Forecast type:Day2 Outlook

Synoptic and convective discussion:

...Synopsis...

Large-scale mid/upper-level ridging has started to build into CNTRL and N Europe by Thursday. Abundant along-flow ageostrophic wind convergence across NE Europe, in between a large-scale ridge over W-CNTRL-N Europe and a trough situated from N Russia into SE Europe, result in a broad belt of surface anticyclogenesis (with 2 distinct cores) stretching from central Europe further eastward into W-CNTRL Russia. Due to the aforementioned upper-level QG convergence a 1025hPa primary surface anticyclone is forecast to develop into the Baltics and Belarus by Friday. 

Further west - into the western flank of the upper-level high pressure ridge - a relatively sharp baroclinic zone elongates from the NE Atlantic Ocean into NW Europe. Somewhat sharper gradient of the thickness field result in 40+kt jet-stream stretching along an eastern flank of a deep quasi-stationary shortwave trough placed over the central North Atlantic.

At the surface, an expansive pool of high theta-e airmass overlapped by steep low to mid-level lapse rates spreads across N France, Benelux into Germany by Friday. This airmass is expected to become strongly buoyant as diurnal heating peakes Friday afternoon...

... N France, Benelux, Germany and Poland ...

A subtle mesoscale shortwave trough is forecast to swing through France into Benelux Friday morning and midday with a sharp dryline centered from west to east across central France with 8g/kg mixing ratio differences. As daytime heating starts to destabilize the moist side the dryline will mix northward toward Paris, Chaumont and Epinal. Further north into the pristine recovered airmass, strong moistening by evapotranspiration and interception and moisture pooling will increase surface mixing ratios up to 18-20g/kg with 10-15g/kg mixed-layer mixing ratios. This will eventually yield to a very strong buoyancy in the order of 2000-4000J/kg MUCAPE. 

Steep low/mid-level lapse rates 7-8°C/km coupled with such favorable moisture quality guarantees vigorous storm updrafts.

Following fine-meshed solutions, there are four prominent areas for CI.

The first across CNTRL-W Germany during midday/evening - linked to the local orography and convergence zones-, the second across the W Alps, the third along a remnant MCV or an outflow boundaty feature noving from W Germany into Poland and finally, and probably the strongest and also the most uncertain storm system, from NW France into Benelux moving NE probably into NW Germany during the evening and night.   

Given a relatively high mid/upper-level moisture content, initially intense water-loaded downdrafts will be the main hazard with strong cores. Due to relatively weak deep-layer shear the storm organization and propagation seems to be strongly dependant on cold-pools and storm crowding effects. Epecially across the Alps fast amalgamation of various thunderstorms will lead to a northward surging cold-pool with eventual CI over S Germany. Strong wind gusts and intense precipitation will be the main risk.

Along a possible MCV/OB across W Germany and W/S Poland a few outflow dominant supercells moving southeast into S Poland capable of large hail and damaging winds are plausible given perhaps an augmented 0-3km bulk shear. A small bowing segment with swath of damaging winds later-on is not ruled out.

It is necessary to note that this scenario will be strongly dependant on a magnitude of nighttime convection moving east across N Germany at night and can be completely abolitionared if it fails to happen. 

Strong downbursts may also be accompanied by thunderstorms forming over W-CNTRL Germany during peak heating given well-mixed boundary layer and inverted-V profiles. A weak mean-wind will result in slow storm motion vectors which elevates the potential for training and one or two flashflood events. Eventual cell interaction could lead into small systems propagating erratically.

Across NW France into Benelux an uncertain scenario regarding CI will unfold Friday evening and night. A substantial spread within convection-allowing model solutions are caused probably by a diffuse nature of the forcing for ascent. As the shortwave trough approaches the highly unstable airmass, augmented LL convergence along the coastline or perhaps sea breeze fronts and also the dryline boundary all of these mesoscale features can be the source for an explosive storm initiation.  For instance, a preceding ICON D2 solution has a strong cold-pool-driven MCS across Benelux propagating NE which was eventually undercut by strong ouflow surges over E Benelux and W Germany.

Despite everything a slight risk was added to account for a development of severe thunderstorms capable of damaging winds should the cold pool propagates into the highly unstable airmass. Weak 0-3km bulk shear and the nighttime BL stabilization will probably yield to an elevated nature and weakening of the storms as the outflow surge undercuts the updrafts failing to form on its leading edge.

... N Spain and N Algeria ...

A similar high-based nature of thunderstorms across Spain and Algeria is expected. Due to steep mid-level lapse rates and well-mixed dry adiabatic boundary layer, outflow dominant thunderstorms are forecast to initiate through a mix of sea breeze circulations and orography across Tunisia and an orography forced circulation across N Spain.

Isolated damaging winds and an instance or two of large hail are all possible.