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Avalanche Forecast

Archived

Feb 22nd, 2020–Feb 23rd, 2020

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

Regions

Purcells.

Small wind slabs may be more reactive than expected where they are sitting on top of a fragile layer of surface hoar.

Confidence

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

Weather Forecast

SATURDAY NIGHT: Few clouds, light southwest wind, alpine temperature -11 C, freezing level below valley bottom.

SUNDAY: 3-10 cm. snow, moderate southwest wind, alpine temperature -5 C, freezing level 1000 m

MONDAY: Mostly cloudy with possible flurries; 0-3 cm, light west wind, alpine temperature -6 C, freezing level 900 m.

TUESDAY: Mix of sun and cloud, light to moderate southwest wind, alpine temperature -6 C, freezing level 900 m.

Avalanche Summary

There have been a few sporadic large avalanches over the past week. Most recently, a large (size 3) deep persistent slab avalanche was remotely triggered last 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

5 cm of new snow has buried a widespread layer of surface hoar (size 5-15 mm.). This layer exists on all aspects and elevations except for steep solar aspects where it was melted by sun and windward alpine slopes where the layer was flattened by wind before being buried. Surface hoar sitting on a thin sun crust may exist on lower angle solar aspects which is a particularly nasty combination. 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.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • 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.

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.