Lots of snow just before Christmas

A significant amount of snow is coming just before Christmas, with PA#2 approaching, but the snow line remains a point of concern. However, the latest model calculations seem much more positive. Let’s take a closer look.

Snowfall returns on Thursday

The next few days will remain mild, with the freezing level between 2500 to 3000 meters across much of the Alps. Thursday will bring slightly warmer conditions, especially in the Northern Alps, where a föhn wind will bring temperatures up to 10°C at 1500 meters. In the Western Alps, the first precipitation will begin to fall in the morning with a fairly high snowline (1800-2000m), which will then move eastward. By the evening, a cold front will approach, quickly bringing cooler temperatures. The snowline will drop to below 1000 meters by Friday morning/afternoon.

The heaviest snowfall is still forecast between the Écrins and Western Switzerland. Up to 15 to 30 cm of snow can fall at higher altitudes by Friday morning, and locally, up to 40 cm can accumulate from Écrins through Belledonne to Mont Blanc. Further east, snowfall amounts will decrease, with 10 to 20 cm up to Vorarlberg and even less to the east. In the Southern Alps, both French and Italian regions will see a significant increase in wind due to the inflow of cold air. Lift closures are possible here, and wind slabs will become a bigger issue, especially along the Alpine main ridge.

A sunny and cold start on Saturday

As I mentioned last Sunday, Saturday seems like a great day to enjoy the snowfall. On Friday night into Saturday, the skies will clear widely from the west. Moderate to severe frost is possible, with temperatures dropping below -20°C in inner-Alpine regions such as Engadin.

Saturday will start cold and sunny, but cloud cover will soon move in from the west, bringing milder air. A second (and more intense) snowfall phase is on the way!

Snow or rain?

There is some uncertainty in the recent model outputs regarding the precipitation just before Christmas. I’ll try to explain this using a new map. Over the next few days, Chris and I will dive deeper into the snowfall amounts and the latest developments regarding this weather event. PA#2 is approaching!

The map below (from last night) shows the temperature differences at 1500 meters and the large-scale pressure distribution between the two weather models, GFS (blue) and ECMWF (red). A key note is that these are the main model runs, and any uncertainty seen in the ensemble models is not shown here.

As of last night: The pressure distribution shows that the ECMWF model predicts a deeper trough with a north-south oriented trough axis, which leads to the flow turning more to the north compared to the GFS model. In this case, cold air can flow with more ease towards the Northern Alps, and the risk of rising temperatures from the west is limited. This is reflected in the temperatures, which are 5 to 10 degrees lower in the ECMWF model compared to the same GFS calculation at that time.

The differences with GFS are clear. This model shows the trough further east, with the trough axis slightly tilted towards the northwest-southeast. Cold air masses are quickly pushed further east, and warmer air can flow in behind with winds from the west. In this scenario, the snowline could temporarily be between 1600-1800 meters.

As of this morning: Mostly positive news! In the latest calculations, GFS is increasingly following the European model, so the high snowline seems less problematic. It will only warm up quickly after the snowfall, and rain at higher altitudes looks to be limited to Sunday. This rain at elevations up to 1500-1800 metres will quickly turn to snow as the cold air moves in, and a strong northwestern flow will bring substantial snowfall all the way down to many valleys.

Accumulations of up to one meter of snow are expected, so it looks like a great PowderAlert for Christmas. During the Christmas days, a strong high-pressure system will take over, bringing rapid warming from the west, but it will remain dry. More information on these two snowfall phases tomorrow!