During the winter, the Alps experience heavy snowfall due to several weather patterns and storm tracks. Five key phenomena, the Nordstau, Sudstau, Westau, Nordweststau, and Retour d'est, greatly influence the alpine climate in the winter. The storm type I will discuss today is the Weststau.

The Weststau is similar to the Nordstau, except the winds and jet stream are out of the west instead of the north. Refer to the introductory on Alps storm types here. 

During the Weststau, westerly winds pick up moisture from the Atlantic Ocean and then collide with the western Alps, producing heavy snowfall during the winter. The Weststau favors the regions shown below.

Check out a typical mid-level pattern as well as the jet stream for a Weststau below.

(Reanalysis maps courtesy of www.meteociel.fr)

The northern and western French Alps benefit most from the Weststau, though snow levels can be an issue with these events due to subtropical warm fronts associated with the westerly flow. The higher peaks in the Haute Savoie (Mt. Blanc) and the eastern Savoie (Val d’Isere, Les Arcs, Val Thorens) regions are often the deepest during the Weststau. 

The Weststau typically features an area of low pressure positioned to the north of an area of high pressure in the northeast Atlantic Ocean, creating an offshore westerly flow. You can see this set up in the mid-level pressure pattern figure above. A positive NAO is more likely to produce the Weststau pattern.

The Weststau can produce large snow events for the western Alps, especially if temperatures are cold. This storm type also has a major influence on the region's climate during the winter months.

Luke Stone
Forecaster, OpenSnow