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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 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

South Coast Inland.

Heads up hockey! Despite benign weather, a buried weak layer is becoming more unstable. Large human-triggered avalanches are likely. These dangerous conditions warrant 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.

Weather Forecast

Thursday night: Clearing overnight, no new snow expected, light west winds, low treeline temperatures near -16 C, freezing level at valley bottom.

Friday: Increasing cloudiness, no new snow expected, moderate southwest winds, high treeline temperatures near -10 C.

Saturday: Overcast, 5-10 cm of snow accumulating by the end of the day, strong southwest winds, treeline temperatures rising to -3 C and freezing level rising to 500 m by late afternoon.

Sunday: Cloudy, up to 5 cm of snow, light variable winds, high treeline temperatures -6 C, freezing level around 200 m.

Avalanche Summary

The likelihood of human-triggering large, destructive avalanches is increasing. An operator in the north of the region on Thursday observed a widespread, large natural avalanche cycle, releasing on the persistent weak layer on southerly aspects at upper elevations. In the neighbouring Sea-to-Sky region, we received reports of two large (size 2-3) persistent slab avalanches 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. 

On Wednesday, small to large avalanches in the recent storm snow were triggered with explosives.

We have few observations from the south of the region. Check out this recent MIN report of a naturally triggered size 3.5 storm slab avalanche reported last Sunday near Mt. Pierce.

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 50-100 cm that formed in early December. On Thursday, this persistent slab problem produced a natural avalanche cycle in the north of the region and surprised several recreationists in the neighbouring Sea-to-Sky region 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 natural, 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.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.

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.