Avalanche conditions remain very dangerous with freezing levels rising to 2600 m, a reactive storm slab and a persistent weak layer in the snowpack.
Cornices are reactive to human traffic with the warm temperatures and natural cornice falls become more likely. Large cornice falls are dangerous on their own.
A failed cornice can trigger buried persistent weak layers and result in very large and destructive avalanches.
A persistent slab problem formed by a crust down 120-200 cm has created a low likelihood, high consequence scenario that is difficult to forecast. The deeply buried weak layer has produced a number of large and surprising avalanches in the past two weeks.
This problem is most likely to be triggered from thin or variable depth snowpack areas such as wind affected features, ridge crests, and near rocky outcroppings. Large additional loads like small avalanches or cornice falls can trigger deeper weak layers. The same feature or path can slide repeatedly when the persistent weak layer is reloaded with new snow.
10-30 cm snow fell with warm temperatures, forming a dense and reactive storm slab. Be especially mindful around steep and convex openings in the trees and ridge features in the alpine.
The new and recent storm snow may overlie a weak interface which could result in deeper and touchier than expected slabs.Smaller avalanches might step down and trigger deeper weak layers, resulting in large and destructive avalanches.
Wednesday night: Cloudy, up to 3 cm new snow, moderate southwest wind, treeline low around -1 °C, freezing level around 2600 m.
Thursday: Mostly sunny, light to moderate westerly wind, treeline high around +2 °C, freezing level around 2000 m.
Friday: Mix of sun and cloud, trace of new snow, light southwest wind, treeline high around -2 °C, freezing level around 900 m, possible inversion.
Saturday: Cloudy, trace of new snow, moderate southwest wind, treeline high around -1 °C, freezing level around 1000 m.
By the time of writing on Wednesday, explosives had triggered numerous large (size 2-2.5) storm slab avalanches. A large natural size 2.5 slab avalanche was reported in this MIN post. The avalanche was 30 cm deep and on a weak layer consisting of soft sugary snow and feathery surface hoar from early January. A few large (size 2) and many small (up to size 1.5) natural avalanches were observed.
On Tuesday and Monday, several small (up to size 1.5) storm slab avalanches released naturally and were triggered by skiers.
On Sunday, explosives triggered several avalanches of size 2 to 3. A few of these were persistent slab avalanches and released on the early December layer. A large size 2.5 wind slab avalanche released naturally.
There has been an alarming pattern of large, persistent slab avalanches being consistently reported over the past two weeks. Almost all of these avalanches ran on the early December weak layer. Deeply buried persistent problems like these don't go away overnight, and it remains a serious concern.
10-30 cm new snow fell with warm temperatures and formed a dense slab. This new snow sits on a crust on steep solar aspects and wind affected surfaces in exposed and open areas. In sheltered areas the new snow might sit on a weak layer of feathery surface hoar. An accumulated total of 70-100 cm of new and recent snow now sits over variable and potentially weak snow surfaces including widespread facets, wind affected snow, and/or surface hoar up to 5 mm in sheltered areas.
The early December crust/facet layer has been responsible for sporadic but very large, persistent slab avalanches over the past two weeks. The crust is now buried 120-200 cm deep except in thin, wind affected areas near ridgetops where nearly all of the recent avalanches have been triggered.
