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Avalanche Forecast

Archived

Feb 20th, 2020–Feb 21st, 2020

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Purcells.

Large triggers, such as falling cornices, have the potential to trigger very large avalanches on a deeply buried weak layer at the bottom of the snowpack. Steep, rocky, and shallow snowpack areas on large alpine faces are the most likely slopes for this to occur.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

THURSDAY NIGHT: Clear, moderate southwest wind, alpine temperature -15 C, freezing level below valley bottom.

FRIDAY: Mix of sun and cloud, moderate southwest wind, alpine temperature -8 C, freezing level 1100 m.

SATURDAY: 2-5 cm snow, moderate southwest wind, alpine temperature -9 C, freezing level 1000 m.

SUNDAY: Cloudy with flurries; 2-5 cm., light to moderate south wind, alpine temperature -11 C, freezing level 700 m.

Avalanche Summary

A couple naturally triggered size 2 and several skier triggered size 1 wind slabs were reported on easterly aspects at treeline and above on Tuesday. 

There have also been a few sporadic large avalanches over the past week. Most recently, a large (size 3) deep persistent slab avalanche was remotely triggered on Sunday just outside the forecast region near Mt. Seven. Photos in the Mountain Information Network report show this avalanche was initiated by a skier causing a whumpf on a ridge that released a small wind slab on an east-facing alpine slope which then stepped down to the facets at the bottom of the snowpack. Another size 2.5 deep persistent slab avalanche was observed on a southwest aspect in steep and rocky alpine terrain near Golden on Thursday (see MIN report). Additionally, a large snowmobile triggered avalanche was reported on a convex roll at treeline in Quartz Creek on Wednesday (see MIN report). These isolated, but large avalanches suggest that the Purcells region is currrently in a low probability, high consequence scenario that requires careful snowpack and terrain evaluation.

Snowpack Summary

Surface conditions are a mix of low density snow in sheltered terrain, sun crusts on solar aspects, and isolated wind slabs at higher elevations. At lower elevations (below 1600-2000 m) 20-40 cm of snow sits above a hard rain crust. The middle of the snowpack is strong, but the base of the snowpack contains basal facets that have resulted in sporadic, large deep persistent slab avalanches, especially in shallow rocky start zones. 

Terrain and Travel

  • Be mindful that deep instabilities are still present and have produced recent large avalanches.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
  • Cornice failure may trigger large avalanches.

Problems

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.