Up to 40 cm of snow came down from the fog on the north side of the Alps and the southern and western Piedmont the last couple of days. On Tuesday I wrote 'the incoming cold air will result in snowfall from the humid air and might result in some local surprises', and that happened. The fog that sometimes was present up to a altitude of 2500 meters has disappeared by now. It was sunny today, but you can expect some snowfall again in the night to Friday on the north side of the Alps. The sun will come out on Friday and it will be sunny on Saturday and Sunday as well. The current turns to the west after the weekend. In this forecast:
Something special happened last week. Humid air was pushed into the Alps from the southwest during the weekend. This resulted in beautiful powder in the southwest and south of the Alps last weekend. At the same time, cold air from the northeast was on its way to the Alps. This created an interesting but complex situation above the Alps (remember this video forecast?)
The warmer and more humid air passed the Alps after Sunday and was located north of the Alps. At the same time, cold arctic air came down towards the Alps (going south) from the northeast. The mild humid air was trapped between the cold air from the north and the Alps in the south. This cold but dry air was forced to the south with a current from the northeast and crept over the moist air. This humid air thereby literally got stuck in the alpine valleys. This happened in the western and southern Piedmont, but remarkably as well on the northern side of the Alps. Especially the latter is a special phenomenon.
Due to the absence of a western jet stream, the current was east northeast the last couple of days and the supplied air became colder and drier. At the same time, there was that warmer bubble with moist air hanging in the alpine valleys. As a result, light, cold snow came down every day. Strangely enough, it was sunny and dry in the high alpine.
Everything came together in a sublime way around Engelberg and Hasliberg on the 20th and 21st of March. Cold air penetrated the air layers at lower altitude and the temperature dropped significantly on the 20th of March. This cold air drifted over the Vierwalstättersee to pick up some moisture and forced itself into the valleys towards Hasliberg and Engelberg.
With an increasing northeastern wind and the incoming cold air, the clouds literally got stuck. With a lot of 3000 meter+ peaks south of Engelberg and Hasliberg, the clouds couldn't go anywhere. The clouds started rising slowly (we saw the clouds rise from 2000 meters to 2800 meters during the day), it started to cool down and it started to snow. At the same time, the snow caused the humid layer to cool down, resulting in even more condensation and more heavy snowfall. This turned out to be a self-feeding process until the lower layers emptied themselves. That resulted in 20-40 cm of very dry and cold powder. In addition, there was more snow in the valley and at mid-station, while only a few centimeters came down on the peaks or it was sunny and dry.
Tuesday, Wednesday and especially Thursday were therefore perfect with cold, dry powder as a result of the snow that came down from the fog. A special phenomenon that not a single precipitation map could predict. The sun was shining above the clouds, but it snowed heavily in the valleys.
A weak front will pass the north side of the Alps in the night to Friday. A depression with a core above Denmark sends a front towards the Alps. This results in 3-10 cm of snow. With the wind turning to the west, it becomes milder and the real cold air has disappeared.
The sun will return again on Friday. A high pressure system ensures sunny and dry weather. It is cold, but milder than the days before and because we are approaching the end of March, the sun is a lot more powerful. This means that the snow is rapidly transforming on southern faces, but those who arrive in time will definitely find powder on Friday and Saturday close to lifts and slopes. In addition, Saturday is a great day to go touring. Pay attention to the rising temperature during the day which makes the snow heavier and avalanches can occur. Sunday also starts sunny. A storm tries to enter the Alps from the west, but is sent to the Mediterranean by a high pressure system. This might result in some clouds on the south side of the Alps, but it won't produce really significant snowfall. That will happen later when the current turns to the west.
According to the latest calculations, the chances are that the current will turn to the west-northwest after the weekend with possibly the first precipitation on Sunday by the end of the day. That could be the start of a new storm cycle. But we're not there yet.
Stay stoked Morris