The first 'retour d'est'?


By meteomorris on 27 September 2015 · 0

Earlier I was looking back at the first period of winter 15-16 in the Alps with cold temperatures. Snow fell far below the tree line and the snow line even dropped temporarily below the 800 meters in the Gasteiner Valley. The glacier resorts have opened their lifts and you can make your first turns of the season there. The storm depression that was responsible for all of this disappeared from the weather maps and a high pressure system called Netti attempts to dominate the weather in the Alps.

High pressure area Netti

High pressure results in beautiful weather and fronts that keep their distance. The days are sunny with a clear blue sky and the nights clear and cold and that prevents the snow from melting immediately. You can partially see that when you have a look at the current satellite image. The valleys In the Alps and the pre-Alps are filled with low clouds and you can spot the clouds as well in the Italian Po Valley. The clouds will disappear the next couple of hours and days in the northern Alps, but this won't be the case in the southern Alps.

Image of Sunday morning 10 AM

Upcoming eastern current

The maps below date back to Thursday (as I'm staring to these maps and calculations since Thursday) and nothing significant has changed. The high pressure area (H) settles north of the Alps (and is getting stronger), while the air pressure south of the Alps is much lower (L). The result is an emerging current in the Italian Po Valley (blue lines). And those blue lines determine the direction of the snow. Snow from the East, or a 'retour d'est'.

European model (ECMWF) and American model (GFS)

The difficulty with such calculations lies in the fact that an active storm depression is missing. The air pressure south of the Alps fluctuates around 1020-1025 hPa at sea level and that normally means blue skies, but in this case something special happens.

The air coming in is pretty cold, where the Mediterranean supplies all the moisture. The third peculiarity is that the Po-valley is as flat as a pancake in combination with the three thousand meter peaks emerging out of nowhere. Humid air, which is pushed towards the Alps by an eastern current, is forced to rise and hits the cold upper air. The result is called snow.

Humid air is forced to rise

Normally you won't see that much precipitation with an air pressure of 1025 hPa, but the mountains here are too much of an obstacle.

Snow from the east (you'll find the latest snowmaps here)

This pressure distribution, which will bring nice weather to France and Germany, is expected to continue for a week. But because of the lack of clear fronts, it is difficult to make exact calculations for the amount of precipitation. They were very positive yesterday, when over half a meter of snow was forecasted, but that's reduced to 25 centimeters today (actual calculations can be found here). But a retour d'est is known for its surprises and last year we've seen more snow fall from retour d'ests than was promised by computer models.

Anyway, it will snow south of the Alps. I'll post the development as comments this week, so we don't have to miss a single flake of snow.

Stay stoked

Morris


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