I haven't seen any snow reports out of the Alps just yet, but if I do, I will post an update with that information. The first phase of this storm is mostly over outside of some lingering snow showers in the eastern Austrian Alps. Those should wrap up later this (Saturday) afternoon. 

The second storm is fast approaching and will begin to impact the western Alps on Saturday evening. The key point regarding this phase of the storm cycle remains the westward shift of the storm track over the last few days. This change will significantly decrease the snow totals in the northern/western Alps and increase the snow totals in the Pyrenees. Let's take a quick look at this shift in the track of the storm that has occurred over the last few days. 

First, we have the model run from 12/3 from the European model. The storm drops down from the northwest and crosses the western Alps, with predominantly northwest winds over most of the Alps as it does. The storm track and winds are shown in the box. 

During today's model run (12/7), the storm tracks through the eastern Pyrenees between the Alps and the Pyrenees, bringing northwest winds to western France and over the Pyrenees. You can see this happening in the box in the GIF below.

As I mentioned yesterday, this shift results in different winds for different regions and different durations of certain wind directions. This storm is producing snow primarily because of orographics, so wind direction is crucial.

The more westerly track will decrease the amount of time the Alps receive northwest winds and lead to more time with southwest and northwest winds. Northwest winds are ideal for the western and northern Alps, while southwest and northeast winds are not favorable.

The additional time with southwest winds will benefit the central and southern French Alps, however, so I do expect more snow there than with the storm tracking farther east. Northwest winds aren't best for the central and southern French Alps, but parts of the central French Alps do alright with that wind direction. 

However, the area that benefits most from this shift in the track is the Pyrenees, as the length of time with favorable north/northwest winds increases considerably. With the more easterly storm track, the winds were staying out of the west-northwest longer, which is less favorable for the northern side of the Pyrenees. 

Finally, the evolution of the upper-level low after it moves south of the Alps has changed as well. The latest guidance takes this low farther south and has it stall just off the coast of Italy. The moist east/northeast winds that result will produce additional snowfall in the eastern side of the southern Alps and the northern Appenines. 

This will be similar to the Retour d'est storm type that is common for the Alps, although the location where this upper-level low stalls will benefit the Apennines even more than the southern Alps. The counterclockwise flow around the low will push moist air from the warm Adriatic Sea across the Po Valley in Italy into the northern Apennines and the southern Alps. You can see this flow around the low, across the Adriatic, and into the Apennines and southern Alps in the GIF below. 

Below is the latest snow forecast from the high-resolution WRF model. The new regions of greatest snowfall as a result of the enhanced orogrpahics are shown in the boxes.

The additional snow from this system will be 25 - 50 cm for much of the northern/western Alps from the northern French Alps through the western Austrian Alps. The part of the French Alps that gets some benefit from southwest, west, and northwest winds could see a bit more, in the 40 - 80 cm range.

As a result of the storm tracking a bit farther south and stalling, the southern and eastern Alps totals have gone up. At least 15 cm are now expected for all of the eastern and southern-facing parts of the Alps, with 25 - 50 cm possible for the north/east-facing parts of the southern Italian Alps (south/west of Cuneo) as well as the south-facing Italian Alps north of Brescia and especially Verona.

The northern Apennines, closest to the moisture source and with the fewest topographical barriers between them and the moisture source, will see 40 - 80 cm, with the higher elevations approaching 1m.

The forecast for the Pyrenees continues to look very deep, with over 75 cm possible on the northern side of the western part of the range and some favored locations surpassing 1m.

The models are starting to pick up on the possibility of another round of moderate to heavy snow for the southern Alps and northern Apennines as this cut-off low starts to make its way east on Friday. The next chance for snow after that should be around the 18th as another solid storm from the west is becoming more likely. 

My next post will be on Sunday.

Luke Stone
Forecaster, OpenSnow

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