The end of persitent and very mild high-pressure weather is in sight! The weather models are working on something nice for next week, but how exactly it will turn out is hard to say at this point. A cut-off low could cause some surprises, but to be clear right away: for details on snow amounts and distribution it is of course still too early.
Extraordinarily warm
It was extraordinarily warm in the Alps last week. Especially higher up, the air masses were very warm due to the inversion weather and descending air movements under the dominant high-pressure area. The freezing level rose to well above 4,000 metres. The radiosonde measurement in Payerne, Switzerland, measured a freezing level as high as 4284 metres, something that has never happened in November in the 82-year measurement series here.
This record value came not alone, as November records were also broken at several well-known weather stations including the Jungfraujoch (CH), the Säntis (CH), and Sonnblick (AT).
With such a prolonged, stable, mild weather phase, you can hardly help but look far ahead and search for some silver linings. The first ones are now well visible in the weather models.
Cut-off low
Both the GFS and ECMWF are showing a weakening of the dominant high-pressure area over Europe. Instead of a Euro blocking pattern, we will face an Atlantic blocking pattern. At the same time, cut-off lows may reach Central-Europe. Where and how? That is impossible to say at the moment, but from the middle of next week onwards, the Alps will likely enter a more changeable weather type. We’ll take a quick look at one of the model calculations.
Source: wxcharts
What do you see on these maps? To start with: we are looking at maps showing the geopotential height at about 5.5 kilometres high (500 hPa level). In brief, but sufficient for now: red colours (higher altitudes) correspond more or less to high air pressure and green/blue colours (lower altitudes) to low air pressure. The white contours indicate surface air pressure (mean sea level pressure).
There is currently a strong high pressure area over central Europe (orange/red colours), but in the upper left image (next Sunday) you can already see that the high is weakening and splitting in two, so to speak, with a core west of Europe and a core over Eastern Europe/Baltic States. The left core may strengthen again in this process. The dark red colours west of Europe and over the Atlantic indicate a strong (but temporary) high-pressure blocking pattern.
This blocking makes it possible for a blob of cold air to become isolated from the main flow (the jet stream). You can see this process by the trough deepening. Top left, you can see the trough with its axis from Iceland to Britain. This gets ‘filled with cold air’ and a drop of this cold air then falls ‘down’ towards the south. A cut-off low is thus born. In German, such a cut-off low is also called a cold drop.
You can clearly see from the green colour where the cut-off low is moving towards over the course of next week. On Wednesday afternoon (bottom centre), it will be over the Gulf of Genoa, according to this calculation. Partly due to the large temperature difference (green colours indicate a very cold upper air) and the relatively warm sea water, a Genoa Low may form, resulting in a lot of precipitation. We all know what such a low pressure area can cause: a lot of snow for the southern Alps. We also see this in this model run in the accumulated precipitation map below.
So tonight’s EC oper run leans towards the development of a low pressure area over Italy with a possibility of a Genoa Low, but the precipitation distribution in both runs already differ a lot. For now, this calculation is purely indicative and not a reality of what will happen. The morning run of the same model already looked completely different (with only some precipitation in the Northern Alps, for instance) and a lot can still change in the coming days. However, it is of course great to see these maps after a long period of stable high-pressure weather!
More details on the developments later this week!