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Avalanche Forecast

Archived

Feb 2nd, 2021–Feb 3rd, 2021

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

Human triggering of large storm slabs remains likely on Wednesday. Storm slabs may be surprisingly large and easy to trigger due to the presence of buried weak layers of surface hoar.

Confidence

Moderate - Uncertainty is due to the limited number of field observations.

Weather Forecast

  

TUESDAY NIGHT: Flurries; 3-10 cm. / Moderate, west ridgetop wind / alpine low temperature -10 / Freezing level valley bottom.

WEDNESDAY: Mix of sun and cloud / Light, northwest ridgetop wind / alpine high temperature -6 / Freezing level valley bottom.

THURSDAY: Cloudy / Moderate, west ridgetop wind / alpine high temperature -7 / Freezing level valley bottom.

FRIDAY: Cloudy / Moderate, northwest ridgetop wind / alpine high temperature -7 / Freezing level valley bottom.

Avalanche Summary

On Tuesday, several skier triggered storm slabs up to size 2 and numerous explosive triggered storm slabs up to size 3 were reported near Golden.

Check out this MIN on the 29th and this MIN on the 28th from near Kicking Horse.

Touchy storm slabs are sitting on a weak layer of surface hoar which is likely to be reactive to human triggers. Avalanches may be larger than expected due to the presence of this weak layer.

Snowpack Summary

15-25 cm. of new snow brings recent snow totals to 40-50 cm. The recent snow fell with warm temps and moderate southerly winds, both of which increased slab development and reactivity within the new snow. This recent snow may be sitting on top of a layer of surface hoar which would greatly increase the reactivity of the newly formed slabs to human triggers, as well as the size of avalanches. However, the size and distribution of this layer is uncertain. 

50-90 cm. down is an older surface hoar and/or a thin melt crust from mid January. This layer was more prominent in the north Purcells in sheltered, open slopes at treeline and may become reactive with the recent snow load.

Two deeper layers persist, though they have been recently unreactive. Down 100-150 cm lies the December weak surface hoar / crust / facet layers. Closer to the ground lies the early November crust facet complex. Though unlikely to trigger, caution for these layers remains with large loads in steep, shallow, rocky areas with a thin to thick snowpack or shallow areas below treeline.  

Terrain and Travel

  • Be aware of the potential for larger than expected storm slabs due to the presence of buried surface hoar.
  • In times of uncertainty conservative terrain choices are our best defense.
  • Avoid open slopes and convex rolls at and below treeline where buried surface hoar may be preserved.
  • Be careful as you transition into wind affected terrain.
  • 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.

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.