An analysis of the perfect storm (November 2017)

A perfect storm hit the Alps on the 5th of November. This storm called Karl brought a lot of snow to almost the entire Alps and that's pretty unique. We'll analyze storm Karl in this article and explain why this actually was a perfect storm.

A unique storm

The Alps have a special shape. You can draw a straight line through a lot of mountain ranges, but not through the Alps. The Pyrenees, for example, are mountains that run from west to east in a straight line, while the Rocky Mountains, the Andes and the Japanese Alps are stretched fairly straight from the north to the south. The Alps are different, you can draw both lines from the east to west and from the north to the south, like the shape of a hockey stick.

Storms usually come from one direction,and it usually snows on one side of the mountain (Stau), while it wind is stronh on the other side (Föhn). A northern Stau will result in a lot of snowfall on the north side of the main alpine ridge. It won't snow on the south side of the main alpine ridge in Italy. The same goes for a retour d'est where it can snow heavily in the Italian Piedmont, but it won't snow at all across the border in France (the storm is simply blocked by the high peaks that form the border). But Karl was quite unique because it snowed in the entire Alps.

We forecasted a lot of snowfall on our maps and it pretty much all happened, except for the French southern Alps, where the storm arrived with too much heat and moved on too fast.

What's special about Karl

Karl was not that special at first. It was a slow, somewhat rude storm depression that arrived in Portugal and Spain from the west. Karl was fed with warmer air from the south and mild air from the Atlantic on the north (the first image below). Karl was being pushed to the east by a high pressure area on the Atlantic the. Once above Spain, Karl appeared to have the keys to open the door to cold air from the north. A severe storm depression close to Iceland sent cold air to the south. At the same time, Karl moves towards the Mediterranean and with that action it opened the door for the cold air to come in (the second and third image). Karl was sucking up cold air from the north and at the same time was fed with warm air from the south and moisture from the Mediterranean. A Genoa storm depression was born (the fourth image).

The snow came down throughout the Alps in a number of steps:

1. Karl first passed the French southern Alps. A front with relatively warm air passed the French southern Alps in the night from Saturday to Sunday and brought 10-20 centimeter of fresh snow above 2100 meters. At the same time it started snowing on the south side of the Alps on Sunday morning.
2. The cold air reached the French northern Alps and it started to snow over there.
3. The Alps ended up in some kind of snow sandwich. Warm air hit the Alps from the south and it started to snow heavily. At the same time, the wind on the northern side of the Alps was turning north to north-east, and the clouds were trapped. They snowed empty on the border of Italy and Austria in the night to Monday with a spectacular 100 cm of fresh snow on the Stubaier glacier in less than 24 hours.

It's been snowing in the complete Alps by then, but there's one big exception. The Italian Piedmont normally never benefits from such a snow sandwich. This is because storms tend to move from west to east. But because Karl got trapped between high pressure in the west and high pressure above Russia the storm it simply couldn't go anywhere. It wasn't moving at all and an eastern current emerges.

4. And that resulted in a 'retour d'est'. It was snowing from Monday the 6th of November till Friday the 9th of November and the 'retour d'est' brought a meter of fresh snow to the Italian Piedmont. Karl has done an outstanding job and brought snow to the complete Alps.

Base for the rest of the season

The storm cycle Karl brought an average of 20-60 cm of fresh snow above the 2000 meters between November 4th and November 9th, and locally even 100 cm of snow. This is the base above the tree line in large parts of the Alps for the entire season. Especially in the northern Alps where it snowed heavily again in weekend of 10-13 November.

From Föhn to Stau in a couple of minutes

The 100 cm of fresh snow that came down in the high alpine in Austria deserves some extra attention. You'll find the data of a measuring station in the proximity of the Stubaier glacier below. The red arrow is important. You see successively from top to bottom: the thickness of the snow cover, the air temperature, the wind speed and the wind direction. In the course of Sunday afternoon/evening, the 6th of November, the wind (the red arrow) suddenly drops, it turns from south to north, the temperature drops 10 degrees and it starts to snow heavily.

You will find an explanation based on the air pressure maps below. The northern Alps and therefore also the Austrian high alpine first had to deal with Föhn winds from the south. The red arrow creates a warm and stormy wind. But with the incoming of the cold air the warm air is pushed away, the wind calms down, the current suddenly turns to north and even east (check the blue arrows) and the clouds get stuck. This relatively warm air contained a lot of moisture, can't go anywhere and all the moisture is squeezed out in the form of snow.

And that's how Karl become a perfect storm. Let's hope he'll visit the Alps again a couple of times this season!