Storm Forecast
Valid: Sat 26 Aug 2023 06:00 to Sun 27 Aug 2023 06:00 UTC
Issued: Fri 25 Aug 2023 22:37
Forecaster: PUCIK/KUZMENKO
A level 3 was issued for Catalonia and extreme NE Spain mainly for very large hail, damaging wind gusts, heavy rainfall and tornadoes.
A level 3 was issued across NW Italy mainly for very large hail, damaging wind gusts, heavy rainfall and to the lesser degree for tornadoes.
A level 3 was issued across SE Germany and NW Austria mainly for damaging wind gusts, large to very large hail and to the lesser extent for heavy rainfall and tornadoes.
A level 2 was issued across SE France mainly for heavy rainfall, large hail and to the lesser extent for tornadoes and severe wind gusts.
A level 2 was issued across S and E Czechia mainly for heavy rainfall, severe wind gusts and large hail.
A level 2 was issued across S Poland and N/NW Slovakia mainly for large to very large hail, severe wind gusts and heavy rainfall.
A level 1 was issued across Poland, Lithuania, W Belarus, NW Ukraine mainly for heavy rainfall, large hail and isolated severe wind gusts.
A level 1 was issued across Greece, Northern Macedonia, S Albania mainly for large hail and heavy rainfall.
A level 1 was issued across SW Turkey mainly for large hail and severe wind gusts.
SYNOPSIS
At mid to upper troposphere, main synoptic-scale feature will remain to be a deep cyclone centered over the UK. On its forward flank, strong flow is forecast, exceeding 25 m/s at 500 hPa. Base of the trough will rotate across Iberia towards the Mediterranean Sea, providing strong synoptic-scale lift over the region. Towards the surface, a pronounced wavy cold front will run from Spain through SE France, the Alpine Region into Czechia, Poland and the Baltic Sea. A warm front will run from Estonia towards W Russia. The frontal boundary will lift NE-wards during the day. Widespread thunderstorms, including isolated to scattered severe storms, are forecast along and ahead of the cold front, where abundant low-level moisture is present. Further storms are forecast under a cut-off low centered over S Greece.
DISCUSSION
*** An outbreak of severe thunderstorms is expected across a large area from E Spain to Poland. Catalonia, NW Italy and SE Germany/ NW Austria will be under the highest probability of extremely severe thunderstorms ***
... NE Iberia, Balearic Sea ...
MLCAPE values well above 2000 J/kg and 0-6 km/0-3 km bulk shear above 25/20 m/s will create conditions very favorable for intense and well-organised storms, including bow-echoes and supercells. The first storms will be initiated by orographic lift and coverage is forecast to increase rapidly towards the evening and night hour as the cold front and the short-wave trough approach. All severe weather hazards will be possible, very large hail and damaging wind gusts perhaps the most pronounced ones. Some models show window of opportunity for tornadoes with increasing low-level shear and storm-relative helicity towards the evening hours along the coastline before the storms become elevated. During the nighttime, the dominant type of severe weather threat will shift more towards the heavy rainfall. Severe storms will then spread SE-wards, possibly reaching the Balearic Islands with very large hail and damaging wind hazards.
... SE France ...
Multiple rounds of (severe) thunderstorms are forecast across the region. During the afternoon hours, more isolated storms, including supercells will be possible across the area, posing threats of large hail and severe wind gusts. Convective coverage will increase rapidly towards the night as the cold front crosses the area. Substantial MUCAPE will be present inland and combined with moist profiles will lead to high rainfall rates. Persistent and rainfall efficient storms may locally produce very high rainfall amounts, even well above 100 mm. Tornado threat will be present along the coastline, where low LCLs will combine with 0-1 km shear exceeding 10 m/s, curved hodographs and at least some SBCAPE.
... N Italy ...
Across NW Italy, models unanimously agree on MLCAPE exceeding 2000 J/kg, combined with 0-6 km bulk shear above 25 m/s. Multiple intense supercells are forecast to initiate across the W Alps by high-resolution models and move NE-wards across the lowlands with risks of very large hail and severe wind gusts. An upscale growth is likely towards the evening hours, especially near the mountains, with an increase in damaging wind gusts and heavy rainfall risk. An increase in low-level shear is forecast towards the evening hours, yielding a tornado threat before the storms become elevated.
... SW Germany, N Austria, Czechia ...
High-resolution models agree on the development of intense supercells along the N Alpine rim in the environment of MLCAPE around 1500 J/kg and 0-6 km shear up to 30 m/s. These supercells will pose threats of very large hail and damaging wind gusts. It's likely that the storms will grow upscale as they move E towards Austria and Czechia, increasing the risk s of severe wind gusts and heavy rainfall even further. Between 15 and 18 UTC, models simulate an increase in the low-level shear and hodograph curvature across N Austria, increasing the risks of tornado, especially if isolated supercells will still be present at that time. After that, the storms will move further NE into Czechia and S Poland, where higher LCLs will allow for maintenance of severe wind gust risk despite substantially weaker CAPE compared to N Austria and SW Germany, at least initially. Heavy rainfall is also likely locally with abundant elevated storms forming just W of the frontal boundary. Elevated storms, including supercells, will be possible through the night, posing the risks of large hail and perhaps isolated severe wind gusts as the MUCAPE is forecast to increase across the extreme E Czechia.
... Slovakia, Poland ...
The highest severe weather risk is forecast in the afternoon to late afternoon hours with storms forming across the Carpathians and spreading into the lowlands of S Poland. Well-organised multi or supercells are forecast with threats of (very) large hail and isolated severe wind gusts. At the same time, isolated to scattered storms are also possible further N and NW. However, these storms will form in lower CAPE-shear overlap and will pose lesser risk. Further storms are expected in the evening and overnight, likely connected to large convective system arriving from Austria and Czechia. Severe wind gusts, heavy precipitation and also isolated large hail threats will persist through the night.
... Greece, Northern Macedonia, S Albania ...
Isolated to scattered storms will be initiated by orography during the early afternoon hours. Substantial CAPE below -10 deg C isotherm along with 0-6 km bulk shear around 15 m/s point to the threat of large hail across the area. Slow-moving storms may also locally pose threat of heavy rainfall.
... W Turkey ...
Isolated storms are forecast to form in the afternoon with threats of large hail, due to the substantial CAPE in the cold part of the cloud and severe wind gusts, due to the Inverted-V profiles.
