Avalanche Forecast
Regions: Northwest Coastal.
Storm slabs may be reactive to human triggers at upper elevations. Reverse loading from easterly winds may have formed fresh wind slabs on opposite slopes. Conditions change with elevation and a conservative approach in the alpine is warranted.
Confidence
Low - Uncertainty is due to limitations in the field data.
Weather Forecast
The cooling and drying trend continues with a mix of sun and cloud on Friday/ Saturday then overcast conditions by Sunday accompanied by light snowfall amounts. Alpine temperatures will remain cool at -5 or less and freezing levels will likely be near 500 m. Ridgetop winds will blow moderate from the northeast through the forecast period.
Avalanche Summary
No new observation reported on Thursday.
On Wednesday, human and explosive triggers saw up to size 1 running within the recent storm snow.
On Tuesday, a widespread natural avalanche cycle and to size 3 and reports of a size 4 in the Bear Pass area. These larger slab avalanches are said to be mostly storm instabilities with some active persistent slab avalanches. As conditions start to get cooler and dryer the natural avalanche activity will likely decrease, however; uncertainty sits with the persistent slab problem which should remain on your radar.
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Snowpack Summary
The recent storm brought over a metre of snow accompanied by strong southwest winds to alpine elevations. Treeline elevations saw quite the mix of snow and rain depending on where the freezing level sat (mostly around 1200 m during the storm). Below treeline the snowpack has likely refrozen into a firm crust. Cornices have likely grown large along ridgelines. All and all, this storm provided new load and stress to the existing snowpack and a widespread avalanche cycle was the response.
The current cooling and drying trend will likely lock-up and stabilize the snowpack. However; deeper instabilities may still exist at treeline and above elevations. The early December crust is now down 100 cm plus in the alpine and just below the surface at treeline. The early November crust is even deeper in the alpine and down 70-120 cm at treeline. Basal facets exist and have produced large full depth avalanches pre-storm and during the storm. The extent of this problematic snowpack structure in the region is not well-known, but it likely exists in colder, shallower snowpack areas.
Snowpack depths are highly variable across aspects and elevations as a product of wind scouring, above-freezing temperatures, and rain. Snow has melted fast at lower elevations, and snowpack depths have seen rapid settlement.
Terrain and Travel
- Keep in mind that human triggering potential persists as natural avalanching tapers off.
- Storm slabs in motion may step down to deeper layers resulting in large avalanches.
- Recent wind has varied in direction so watch for wind slabs on all aspects.
- Be alert to conditions that change with elevation.
Avalanche Problems
Storm Slabs
Storm slabs may be reactive to human triggers. These will be more sensitive to triggering and larger in size in areas where strong winds have transported the snow into deeper slabs.
Reverse loading on Thursday may have formed wind slabs on opposite slopes for Friday.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Possible - Likely
Expected Size: 1 - 2.5
Persistent Slabs
Large avalanches earlier this week provided evidence that a buried crust formed in early December and November combined with weak sugary snow at the ground are reactive to large loads and stress to the snowpack. This problematic snowpack structure may be more prevalent in colder, shallower areas North and in the East in the region.
Cornice falls may have the potential to step-down to these deeper layers, producing large, destructive avalanches.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Unlikely - Possible
Expected Size: 2.5 - 3.5