Storm Forecast
Valid: Wed 20 Aug 2025 06:00 to Thu 21 Aug 2025 06:00 UTC
Issued: Tue 19 Aug 2025 22:31
Forecaster: PUCIK
A level 2 was issued across northern Italy, southwestern Slovenia and northwestern Croatia mainly for large to very large hail, heavy rainfall, severe wind gusts and to the lesser degree for tornadoes.
A level 1 was issued across southeastern France mainly for heavy rainfall, large hail and to the lesser degree for severe wind gusts.
A level 1 was issued across central France, Switzerland, south Germany and Austria mainly for heavy rainfall.
A level 1 was issued across eastern Spain and the Balearic Sea mainly for large hail and severe wind gusts.
A level 1 was issued across SE Lithuania and N Belarus mainly for severe wind gusts.
A level 1 was issued for western Caucasus mainly for heavy rainfall.
DISCUSSION
... Italy to Istria ...
A complex scenario will unfold across Italy during the forecast period with multiple rounds of severe storms possible. In general, a frontal system with warm air advection regime ahead of it will impact the area. Mixing ratios between 12 and 14 g/kg will overlap with about 6.5 K/km of mid-tropospheric lapse rates increasing to the southwest of the area. Ahead of an approaching mid to upper tropospheric low over France, vertical wind shear is forecast to increase. An overlap of 1000 to 2500 J/kg of MLCAPE with 15 to 25 m/s of 0-6 km bulk shear is forecast across the area with the best CAPE-shear overlap in the southern extent of the Lvl 2.
The first round of storms will cross northern Italy in the morning hours, arriving in Istria and Slovenia. Across Liguria, slow-moving system is likely with threat of heavy rainfall and perhaps isolated large hail with environment characterized by low LCLs, plentiful CAPE and modest wind shear.
The second round of storms will likely follow on the Appenines towards the noon with isolated supercells spreading towards the lowlands and the Adriatic coast. This activity is shown by all convection allowing models, though the coverage and the exact track of the storms vary from one model to another. Environment will feature around 2000 J/kg of MLCAPE with substantial CAPE in the upper troposphere and 20 to 25 m/s of 0-6 km bulk shear. Low LCLs will somewhat diminish the hail/wind risk, but still stronger storms will be capable both of very large hail and severe wind gusts.
The third round of storms would be associated with the storms crossing from France or evolving over the Alps and moving over the plains of Piemonte and further east in the evening hours. While this activity is not reproduced by all models, there would be a considerable severe weather risk with MLCAPE around 2000 J/kg and 0-3 km bulk shear between 15 and 20 m/s. High-resolution models suggest a mix of supercells and/or linear segments capable of large hail and severe wind gusts. These storms would spread in the early night hours towards Slovenia and NW Croatia (Istria), maintaning their intensity.
The fourth round of storms would begin with development of new storms over Liguria that would spread towards south into the pool of high CAPE and reducing CIN with the approach of the cold front. The storms are also forecast to spread east and northeast, reaching NE Italy, SW Slovenia and NW Croatia in the early morning hours. The storms will most likely be elevated, fed by a developing low-level jet with strongly curved hodographs and large low-level shear. A mix of heavy rainfall threat, large to very large hail and severe wind gusts is likely with the stronger storms. The threat of very large hail will be the greatest over the coastline of W Italy, where the most CAPE in the upper troposphere is forecast. Tornado risk will be present should any developing storm become surface-based.
... Southeastern France ...
A deliberation between lvl 1 and lvl 2 was made across this area. Found first ahead and then within the passing low and near the advancing cold front, there will be a potential for several rounds of storms across the area with the aid of the orographic lift. The environment will be characterized by MLCAPE around 1000 J/kg and 0-6 km bulk shear at first up to 20 m/s and decreasing towards the afternoon and evening. Some supercells will be possible in the late morning hour with threat of isolated large hail and severe wind gusts, combined with heavy rainfall wherever cells train over the same spot. Heavy rainfall threat will persist into the afternoon hours. Questionable coverage of storms with large differences among the models precluded Lvl 2 issuance at this moment.
... East Spain to the Balearic Sea ...
Another region with quite complex setup. Starting from south, in the Valencia region, a dryline is forecast to set up in between of the mountains and the low terrain with easterly moist flow over the lower terrain and westerly dry flow over the higher terrian. Isolated convective initiation is expected on the dryline with storms forming in a strong vertical wind shear between 20 and 30 m/s. A lvl 2 was considered for large to very large hail, but there's an uncertainty regarding the CAPE profiles. ECMWF shows pronounced stable layer in the mid-troposphere, which would reduce the updraft magnitude in the hail growth layer. Nevertheless, expect one or two supercells capable of large hail and severe wind gusts that may move over the sea. This option is not indicated by any model, but the forecast profiles do not show significant CIN or dry layer near the LFC between the coastline and Ibiza.
Further north/northwest, towards Mallorca and Catalonia, a convergence zone is forecast over the area during the day that may trigger isolated to scattered storms in MLCAPE around 1000 J/kg and 0-6 km bulk shear exceeding 25 m/s. Some higher resolution models indicate right-moving supercells within the mix of the storms developing here, which would be capable of large hail and severe wind gusts.
Over Catalonia, lower CAPE is forecast, but the wind shear will remain strong, exceeding 20 m/s in the 0-6 km bulk shear. Orography and the cold front will initiate storms, some of which may become low-topped supercells capable of marginally large hail and severe wind gusts.
... central France through Switzerland into S Germany and Austria ...
Forecast profiles show slow moving storms developing in an environment of skinny CAPE, low LCLs and deep warm cloud depths. Expect isolated heavy rainfall events.
... SE Lithuania, Belarus ...
A cold front will cross the area with an environment characterized by marginal CAPE, 0-6 km bulk shear exceeding 20 m/s and mean flow of around 15 m/s in the low to mid troposphere. Upscale growth into a line is expected given parallel flow to the front. Isolated severe wind gusts along a squall line are forecast.
... western Caucasus ...
Continuous weak upslope flow combined with deep warm cloud depth, low LCLs and substantial low-level moisture will elevate the risk of heavy rainfall with the storms.
