Avalanche Forecast
Issued: Dec 7th, 2020 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs, Persistent Slabs and Loose Wet.
swerner,
Avalanche Canada
Natural avalanches are likely. I would stick to simple terrain and stay well away from overhead avalanche prone terrain and hazards like cornices. The snowpack continues to be stressed under the new wind-loaded snow up high and rain down low.
Summary
Confidence
Weather Forecast
Monday Night: 5-15 cm expected overnight. Alpine temperatures near -1 and ridgetop wind strong from the southwest. Freezing levels 1000 m.
Tuesday: 5-20 cm and alpine temperatures near -2. Ridgetop wind will be strong from the southwest. Freezing levels 900 m.
Wednesday: 5-10 cm and alpine temperatures near -5. Ridgetop wind will be strong southwest. Freezing levels 700 m.
Thursday: Some flurries up to 5 cm. Alpine temperatures near -5 with freezing levels falling to 660 m. Cranking southwest wind is forecast.
Avalanche Summary
On Monday, a natural cornice fall size 2 was reported. This only pulled a slab avalanche within the recent storm snow and did not step down deeper to suspect weak layers. Natural avalanche activity will likely continue with the storm on Tuesday.
Over the weekend, observers reported a natural storm slab avalanche cycle (size 2-3). One storm slab reportedly released from a corniced alpine ridge feature. Additionally, numerous small to large (size 1-2) wet loose avalanches were observed on all aspects and up to alpine elevations during peak warming.
Areas near Ningunsaw continue to see large to very large deep persistent slab avalanches (size 2-3.5) releasing on weak snow at the ground.
Last week, operators near Bear Pass reported natural and explosive triggered avalanches releasing on the early November facet/crust later. Easy-to-trigger storm slab avalanches have the potential to step-down to these deeper layers and produce large, destructive avalanches.
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Snowpack Summary
As of Monday morning, weekend storm totals ranged from 55-95 mm of water in 62 hours and above freezing temperatures up to 1900 m and now dropping. At summit elevations and in areas further north in the region, precipitation fell as snow and some operators are reporting up 90 cm throughout this stormy period. Strong winds from the south have rapidly loaded areas receiving snow with touchy storm slabs. More snow and strong southwest wind will continue to build these storm slabs over the next few days. Cornices may also become overloaded and weak. Below the freezing level, the snowpack is saturated at this point but may start to freeze into a crust as the freezing level drops.
The intense periods of precipitation has the potential to bring buried weak layers to their breaking point. Two early season weak layers are still on our radar: a facet/crust combination that formed in early November and a layer of facets near the ground. In the Ningunsaw area, basal facets have recently produced very large deep persistent slab avalanches. The early November crust also been reactive in the Bear Pass area within the last week. 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
- Avoid the alpine during periods of heavy loading from new snow wind and/or rain.
- Avoid exposure to overhead avalanche terrain during periods of heavy loading from new snow, wind or rain.
- Storm slabs in motion may step down to deeper layers resulting in large avalanches.
- A moist or wet snow surface, pinwheeling and natural avalanches are all indicators of a weakening snowpack.
Problems
Storm Slabs
40-90 cm of new snow with strong winds from the southwest have formed reactive storm slabs at upper elevations. These will be more sensitive to triggering and larger in size in areas where strong winds have transported the snow into deeper slabs. Natural triggers from heavy snowfall, wind, and cornice falls are a concern.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Persistent Slabs
Large to very large avalanches over the past week provide evidence that a buried crust formed in early November combined with weak sugary snow at the ground are reactive persistent and deep persistent weak layers. Observations suggest that this problematic snowpack structure may be more prevalent in colder, shallower areas North and in the East in the region. Keep in mind that storm slab avalanches and cornice falls have the potential to step-down to these deeper layers, producing large, destructive avalanches.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Loose Wet
Above freezing temperatures and periods of heavy rain at lower elevations may trigger loose wet avalanches. Pinwheeling and rollerballs are good indicators of this type of instability. Avoid steep slopes with saturated snow, as wet loose avalanches can be surprisingly destructive due to their density.
Aspects: All aspects.
Elevations: Below Treeline.
Likelihood
Expected Size
Valid until: Dec 8th, 2020 4:00PM
