Storm Forecast
Valid: Mon 18 Aug 2025 06:00 to Tue 19 Aug 2025 06:00 UTC
Issued: Sun 17 Aug 2025 22:35
Forecaster: PISTOTNIK

A level 2 and level 1 are issued for Corsica, parts of Italy and the Balkans for excessive convective precipitation, large hail and severe convective wind gusts.

A level 2 is issued for E Spain for large hail, excessive convective precipitation and severe convective wind gusts.

A level 1 is issued for central Spain mainly for severe convective wind gusts.

SYNOPSIS

Weak pressure and temperature gradients prevail across Europe under a quite fragmented circulation pattern. Mid-level lows are placed over W Russia, N Scandinavia and - weaker ones - over Romania and the Celtic Sea, all of them slowly moving eastward. Slightly lower 500 hPa geopotential also extends into the central Mediterranean region. In contrast, a subtropic ridge stretches from Morocco to the North Sea. Very warm to hot conditions continue in the range of this ridge, whereas slightly cooler, dry continental air has spread over large parts of the continent.

DISCUSSION

... Corsica, Italy to Greece, the S Balkans and Romania ...

Though slowly receding, hot air with steep lapse rates still resides across the southern third of Europe. Low-level moisture is limited in inland areas, owing to little evaporation from the dry surfaces and deep vertical mixing, and keeps CAPE mostly confined to values below 800 J/kg. Much richer low-level moisture and CAPE well in excess of 1000 J/kg are in place in coastal and offshore areas (e.g., almost 2000 J/kg were sampled by the Sun 12 UTC Ajaccio and Trapani soundings, and even almost 4000 J/kg by the Pratica di Mare sounding), though a strong capping inversion remains in place.
In the absence of noteworthy synoptic lift and under weak deep-layer shear around or below 10 m/s, scattered thunderstorms are mostly tied to orographic features and the afternoon to evening hours. The strongest pulse storms can produce excessive rain, large hail and severe downbursts. These events are expected to be rather widely scattered from Greece to central Romania, covered by a level 1. However, Sunday's widespread and explosive storms over e.g. central Italy and Corsica suggested that it was easier than anticipated for air parcels with rich coastal moisture to break the capping inversion when picked up by upvalley and upslope circulations. Since a similar scenario is foreseen again for Monday, Corsica and the southern half of Italy are upgraded to a level 2, even more as the weak northerly background flow over thermal onshore and upslope circulations could bring deep-layer shear into the 10-15 m/s range and allow multicellular or even transient supercellular storm organization in these areas. The dominant hazards are flash floods and large hail (isolated very large hail not ruled out in case of a supercell), but severe downbursts may occur as well.

... Spain ...

A similar setup is also present in Spain: very hot and dry, deeply mixed air with little CAPE well inland, but sea breezes with abundant low-level moisture nibbling at the east coast. Sunday's soundings from Barcelona and Murcia showed only little and no CAPE yet, respectively, but with the gradually approaching Atlantic mid-level low and more backed surface flows, the moisture is expected to penetrate farther inland on Monday, also including the lower Ebro valley. The degree to which this happens is still somewhat unclear, though, as some models still show only a few hundred J/kg CAPE over land, whereas e.g. ICON-EU and WRF even bring a plume of 2000-4000 J/kg CAPE a little distance inland.
Current thinking is that the areas well inland will see another round of scattered afternoon to evening storms that are very high-based and pose a main hazard of a few strong to severe, perhaps even dry downbursts. These winds could at least temporarily aggravate the wildfire situation, and new wildfires could be sparked by lightning strikes.
More plentiful and more vigorous storms are expected to form over the Sistema Iberico and the Pyrenees, depending on how much coastal moisture can indeed be fed into their updrafts as described above. Vertical wind shear increases to widespread 15 m/s by evening, perhaps even 20 m/s in the Ebro valley and other zones of well-pronounced onshore, upvalley and upslope circulations. Storms can organize into multi- or supercells, perhaps later also into one or two large clusters that could propagate towards the coast before the capping inversion becomes too strong. Large to very large hail and excessive rain with a possibility of dangerous flash floods are the main hazards, next to severe wind gusts as a secondary threat.
Convection can continue into the first half of the night before it finally dissipates. Elevated and non-severe storms can also travel into France then, and scattered lightning is possible in the entire band of steep lapse rates and elevated CAPE that fans out as far as SW England and Ireland.