What caused this incredible amount of snow?

First storm Dillon, and then Eleanor, battered the UK with Arctic air fuelled weather systems and caused havoc with our domestic infrastructure. The weather systems that rocked the home countries impacted much of Europe at a similar time, including the Alps. The ferocious and fast-moving storms brought on gusts of over 200kph in some resorts, causing many to suspend lift operations.

However, whilst these systems alone are enough to bring a healthy amount of snowfall they will often pass over the Alps with great speed. The excessive amounts of snow that the Alps have experienced at the start of this season were caused by a rare localised weather phenomenon, retour d’est.

For European powder hunters in the know, a mention of retour d’est sends an excited shiver down the spine. But what is this meteorological magic, and how does it work? How can only certain areas of the Alps be affected?

Introducing the retour d’est

Translated from French, retour d’est literally means return from the east. In short, retour d’est is an easterly weather front that causes intense and sustained period of precipitation around the border between France and Italy.

The route of the Retour d'Est

Retour d’est occurs when cold air from the Arctic makes it way south through the Rhône Valley in France and collides with relatively warm and humid Mediterranean air over the Gulf of Genoa. The cold air is forced below the warmer air forming a small but potent depression, or area of low pressure, often called the Genoa Low.

The unstable storms that form in this area tend to stay put for days due to stable high pressure zones to the east and west, or more commonly due to the absence of any strong prevailing jet stream across this area of the Mediterranean.

Radiating from the epicentre over the Gulf of Genoa, the storm fronts move anticlockwise on to the planes of the Po Valley. The counter-clockwise movement then continues over the low elevation Po Valley from east to west with high velocity, this wind is often called the Lombarde (illustrated on the above map).

The humid Mediterranean air is funnelled up the Po Valley, unable to escape to the south due to the Italian Apenine hills, the north is blocked by the Alps and the Lombarde pushing from the East, the humid air hurtles towards the high altitude Italian Piemonte range in the western Alps. As there are no foothills when approaching the Alps from the Po Valley, the Lombarde backed humid air must rise exceptionally quickly to squeeze over the 3000m+ Italian Piemonte mountains. The damp air rises over high altitude ranges like the Gran Paradiso, cooling and condensing as it rises, until it eventually gives and releases monumental amounts of moisture.

Et voila! Heavy localised precipitation on the France-Italy border. Due to their location, Italian Piemonte resorts bear the brunt of the snowfall, as well as parts of the Tarentaise, in particular Tignes/Val d’Isere, where the retour d’est spills over the border in to the fringes of France via the Isere and Arc river valleys.

Stay up to date with the latest snow conditions and forecast with the Club’s Snow and Weather reports.