Storm Forecast
Valid: Tue 26 Aug 2025 06:00 to Wed 27 Aug 2025 06:00 UTC
Issued: Mon 25 Aug 2025 22:00
Forecaster: PISTOTNIK

A level 1 is issued for parts of the Italian mainland mainly for excessive convective precipitation and large hail, and for parts of its E coast mainly for tornadoes.

A level 1 is issued for parts of France mainly for excessive convective precipitation and large hail.

A level 1 is issued for parts of Spain mainly for severe convective wind gusts, and to a lesser degree for large hail and excessive convective precipitation.

SYNOPSIS

Three large-scale features dominate the weather maps in Europe: (1) a pronounced and largely barotropic mid-level low over NE Europe, (2) ex-hurricane Erin NW of the British Isles, and (3) a highly amplified mid-level ridge in-between from Algeria to the Norwegian Sea. As ex-Erin has morphed into an extratropical cyclone and its extensive circulation tapped a plume of hot subtropic air from NW Africa, a pronounced warm sector spans central Europe. It is opposed by autumnal polar air in NE Europe and cooler maritime air over the British Isles, which slowly advances onto the continent behind a long cold front.

DISCUSSION

... greater Mediterranean region plus France, Switzerland and SW Germany ...

Notable CAPE is mostly confined to the Mediterranean Basin and its coastal areas. While it stays largely below 800 J/kg towards the East, a pronounced and extensive elevated mixed layer (EML) from the Sahara allows higher CAPE values over the W Mediterranean Sea, peaking beyond 2000 J/kg near the Algerian coastline. Aided by strong insolation and regionally by subtle synoptic lift, a few hundred J/kg CAPE will also build well inland in France, Switzerland and the Alpine region. However, fairly strong capping remains an issue everywhere within reach of this EML, which will spread deeply into France and Germany by the end of this forecast period.
Scattered thunderstorms will mostly be tied to orographic features, most notably the southern half of the Italian mainland, the French Massif Central, the Pyrenees and parts of the Spanish mountains. Vertical wind shear is mostly weak, therefore the majority of storms will be single- to multicellular. The main hazards are localized excessive rain, along with large hail in initiating stages. Stronger deep-layer shear up to 20 m/s is present east of a sea breeze front / dryline along the Spanish east coast and in the Ebro valley. A better storm organization into stronger multicells or a few transient supercells is not ruled out here, increasing the hail threat to some degree. However, since it is not clear if coastal air parcels with higher CAPE and better shear can be hauled across the fairly robust cap, no upgrade to a level 2 seems necessary. In the Spanish interior, the capping inversion can be broken more easily, but a dry and deep convective boundary layer keeps the CAPE magnitude lower. In this environment, the main hazard shifts to localized severe downbursts. (NB: These winds and lightning strikes are still a concern with respect to the ongoing wildfire situation.)
Most of the storms will decay around sunset, but a few vorticity maxima could keep activity going well into the night from NE Spain to S France, Switzerland, and later even SW Germany. These storms will likely be elevated and should not produce severe weather.

In some coastal waters around Italy and Greece, a little convection may also be active at nocturnal land breeze fronts in the first hours of this forecast period. This convection - not necessarily electrified or not even precipitating - may spin up a few non-mesoscyclonic tornadoes in case of well-developed convergence zones over the warm waters. The level 1 is therefore expanded a bit offshore along the Italian east coast where the model pool shows some precipitation signals.

... NE Europe ...

Scattered to widespread, locally thundery showers are expected in marginally unstable polar air and under weak to moderate vertical wind shear. Activity largely follows the diurnal cycle over land, but may occur around the clock over the Baltic Sea. Severe weather is unlikely.

... Ireland ...

After 21 UTC, a feature moves onshore in the westerly background flow that can either be interpreted as a comma or a secondary frontal wave within the maritime airmass. High-resolution models predict a shallow convective line. Severe wind gusts appear unlikely, as the pressure gradient quickly relaxes with its passage, but it is not ruled out that a few weak tornadoes spin up in the very moist air as it is temporarily covered by a strongly sheared and helical inflow. Considering the shallow nature of this convection, this scenario appears just a bit too unlikely for a level 1, but could be kept in mind.