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.