By Luke Stone, Forecaster Posted 1 month ago October 23, 2023
Alps Storm Type #4: The Sudstau
The Sudstau is similar to the Nordstau, Nordweststau, and Weststau, except the winds and jet stream are out of the South instead of the north, northwest, and west, respectively. Refer to the introductory on Alps storm types here.
During the Sudstau, southerly winds pick up moisture from the Mediterranean Sea and then collide with the southern Alps, producing heavy snowfall during the winter. The Sudstau favors the regions shown below, on the southern side of the Alps.
Below is a jet stream and mid-level pressure map typical during the Sudstau.
(Images courtesy of meteociel.fr)
For the Sudstau to setup, we typically need an area of low pressure close to central Europe to supply cold air. We often see a strong low-pressure area over the British Isles or further south and high pressure to the east and west of the Alps. The jet stream in this case is a lot further south than the normal east to west zonal flow.
While the Sudstau typically has even more moisture than the Nordstau (due to its origins in the Mediterranean Sea), the southerly winds over these warm waters can result in higher snow levels. The Sudstau tends to be even stronger during the early season, when the surface of the Mediterranean Sea is still warm.
When the flow is due south, the Reallon in the Hautes-Alpes and Alpes-Maritimes (Isola 2000) in France do well. If the wind is more from the southwest, Serre Chevalie, Puy St. Vincent, and La Grave in the Hautes-Alpes can get nailed.
(Image courtesy of Alta Badia)
The Dolomites (Cortina d'Ampezzo, Val Gardena, Alta Badia, Arabba, Kronplatz) and the Udine region (Tarvisio, Selle Nevea) in Italy often see big snow totals during the Sudstau.
The geography of the southern Alps lends itself to two main convergence zones with enhanced snowfall. Winds are forced to converge, and then rise, due to the shape of the southern Alps, creating even more lift and enhanced orographic precipitation.
The massive snow storms that result from this phenomena impact the Ticino region (Airolo, Bosco Gurin, Monte Tamaro) in Switzerland, and the Julien Alps (Kanin, Vogel, and Kranjska Gora) in Slovenia. Other hotspots with a Südstau include Osttirol, Carnic Alps, Adamello, Madesimo, Monte Rosa.
The Sudstau can produce large snow events for the southern Alps, especially if temperatures are cold. This storm type has a major influence on the region's climate during the winter months, providing some of the biggest snowstorms every year.