Avalanche Forecast
Issued: Dec 13th, 2021 1:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
JSmith,
Avalanche Canada
Forecast snow and wind overnight on Monday are expected to form reactive wind slabs on lee features at treeline and above.
The presence of a persistent slab problem requires continued conservative terrain choices.
Summary
Confidence
Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.
Weather Forecast
MONDAY NIGHT: Snow, 10-20 cm / Strong, southeasterly winds / Low of -7 C / Freezing level surface.
TUESDAY: Mostly cloudy with flurries; 0-3 cm / Light, northwesterly winds / High of -5 C / Freezing level 400 m.
WEDNESDAY: Mix of sun and cloud / Moderate, southerly winds / High of -5 C / Freezing level 500 m.
THURSDAY: Mix of sun and cloud / Light, northwesterly winds / High of -5 C / Freezing level 500 m.
Avalanche Summary
On Sunday, operators reported numerous small to large (size 1.5-3) human and explosive-triggered avalanches releasing within in the storm snow on a variety of aspects.Â
We also received reports of four large (size 2-2.5) explosive-triggered persistent slab avalanches failing on the early December crust/facet layer. These occurred on northwest and northeast aspects around 2000 m. A large (size 2.5) explosive-triggered cornice fall was also reported.Â
Snowpack Summary
Large amounts of recent snow and extreme southeasterly winds have formed deep wind slabs on lee features in open terrain. Extreme winds can form reactive wind slabs much further down slopes than is typical.
Pay attention to conditions that change with exposure to wind and monitor for cracking or drum-like sounds.
An impressive storm over the weekend brought 40-70 cm of new snow to the mountains with strong southwest winds. Below the storm snow (down 60-100 cm), there is a layer of weak facets (sugary snow) above a crust that formed in early December.Â
Over the weekend, snowpack tests at upper treeline elevations produced sudden results at this interface (see this, this, and this MIN report). On Sunday, operators reported large, explosive-triggered avalanches releasing on this layer in alpine start zones. This persistent slab problem warrants careful evaluation and conservative route-finding as the recent snow consolidates into a more cohesive slab over the weak layer.
Terrain and Travel
- Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
- Back off if you encounter whumpfing, hollow sounds, or shooting cracks.
- Be aware of the potential for large avalanches due to the presence of a persistent slab.
- Cornice failures could trigger very large and destructive avalanches.
- Uncertainty is best managed through conservative terrain choices at this time.
Problems
Wind Slabs
New snow and shifting winds continue to form fresh wind slabs that are possible to human trigger. Pay attention to changing conditions with elevation/aspect and avoid slopes that have been freshly wind-loaded.
Aspects: North, North East, East, South West, West, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Persistent Slabs
A layer of weak facets (sugary snow) can be found down 60-100 cm near a crust layer that formed in early December. The persistent slab problem has demonstrated recent instability, and it may be possible to human trigger.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 14th, 2021 1:00PM
