+++ A severe thunderstorm outbreak is expected across parts of Po valley across the Adriatic sea into W Croatia this afternoon and tonight. Intense supercells with very large hail, severe damaging winds and tornadoes are likely! +++
A large long-wave upper trough is centered over W Russia and E Europe with a sharp short-wave pushed across the Alps and the N Mediterranean, while a shallow upper ridge is located over the S Mediterranean. In between, a very robust jet stream is providing both large-scale forcing and strong shear for widespread convective activity. To the west, a weakening upper low is centered over Iberia while a ridge builds up over W Europe.
HIGH risk has been issued for Emilia-Romagna, S Veneto and N Marche regions (Italy) across the Adriatic sea into the S Kvarner and N Dalmatia regions (Croatia) with threat for severe storms, capable of producing severe damaging winds, very large hail, tornadoes and torrential / excessive rainfall. A very dangerous setup is shaping up on the models with strong to extreme instability (MLCAPE of 2000-3500 J/kg) overlapped with both strong (50-60 knots) shear and strongly enhanced helicity (200-400 m^2/s^2 of 0-3 km SR helicity). Storm initiation is expected off the Apennines by early to mid-afternoon hours with cells rapidly forming into discrete supercell storms. Strong low-level E-SE moist inflow from the Adriatic region will result in enhanced SR helicity, favorable for tornadic storms as well. Together with very large hail (5-8 cm in diameter) and severe damaging winds as bowing segments are possible with some storms. High PWAT values (42-44 mm) should also lead to locally very intense rainfall and therefore flash floods. Storms will gradually cluster towards the evening hours when a large MCS is expected over the Adriatic sea, moving ESE towards the Croatian coast of Adriatic sea. Both very large hail, extremely severe damaging winds with local gusts in excess of 120 km/h are possible along the leading line of intense cells within the large cluster. Tornado threat cannot be ruled out either, given the maintaining very strong helicity ahead of the storms along the Adriatic coast of N Dalmatia.
MDT / ENH / SLGT risks have been issued for areas surrounding the MDT risk including parts of N-CNTRL Italy, Croatia and Bosnia with threat for severe storms, capable of producing severe winds, large hail and torrential rainfall. While the eastern parts of MDT / ENH risk area are placed where the potentially robust MCS with damaging winds, flash floods and large hail threat will travel overnight, the western parts will see scattered supercell storms under moderately strong instability and strong shear. ENH risk has also been expanded across S France and NE Spain where a few large hail and severe wind events are likely as well.
ENH / SLGT risks have been issued for parts of W Russia with threat for severe storms, capable of producing severe winds, large hail, torrential rainfall and tornadoes. Storms are expected to form across the warm sector under moderate instability and shear with relatively strong helicity near the center of the surface low. Supercells are expected, supporting some tornado and very large hail threat as well.
SLGT risk has been issued for NE Turkey into Georgia with threat for severe storms, capable of producing severe winds, torrential rainfall and large hail under moderate strong shear and instability.
TSTM risk areas have been placed where convective storms are likely to occur but should remain sub-severe.