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Avalanche Forecast

Archived

Dec 24th, 2020–Dec 25th, 2020

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

Regions

Northwest Inland.

Heads up! Winds are changing from the southwest to southeast which means fresh and reactive wind slabs may be found on all aspects at upper elevations. Wind slabs may step-down to deeper persistent weak layers and produce a large avalanche. Dig down and test those layers. 

Confidence

Moderate - Uncertainty is due to the extreme variability of wind effect on the snowpack.

Weather Forecast

A few flurries for Christmas as the low moves off and a bit of sunshine brings in the afternoon.

Friday: Mix of sun and cloud with isolated flurries 3-5 cm. Alpine temperatures near -4 and freezing levels valley bottom. Ridgetop wind moderate with strong gusts as its switching from southwest to southeast. 

Saturday/Sunday: Unsettled conditions and cloudy with some flurries. Alpine temperatures near -7 and freezing levels 500 m. Ridgetop wind strong from the southeast. 

Avalanche Summary

No new reports on Thursday.

A MIN report submitted on Tuesday showed a deep release from a leeward high alpine feature. The deep persistent slab problem may be isolated, however; a high consequence exists if you find yourself involved in an avalanche of this size.

Last 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. 

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

Snowpack Summary

Recent winds and up to 10 cm of new snow have scoured some areas have built stiff wind slabs in others. Locations that have not been stripped by the wind likely have 30-50 cm of snow above 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 has been reactive to human triggering in the last week and produces size 2 avalanches.

A crust that was buried in early November sits 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. This MIN report from December 22 highlights a deep release avalanche and the variable snowpack depths.

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.
  • In areas where deep persistent slabs may exist, avoid shallow or variable depth snowpacks and unsupported terrain features.

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.