Forecast type:Day1 Outlook
 

Synoptic and convective discussion:

...Synopsis...

A broad cyclonic vortex is placed across Greenland while an expansive Bermuda High is stretching across the southern North Atlantic. Between these features a wave train, consisting of several mobile troughs embedded in westerlies, is pointed toward W Europe. A fast moving neutrally tilted short-wave trough embedded within an opening cold-core low across the Celtic Sea and NW Iberia is turning negatively-tilted and ejecting toward the Pyrenees with an attendant unseasonable strong 90+kt 500mb jet-streak centered across CNTRL-E Spain.

As a barotropic high pressure area over the Mediterranean Sea and N Africa remains quasi-stationary the aforementioned trough with several DPVA maxima creates a large-scale height falls regime across CNTRL and S Europe on Friday. As the horizontal temperature gradient along the trough weakens, the thermal wind will also weaken hand in hand with it during the day. The coupled jet-stream is forecast to split in two jet-streaks one over N Germany and one over SE France and NW Italy as another short-wave trough moves into France Friday afternoon/evening. 

This creates an area of positive upper-level ageostrophic divergence from N Italy into W Poland which will enhance the quasi-geostrophic forcing for ascent. 

At the surface, frontogenesis is expected to take place along the main baroclinic zone from S France into W Poland Friday afternoon as the mentioned UL divergence strengthens. This could further reinforce the foci for storm initiation from SE France into CNTRL Europe. 

...W France, Benelux, W Germany...

Along a northeastward shifting secondary cold front, moving into sufficiently unstable warm sector, numerous thunderstorms are forecast to form. Within an attendant mesoscale low-level short-wave trough, 900-1000J/kg MLCAPE is forecast across Benelux and NW France into W Germany given ample filtered daytime heating and abundant moisture plume characterized by 12-14g/kg LL mixing ratios.

40+kt deep-layer shear and elongated downwind vectors coupled with strong BL flow favors splitting supercells or linear segments with forward propagation capable of large hail and brief tornados as supercells and severe wind gusts as lines. This activity should start early in the morning hours given strong DPVA maxima and not much heating needed due to uncapped profiles amidst the UL cold-core low.

A vorticity rich airmass can easily be tilted into the vertical given sufficient 3CAPE. Later in the day, a comma structure can form across Germany as storm activity increases along the cold front. The entrainment given northeast stretching dry slot may be a problem but mature storms or systems affected by this feature could see stronger downdrafts as a result of these processes.

...S and CNTRL Europe...

The main thunderstorm activity should be focused along the main baroclinic zone under strong mid/upper-level flow and in more capped atmosphere. A few clusters or stratiform rain bands should move NE from the previous night amid mesoscale diabatic geopotential waves which are not reflected well in the numerical models yet. The strongest buoyancy should be confined along the confluence zone north of the Alps across S Germany and over N Italy where moisture pooling of 15g/kg sfc mixing ratios takes place yielding 2300J/kg MUCAPE and 1700J/kg MLCAPE.

Capping inversion will be present within these CAPE regimes on both side of the Alps. Over Germany given an expanding alpine EML plume and over N Italy given the close proximity of a heat dome within the barotropic ridge. Substantial uncertainties and spread exist regarding storm initiation, longevity and activity bullseye among convection-allowing guidance. Given very strong kinematics, characterized by 50+kt deep-layer shear amid the baroclinic zone and passing thunderstorm modified mesoscale waves along it, a few thunderstorms are expected to initiate over S Germany and NE Switzerland during the day. They will quickly turn supercellular capable of large to very large hail and severe wind gusts given a very strong shear and abundant CAPE within the hail-growth zone. A tornado is not ruled out. 

Another storm foci will be along the Alps across SE Germany and N Austria where one or two strong supercells are simulated to deviate east. Large to very large hail and significant damaging downbursts will be the main risks as their combination creates conditions favorable for hail storms (wind-driven hail events). The potential for further storm initiation across Bavaria, Czech Republic and N Austria will increase as the day progresses and upper-level ageostrophic divergence between coupled jet-streaks emerges and LL frontogenesis strengthens. Several supercells or clusters can form or move into Czech Republic and N Austria Friday evening. 

A trailing/ana-frontal portion of the boundary across E France will be oriented more parallel toward shear vectors for much of the day before the secondary short-wave trough axis impinges upon the W Alps from the west. This will generate conditions favorable for long-lasting CI along the W Alps where storm will have potential for training over E France and W Switzerland given interaction of the boundary with the orography. An upslope storm initiation across N Italy is possible as well but the uncertainty regarding storm initiation arises toward the Po Valley as the upper-level forcing will be nebulous given the poor timing of the approach of the ML short-wave trough.

Should storms form also just south of Milan, which some guidance simulates, they will quickly become supercellular capable of large to very large hail and severe wind gusts which could lead to an MCS event later on with the potential for a bow-echo shifting east sweeping through the Po Valley into Veneto.