Storm slabs likely remain triggerable in wind loaded deposits and where recent snow sits over a weak layer. We have uncertainty around reactivity and distribution of a recently buried layer at treeline and below. Approach steep slopes and convexities cautiously at all elevations.
Rapidly cooling temperatures help to stabilize the recent storm snow at low, previously wet elevations. At elevations where dry snow fell, storm slabs may remain reactive in wind loaded terrain features and where recent snow sits over a weak layer of surface hoar.
The early December crust can be found down 100-200 cm below the surface, with weak, sugary crystals (facets) on top of it. The likelihood of triggering an avalanche on this layer is decreasing but still remains a concern for avalanches stepping down. We are now in a low probability/high consequence scenario with this layer.
Monday night: Snowfall tapering, around 5 cm. Moderate SW wind switching NW. Treeline temperature around -5 °C. Freezing level dropping to valley bottom.
Tuesday: A mix of sun and cloud. Light NW wind. Treeline high around -7 °C. Freezing level valley bottom.
Wednesday: Mainly sunny. Light variable wind. Treeline high around -8 °C. Freezing level valley bottom.
Thursday: Snowfall 5-15 cm. Moderate SW wind. Treeline high around -5 °C. Freezing level 800 m.
On Monday in neighboring Glacier National Park, storm slabs were reactive naturally to size 3 at elevations below 1900 m, where they are suspected to have run on a recently buried layer of surface hoar. Many of these avalanches ran to valley bottom. Preliminary observations within the region from the storm Sunday/Monday include a natural cycle size 1.5-2.
A warm storm on Wednesday and Thursday last week triggered a widespread natural avalanche cycle up to size 3.5 (very large) at upper elevations.
The large loads applied to the snowpack during these storm cycles, in combination with warm temperatures has enough to trigger the deeply buried December layer, producing numerous very large avalanches. As of Monday, observations continue to roll in from throughout the region of deep persistent slab avalanches. These avalanches are generally around 2100 m, size 2.5-3.5 (very large), 100-150 cm deep and on NE-NW aspects.
20-30 cm of new snow has likely been redistributed into leeward terrain features at upper elevations. A few concerning layers can be found in the upper snowpack. We have uncertainty about if/when these layers will become active (or if they have been during the most recent storm) so we will continue to track them. Most notably:
The early December crust/facet interface is now typically down 80-140 cm, but as deep as 200 cm in wind loaded terrain. It consists of faceted grains above a decomposing crust formed by the Atmospheric River rain event at the end of November. There has been avalanche activity on this layer as recently as recently as Friday in the region. Heavy triggers like natural cornice falls and storm slab avalanches may still have potential to step down to this layer resulting in very large avalanches.
