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Avalanche Forecast

Archived

Dec 29th, 2020–Dec 30th, 2020

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

Regions

Northwest Inland.

Freshly formed wind slabs may form lee of ridgecrests at treeline and above. Continue to make conservative terrain choices and be extra cautious of shallow and / or rocky slopes where it is possible to trigger a deeper weak layer. 

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack.

Weather Forecast

Tuesday night: Isolated flurries, alpine low temperatures -6C, freezing levels valley bottom & ridgetop winds moderate southwest.

Wednesday: Scattered flurries, trace - 10 cm new snow, alpine high temperatures -6C, freezing levels valley bottom & ridgetop winds light southeast. 

Thursday: A mix of sun and clouds, alpine high temperatures -5C, freezing levels valley bottom, ridgetop winds moderate west. 

Friday: Scattered flurries wth trace - 10 cm new snow, alpine high temperatures -7C, freezing levels valley bottom, ridgetop winds strong southwest. 

Avalanche Summary

Fresh wind slabs continue to produce avalanches (size 1-2) when tested with explosive and with ski-cutting. In the days following Christmas, there were numerous natural and explosive triggered large (size 2) wind slab avalanches and loose dry avalanches in steep terrain. A few large (size 2) avalanches involved the December crust. 

A significant avalanche from the deep basal facets was reported in this MIN report from Dec 22nd showed a deep release from a leeward high alpine feature off the north side of Hudson Mountain. The deep persistent slab problem may be isolated, however it remains a very high consequence if you find yourself involved in an avalanche of this size.

Snowpack Summary

Isolated flurries add to recent snow fall amounts (10-30 cm) from previous days. Recent snow has buried wind affected snow in alpine and exposed tree line regions. Underneath this soft snow you may encounter scouring, buried wind slabs, cross loading or reverse loading patterns.  

The December crust layer is buried 30-80 cm and featured in several avalanche reports from Dec 19th & Dec 17th. Although it appears to be gaining strength, It remains a notable weak layer. We continue to monitor it, particularly in shallow or rocky snowpacks. This MIN from the 27th found the crust down 65 cm.

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 why we continue to advise caution around shallow rocky areas in the alpine.

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

Terrain and Travel

  • Be alert to conditions that change with elevation, aspect and exposure to wind.
  • 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.