Forecast type:

-  experimental

Dynamic and Thermodynamic analyses:      

...   This is an experimental and somewhat unique product covering expected unsettled winter weather conditions across western and central Europe for this time of year including both the mountain and lowland regions.

... Overview ..

A large scale system/Rossby wave is established across the whole continent with a strong jet stream rounding its periphery with an impressive thickness gradient and quasi geostrophic forcing. Multiple short waves are pronounced in low levels within the main trough but the most important one will be a LL impulse - now forming across the N Atlantic Ocean - with a dramatically looking deformation zone and shearing contributing to a sharpening of a horizontal temperature gradient and strengthening of the thermal wind.

An upper-level jet streak 80m/s upstream from the base of the longwave trough across S EU is elongated from CNTRL France into the British Isles with a decaying tendency which is a response to a further tightening of a sharp density gradient between SW and NW Europe. Similar strengthening will be pronounced upon the LL baroclinic wave with significant differential temperature and vorticity advection which will lead to a strong LL mass response and mid-level height falls across the English Channel by forming a synoptic frontal low by Thursday forenoon.

As a bullseye of the dynamic tropopause emerges and this LL wave slides under the forming, positive upper-level jet streak dynamics - working in an impressive synergy -  further deepening is forecast with central MSLP expected to drop around 980hPa Thursday afternoon across western France. Ample veering wind with altitude within the warm sector of the low and closely packed isentropes within the strong geostrophic momentum advection creates an isentropic ascent of parcels with high energy values above the cold air at the surface which will lead to a TROWAL situation. Rich fresh snowfall situation will potentially cause some problems from W into CNTRL France toward the Alps. High snowfall rates are expected across the mountainous regions especially W/SW facing slopes of the Alps.

As the air within the TROWAL contains an ample moisture content, heavy and large snowflakes are expected to fall and in cause of windy conditions there is a high potential for snow drifts and the windblown snow on the opposite sides of the mountain peaks which also creates an avalanche risk especially there where the snow cover is not consolidated and is pre-melted by previous warmer conditions. As the low pressure system interacts with The Alps (isobars facing with a strong parallel component toward the mountain range) that will lead to a increased friction and deformation of the warm sector. A coherent wave will thereafter form on the southern side of the mountain range with strong Ponente wind across the Ligurian Sea.

A subsequent warm sector formation is then expected upon the CNTRL Mediterranean region with a theta-e tongue stretching from the Adriatic sea farther into Hungary and Slovakia by Friday morning within an effective warm front, but it wont be that steep and well-structured as the one performing throughout France during Thursday. The same as in the aforementioned case ... an impressive moisture content within the dendritic growth zone is forecast with ample veering and omega values. Heavy snowflakes in both lowland and mountains areas with larger snow accumulation along windward mountain slopes are probable given strong isentropic ascent of higher theta-e parcels with trajectories originated in the Adriatic sea region (S Slovakia, SE Alps). Potential serious disruption to the traffic is considerably likely.