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Avalanche Forecast

Archived

Feb 21st, 2020–Feb 22nd, 2020

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

Regions

Purcells.

Small storm slabs may be more reactive than expected; especially on slopes where it has been wind loaded and is sitting on a layer of fragile surface hoar. Recent cornice falls have produced large avalanches on deeply buried weak layers in this region.

Confidence

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

Weather Forecast

FRIDAY NIGHT: 3-5 cm. snow, strong southwest wind, alpine temperature -10 C, freezing level below valley bottom.

SATURDAY: 3-5 cm snow, moderate southwest wind, alpine temperature -5 C, freezing level 1100 m.

SUNDAY: 3-5 cm. snow, light to moderate southwest wind, alpine temperature -6 C, freezing level 800 m

MONDAY: Cloudy with possible flurries; 0-3 cm, light west wind, alpine temperature -5 C, freezing level 1000 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 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

Clear and sunny skies have formed new surface hoar (size 3-10 mm.) on all but solar aspects where a new sun crust has been formed. This new interface will need to be carefully monitored as it gets buried. New snow falling on this layer will be more reactive than previous storms where the new snow did not fall on a persistent weak layer. 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 aware of the potential for larger than expected storm slabs due to the presence of buried surface hoar.
  • 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

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

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.