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Avalanche Forecast

Archived

Dec 22nd, 2020–Dec 23rd, 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 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

Northwest Inland.

New snow and strong wind on the way. Storm slabs will likely remain reactive to human triggering. Its a good time to dig down and test weak layers, especially with an active persistent slab avalanche problem. Be conservative with your terrain selection.

Confidence

Moderate - Uncertainty is due to the limited number of field observations. Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Light to moderate snow and strong wind will arrive with the forecast strong Pacific frontal system.

Wednesday: Snow 5-10 cm accumulating through the day. Alpine temperatures near -3 and freezing levels valley bottom. Ridgetop wind strong from the South.

Thursday: Snow 10-20 cm. Alpine temperatures -3 and freezing levels 900 m. Ridgetop wind switching to the southeast.

Friday: Cloudy with isolated flurries. Alpine temperatures near -5 and freezing levels 600 m. Ridgetop wind moderate from the southeast.

Avalanche Summary

A new 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.

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.

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

Snowpack Summary

Incremental loading of new snow has 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

  • In times of uncertainty conservative terrain choices are our best defense.
  • 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

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.

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.