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Avalanche Forecast

Archived

Dec 21st, 2020–Dec 22nd, 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 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.

Regions

Northwest Inland.

Recent storm snow and wind has formed stiffer slabs that continue to be reactive to human triggers, especially where they sit above the early December crust. A skier triggered a size 2 avalanche yesterday and was partially buried. The December crust seemed to be the culprit.

Confidence

Moderate - Confidence is due to a stable weather pattern with little change expected. Uncertainty is due to the limited number of field observations.

Weather Forecast

The lull before the next storm. This region can expect some light precipitation as a weak trough crosses the north.  

Tuesday: Mix of sun and cloud. Alpine temperatures near -4 with moderate wind from the southwest. Freezing levels valley bottom.

Wednesday: Mix of sun and cloud with flurries. Alpine temperatures near -4 and ridgetop winds strong from the southwest. Freezing levels 500 m.

Thursday: Snow 5-10 cm with a strong southwest wind. Freezing levels 1000 m.

Avalanche Summary

On Sunday a human triggered slab avalanche size 2 was reported which started as a wind slab or storm slab and then stepped down to the early December persistent weak interface. The person was caught and partially buried. Reports from Saturday continued to highlight the instability of slabs that have formed above the early December rain crust. Check out this notable step-down release from the Hankin-Evelyn area.

Friday's reports likewise suggested continued snow accumulations are keeping the snowpack near the tipping point. Check out Aaron's MIN here, and this one reporting whumfing (a really important obs about deeper layers). 

Please consider sharing your observations to the Mountain Information Network. Thank you to those that have already submitted this winter!

Snowpack Summary

Successive rounds of light to moderate snowfall have given us 30-50 cm of recent snow. This was redistributed by recent southwest winds in exposed areas but likely remains lower density in more sheltered areas.

This recent snow sits on an early-December melt-freeze crust with weak and sugary faceted grains around it. This layer has featured in several avalanche reports that can be found on the MIN. This interface is currently reactive to human triggering and producing size 2 avalanches.

 

A crust that was buried in early November is near the base of the snowpack. This crust has weak facets associated with it. These facets have produced large avalanches in both the south and north of the region. Friday's reports of whumpfs suggest this layer should still play a role in guiding your terrain selection.

Snowpack depths vary substantially with aspect, elevation, and wind exposure. 

Terrain and Travel

  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
  • In times of uncertainty conservative terrain choices are our best defense.

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.