Regional dump in the southern Alps tomorrow

It is bone dry in the Alps; in many places no precipitation has fallen for two weeks with predominantly positive temperatures. Off-piste snow conditions are increasingly deteriorating. Especially in the Northern Alps, a lot of snow has melted below 2000 meters, partly due to the föhn wind. In a small part of the Alps this will change starting tonight. Afterwards, high pressure returns, but towards the end of the week the chance of precipitation in the Western and Southern Alps increases again. Today we look at which areas have a chance of a (small) dump in the coming 24 hours and how things will develop towards Christmas.

Alpes-Maritimes in the firing line

As announced for days, snow will finally return. This evening it will become cloudy in the west, and from midnight the first precipitation will reach the French Southern Alps and Piemonte and Liguria in Italy. Initially this is Südstau, caused by a depression over Spain. During the night a retour d’est sets in, bringing precipitation to the Alpes-Maritimes and the Italian inner arc.

Today I use the ECMWF precipitation chart as the main reference, because our model (based on GFS) is rather too enthusiastic about precipitation. GFS pushes the precipitation far north, with snowfall in large parts of the French and Italian Alps. Other weather models, such as ICON and ECMWF, are less optimistic and limit significant precipitation to the far south. I assume this scenario, with up to half a meter in the Alpes-Maritimes (Isola 2000, Limone Piemonte, Prato Nevoso), but only 10-30 cm in the French Southern Alps and the northern half of Piemonte. Below is a precipitation comparison between various weather models through Wednesday.

Snowline between 1000 and 1600 meters

The first precipitation falls with a snowline around 1500 meters, but thanks to isothermal cooling this drops during the night to 1000-1300 meters in areas with intense precipitation. In the rest of the Alps it remains dry, with a freezing level around 2000 meters.

Mild Tuesday with more cloud cover

On Tuesday afternoon, the intense precipitation in the south shifts eastward, and many places around and south of the main Alpine ridge become cloudy. Scattered light rain (snow above 1800 meters) may fall in the Southern Alps, with a maximum of 1–3 mm/cm. On the northern side, the Südfohn blows and it is very mild, with thaw up to about 2400 meters.

Calm and mild again on Wednesday and Thursday

The mild high-pressure weather that has dominated this month so far will return from Wednesday. The high-pressure bridge between the Azores and Eastern Europe closes again, keeping disturbances at bay. There will be broad clearings, especially above 1000 meters, and conditions remain mild with a freezing level around 2000 meters. Nights in the valleys will bring some frost.

Colder run-up to Christmas, high uncertainty

From Friday, another break seems to develop in the previously mentioned high-pressure bridge. Disturbances tend to move from the British Isles onto the continent. This is not yet very convincing, meaning mainly the Western Alps and the western Southern Alps have a chance of precipitation. This may already occur on Friday, with a snowline around 1800 meters. After that, uncertainty increases significantly.

Precipitation most likely in western parts

For illustration, above is the 15-day GFS ensemble for Grenoble and Salzburg. We see that in both the western and eastern Alps the average temperature at 1500 meters (white line) drops to around the normal level (red line). In the west, the chance of precipitation is quite high between December 20 and 25, while conditions in the east are much drier. The spread is still too large for concrete forecasts, but there is certainly potential for a more wintry weather pattern in the final week of December.