Avalanche Forecast
Issued: Dec 23rd, 2020 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Persistent Slabs.
MBender,
Avalanche Canada
The load sitting on a variety of buried weak layers has reached a tipping point. If triggered avalanches running on these weak layers will likely be large and destructive. Choose conservative terrain in the coming days.
Summary
Confidence
Weather Forecast
WEDNESDAY NIGHT: Mostly clear skies, light to moderate southwest wind, alpine temperature near -6 C.
THURSDAY: Mainly sunny, light southerly wind, alpine temperature near -3 C with a chance of a weak temperature inversion.
FRIDAY: Mix of sun and cloud, light to moderate southwest wind, alpine temperature near -5 C.
SATURDAY: Mainly cloudy with scattered flurries, light west wind, alpine temperature near -7 C.
Avalanche Summary
Reports on Tuesday show a widespread storm slab and persistent slab avalanche cycle in the alpine and at treeline. Avalanches sizes ranged from size 1.5 to 3 on all aspects. Additionally reports from Tuesday in Rogers Pass indicate several natural and explosives controlled persistent slab avalanches to size 3.5-4 running full path from the alpine to the valley floor Monday night.
On Monday there were several reports of persistent slab avalanches running size 2.5 to 3 at treeline and in the alpine on east and west aspects.
Snowpack Summary
An active storm cycle has produced 60 to 100 cm of storm snow over the last week. Underneath this storm snow there is a sandwich of weak layers which are widespread throughout the region.
Just under the new snow, down about one meter below the surface is the mid December surface hoar. This layer has been failing naturally and has been quite sensitive to human triggering throughout the storm cycle. Large avalanches have been running on this interface connecting big terrain features.
Below that lies the early December persistent weak layer, it's about 80 to 140 cm below the surface. This weak layer is a mixed bag that often presents as surface hoar and facets sitting on top of a supportive crust. This is a nasty weak layer, when avalanches fail on it, they have been large and destructive. Smaller avalanches in motion can step down to this one too. This weak layer is going to haunt us for the foreseeable future.
Terrain and Travel
- Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
- Remote triggering is a concern, watch out for adjacent and overhead slopes.
- Uncertainty is best managed through conservative terrain choices at this time.
- Be aware of the potential for large avalanches due to the presence of a persistent slab.
- Storm slabs in motion may step down to deeper layers resulting in large avalanches.
Problems
Storm Slabs
20 to 40 cm of new snow fell Monday night with moderate north wind. Watch for the formation of fresh sensitive slabs in the new snow that will likely continue to be sensitive to human triggering.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Persistent Slabs
Recent storms have pushed the persistent slab problem over the tipping point resulting in very large avalanches. A slab rests on a persistent weak layer which has the capacity to propagate avalanches across terrain features.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 24th, 2020 4:00PM
