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Avalanche Forecast

Archived

Mar 3rd, 2020–Mar 4th, 2020

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

Regions

Purcells.

There is a high degree of variability through the region. Avalanche danger is highest in the north and west where a persistent weak layer is active and overnight snow and wind are forming fresh and reactive wind slabs. In drier eastern areas, avalanche danger may be a step lower.

Confidence

Moderate - Uncertainty is due to extremely variable snowpack conditions reported through the region.

Weather Forecast

Tuesday night: 10-15 cm new snow in the north of the region, trace elsewhere. Strong west wind. Freezing level 1400 m.

Wednesday: Up to 5 cm new snow. Moderate to strong northwest wind. Freezing level 1000 m.

Thursday: Flurries. Moderate southwest wind, strong at ridgetop. Freezing level 1700 m.

Friday: 5-15 cm new snow. Light southwest wind. Freezing level 1500 m.

Avalanche Summary

Over the weekend and through Tuesday, natural and skier triggered wind slabs size 1-1.5 were observed. 

Persistent slab avalanche activity is starting to pick up in the west of the region, were snowfall amounts have been greatest over the surface hoar layer which is now reaching a tipping point. In this part of the region on Sunday, skier accidental persistent slabs size 1-2 were reported on east aspects treeline and below. Heli remotes were reported up to size 2.5. Natural persistent slabs 2-3 were observed on south aspects around treeline.

Snowpack Summary

Light overnight snowfall and strong winds are building fresh wind slabs in the alpine and open areas at treeline.

A weak layer of surface hoar sits 20-60 cm deep. The overlying snow has been cohered into slabs by incremental loading through successive storms, wind at upper elevations and mild temperatures at lower elevations. As slab character increases, so do reactivity and size of avalanches failing on the weak layer (surface hoar). Read more about surface hoar on our forecaster blog!

Deep basal facets lurk near the bottom of the snowpack. This layer has been responsible for occasional and hard to predict deep persistent slab avalanches usually triggered from shallow, rocky start zones. No activity has been observed on these layers in the past week.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Shooting cracks, whumphs and recent avalanches are strong indicators of an unstable snowpack.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

Problems

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

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.