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Avalanche Forecast

Archived

Feb 23rd, 2020–Feb 24th, 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 possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Purcells.

Touchy storm slabs are developing over a widespread layer of surface hoar at all elevations. Expect these slabs to be most reactive on wind affected slopes.

Confidence

Moderate - Uncertainty is due to field data and reports showing a wide variation in conditions throughout the region.

Weather Forecast

SUNDAY NIGHT: Cloudy with flurries; 3-5 cm, light to moderate west wind, alpine temperature -9 C, freezing level below valley bottom.

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

TUESDAY: Mostly cloudy, light to moderate northwest wind, alpine temperature -5 C, freezing level 900 m.

WEDNESDAY: Mix of sun and cloud, light to moderate northwest wind, alpine temperature -3 C, freezing level rising to 1300 m.

Avalanche Summary

On Saturday the new snow was reported as being reactive to skier traffic as a very thin soft slab in wind affected terrain. These slabs will become increasingly touchy as the new snow stacks up and settles into a cohesive slab. There have been a few sporadic large avalanches over the past 10 days. 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. 

Snowpack Summary

10-25 cm of recent snow has buried a widespread layer of surface hoar (size 5-15 mm.). This layer exists at all elevations. This same layer of surface hoar may be sitting on a thin sun crust on solar aspects which is a particularly nasty combination. The new snow will likely become increasingly reactive to human triggers as it stacks up and settles into a cohesive slab. 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

  • Shooting cracks, whumphs and recent avalanches are strong indicators of an unstable snowpack.
  • Be alert to conditions that change with elevation and wind exposure.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

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.