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Avalanche Forecast

Archived

Mar 5th, 2020–Mar 6th, 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 recent snow and wind have formed 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

Thursday night: Mostly cloudy, convective flurries with up to 5 cm of snow, moderate southwest winds, alpine temperature -5 C, freezing level 1000 m.

Friday: Cloudy, convective flurries with up to 10 cm snow, light to moderate southwest wind, alpine high temperature -1 C, freezing level 1500 m.

Saturday: Cloudy, 5-10 cm new snow, light southwest wind, alpine high temperature -4 C, freezing level 500 m.

Sunday: Mostly cloudy, another 5-10 cm of snow overnight, light southwest winds, alpine high temperature -5 C, freezing level 500 m.

Avalanche Summary

Over the past several days, small to large (size 1-2) natural, human-triggered, and explosive-triggered avalanches have been observed breaking 25-80 cm deep in the recent storm snow. A few large (size 2) cornices were also observed failing naturally.

Since the weekend, persistent slab avalanches have been observed in the west of the region, where the most snow has accumulated over the surface hoar. Both human-triggered and remotely-triggered avalanches have been reported, even as professionals tiptoe around and avoid suspect terrain features. The problem is touchy but tricky. In many instances, slopes were ski cut with no results, only to have the third or fourth skier in the group accidentally trigger the slab. 

Snowpack Summary

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

A weak layer of surface hoar sits 30-70 cm deep. It may sit over a crust on solar aspects. The distribution of this weak layer and the amount of snow above it varies widely across the region. Incremental loading from successive storms, strong winds, and mild temperatures have strengthened the slab character and increased the depth of the snow above, creating the potential for larger avalanches with serious consequences. 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 sporadic deep persistent slab avalanches, usually triggered from shallow, rocky start zones. No activity has been observed on this layer since Feb 21.

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.