Avalanche Forecast
Issued: Dec 4th, 2020 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet, Storm Slabs and Persistent Slabs.
astclair,
Avalanche Canada
Rain, snow, and wind will further load an already stressed snowpack. Stick to simple terrain and maintain conservative margins from overhead hazard.
Summary
Confidence
Weather Forecast
The storm continues through the weekend.
Friday night: Overcast, 20-30 cm of snow above 1700 m, heavy rain below, strong southwest winds with extreme gusts.
Saturday: Mostly cloudy, 5-10 cm of snow above 1500 m, strong southwest winds.
Sunday: Mostly cloudy, 10-20 cm of snow above 1400 m, strong southwest winds, freezing level dropping to 1200 m.
Monday: Mostly cloudy, 20-30 cm of snow above 1200 m, moderate southwest winds freezing level dropping to 1000 m.
Avalanche Summary
On Friday, observers reported several small (size 1) wet loose and storm slab avalanches releasing in the recent storm snow above the Dec 3 crust.
In the aftermath of last weekend's storm, a series of notable avalanches have been reported further north in the region. These were very large deep persistent slab avalanches (size 3-3.5) that were both explosive-triggered and natural releases, running on an early November crust and weak snow at the ground. The extent of this snowpack structure in the region is not well-known.
Observations in this region are very limited. If you're out in the field, please consider sharing a photo or description via the Mountain Information Network (MIN).
Snowpack Summary
The storm will continue overnight and is expected to taper into Saturday morning. An additional 25-40 mm of water is forecast to saturate the snowpack below 1700 m, which is likely to trigger widespread wet loose avalanches. The precipitation may fall as snow on the high peaks. Strong southwest winds are expected to rapidly load areas receiving new snow into reactive slabs. Temperatures are rising during the storm event, increasing instability.
The significant load from intense periods of precipitation has the potential to bring buried weak layers to their breaking point. A layer of buried surface hoar and facets that formed in late November may still linger in isolated areas. A widespread layer of facets on a crust from early November warrants concern. Further north in the region, we're learning that weak snow developing near the ground has produced very large deep persistent slab avalanches. As the snowpack becomes more saturated, smaller wet loose or storm slab avalanches have the potential to step-down to these deeper layers and produce large, destructive avalanches.
Snowpack depths range from roughly 150 to 200 cm at upper treeline and alpine elevations.
Terrain and Travel
- Watch for rapidly changing conditions during periods of heavy loading from new snow, wind, or rain.
- If triggered loose wet avalanches may step down to deeper layers resulting in larger avalanches.
- The more the snow feels like a slurpy, the more likely loose wet avalanches will become.
- Avoid areas with overhead hazard.
Problems
Loose Wet
Above freezing temperatures and periods of heavy rain 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: All elevations.
Likelihood
Expected Size
Storm Slabs
Periods of heavy snowfall combined with strong winds are expected to form reactive storm slabs at upper elevations. These slabs will be more sensitive to triggering and larger in size in areas where strong winds have transported the snow into deeper, more cohesive drifts. Natural triggers from heavy snowfall, wind, and cornice falls are a concern with this rapid load.
Aspects: All aspects.
Elevations: Alpine.
Likelihood
Expected Size
Persistent Slabs
Intense periods of precipitation have the potential to bring buried weak layers to their breaking point. A layer of buried surface hoar and facets that formed in late November and a widespread facet/crust layer from early November have the potential to become sliding surfaces with the stress of the new snow and rain. Easy-to-trigger wet loose or storm slab avalanches have the potential to step-down to these deeper layers and produce large, destructive avalanches.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 5th, 2020 4:00PM
