A warm front from a low over the Bay of Biscay will reach us from the west on Thursday night, bringing widespread snow in the Swiss Plateau, especially in German-speaking Switzerland, and thus an onset of winter. Before the snowfall turns to rain, we can expect around 5 to 20 centimetres of fresh snow. On Friday, the low pressure system will move to northern Italy and bring further precipitation in the Swiss Plateau, mostly in the form of rain. During the night to Saturday, cold air will flow in again, causing the snow line to drop to low altitudes. Further large amounts of fresh snow are expected, especially in the Alps and particularly in Graubünden.
Interim assessment of fresh snow on Thursday afternoon
As announced, the snow began to fall from the west last night. While the snow line rose rapidly in the southwest (it is also green here at the moment, see the example of Lausanne in Fig. 1), it was still snowing down to low altitudes in many places in the very north of Switzerland - from Aargau to Lake Constance - until midday. However, the snow line will soon rise to over 1000 meters here too.
Fig. 1: There is no snow around Lake Geneva, where the snow line has risen rapidly.; Source: Roundshot-Cam
The most snow has fallen in the western Alps since midnight. Up to 50 cm locally in the lower Valais, but only at altitudes above around 1000 meters. At the bottom of the valley there was little or only a few centimetres of snow. 10 to locally 20 cm also fell in places in the Jura and in the western foothills of the Alps. There was also enough snow cover in many places on the Central Plateau, where a few centimetres fell in many places, but usually less than 10 cm. However, there were a few exceptions where more than 10 cm of snow fell, mainly from the canton of Fribourg to Bern, Solothurn and Aargau.
Fig. 2: A few centimetres of snow over the Federal Palace in Bern.; Source: Roundshot-Cam
Fig. 3: Winter landscape in Schupfart with around 10 cm of fresh snow.; Source: Roundshot-Cam
The following map shows the analyzed amount of fresh snow up to 11:00 a.m. today. The values on the map (Fig. 4) do not correspond to the current snow depth; some of the fresh snow has already collapsed or melted again (see Fig. 5).
Fig. 4: Fresh snow until Thursday noon 11:00 am; Source: MeteoNews / UBIMET
Fig. 5: Snow depth at 11:00 am; Source: MeteoNews / UBIMET
Large amounts of fresh snow will continue to fall in the mountains until Saturday, more on this in the latest MeteoBlog.
Initial situation and further development at a glance
After a fairly sunny day today (Wednesday), a warm front from a low over the Bay of Biscay will reach us tomorrow night (Thursday), bringing widespread snowfall. During the day, an air mass boundary will form over Switzerland, which will slowly make it milder at higher altitudes. The air mass boundary will remain in place on Friday, while the aforementioned low pressure system moves into the Mediterranean region. As a result, the south-eastern parts of the country will experience a counter-current, which will bring a lot of fresh snow, especially in the mountains. On Saturday, the low pressure system will move further to the north-east, but on its backside, cold and initially quite humid air will flow towards us over the weekend, which will accumulate in the Alps, bringing some more fresh snow to the valleys. On Sunday, an intermediate high will take over and the snowy spell will be over everywhere.
Some fresh snow in the north by Thursday morning
Before midnight tomorrow night, snowfall will begin in the west as part of the warm front mentioned above, which will then spread all the way to the east by the morning. On Lake Geneva, however, the snowfall will change to rain by the morning. By daybreak, we expect around 2 to 4 centimetres of fresh snow in the Swiss Plateau in the east and around 4 to 7 centimetres in the central and western Swiss Plateau. There will be significantly more snow in the Jura and the western Alps, including the Valais mountains (see Fig. 1).
Fig. 1: Expected amount of fresh snow until Thursday morning; Source: MeteoNews, UBIMET
This fresh snow in the Swiss Plateau should be enough to make the roads partially snow-covered and therefore slippery, causing major traffic obstructions in the morning rush hour. In any case, it is advisable to plan more time for your commute tomorrow and to use public transport if possible.
First snow during the day, then rain from the west
During the day, it will continue to snow in German-speaking Switzerland at first, but a layer with slightly positive temperatures will increasingly form from southwest to northeast over a still frosty and windless base layer at medium altitudes with some southwest winds. As soon as this layer is thick enough, the snowfall in the Swiss Plateau will suddenly change to rain and the snow line will rise from the low altitudes to around 1500 meters. The fresh snow in the Swiss Plateau will increasingly turn into a "soggy" affair. This is likely to be the case in the western Swiss Plateau, in central and lower Valais and in central and north-western Switzerland by midday. In the course of the afternoon, the snow is also likely to change to rain in the east, with the longest duration in the north-east. This is also well illustrated by the calculated temperature profiles for Bern and Zurich, which already show a very pronounced positive temperature layer in Bern at midday, while at the same time no positive temperatures are calculated at altitude in Zurich (see Figs. 2 and 3). Without going into details, a positive temperature layer is present when the red curve runs to the right of the orange line.
Fig. 2: Calculated temperature profile at altitude on Thursday at 1 pm in Bern; Source: MeteoNews, UBIMET
Fig. 3: Calculated temperature profile at altitude on Thursday at 1 pm in Zurich; Source: MeteoNews, UBIMET
From 7 a.m. in the morning until 7 p.m. in the evening, the amounts of fresh snow calculated in Fig. 4 can still be expected tomorrow Thursday. On the Swiss Plateau, another 2 to over 5 centimetres of fresh snow is likely to fall, with higher totals at somewhat higher altitudes and in the Alps.
Fig. 4: Expected amount of fresh snow from Thursday evening to Saturday evening; Source: MeteoNews, UBIMET
Freezing rain or sleet?
If the precipitation falls through a positively temperate layer, melts in the process and then falls into the cooler and partly still frosty base layer, as will be the case tomorrow during the course of the day, the question arises as to whether the rain can freeze again and ice grains can form, i.e. whether there will be freezing rain in the Swiss Plateau (for the schematic situation of the formation, see Fig. 5). However, the base layer is probably not thick enough and cold enough for this to happen tomorrow. There is also a risk that the supercooled rain will freeze on the ground in this situation. This is also unlikely to happen tomorrow, as the temperatures on the ground are likely to be just positive when the rain starts and the rain will fall into the freshly fallen snow. Where the roads have been cleared of snow, there is likely to be so much salt on the roads that the rain will hardly freeze to the ground; this is only possible locally in exposed locations such as on bridges.
Fig. 5: Situation in freezing rain and sleet; Source: MeteoNews
More precipitation on Friday and Saturday
As mentioned above, a northern Italian low will form on Friday, bringing further widespread precipitation, which will be heavy, especially in Graubünden. Rain will mostly fall at low altitudes, with the snow line then dropping to low altitudes during the night to Saturday, meaning that there could also be some more snow in the Swiss Plateau. In Graubünden in particular, very large amounts of fresh snow, sometimes over half a meter, are to be expected (see Fig. 6). The weather development, including the expected amounts of snow on Friday and Saturday, will be described in detail here tomorrow.
Fig. 6: Expected amount of fresh snow from Thursday evening to Saturday evening; Source: MeteoNews, UBIMET
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