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Avalanche Forecast

Archived

Mar 1st, 2020–Mar 2nd, 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 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 possible, human triggered probable.

Regions

Purcells.

Avalanche danger will be one step lower in the central part of the region where less than 10 cm of snow fell on the weekend. Recent snow has been blown into wind slabs which may sit over a weak layer. Watch for signs of instability as you travel.

Confidence

Moderate - Recent weather patterns have resulted in a high degree of snowpack variability within the region.

Weather Forecast

Sunday night: Cloudy periods. Moderate to strong northwest wind, extreme at ridgetop by early morning. Freezing level 500 m.

Monday: Flurries bringing up to 5 cm new snow. Strong northwest wind. Freezing level 1400 m.

Tuesday: 5-15 cm new snow overnight. Moderate southwest wind. Freezing level 1700 m.

Wednesday: 10-20 cm new snow. Moderate west wind. Freezing level 1200 m.

Avalanche Summary

On the weekend, natural, skier and snowmobile triggered storm and wind slab avalanches were reported size 1-2. 

A few skier triggered avalanches have been associated with the persistent surface hoar problem. Activity on this layer is expected to increase with additional snow loads forecast later in the week.

Snowpack Summary

Recent wind has redistributed surface snow in the alpine and exposed treeline, loading snow into lee terrain features. Fresh wind slab development will be deepest and most reactive in the north of the region, where 20 cm of snow fell on the weekend.

A weak layer of widespread surface hoar sits 20-40 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.

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.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.

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.