Avalanche Forecast
Issued: Dec 21st, 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.
mconlan,
Avalanche Canada
A substantial storm impacted the south of the region. Avoidance of avalanche terrain and highly conservative terrain travel is a must until we have clear evidence that the snowpack has stabilized. The hazard may be one step lower in the north of the region where less snow fell.
Summary
Confidence
Weather Forecast
MONDAY NIGHT: Snow, accumulation 10 to 20 cm in the south of the region and 2 to 5 cm in the north, 30 to 50 km/h north wind, alpine temperature -10 C.
TUESDAY: Clear skies, 30 to 50 km/h north wind, alpine temperature -11 C.
WEDNESDAY: Mix of sun and cloud, 10 to 20 km/h southwest wind, alpine temperature -2 C, above-freezing layer forming around 2000 m.
THURSDAY: Clear skies, 20 km/h southwest wind, alpine temperature 1 C, freezing level 1700 m.
Avalanche Summary
Large wind slab avalanches were triggered naturally on Sunday at treeline and alpine elevations from southwest to north aspects. An avalanche cycle is expected to have occurred in the south of the region during Monday's storm, but observations were not available at the time of this report.
Snowpack Summary
Over 50 cm of snow accumulated on Monday in the south of the region (e.g., Manning Park, Coquihalla Summit). Storm and wind slabs are expected to be reactive to human traffic for a few days until the snow bonds to the snowpack. In the north, wind slabs may still be found in exposed terrain. The wind is forecast to be from the north, so slabs may be found on atypical slopes.
Two concerning weak layers are present in the snowpack. The shallower layer, being around 30 to 70 cm deep, includes feathery surface hoar crystals. There is uncertainty in the distribution of this layer, but the most likely place to find it would be in terrain features sheltered from the wind around treeline and lower alpine elevations. The more widespread layer in the snowpack is a melt-freeze crust from early December, which may have a weak layer of sugary faceted grains above it. This layer is generally found around 40 to 90 cm deep and has produced varying test results in the region. This layer should be treated as suspect were you find faceted grains above the crust.
The remainder of the snowpack is well-settled in the south of the region. In the north, another melt-freeze crust with associated faceted grains around it may be found near the ground. With a lack of recent avalanche activity, the layer appears to be inactive at this time.
Terrain and Travel
- Travel in alpine terrain is not recommended.
- Give the new snow several days to settle and stabilize before pushing into bigger terrain.
- Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
- Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
- Storm slabs in motion may step down to deeper layers resulting in large avalanches.
Problems
Storm Slabs
Over 50 cm of snow accumulated in the south of the region and over 15 cm in the north. This snow will take a few days to bond to the snowpack and may be reactive to riders on Tuesday, particularly in the south. Up high, wind slabs are likely to be found adjacent to ridges. The wind direction has switched, so slabs may be found on all aspects.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Persistent Slabs
A couple problematic layers exist around 40 to 90 cm deep, including weak layers of surface hoar and faceted grains above a hard melt-freeze crust. Where these layers exist, they are at a prime depth for humans to trigger them.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 22nd, 2020 4:00PM
