Storm Forecast
Valid: Wed 27 Aug 2025 06:00 to Thu 28 Aug 2025 06:00 UTC
Issued: Tue 26 Aug 2025 21:14
Forecaster: PUCIK
A level 2 was issued from N Spain to central France mainly for large to very large hail, severe wind gusts and heavy rainfall.
A level 2 was issued from SE France to NW Italy mainly for heavy rainfall and to the lesser degree for large hail and severe wind gusts.
A level 1 was issued across E France, SW Germany, Switzerland and N Italy mainly for large hail, severe wind gusts and heavy rainfall towards the night hours.
A level 1 was issued for England for tornadoes and marginally severe wind gusts.
DISCUSSION
... NE Spain ...
A cold front will stagnate across NW Iberia with heat low remaining over the interior of Spain with hot airmass and steep lapse rates within the boundary layer. Towards Ebro valley, weak easterly winds will advect moister air further inland with specific humidity of around 12 g/kg. Lack of overlap of steep lapse rates over the continental part and the regions with moist airmass will keep CAPE values in the range of 1000 to 2000 J/kg. 0-6 km bulk shear will reach 15 to 25 m/s, increasing towards the afternoon and evening as a trough approaches.
In the afternoon hours, storms will be initiated near convergence zones on the drylines and the upslope flow of the Pyrenees. Towards the evening, further initiation over the Ebro valley is expected first from the cold front and then a potential cold pool running across the valley from the surrounding topography. First, surface-based multi to supercells are expected, capable of large to very large hail and severe wind gusts. Eventually, storms will become elevated in the night hours, but the severe weather risk will persist throghout the night as storms progress over Catalonia.
... S France ...
Stalling weak cold front will serve as a focus for the storm development in the afternoon to evening hours with an increase in the coverage expected as the trough approaches from the west. Storms will continue into the overnight hours and will spread towards SE France. Further storms are expected to arrive from the Bay of Biscay towards Thursday morning on the another cold front.
The afternoon to early evening storms will be forming in an environment of 500 to 1500 J/kg of MLCAPE with rather skinny CAPE profiles and 0-6 km bulk shear of 15 to 20 m/s. A mix of multicells and supercells is expected with threats of large hail and severe wind gusts. A convective system may form over Pyrenees and track over S France towards Massif Central in the evening hours, bearing the risk of severe wind gusts. Towards the night, southerly low-level flow is forecast to increase ahead of the cold front, feeding the convective system and redevelopment of cells as a system spreads towards SE France with marked increase in the heavy rainfall threat overnight and till morning hours. Over extreme SE France, impressively curved hodographs are forecast but with stable boundary layer, yielding the low-level shear "ineffective" for the storms. Still, strong deep layer shear and MUCAPE around 1000 J/kg may result in threat of large hail and isolated severe wind gusts even in the early morning hours.
Storms advancing on the cold front from Bay of Biscay will form in marginal CAPE environment, combined with 20 m/s of 0-6 km bulk shear and 15 m/s of mean flow in the bottom 3-4 km. This will result in an isolated severe wind gust risk.
... NW Italy ...
Scaterred to widespread storms are expected over the area starting in the afternoon hours. With MLCAPE around 1000 J/kg and 0-6 km bulk shear around 15 m/s, stronger storms may become supercells capable of large hail and severe wind gusts. Both threats will be limited by rather skinny CAPE and/or low LCLs with rather moist profiles. The main hazard will be heavy rainfall, which will ramp up in the late evening to overnight hours with an increase in the southerly to southeasterly low-level flow both over the Alps and Liguria. Moist profiles with skinny MUCAPE will result in efficient heavy rainfall processes and local accumulations till 6 UTC may exceed 200 mm. As hodographs elongate due to the increase in the mid-tropospheric flow towards Thursday morning, some elevated supercells may be ongoing over northern Italy with isolated threat of large hail.
... E France, Switzerland, Germany ...
Along and ahead of the cold front, modest mid-tropospheric lapse rates of around 6 K/km will overlap with specific humidity of around 12 g/kg. Environment for the storms will feature 500 to 1500 J/kg of MUCAPE. 0-6 km bulk shear will reach 15 to 25 m/s with 0-3 km bulk shear locally up to 20 m/s. Initiation is expected in the afternoon hours with a mix of supercells and short linear segments bringing threats of large hail and severe wind gusts. The most likely corridor of storms will lie between Massif Central to NW Switzerland and SW Germany. Storms will spread NE during the early night hours, eventually forming a parallel stratiform system with heavy rainfall being the dominant threat. S Switzerland will also be affected by storms forming over the Alps with very heavy rainfall threat especially towards Thursday morning.
... England ...
Ahead of the advancing cold front, skinny CAPE profiles are forecast with MLCAPE on the order of hundreds J/kg. Simulated hodographs show curvature in the low-levels and modest deep-layer shear yielding 0-1 km bulk shear exceeding 10 m/s, 0-6 km bulk shear around 15 m/s and 0-500 m SRH locally exceeding 100 m2/s2. Expect a mix of small linear segments and low-topped supercells along the front with a risk of tornadoes and marginally severe wind gusts.
