The poor base in a large part of the Northern Alps will at least temporarily be a thing of the past, because a nice pack of snow will arrive from tonight. Still, my attention is already going to the days after Christmas, because the developments for the Southern Alps are even more interesting. Up to a meter of snow is possible for the eastern Southern Alps, but the other regions can also look forward to a nice dump. Anyway, it is certain that we will end the bizarre year 2020 with a colorful forecast map. Let's hope that this may be a prelude to the new year in which we can all fully enjoy this southern Alps winter in the making.
Today is the last day with temperatures well above average for the Alps. The Northern Alps in particular have to deal with temperatures of 8 to even 10 degrees above average. In addition, it remains gray with quite a bit of clouds, but fortunately it is dry. Yesterday, temperatures of around 15 degrees were already measured on the north side on a large scale and today these temperatures will again be possible in the entire Northern Alps.
In the course of tonight, the first snowfall from the northwest reaches the Alps. With about 10 centimeters on the Swiss and French north sides, this first snowfall isn't going to bring the motherload, but it is a prelude to the more intensive snowfall that will follow during the course of the day tomorrow. The snow line is initially still above 1500 meters, but drops steadily during the first snowfall.
Tomorrow the sun will temporarily come out in the course of the morning and afternoon between the two snow fronts. For a short period, the sun can shine through between the two precipitation fronts on the north side, but in the afternoon the second front quickly follows. Moist air masses of arctic origin flow out over the Alps at the passage of the cold front, causing the amount of precipitation to increase considerably and it will snow into the lowest valleys.
From the French Northern Alps to the Arlberg, about 20 to 50 centimeters of fresh snow will fall until Saturday, taking into account that the inner alpine areas will receive significantly less. Further to the east, the Salzburgerland in particular seems to benefit well with quantities of up to 30 centimeters. The Austrian areas close to the main alpine ridge (with the exception of the Hohe Tauern) will not reach these quantities. Here I expect a maximum of about 10 to a maximum of 20 centimeters of snow until Saturday.
The Southern Alps will also benefit from the snowfall from the northwest, but this will not result in more than 10 centimeters in Lombardy and Adamello-Brenta and around 20 centimeters in the Julian Alps. For the rest, it's mainly a cosmetic layer of snow. The real work for the Southern Alps will follow after Christmas.
Due to the tilt of the high-pressure area over the Atlantic Ocean, the high-pressure area expands across Western Europe and the current temporarily becomes more zonal (from west to east), causing the snowfall to slowly decrease in intensity and the temperatures to rise again. The sun comes out from the west on Saturday, but Austria in particular may have to deal with snowfall a bit longer on Boxing Day. Much more than a few centimeters is no longer possible on this Saturday.
On Sunday, the Eastern Alps will also temporarily enter calmer waters. The sun comes out to shine in a lot of places and with the fresh snow it becomes a bluebird picture perfect day. The Western Alps soon have to deal with clouds from the west, because there is much more snow on the way!
The aforementioned tilt ensures that the low-pressure area above Iceland can descend towards Great Britain and France on Saturday and Sunday, which could possibly cause a lot of snow in the Southern Alps. Under the influence of this low-pressure area, a southerly current sets in during the course of Sunday that transports moist air from the Mediterranean to the Southern Alps. Both the French Southern Alps and Italy will get hammered, but also the areas in the Northern Alps that are close to the main alpine ridge and that picked up less snow during Christmas (e.g. the Stubai and Ötztal Alps) will also benefit from this snowfall from the south. .
Both models have been hinting at a Southern Stau for a number of days and gradually we can go into more and more detail, but there are still differences between the models, but also between the individual runs. While yesterday the GFS also calculated a lot of precipitation more westerly (French Southern Alps & Ticino) (about 80 to 120 mm), today the model shows a somewhat more easterly course for the center of gravity of the precipitation. The Dolomites and Julian Alps can get more than a meter of snow according to the American model, but further west the quantities decrease somewhat. I must add that we are still talking about large amounts of 40 to possibly 80 centimeters of snow for Lombardy and the French Southern Alps. The ECMWF shows 80 to 100 mm of precipitation from Engadin to the Julian Alps. Similar to the American model, the French Southern Alps can expect 50 to 80 mm of precipitation.
At the same time, the pressure gradient between the Northern and Southern Alps is increasing considerably, as a result of which the Northern Alps will have to temporarily say goodbye to the deep winter conditions due to the incoming Südföhn, but due to the influx of cooler air from the west, it is also possible that it starts snowing here from the south in the course of Monday. Exactly how much is still too uncertain and strongly depends on the moment when the Föhn situation 'collapses'.
Anyway, merry Christmas everyone. Let's hope that we can all enjoy the powder again soon.
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