Forecast type:

- convective outlook

Dynamic and Thermodynamic analyse:      

... SYNOPSIS ...

A swiftly shifting pattern has evolved across Europe and the NE Atlantic. A gradually weakening strong ridging over the CNTRL Mediterranean is an indisputable precursor of a short future troughing from the north. A strong baroclinic instability prevailing in-between a very hot and humid subtropical airmass located over the Mediterranean and N Africa, the Iberian Peninsula and the Balkans and a cooler airmass spread out across N Europe into the Baltics. Initial somewhat calm westerlies across W-CNTRL-E Europe start to break-down into a longwave trough over the British Isles and W Europe in response to a southeastward oriented strong geostrophic channel upstream from the trough axis. The leading shortwave trough will cross the British Isles by Sunday morning (6Z) and slow down over France afterwards (17Z). Another small scale wave emerges along/behind the Alps as a stronger SWerly flow impinges the mountain range downstream. A substantial amplification is expected at the end of the forecast period given strong energy flux/group velocity within a secondary shortwave trough approaching from NE and crossing Ireland Sunday night. As this wave catches-up with the leading shortwave trough a positive interference/wave superposition is forecast driving the amplification process. A large quasi-stationary Rossby wave is forecast to stall across a large part of Europe next week bringing cooler and unsettled weather there. 

Inspecting the isentropic potential vorticity field a textbook wave evolution is visible within this chart with multiple filaments trying to overturn and cut-off from their source which will eventually lead into a large scale cut-off cyclone weather system over Europe as another strong energy flux emerges from the Atlantic and ridging will overspread W and N Europe. 

At the surface a complex frontal zone will start to stretch more SW-NE across W/CNTRL Europe as troughing reaches the Alps. Strong frontogenetic forcing is forecast from E France across the Alps into CNTRL Europe. Warm air advection downstream the trough axis will rise heights across E Europe with temporal ridging across E Europe/W Russia shifting NE. A baroclinic wave is expected to consolidate just behing the Alps Sunday afternoon within a low-level large stationary trough. This wave is forecast to move northeast into Poland and Belarus Monday early morning while a cold front from this disturbance will push east through Poland, Slovakia, Hungary with its trailing portion into the Balkans where becomes undulated and a baroclinic zone. 

... DISCUSSION ...

.. N Italy, the Balkans and central Europe ..

The center of the severe thunderstorm activity for Sunday will be located within the Alps and their adjacent areas given ample cyclogenetic forcing/convergence within this region. As the shortwave trough approaches from NW the flow at its eastern portion induces warm advection far into central and eastern Europe. This scenario will induce low-level height falls and 0-3km SSWerly flow strengthening up to 17m/s. An impressive 700mb negatively tilted trough will form over the E Alps with a local gradient wind maxima. They can act as sources for lift especially as they impinge the mountains creating an upslope flow.

Strong pressure falls across N Italy from the morning hours will induce moisture return from the Adriatic and Ligurian Sea leading to adequate moisture pooling with mixing ratios up to 14g/kg. Local moisture maxima will also be located along the frontal structure especially in E Austria and SW Slovakia. 

Given ample forcing for ascent, at the hand of frontal/trough convergence and strong surface heating which will yield to strong buoyancy, multiple severe thunderstorms are forecast within N Italy, S Austria, N Croatia and Slovenia. As the overall background flow and bulk wind difference vectors turn more paralell along the front, thus the overall storm mode looks to be more widespread and clustery. The rate of destabilization could be very high as unimpeded heating recovers the moist airmass in the order of 1200-2000J/kg MLCAPE across N Italy.

Current thinking is that discrete storms will start to form around noon across the Alps where many of them will organize into supercells capable of very large hail and severe downbursts (wind-driven hail) given strengthening of the mid-level flow up to 25m/s leading to a 15-20m/s deep layer shear. As the height falls gradually augments low-level convergence across W Hungary, E Austria and N Italy across the Po Valley and the Adriatic coastline scattered initiation is also expected here. Initial discrete supercells capable of all severe including a few tornado events close to the coastline, due to backed winds within the S region of the lee trough, are possible. 

As mentioned above at the hand of strong forcing for ascent and positive thermodynamics, multiple storm interactions will lead to an upscale growth into multiple mesoscale convective systems. One is expected to form along Como, Milano and Pavia vicinity moving E into Bergamo, Brescia and Trento later on. Embedded supercell can produce downbursts leading to bowing segments and widespread severe straight line winds. Another system can form within a dryline circulation from the N Apennine Mountains. Thunderstorms will initiate along this zone and move into lowlands where they will propagate forward along a leading gust front into Padova, Venice and Treviso.

Strong wind gusts and large hail will be the main risk next to heavy precipitation. In the eastern portion of the height falls scattered thunderstorm development is foreseen within W Hungary and Slovenia as well as E Austria within a tad lower instability and moisture. Due to the strong low/mid-level flow clustering into one or two MCSes are certainly propable here. Higher dew point - temperature spread within somewhat mixed boundary layer will augment the evaporative cooling of downdrafts yielding to quick cold pool development and subsequent wind-driven initiation. 

Inverted-V profiles where 10°C wet-bulb temperature gradient is located will nose far NE into SW Slovakia. This infers that storms developed south and organized into an MCS(es) can continue to propagate NE into Slovakia and even S Poland given still present MUCAPE in the order of a few hundreds of J/kg. Strong wind gusts and excessive precipitation will be the main risk. As the cold front finally crosses the Alps during the night and early Monday morning, a secondary round of storms is expected within still moist and converging airmass along the Po Valley and NE Italy into SE Austria.

Multiple embedded supercells are forecast to ride the quasi-stationary boundary from Treviso into Klagenfurt and grow upscale into a squall line afterwards along Slovenia as it pushes east Monday morning. Isolated severe wind gusts, large hail and heavy rain will be the main risk next to an isolated tornado event.