Avalanche Forecast
Issued: Dec 26th, 2020 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
cmortenson,
Avalanche Canada
Look for wind affect on all aspects at exposed treeline & above. Deep & reactive windslabs have formed from recent east winds that might be less obvious as the winds shift back to the southwest/west. Remember: a small slab might trigger deeper layers in a step-down avalanche.
Summary
Confidence
Weather Forecast
Continued cloudy conditions with light snowfall accumulations and cool temperatures for Sunday and then a high pressure influences the area for Monday / Tuesday.
Saturday night: Cloudy with clear periods with isolated flurries, trace - 5cm new snow. Alpine low temperatures near -7C and freezing levels valley bottom. Ridgetop wind light from the southwest.
Sunday: Mainly cloudy with isolated flurries, trace - 5 cm new snow. Alpine high temperatures near -5 and freezing levels valley bottom. Ridgetop wind increasing moderate to strong from the southwest.
Monday: A mix of sun and clouds. Alpine temperatures near - 5 and freezing levels valley bottom. Ridgetop winds easing to light and moderate southwest. Mild alpine temperature inversion.
Tuesday: Mainly cloudy with isolated flurries, trace to 10cm new snow late in the day, alpine high temperatures near -5C and freezing levels valley bottom. Ridgetop wind light from the southwast. Mild alpine temperature inversion.
Avalanche Summary
We suspect a widespread natural avalanche cycle with the recent Christmas Eve storm that delivered 30-50cm of new snow with moderate to strong southwest winds that shifted east as the storm left. This natural cycle has tapered, but human triggered avalanches remain likely, especially where wind has stiffened recent snow into windslabs or where storm slab sits atop weak layers. Step-down avalanches are a significant possibility where a small wind slab or cornice fall may trigger a large and destructive avalanche.
On Friday morning reports of natural and explosive triggered wind slab avalanches to size 2. In addition, a very large (size 3.5) natural avalanche was reported from reverse loading from east winds. This MIN from the Shames back-country reports a couple small (size 1) avalanches.
On Thursday, numerous explosive triggered size 2 storm slab avalanches were reported. On Tuesday reports from the Beaupre sled area and the Nass Valley showed that storm slabs were easily triggered by humans up to size two. Check out the MIN posts here. Reports indicate that the storm slab may be sitting on a weak layer of surface hoar down 40-60 cm which is why the slab is so touchy.
Please consider sharing your observations with the Mountain Information Network; even just a photo of your day helps. Thank you to those that have already submitted this winter!
Snowpack Summary
Recent low density 30-50 cm of snow from earlier in the week has been blown by moderate to strong winds in the alpine and exposed treeline locations building windslabs. Winds have been variable in direction: initially from the south and west but then switching to east before returning to southwest/west. Look for signs of wind effect for clues to how the wind has impact the snow: scoured surfaces, sastrougi, reverse wind loading, cross loaded gullies and deep wind slabs (100-150cm deep) in lee features.
In sheltered locations recent snow is settling rapidly with mild temperatures. New snow may sit atop a crust or surface hoar layer that formed before this recent storm down 40-70 cm. The buried surface hoar has been evident in the Nass Valley riding area and more recently in Shames back-country.
A crust that was buried in early December is now down 100 cm plus and counting in the alpine but is closer to the surface at and below treeline elevations. Around this crust are facets and potentially surface hoar in sheltered areas. Storm slabs may step-down to this layer.
The early-November crust is buried around 170-200 cm at treeline. This crust may have weak and sugary facets above it in parts of the region. This layer is currently dormant but remains on our radar.
Terrain and Travel
- Be aware of highly variable recent wind loading patterns.
- Be especially cautious as you transition into wind affected terrain.
- If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
- Avoid open slopes and convex rolls at and below treeline where buried surface hoar may be preserved.
- Don't let the desire for deep powder pull you into high consequence terrain.
Problems
Wind Slabs
Fresh wind slabs will likely remain reactive, especially where they have been stiffened by the wind or where they sit atop a surface hoar layer that was reported from just before the recent storm. Recent winds have been highly variable. Initially winds were from the southwest but then shifted strong to extreme from the east before returning to a more typical west flow. Thus all aspects near ridgecrest or wind affected terrain should be assessed for potential windslabs or buried windslabs.
Natural avalanche activity may begin to taper but human triggering remains likely. A conservative plan and terrain selection is key while avoiding overhead hazards like fresh cornices and large avalanche paths.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Persistent Slabs
A weak layer of surface hoar is buried 40-70cm below recent storm snow. It was more reactive in the Nass Valley, but also reported at ridgecrests and below in the Shames area. At the moment the reports are localized, may include two different layers & we are unclear where else it exists regionally. New snow is not expected to bond well to this surface hoar.
A crust that was buried in early December is now down 90-200 cm and counting. Around this crust are facets and potentially surface hoar in sheltered areas. Human triggered avalanches and storm slabs may step down to this layer.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 27th, 2020 4:00PM
