Real risk of remote triggering, danger rating is now level 4 nearly everywhere

Henry explains in this video

Recently we have experienced some very cold weather -20C and colder at 2000m. This has rotted the old snow and created a very weak layer of faceted grains. In the past 24 hours we have seen between 20cm and 1m of new snow in different parts of the Northern French Alps due to snowfall and wind. This is now very unstable and the avalanche danger has been increased to danger rating 4. The reason it is a danger rating of 4 is that there is a real risk of remote triggering.

Remote triggering means you can be on a low angle slope of 20 degrees or less and trigger an avalanche on steeper slopes that are 50m 100m or 300m above you. Professionals all over the region have confirmed that this weak layer exists. Remote triggering is now a real danger that is being emphasised by Meteo France. So be very careful where you go as the weather improves. If you don’t understand this or don’t know how to judge it, then only go off-piste with someone who does.

Henry’s technical explanation

a) The recent cold has created a seriously fragile snowpack (best to use that term ‘fragile snowpack’ rather than ‘weak layer’ because there are some different weak layers and the main weak layer presents itself in many different forms in different places).
b) in many places there’s a weak layer sitting on top of a hard layer now with fresh snow on it. This is characterized as a textbook example of the most susceptible type of scenario for triggering big slabs – the slabs that go will be facilitated by this hard layer which forsm a sliding plane. The sliding plane provides a secondary magnifying effect albeit significant in the current situation.
c.) Remote triggering is a serious threat/trap for professionals over the next few days. Thierry Arnaud from Meteo France said that this is why he put the rating up to 4. and warned that on slopes of 20 degrees expect to able to trigger steep slopes above if there are any. He emphasized the serious propagating effect that fragile snowpacks have due to facets / unconsolidated depth hoar layers.
d) There were two snowpacks before this snowfall. This was due to mild weather and rain at Christmas that affected snow below 2500m: So there was one snowpack above 2400/2500 m and one more stable below. But now the lower bit is rotting out more in some places especially shaded ‘when it’s that cold, it happens fast’. So we have to be careful everywhere. The avalanche danger rating 4 applies to all slopes at all altitudes.