The 'retour d'est' is a term I sometimes use in my weather forecast and passionate riders will get instantly stoked. A 'retour d'est' is something mythical and iconic. According to the tradition: up to 2 meters of fresh snow can come down in a short period of time.
But what is a 'retour d'est' exactly? And where does it happen? A 'retour d'est' is a French name for a phenomenon that is mostly happening in the Italian Piemonte. Literally translated, it means a return from the east. A true 'retour d'est' is created as cold air finds its way to the Gulf of Genoa through the Rhône valley and the French Alps. Once it arrives in the Gulf of Genoa this cold polar air hits the (relatively) warm and humid air above the Mediterranean and small, but very active storm depression is formed. This is called a Genoa-low.
Sometimes those Genoa lows simply can't go anywhere, because they're stuck between high pressure areas in the west and the east. But especially because of the lack of a strong (zonal) jet stream from the west it can stay foot in the Gulf of Genoa for days. And because storm depressions rotate counterclockwise, there's a current from the east forming in the Po-valley.
The map above deserves an explanation. There's a storm depression in the Gulf of Genoa and the orange arrows indicate the high winds/currents. This wind that is also called the Lombarde. It is this eastern current that makes a 'retour d'est' unique. Normally, storm depressions pass with high speed and the wind pretty much always has a western component in it. Not with the 'retour d'est'. It's not a common phenomenon, but it results in great colors on the snow maps.
A 'retour d'est' may be rare, the amount of snow it delivers is amazing. That has three reasons.
Check out the Po-valley below. It's fenced by mountains on the north, west (the Alps) and south (the Apennines). The clouds are trapped when there's a current from the east and can't go anywhere. They just have to hit the mountains, no escape possible.
The Po-valley is situated at an elevation of 150-200 meters and mostly is as flat as a pancake. But if you drive to the Alps from that flat Po-valley, you can by next to a 3000 meter high peak within 40 kilometers. The mountains are instantly huge, no such thing as pre-Alps on this side of the main alpine ridge. The clouds have no choice and have to rise very fast. The rapid rise causes rapid cooling and therefore also a lot of snow.
The snow will mainly come down in the Italian Piedmont and partly in the nearby regions. The clouds hit the Alps from the east and it will snow (heavily), but once the passed the highest peaks (the main alpine ridge) they , making it snow. But once they have passed the highest peaks of the Alps (also the Alps headquarters) they resolve.
The Piedmont is clear. But what are the nearby regions?
But a 'retour d'est' is a very local phenomenon and the devil is in the details. The exact direction determines which part of the Piedmont hits the jackpot. If the current is coming more from the south, the northern Piedmont will get hammered and a more northern component will bring more snow to the southern Piedmont.
There are all kind of mixtures possible as well. There are some variants where there is no Genoa-low, but due to pressure differences between low pressure on the Mediterranean and high pressure just north of the Alps also create a current from the east. A lot of snow can come down because of all the moisture in the Po-valley even with relatively high pressure. Forecasting a 'retour d'est' on the weather maps requires some experience. The more often you see them (and experience them yourselves) the better you will succeed in identifying the impact of a current from the east for this part of the Alps.
There's no precipitation in the rest of the Alps. The sun is shining in the French Alps with a strong Föhn in the valleys (the Lombarde). The weather differs per storm for Austria, Switzerland and the northeast of Italy, but you can assume that the most snow will come down in the Piedmont. Time for an espresso, pasta and larch trees.
The most recent 'retour d'est' is from the 6th of November 2017 and it delivered between 50 and 100 cm of snow in the southern Piedmont and the east of the French Queyras.