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Avalanche Forecast

Archived

Dec 19th, 2021–Dec 20th, 2021

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

Regions

Sea To Sky.

Conditions are more complicated than meets the eye. Wind-drifted snow and a buried weak layer warrant a cautious approach and diligent decision making. 

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Sunday night: Clear, no new snow expected, light north winds, low treeline temperatures near -13 C.

Monday: Sunny, no new snow expected, light north winds, high treeline temperatures near -8 C.

Tuesday: Increasing cloudiness, isolated afternoon flurries with trace accumulations, winds becoming southwest and increasing to strong, high treeline temperatures near -3 C, freezing level rising to 500 m.

Wednesday: Overcast, 20-40 cm of snow, strong southwest winds, treeline temperatures near -3 C, freezing level around 500 m.

Avalanche Summary

Although avalanche activity is tapering, it is still possible to trigger large avalanches on Monday. Avalanches in the recent snow have the potential to step-down to recently reactive buried weak layers, with the potential to produce very large, destructive avalanches. Cornices may also reach their breaking point, triggering persistent slab avalanches on slopes below. 

On Sunday, operators reported several large (size 2-2.5) explosive-triggered avalanches releasing 50-75 cm deep in the recent storm snow. On Saturday, operators reported numerous small to very large (size 1-3) natural avalanches releasing in the storm snow across elevations and aspects. 

On Thursday, we received reports of two large (size 2-3) avalanches that released on a persistent weak layer on northwest and southwest aspects between 1700-1900 m. There are valuable photos and accounts in this MIN report and this MIN report. Notably, skiers remote-triggered these avalanches while traveling uphill. Several small (size 1-1.5) sympathetic avalanches also released on nearby terrain features during these close calls. 

Snowpack Summary

Saturday's powerful storm brought 40-70 cm of snow to the mountains. Strong south winds have redistributed the new snow into dense wind slabs that may remain possible to trigger at upper elevations, especially where the new snow is poorly bonded with the old snow. The recent snow sits on a variety of snow surfaces, hard wind-scoured surfaces, preserved powder, and possible surface hoar in sheltered areas. Stay alert to changing slab conditions as you move into wind-exposed terrain, and monitor the bond of the new snow to the old snow surface. 

A culprit weak layer of facets (sugary snow) can be found over a widespread crust that formed in early December (down 100-180 cm). This layer seems particularly reactive at treeline elevations, between 1600-2100 m. This problem is particularly hard to predict and tricky to manage. Given the rapid load from new snow and wind along with recent avalanche activity, this persistent slab problem requires wide, conservative terrain margins and disciplined backcountry travel techniques. Get more details and photos in our forecaster blog. 

Terrain and Travel

  • Stay off recently wind loaded slopes until they have had a chance to stabilize.
  • The trees are not the safe-haven they normally are at this time. Terrain at treeline is primed for human triggered avalanches.
  • Remote triggering is a concern, avoid terrain where triggering slopes from below is possible
  • Cornice failures could trigger very large and destructive avalanches.
  • In times of uncertainty conservative terrain choices are our best defense.

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.