Extended Forecast
Valid: Wed 04 Jun 2025 06:00 to Thu 05 Jun 2025 06:00 UTC
Issued: Mon 02 Jun 2025 22:06
Forecaster: PUCIK
A level 2 was issued for southeastern France mainly for heavy rainfall and tornadoes.
A level 2 was issued for northern Italy mainly for heavy rainfall, large hail, severe wind gusts and tornadoes.
A level 2 was issued from northeastern Switzerland, northwestern Austria through southeastern and eastern Germany into Czechia and Poland mainly for destructive wind gusts, very large hail and to the lesser degree for heavy rainfall and tornadoes.
A level 1 was issued for Corsica for large hail.
A level 1 was issued for Sweden for tornadoes and marginally severe wind gusts.
A level 1 was issued for Romania for large hail.
A level 1 was issued for Russia and Ukraine mainly for severe wind gusts.
DISCUSSION
An outbreak of severe storms is likely ahead and along a pronounced cold front from southeastern France through northwestern Italy, Switzerland into southern Germany, Czechia and Poland!
... Southeastern France ...
Strong warm air advection regime is forecast across the area in the morning hours. Skinny CAPE profiles with MLCAPE around 500 J/kg will combine with strongly curved hodographs featuring 0-6 km bulk shear around 25 m/s, 0-1 km low level shear around 10 m/s and 0-500 SRH of around 100 m2/s2. Environment will be conditionally favorable for tornadoes given the strong low-level shear and low LCLs. The condition will be surface-based convection, which is questionable. Besides tornadoes, heavy rainfall will be the main risk. Threat should diminish after 12 UTC, but this may be refined in the later forecast.
... Northern Italy ...
A questionable setup, especially regarding how far south towards the lowlands the convective initiation will spread, as the cold front will not reach this area. Cyclogenesis is forecast across NW Italy, resulting in strong low-level shear and curved hodographs, especially within the Lvl 2 area. With southerly component in the flow at 500 m to 1 km AGL, Alpine foothills will initiate the storms that will rapidly spread NE in strong mean flow. While the window for storms to experience favorable conditions may be short before moving over the main Alpine ridge, all hazards will be possible especially for right-moving supercells. The main threat though will be very heavy rainfall from cells redeveloping over the Alps in persistent southerly flow.
... NE Switzerland through SE/E Germany, NW Austria, Czechia into Poland ...
A wide belt with favourable conditions for (extremely) severe storms will exist in the region, featuring MLCAPE of 1500 to 3000 J/kg in combination with 0-6 km bulk shear of 20 to 30 m/s. The highest conditional threat of severe weather, concerning the CAPE-shear overlap, will exist from NE Switzerland through S Germany and NW Austria into SW Czechia. Here, fat CAPE profiles are simulated within and at the edge of a plume of steep lapse rates generated in the wake of the Alps. At first, high CAPE areas will be capped, but the approach of the front and a short-wave through from southwest will remove the CIN and the drier layer. The first storms will develop along the stalling cold front, the main round will be tied to the passage of the short-wave, E-ward surge of the cold front and storms spreading from the Alps over the lowlands.
Storms forming in this environment will be capable
of very large to giant hail and destructive wind gusts locally exceeding 32 m/s. Given how rapidly the cold front will move E, an upscale growth into a bow echo that will move from Germany through Czechia into Poland is likely in the late afternoon to evening. Such scenario would entail a long-lived convective windstorm with high coverage of severe to extremely severe gusts. Thus it is possible that an upgrade of a part of Lvl 2 area into Lvl 3 will be needed. Besides hail and wind, models show locally enhanced low-level SRH, so that supercells may produce tornadoes, but there is no spatial agreement yet.
Both westward and eastward extents of the Lvl 2 zone are questionable. Concerning the western edge, overnight storms and earlier frontal passage may reduce the risk. Concerning the eastern edge, models simulate that the fohn will mix out the low-level moisture across parts of N Austria, reducing the CAPE significantly. Thus, it's not clear how far E eventual Alpine-hugging supercell could travel. Again, this may be refined in the later outlook.
... Corsica ...
An isolated storm or two forming over the island are possible with threat of large hail given 15 - 20 m/s of 0-6 km bulk shear and decent CAPE profile through the upper troposphere.
... Sweden ...
Both ECMWF and ICON show marginal CAPE environment with decent low-level lapse rates, low LCLs and strong low-level shear. While storms will produce barely any lightning, low-topped supercells capable of a tornado are not ruled out.
... Romania ...
Isolated to scattered storms will initiate across the Carpathians. Given moderately strong shear and decent CAPE profiles, stronger updrafts will be capable of large hail.
... Ukraine, Russia ...
A deepening wave along the front will enhance the low-level shear ahead of an advancing cold front. Combination of strong-level shear and strong mean flow will yield a noteable severe wind gust risk in linear segments. Models show mostly straight hodographs, which should reduce the tornado risk. A Lvl 2 may be considered in the later outlook for the area with the strongest low-level shear.
