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Avalanche Forecast

Archived

Dec 20th, 2020–Dec 21st, 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 unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Northwest Inland.

Our recent snow has been sculpted into wind slabs in exposed areas while bringing a crust layer from early December to its tipping point elsewhere. Keep choosing terrain that is unlikely to produce a shallow release and you'll avoid the potential for a nasty step-down avalanche.

Confidence

High -

Weather Forecast

SUNDAY NIGHT - Clearing. Moderate southwest winds.

MONDAY - Mainly sunny. Light west winds shifting northwest. Alpine high temperatures around -7.

TUESDAY - Cloudy with sunny periods. Light west winds. Alpine high temperatures around -7, warming overnight as a temperature inversion establishes.

WEDNESDAY - Sunny, with cloud increasing in the afternoon. Light south winds. Alpine high temperatures around -4, cooler at lower elevations with a light temperature inversion in place.

Avalanche Summary

Reports from Saturday continued to highlight 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). 

Thursday was also an active day. Check out Bryan's great MIN report from Hankin that outlines a few avalanche observations.

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.

 

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.

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.

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.