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Avalanche Forecast

Archived

Feb 1st, 2021–Feb 2nd, 2021

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
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 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.

Regions

South Columbia.

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

Confidence

Moderate - Uncertainty is due to the timing, track, & intensity of the incoming weather system. Forecast snowfall amounts are uncertain.

Weather Forecast

  

MONDAY NIGHT: Snow; 15-20 cm. / Strong, southwest ridgetop wind / alpine low temperature -4 / Freezing level 1400 m.

TUESDAY: Snow; 5-10 cm. / Moderate, southwest ridgetop wind / alpine high temperature -3 / Freezing level 1300 m.

WEDNESDAY: Mix of sun and cloud / Light, northwest ridgetop wind / alpine high temperature -5 / Freezing level valley bottom.

THURSDAY: Cloudy with flurries; 3-5 cm. / Moderate, west ridgetop wind / alpine high temperature -7 / Freezing level valley bottom.

Avalanche Summary

A widespread natural cycle and numerous skier triggered storm slab avalanches up size 3 were reported in this region on Sunday. These storm slab avalanches were primarily running on a weak layer of surface hoar that was buried by the recent storm. See one of several MIN reports Here and Here.

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

Snowpack Summary

25-30 cm of new snow as of Monday afternoon brings recent snow totals up to 60 cm. Rising freezing levels throughout the storm has caused rapid slab formation. These touchy storm slabs are sitting on multiple weak layers of surface hoar. 

There are a series of surface hoar layers in the top meter of the snowpack that could result in surprisingly large avalanches. The first from Friday night (down 25-50 cm), a second more widespread and unstable layer from the long drought (down 30 - 70) and a much deeper layer from early January (down 40 - 130). This last layer was showing only isolated reactivity in shallow snowpacks in the Valhallas and southern Selkirk's.

Deeper in the snowpack, a couple of older persistent weak layers may still be identifiable. The first is from December consisting of decomposing surface hoar and a crust (buried 100-15cm) and the second from early November consisting of a series of crusts with faceted snow (closer to the ground). 

Terrain and Travel

  • Keep in mind that human triggering potential persists as natural avalanching tapers off.
  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.
  • Use conservative route selection. Choose simple, low-angle, well-supported terrain with no overhead hazard.
  • Avoid exposure to overhead avalanche terrain, avalanches may run surprisingly far.
  • Be aware of the potential for human triggerable storm slabs at lower elevations, even on small features.

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.