Sahara dust, temperature changes: an eventful week – rts.ch

Dust from the Sahara, beautiful days at 20 degrees in the plain, then returning to freshness from Saturday, the coming week should bring us quite changeable weather. Everything must be played out between an anticyclone over Eastern Europe and a depression over the Mediterranean. Here’s why:

The low trains seem well-organized across the Atlantic, but since the beginning of the month they have tended to block against the high pressure that exists between Scandinavia and the Baltic countries. Two scenarios are so possible: to head north or to attempt a passage to the Mediterranean.

We are currently in the second scenario. A solid depression is forming off Gibraltar: it is a cold drop that isolates itself from the western currents to lead a separate existence on the Mediterranean, the event can be recognized by the presence of cold air at altitude, about 5500m ( see below).

Distribution of high and low pressures and temperatures at 500 hPa (5500m) predicted by the European model (ECMWF) on 14 March 2022 [Anita Jaquet - ECMWF/RTS]Distribution of high and low pressures and temperatures at 500 hPa (5500m) predicted by the European model (ECMWF) on 14 March 2022 [Anita Jaquet – ECMWF/RTS]

This cold fall will be accompanied by a strong southerly current between today and tomorrow, which will bring us more and more mild air, but will also allow Saharasand to move to the Alps. .

The latest calculations from the European Center (ECMWF) shows that concentrations of particles from the Sahara can be significant over Spain, Portugal and France between Monday and Thursday, and that Switzerland can also be affected between Tuesday and Wednesday. The most vulnerable regions are expected to be the Jura, North Plateau and Ticino (see below).

Jet current position (left) and particle displacements (right) predicted by the European model for 15 March 2022 [Madeleine Bovigny - ECMWF/Copernicus/RTS]Jet current position (left) and particle displacements (right) predicted by the European model for 15 March 2022 [Madeleine Bovigny – ECMWF/Copernicus/RTS]

Temperatures rise on Wednesday and Thursday

The presence of fairly dense clouds, accompanied by sand from the Sahara during the day on Tuesday, will hardly be favorable for a rise in temperatures. But the return of the sun on Wednesday should change the situation:

Temperatures predicted to 1500m by the European model (ECMWF) for March 16, 2022 [Gaëlle Cotture - ECMWF/RTS]Temperatures predicted to 1500m by the European model (ECMWF) for March 16, 2022 [Gaëlle Cotture – ECMWF/RTS]

Meteoswiss forecasts up to 19 ° C in the Geneva region, 20 ° C in Basel and Delémont, up to 22 ° C in Sion. Mildness also at an altitude of 17 ° C at La Chaux-de-Fonds (991m), 13 ° at Montana (1476m) or 17 ° C at Planfayon (861m). Admittedly, a small breeze could settle on the Plateau. But the mood should be quite springy …

Return of freshness at the end of the week.

All in all, the expulsion of the Depression in Libya, at the end of the week, was to redistribute the cards. The latest results from the European model (ECMWF) show that the distribution of pressure will again be favorable for the eastern currents, which will lead increasingly cooler continental air towards the Alps. (look downstairs).

Circulation of currents at 850 hPa (1500m) predicted by the European model (ECMWF) for Saturday, March 19 at.  06.00 UTC. [Marie-Louise Giroud - ECMWF/RTS]Circulation of currents at 850 hPa (1500m) predicted by the European model (ECMWF) for Saturday, March 19 at. 06.00 UTC. [Marie-Louise Giroud – ECMWF/RTS]

The temperature drop was to begin during the day on Saturday, with temperatures reaching 14 ° C in Geneva and Delémont or 16 ° C in Sion. This drop will also be quite marked in the mountains, with an average of 6 ° C to 8 ° C less than in the middle of the week. Definitely turn back, but nothing extraordinary in a month of March.

Uncertainty in spite of everything

The common presence of an anticyclone over eastern Russia and a depression over the Mediterranean and then over North Africa is unanimously accepted from one model to another. However, the location of the action centers varies from one to the other, which is not without consequences for the strength of the currents and the temperatures.

In some excursions the drop can be around 6 ° C to 8 ° C in the mountains. In others, it may be in the order of ten degrees. We need to be careful so far in the evaluations.

The fact that the depression develops as a cold drop, isolated from the western currents, also makes the forecast quite difficult – the models do not like this kind of situation and sometimes tend to exaggerate the scale of events.

The release of the European model last Saturday is a good illustration of this tendency to exaggerate in situations with cold drops, and predicts on March 20 a temperature of -12 ° C above Switzerland, around 1500m. Today, the same model predicts pretty much 2 ° C for the same deadline. Which seems more in line with the climatology of a month of March …

Philippe Jeanneret

Leave a Comment