It looks like a weekly periodical frontal passage has setled up across Europe, as another trough with cold front affects southern Alpine region overnight from Friday to Saturday. Similar that happened exactly a week ago. Again, a strong Mediterranean ridge with very hot airmass provides stable conditions there, while rather rich boundary layer moisture is present across Northern Mediterranean and southern Alpine region. 10-15g/kg 0-1km mixing ratios are seen from the latest surface observations across Slovenia and northern Italy.
Another broad trough crosses central Europe during the next 24hrs and the weakening frontal boundary, that currently sits on the northern side of the Alps, slowly shifts into southern Alpine region overnight and tomorrow morning. Quite strong trough/frontal forcing should provide widespread convective activity mostly overnight, while a few isolated storms are possible in the afternoon hours.
A strong jet streak accompanied by this trough will support very high deep-layer shear up to 70kts in the 0-6km. Once the trough digs more southwards, a classic Lee weak surface depression should take place over northern Italy and therefore backing wind profiles in the lowest levels are likely. Therefore enhanced LL shear and SREH can be expected, GFS predicts availability of 15-30kt wind shear in the lowest 1km while enhanced SREH values up to 300m^2/s^2 are possible.
Aforementioned rich boundary layer moisture should be enough for MLCAPE release near 1000-1500 J/kg, resulted by strong diurnal heating and moderate mid-level lapse rates in the afternoon. ML lapse rates will be slightly improving later towards the evening/overnight as trough starts to dig in.
Until the evening coverage of the storms and especially storm intiation across northern Italy and Slovenia are questionable given the only weak to moderate mid-level lapse rates as well as pretty strong capping inversion placed there during the last few days. Models do suggest there is a possibility for initiation along the LL convergence across Po valley and Scirocco winds from Adriatic and dry SW-erly winds from Apennini converging together could serve enough pre-frontal forcing for an isolated cell or two. Especially GFS model is more confident with weakening capping inversion over N-ern Italy in the afternoon, but it is often mistaken as well. There, if storms manage to initiate before the evening, the potential for well organized multicells and supercells exist, given the very strong shear and helicity overlaping with moderate instability. The main threat is large hail and damaging winds. Enhanced LL shear/SREH values and rather low LCL heights also suggest LL rotating updrafts and minor threat for tornadoes.
Later on in the evening/overnight, a rapidly clustering storms are expected forming a large MCS across the Alps moving mostly ESE-wards as strong frontal forcing will provide widespread intiation. Potential for locally large hail but mostly damaging winds exists. All models are quite confident about the overnight convective activity across the Alps.
The cold front that will be likely across Slovenia in the morning hours will then move slowly SE-wards onto the Balkan states tomorrow by day and continue to weaken. Cooler conditions are expected behind the front as well as NE-erly winds (Bora), until Monday when Mediterranean ridge with warmer/hot and stable conditions returns.
At ESTOFEX, a level-1 threat level was issued for southern Alpine region until tomorrow morning. Questionable coverage of storms before the nightime MCS limits the higher threat level, but as mentioned in the forecast discussion, southern Alpine region could be upgraded into level-2 threat levels if conditions change dramatically during the day. As given the impressive shear and moderate instability overlaping together, robust severe storms would be possible, if they manage to form earlier.
A level 1 was issued for N-Italy and W-Switzerland mainly for large hail and severe wind gusts.
… N-Italy, Slovenia …
A trough passes by north of the Alps and the attendant forcing affects the area during the forecast period. At the same time, a cold front is gradually shifting southwards. Up to 1000 J/kg MLCAPE along the S-Alps are a reasonable scenario with 35m/s DLS, so multicells and supercells are possible with a large to very large hail and severe wind gust threat, the latter one enhanced by roughly 20m/s 0-3km shear. Models still disagree in the final coverage and we stay with a level 1 for now. An update may become necessary tomorrow.