Storm Forecast
Valid: Mon 02 Jun 2025 06:00 to Tue 03 Jun 2025 06:00 UTC
Issued: Sun 01 Jun 2025 19:47
Forecaster: PISTOTNIK

A level 2 and level 1 are issued for a belt from N Spain to Belarus for excessive convective precipitation, large hail and (mainly towards the NW) for severe convective wind gusts.

SYNOPSIS

Europe is largely influenced by a meandering zonal flow with a progressive, positively tilted trough and ridge pattern. A pronounced mid-level trough stretches from Scandinavia all the way to the Canary Islands, a mid-level ridge from Algeria to the Ukraine. This configuration results in a moderate to strong WSW-erly 500 hPa flow with an axis from Spain to Belarus. Closer to the surface, a long and wavy frontal boundary aligns with this jet axis. Warm air advection prevails SE of it.
Late in the forecast period, a new strong cyclone approaches the British Isles, reinforcing the old trough.

DISCUSSION

... belt from N Spain and SE France to the Alpine region, east-central Europe and Belarus ...

Steep lapse rates from the Spanish Plateau and - to a lesser degree - from the Pyrenees and the Alps spread ENE-ward over enhanced low-level moisture on the warm side of the frontal boundary. Resulting CAPE on the order of a few hundred to up to 1500 J/kg in conjunction with multiple sources of lift - travelling short-wave troughs, thermal upvalley and upslope wind systems, and outflow boundaries of antecedent convection - create another active thunderstorm day across wide regions, though it is once more difficult to go into detail due to a low consensus of the forecast models.
Isolated to scattered, mostly elevated and partly embedded thunderstorms may already be active in the morning, especially near the frontal boundary. Depending on the timing of the short-wave troughs and the amount of insolation, the formation of new and surface-based storms will quickly ramp up between noon and late afternoon. Deep-layer shear increases from 10 m/s well ahead of the frontal boundary, where CAPE is more plentiful, to 20 m/s and more in its proximity, where CAPE conversely decreases. In general, the CAPE-and-shear space is robust enough to allow multi- to supercellular convection.
The strongest vertical wind shear, and enhanced shear also across the 0-3 km layer, are present near the frontal boundary from S France to Switzerland, SE Germany and the Czech Republic. However, limited CAPE and a lack of insolation are an issue here, therefore convection may be limited and/or embedded in frontal clouds and rain shields. Nonetheless, if discrete and surface-based storms manage to form in pockets of sufficient insolation, they can turn into well-organized multicells and supercells and produce large hail and severe wind gusts - more likely in SE Germany and Czechia than further west. Excessive rain is possible as well in case of training activity.
Further ahead of the frontal boundary, convection will predominantly initiate over the orography but move into flatlands later on. Due to rich low-level moisture, the front-parallel background flow and plentiful opportunities for backbuilding convection over orographic features, excessive rain is in general the strongest concern. This possibly includes a few dangerous flash floods in mountainous terrain. Discrete and/or tail-end storms can turn multi- to supercellular with large hail and perhaps isolated downbursts. The hail hazard is highest where steep lapse rates are advected off the mountains over low-lying terrain, where CAPE may locally approach 2000 J/kg: Most notably in the La Rioja, Navarra and Aragon regions in Spain, in the Piemonte region in NW Italy, in E Austria and in SE Poland. Upscale grwoth into several large clusters is anticipated, which can partly persist well into the night with slowly decreasing severe weather hazards.
Due to remaining forecast uncertainties, large regions are covered with a level 1. The regions with the expected highest storm coverage are upgraded to a level 2.

... central Turkey ...

Scattered, daytime driven thunderstorms are expected in the range of a cut-off low in an environment of low to moderate CAPE and mostly weak vertical wind shear. Isolated rain, hail and wind events events may occur, but will likely be too marginal and too widely scattered for a level 1.

... Sweden, Denmark, NW Germany ...

On the cool side of the frontal zone, some thundery showers will form in response to daytime heating of well-mixed maritime air. Severe weather is unlikely.