Fluctuating weather models give us a hard time

The weather models are not making it easy for us in recent days. Fluctuating model outputs from run to run mean that there is no clear picture about the upcoming snowfall. Where will most of it fall? Is there much falling at all? Is it PA-worthy? These are questions we try to answer as best we can in our weather reports today and tomorrow.

Different model outputs

First, we saw big differences in the course of the low pressure core on Saturday, which is the cause of the big snow amounts in the Alps. When the European model more or less followed the American GFS model with a more northerly course, things seemed to slowly but surely get clearer. Precipitation amounts were pushed upwards in both models and a snow dump, just for the north-west Alps, seemed imminent. However, the ensembles showed that it was not yet a done deal.

Yesterday morning, the weather models backtracked a bit. GFS became a disappointment altogether last night as the low was calculated too westerly and then drifted south, which would put the Alps on the wrong side for heavy snowfall. EC still calculated a decent layer of snow for the Northwest Alps and now more for the Eastern Alps, on both sides of the Alpine main ridge.

It just goes to show again why we are not immediately cheering when really big snow columns appear on the weather charts for the first time. We don’t want to issue a Powder Alert too early.

Upcoming snowfall

Before we do another round of models for the weekend, a word about the upcoming snowfall, which follows during the night from Thursday to Friday. As we have indicated in recent days, much of this snow will fall with a fairly high snowline in the north-west Alps. Most areas in the French Northern Alps and western Switzerland should expect a prolonged snowline of around 2,000 metres. On the outer Alpine edge, temporarily also up to 2200-2400 metres.

So I only expect the larger amounts of 20 to 30 centimetres, in the Mont-Blanc massif among others, from an altitude of 2300-2500 metres. Below that, accumulations decrease rapidly. Below 1600-1800 metres, it is only a few centimetres.

Further east, the snowline is a bit lower. In Austria, behind the Arlberg, there is less snow, but the snowline is around 1500 metres inneralpine. The precipitation follows a bit later here, during Friday morning. In the night from Thursday to Friday it will stay dry a bit longer due to föhn.

Some additional snow will follow for Austria (east of Tyrol) on Friday evening. With a northerly flow, moist air will still be pushed against the Alps. For the eastern regions like Dachstein, another 5 to 15 centimetres of snow could fall with a lower snowline below 1,000 metres.

New round of weather models for the weekend

With this knowledge from the past few days in the back of our minds, we will now take a new round of weather models. A few key points I have listed:

• EC and GFS now show the low pressure area moving southeast over Benelux. Ahead of the second snow phase, the Northern Alps temporarily re-enter a föhn flow on Saturday during the day, but the cold front quickly clears this föhn. The initial snowline is at around 1,800 to 2,000 metres, but it soon drops as the cold air moves in. Later on Saturday, the snowline drops below 1,000 metres. The northwest wind will strengthen with the cold front passage.

• GFS is slightly less generous with precipitation for Switzerland in both runs today (00z and 06z). Here it dries quickly behind the cold front. EC lets it snow a bit longer. 20 to 40 centimetres seems possible in large parts of Switzerland and the French Northern Alps. More will follow in the west afterwards (on top of that 20 to 40cm so!), as new precipitation follows in the night from Saturday to Sunday, but it is still uncertain exactly how much this will bring. The French Northern Alps have the best papers for this. More on this tomorrow.

• In the night from Saturday to Sunday, a considerable pressure difference develops between the north side and south side in the Western Alps. The French southern Alps, Piedmont and Aosta will face considerable winds.

• Perhaps the most important development: a Genoa low develops on Saturday according to most models, which from Saturday evening will also start to bring a lot of snow in the (southern) eastern Alps. The upper level flow turns south and very moist air is brought in, which is forced to rise over the cold air mass below.

• GFS calculates the most snowfall for the (eastern) Dolomites, Ost-Tirol and southern Carinthia, EC also shows more snow further north towards the Tauern and Salzburgerland, which is quite possible with such a setting. Due to the cold air present, the snowline seems to be around 500 to 1000 metres in many places.

Should these amounts of 20 to 50 cm in Austria and even more in the Dolomites be confirmed in the next 24 hours, PA#1 could just be issued for two completely different regions (northwest & southeast). We’ll keep you posted!

Loads of snow in the Pyrenees

The Pyrenees should not be forgotten in this setting either. The first big snowfall of the season will follow there. Tonight and Friday they are still in far too warm air there and the precipitation unfortunately falls as rain up to an altitude of 2,500 to locally maybe 3,000 metres. In the course of Saturday, the cold front of the low pressure core over Central-Europe will also cause a strong cooling here. Snow falls to below 1,000 metres by the evening and in the following days too, the stau from the north will continue with snow above 800 to 1,000 metres.

At least through Tuesday, the Pyrenees will continue to benefit fully from cold air due to a north to north-east flow. On a large scale, especially in the northern Pyrenees, the French side, a metre of snow could fall. Some high-res weather models make it all the way with 1.5 metres of snow and even slightly more in the western part, think for instance of areas like Gourette, Artouste, Cauterets, Formigal.