Storm Forecast
Valid: Thu 22 Jun 2023 06:00 to Fri 23 Jun 2023 06:00 UTC
Issued: Wed 21 Jun 2023 23:50
Forecaster: PUCIK
A level 3 was issued across parts of Germany and Czechia mainly for damaging wind gusts, large to very large hail and (regionally) for tornadoes and heavy rainfall.
A level 2 was issued across Switzerland, W Austria and N Italy mainly for very large hail and severe wind gusts.
A level 2 was issued across central Czechia and SW Poland mainly for damaging wind gusts and large to very large hail.
A level 1 was issued across S France mainly for heavy rainfall and large hail.
A level 1 was issued across E Austria, N Hungary, Slovakia and SE Poland mainly for large hail, heavy rainfall and severe wind gusts.
A level 1 was issued across Romania mainly for heavy rainfall and large hail.
A level 1 was issued across Bosnia and Montenegro mainly for large hail and severe wind gusts.
A level 1 was issued across E Estonia, E Latvia, E Lithuania and Belarus mainly for marginally severe wind gusts.
A level 1 was issued across Finland mainly for marginally large hail.
A level 1 was issued across E Turkey, Armenia and Azerbaijan mainly for large hail and severe wind gusts.
SYNOPSIS
A short-wave trough at the mid to upper troposphere will move from Spain towards France and Germany during the day. In response, a surface low will deepen along the waved cold front. The low will translate from France to NE Germany during the forecast period. At Thursday 12 UTC, a cold front will be located in the border area of Germany and France and a warm front will stretch from SE BENELUX through central Germany into Poland. The boundary will turn into a cold front and curve through the Baltic countries into Finland.
A plume of steep mid-tropospheric lapse rates, originating from N Africa, will have overspread the areas with abundant low-level moisture across large parts of Italy, central and southeastern Europe. This overlap will yield an unusually large area with CAPE values exceeding 3000 J/kg, even though the majority of the area will not see any initiation due to the strong capping and lack of forcing. Intense and well-organised storms are expected in the strongly sheared and unstable atmosphere ahead of the short-wave trough and along its attendant frontal system.
DISCUSSION
... Switzerland, Luxembourg, Germany into Czechia and SW Poland ...
*** An outbreak of severe to extremely severe storms is forecast across the area ***
1500 to 4000 J/kg of MLCAPE is forecast to overlap with 20 - 25 m/s of 0-6 km and 15 - 25 m/s of 0-3 km shear. At the same time, scattered to widespread storms are expected to form along either the warm front or the fast-moving cold front that will cross the whole area before Friday morning. Supercells and bow-echoes are forecast with high likelihood of both severe and extremely severe hazards.
The most likely hazard may vary regionally. The highest risk of tornadoes is forecast across W-Central Germany, north of the warm front and near the deepening low. Here, low LCLs will combine with 0-1 km bulk shear > 10 m/s and largely streamwise vorticity in the inflow to the surface-based storms based on the hodograph curvature. Convection-allowing models also simulate supercells with intense rotation and high vorticity tracks in this area. Even strong tornadoes will be possible with the most intense supercells.
The risk of large to very large hail will be the highest in the early stages of the scenario (given more isolated nature of storms) and in the areas with highest concentrations of buoyancy in the cold part of the cloud, which will be the areas more in the south of the Lvl 3 or Lvl 2. Very large hail may be the dominant risk over Switzerland, extreme N Italy or NW Austria.
Severe wind gust risk will ramp during the afternoon hours. Even along the warm front, some of the models show an upscale growth of supercells into bow-echo. The highest risk of severe winds will be more south though, where well-mixed boundary layer is forecast with LCLs up to 2 km and a high potential for strong cold pools. Given that, strong 0-3 km shear and widespread linear lift from the accelerating cold front, there is a very high probability of one or more bow-echoes developing in this setting. The most likely corridor of severe to extremely wind gusts will be SW Germany - NW Czechia.
Heavy rainfall risk will be most pronounced to the NW of the surface low, where strong frontogenetical lift will combine with some elevated CAPE.
... E Austria, Slovakia, N Hungary, SE Poland ...
Isolated to widely scattered storms are forecast to form, forced by an orographic lift. Given around 15 m/s of 0-6 km bulk shear, well organised multi or even supercells are forecast with threats of large hail, isolated severe wind gusts or heavy rainfall. Higher coverage of severe could result from the effect of the overnight or afternoon/evening convective systems and their cold pools that would travel to the area from W/NW. Such a possibility needs to be evaluated during the morning forecast and in the nowcasting setting.
... Romania ...
Scattered to widespread storms are forecast, especially over the Carpathians. Expect predominantly flash flood and large hail risk in stronger multicells.
... Belarus, E Baltics ...
A trough with an ill-defined cold front will move over the region. Scattered storms will form and possibly form one or several short linear segments along the front. Short hodographs will limit storm organisation and only skinny CAPE profiles are forecast. A marginal Lvl 1 is issued for marginally severe wind gusts as the linear segments move E, where higher LCLs may allow for a stronger cold pool and 0-1 km shear is forecast to increase locally to 10 m/s.
... Bosnia to Montenegro ...
A questionable Lvl 1 is issued for a very high CAPE environment with moderate shear. Some models produce weak convective precipitation signals, suggesting initiation over the mountains. Abundant buoyancy and high LCLs suggest large hail and downburst risks.
... Finland ...
Hodographs suggest that at least multicells will be able to form along a weak frontal boundary. Marginally large hail may occur given sufficient buoyancy in the cold part of the cloud.
... E Turkey, Armenia, Azerbaijan ...
Organised storms will form in the environment of high LCL and buoyancy concentrated in the cold part of the cloud, resulting in a threat of large hail or severe gusts. The flash flood risk will increase towards N and W, where the shear decreases along with the strength of the mean wind, rendering slower-moving storms.
