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Avalanche Forecast

Archived

Dec 16th, 2021–Dec 17th, 2021

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

Regions

Sea To Sky.

Heads up hockey! Despite benign weather, a reactive buried weak layer is at its tipping point. Human-triggering large avalanches is likely and warrants careful assessment and wide terrain margins. Get the details in our forecaster's blog here. 

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast. We are confident about the likelihood of avalanche activity, what is less certain are their possible size.

Weather Forecast

Another powerful storm is expected to move in Friday night

Thursday night: Clearing overnight, no new snow expected, light west winds, low treeline temperatures near -6 C, freezing level dropping to 300 m.

Friday: Increasing cloudiness, no new snow expected, light winds becoming southwest, high treeline temperatures near -4 C, freezing level around 500 m.

Saturday: Overcast, 20-35, 25-50 mm of precipitation is expected by the end of the day, strong southwest winds with extreme gusts, treeline temperatures rising to near +1 C, freezing level rising to around 1200 m.

Sunday: Cloudy, 10-20 cm of snow, mm of precipitation, southwest winds decreasing from moderate to light, high treeline temperatures dropping from 0 C to -3 C, freezing level around 700 m.

Avalanche Summary

The likelihood of human-triggering large, destructive avalanches is increasing. 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. 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. Operators also reported a small (size 1.5) explosive-triggered persistent slab on a steep north aspect at 1900 m. 

Additionally, operators on Thursday reported small (size 1) very soft slabs triggered by both skiers, explosives, and loose snow falling off of rocks. Smaller avalanches in recent snow have the potential to step-down to deeper layers and produce large avalanches.

Snowpack Summary

Winds shifted to the north on Thursday and may have created a tricky reverse-loading pattern. Avoid slopes where the snow feels stiff or slabby, and seek out softer conditions in wind-sheltered terrain.

The primary layer of concern is a weak layer of facets (sugary snow) above a widespread crust down 100-180 cm that formed in early December. On Thursday, this persistent slab problem surprised several recreationists with large, remotely-triggered avalanches. This problem is particularly hard to predict, tricky to manage, and will likely continue to surprise complacent backcountry travelers. Given recent remote-triggered and sympathetic avalanche activity, these conditions require wide, conservative terrain margins and disciplined backcountry travel techniques.

Terrain and Travel

  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Remote triggering is a concern, avoid terrain where triggering slopes from below is possible
  • Uncertainty is best managed through conservative terrain choices at this time.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Cornice failures could trigger very large and destructive avalanches.

Problems

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.

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.