Winter has finally reached us in Haute-Maurienne, and it’s been snowing all the way down to the valley, with about 25cm at 12oom.
For sure, the biking season is over !
Even if downhill biking is a good alternative to skiing, we are many this morning looking at the white landscape with a huge craving for ski after those too long weeks of drought and spring-ish temperatures on the southern aspects.
So what now? Should we store the old skis we’ve been grinding on stones for weeks, take out the new gear and go shredding ?
What about a bit of thinking before ?
Be careful for stones !
The snowpack is still thin in spite of last night’s dump, and rocks, roots crevasses and other not-skiable obstacles are not hiding that far under the surface, so beware : the danger is almost the same than a few days ago.The new snow is as thick as 25cm in the valley and a bit more than 60cm higher up (Bonneval Sur Arc automatic station, below). This is nothing more than a good start.
The wind has been blowing last night, 20 to 40 km/h with gusts up to 90, we can expect some great variations in the thickness of the new snow from place to place.
So we’re still “skiing on thin ice” !
What about the stability ?
This morning’s avalanche bulletin gives considerable danger level, 3 on 5. it’s insisting on the possibility of remote triggering even in the low angle slopes , and talks about a “sensible situation, typical of a dry early winter”.
And that is true, little snow doesn’t mean little risk.
Let’s look at the situation as it still was 48h ago :
What do we see (28th décembre, La Norma) :
-the south has very little snow above 2200m, almost nothing below.
-the north has a thin snowpack, made mostly of big crystals, without cohesion (the uphill ski track is visible, but the ski are sliding…not nice).
It doesn’t take much time to dig a little :
Here at about 2000m, fully north, still in La Norma, we can see 20cm of big crystals (facets, depth hoar), typical of the fragile layers. Most of the avalanches are related to the presence of those buried weak layers : it’s their brutal collapse, by overloading, that will destroy the balance of the snowpack and get moving the layers above. They also make possible the triggering from distance, as underlined in the avalanche bulletin. But we”ll talk more about this in another post.
These crystals are appearing when the thermal gradient is high enough (temperature difference/ thickness). So a thin snowpack and a cold weather are the most favorable conditions fro the weak layers to develop.
But it wasn’t that cold recently, was it ? That’s a good lesson to be learned : it can be mild in the valleys, but higher up there will always be places sheltered from the sun that will remain cold enough for big snow crystals to grow. And remember as well that the nights are long in December.
Let’s summarize :
-We have some fresh snow that came directly on the soil, or on fragile layers, or even sometimes on some wind affected and hardened snow.
-The variation in the thickness is important as the wind has been blowing hard.
-We should keep skiing carefully, it is still not thick enough to be safe from rocks and other obstacles!
-A thin snowpack is potentially dangerous, beware of the false impression of safety when there is not a lot of snow!
-What matters most : knowing what is hiding under our skis…and under the powder.
-Winter just begins, keep calm and enjoy it until the end !
The situation will only get better as more storms are on the way.
So leave the bikes and let’s go skiing !