Everything you want to know about a 'retour d'est'


Par meteomorris sur 8 novembre 2017 · 10

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.

What is a 'retour d'est'?

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.

Huge amounts of snow

A 'retour d'est' may be rare, the amount of snow it delivers is amazing. That has three reasons.

  • The proximity of the Mediterranean Sea. A storm depression can therefore continuously absorb humid air to drop, which it can drop in the mountains only 500 kilometers away as snow.
  • The extreme clash of cold and hot air ensures maximum moisture absorption. The cold air (which is heavier) squeezes itself underneath the warm air and pushes it up with rapid condensation as a result.
  • The combination of the Alps and the very flat Po-valley ensures maximum effect of the snow front.

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.

Where will the snow come down?

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.

What about the rest 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 extreme retour d'ests

  • January 1978: 3,05 meter of fresh snow in Saint Véran
  • March 1993: 2 meter of fresh snow on the Mont Viso
  • March 2007: 2 meter of fresh snow on the Mont Viso in just 24 hours (a dump that was missed by all the weather models by the way)
  • December 2008: 2,15 meter of fresh snow in Saint Véran
  • March 2015: over a meter of snow in Sestrière in just 24 hours

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.

Commentaires


  • akollmann
    Beginner
    akollmann op 9 novembre 2017 · 22:58
    Morris, I beg you to give forecast about the Tatra.
  • meteomorris
    Expert
    meteomorris op 11 novembre 2017 · 21:44
    @akollmann, will check what I can do the upcoming winters.
    May the powder be with you.
  • floneu
    Advanced
    floneu op 13 novembre 2017 · 14:38
    As always, interesting and easy to understand. Merci!
  • Chester_Tartsnatcher
    Expert
    Chester_Tartsnatcher op 28 novembre 2017 · 18:29
    In general, is La Grave too far Northwest to benefit from a retourd'Est?

    Are there exceptions?
  • meteomorris
    Expert
    meteomorris op 29 novembre 2017 · 11:23
    @Chester_Tartsnatcher yes. It is too far Northwest and there are no exceptions. Is has the Ecrins in the south and east that will block a retour d'est from moving in.
    May the powder be with you.
  • Chester_Tartsnatcher
    Expert
    Chester_Tartsnatcher op 29 novembre 2017 · 21:00
    well... drat.

    I'm hoping that the Ecrins gets some snow love before mid January. If not, I may have to venture into more Teutonic portions of Yurp then.
  • meteomorris
    Expert
    meteomorris op 29 novembre 2017 · 21:05
    Well that's about a month and a half... but La Grave usually is at it's best mid March...
    *message édité par meteomorris op 29 nov. 2017 21:05 (9% bewerkt)
    May the powder be with you.
  • Chester_Tartsnatcher
    Expert
    Chester_Tartsnatcher op 30 novembre 2017 · 04:08
    Plane ticket has been bought, landing in Basel Jan 12 and heading for pow thereafter for a couple of weeks.

    3 years ago, end of January was fantastic In La Grave. But it was fantastic everywhere with the Gemstock closed, Airolo and Madesimo only running the lower lifts. I hit some brilliant conditions in Grands Montets and Sankt Moritz and they were running.

    A month and a half is a while. I hope things change and they get some cover.

    My inlaws horses here are growing thick coats, so the PNW should be good anyway.

    I'll keep WePowder posted if anyone is crazy enough to meet up.
    *message édité par Chester_Tartsnatcher op 30 nov. 2017 04:09 (2% bewerkt)
  • meteomorris
    Expert
    meteomorris op 30 novembre 2017 · 16:13
    @Chester_Tartsnatcher let's stay in touch. When I am in the neighbourhood we should at least drink a beer and ride a line together.
    May the powder be with you.
  • Chester_Tartsnatcher
    Expert
    Chester_Tartsnatcher op 1 décembre 2017 · 02:45
    @meteomorris, I would be honored.

    I'll update plans as they congeal near the liftoff date.

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