The Retour d'est is similar to the Nordstau, Nordweststau, Weststau, and Sudstau, except the winds are out of the east instead of the north, northwest, west, and south, respectively. Refer to the introductory on Alps storm types here. 

The Retour d'est (translated "return from the east" in French) can produce some of the most intense snowstorms in the southwest Alps, and occurs when a moist air mass coming from the east collides with the western side of the Alps. This phenomenon is produced by a low-pressure system moving away from the Mediterranean Sea to the east, creating an easterly flow over the Po Valley in Italy.

The part of the western/southern Alps that face east, including the piedmont of Italy, can get absolutely buried with snow during these less common events. 

In order for the Retour d'est to occur, two ingredients are necessary. First, a low pressure system must track through the Gulf of Genoa. This track often leads to the second ingredient, the Adriatic Sea feeding abundant moisture into the storm. 

The Po Valley is a large and flat plain located in northern Italy. There are no obstacles to impede the easterly flow that occurs during the Retour d'est. Further, the boundary between the Po Valley and the eastern edge of the Alps is sharp, with the terrain rising from a few hundred meters above sea level to over 2500 m in just a few kilometers. This sudden transition forces moist air to ascend even more abruptly and rapidly, enhancing the orographic precipitation. 

During a Retour d’est, resorts along the border of France and Italy, as well as those in Italy east of the border, do best. The Torino (Prali, Bardonecchia, Sestrière) and Cueno (Limone) regions can see huge snow totals. In the French Alps, the Queyras (Abries, Ristolas, Arvieux,  Molines-en-Queyras, Saint-Veran) and La Haute Maurienne Vanoise (Val Cenis, Bessans, Bonneval-sur-Arc, La Norma) regions, as well as Montgenevre in the Hautes-Alpes often receive major snow totals during these events. 

The track of the low pressure system is crucial, and when it produces a more southeasterly flow, the bullseye for snow totals can shift. The Valle d’Aosta (Monterose) and Anzasca Valley (Macugnaga) regions in Italy, as well as the Wallis (Zermatt, Saas-Fee) region in Switzerland, benefit from this wind direction. If the track produces a more southerly or southwesterly flow, the aforementioned regions will see very little snow.

The Retour d’est can produce large snow events for the southwestern 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.

Thanks for reading this series on the types of storms that impact the Alps during the winter months.

Luke Stone
Forecaster, OpenSnow