Storm Forecast
Valid: Tue 19 Aug 2025 06:00 to Wed 20 Aug 2025 06:00 UTC
Issued: Mon 18 Aug 2025 23:34
Forecaster: PISTOTNIK

A level 3 is issued for E Spain mainly for large hail and to a lesser degree for severe convective wind gusts.

Level 2 and level 1 areas are issued for NE Spain, S and E France, parts of Switzerland, Italy, Greece and Bulgaria mainly for large hail, excessive convective precipitation and to a lesser degree for severe convective wind gusts.

A level 1 is issued for parts of Latvia, Estonia and far-NW Russia mainly for severe convective wind gusts and to a lesser degree for tornadoes.

SYNOPSIS

A pronounced mid-level low filled with polar air over Scandinavia gradually extends its influence SE-ward. Its cold front reaches the Baltic States and the Polish and German coasts by the end of this forecast period, bringing a first hint of the coming autumn.
Further south, rather weak geopotential / pressure and temperature gradients prevail with still very warm air. Two cut-off lows slowly travel E-ward over the Bay of Biscay and over Romania, respectively. They are separated by a mid-level ridge that stretches from the central Mediterranean region to Germany to Iceland.

DISCUSSION

... E Spain ...

Abundant low-level moisture is in place over the Mediterranean Basin, capped by an elevated mixed layer (EML) that has been advected off the Spanish Plateau. Monday's soundings did not sample the moisture maximum along the Spanish east coast between the Ebro Delta and Alicante, but observed 2m dewpoints between 23 and 27C suggest that the forecasted CAPE magnitude of 3000-4000 J/kg could indeed have materialized. The depth of the moisture in coastal areas undergoes a characteristic diurnal cycle, dictated by its removal under downslope and offshore winds overnight and by its re-invigoration with onshore and upslope flow during daytime. Monday's observations showed that dewpoints up to 20C crept up to altitudes around 1000m above sea level in the Valencian hinterland. Consequently, scattered convective initiation occurred over orographic features that had access to this rich moisture inflow.
A similar setup establishes again on Tuesday, with a bit stronger mid-level flow that drives vertical wind shear to 15-20 m/s across the 0-3 km layer and slightly beyond 20 m/s across the 0-6 km layer, and a short-wave trough that should erode the cap and facilitate convective initiation. While forecast models do not show such a concentrated extreme CAPE maximum anymore, robust CAPE values on the order of 1000 to 3000 J/kg should be present across wider areas and advect also some distance inland, especially in the Ebro Valley. By afternoon, scattered to widespread thunderstorms are expected to initiate over Catalonia. Supercells with large to isolated very large hail are possible in the first hours, followed by upscale growth into several large clusters. In these later stages, excessive rain becomes the primary hazard, and severe wind gusts are possible as well, especially at the leading edge of clusters.
The most interesting environment on the mesoscale is once more in place along the sea breeze front in the Valencia province and its northwestward extension in form of a dryline at the NE slopes of the Sistema Iberico towards the Ebro Valley in adjacent Catalonia. This boundary is expected to be extremely pronounced with 2m dewpoint differences well in excess of 10K across few tens of kilometers or even smaller distances. Convection-resolving models (WRF and AROME) uniformly agree on isolated to scattered storms that should initiate in the mid-afternoon over the mountains and tap into CAPE in excess of 2000 J/kg and strong deep-layer shear as soon as they detach from them and descend into the lowlands. A handful of strong supercells with large to very large hail appears likely, and even localized giant (8-10 cm sized) hail is not ruled out. Overall, confidence in a few extreme hailfalls is high enough for a confined level 3. Severe downbursts pose a secondary hazard, but should stay rather isolated. With weaker synoptic lift than further north and a stronger cap, upscale growth into clusters is less likely, though some remnants of gradually weakening and clustering storms could still affect the coast overnight.
Further south along the sea breeze front, convective initiation becomes less and less likely towards southern Valencia and Murcia provinces, but is not ruled out. Any updraft that turns robust enough to withstand the strong entrainment and break the cap can acquire rotation and produce large hail, though. This conditional scenario is reflected by a southward extension of the level 2 and level 1 areas.

... France, Switzerland, Italy, Greece, Balkans ...

Scattered afternoon storms are expected in an environment of moderate CAPE up to 1000 J/kg (locally more in coastal areas, but with the characteristic issue of a stronger cap that may make convective initiation fail) and deep-layer shear around 10 m/s. Primary initiation will mostly happen over orographic features, secondary initiation along outflow boundaries or other convergence zones in the later afternoon or evening. An enhanced storm coverage is expected over Bulgaria and S Romania, which are crossed by a short-wave trough at the flank of the eastern cut-off low, and over S to central France, which is overspread by both warm air advection and positive vorticity advection ahead of the western cut-off low. Corsica and central Italy will be another hotspot, as well-established onshore and upslope circulations manage to break the cap even in the absence of synoptic lift.
Single cells, multicells and (later) large and rather poorly organized clusters are the expected storm modes. The main hazards are excessive rain and large hail (the latter mainly in early stages of strong pulse storms) as well as a few severe wind gusts.
The majority of the storms will subside around sunset, but persistent synoptic lift can carry some activity well into the night in E France, Switzerland and NW Italy. Another level 2 is added for N Italy, where the model pool largely agrees on an MCS towards Wednesday morning, which can bring abundant rain with flash flood issues in the southern Alps and perhaps large hail and severe wind gusts at its tail end that works its way eastward over the Po Valley.

... Estonia, Latvia into far-N Russia ...

Synoptic lift and daytime heating create a few hundred J/kg CAPE ahead of the SE-ward moving cold front and mid-level trough. Scattered low-topped thunderstorms are likely in the 12 to 21 UTC time frame and can organize into shallow multi- and supercells or line segments. Under moderate to strong vertical wind shear across all layers, a few severe wind gusts are possible and one or two tornadoes not ruled out.