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Avalanche Forecast

Archived

Dec 12th, 2021–Dec 13th, 2021

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 unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
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

South Coast Inland.

Pay attention to conditions that change with exposure to wind, and maintain margins around wind-drifted features. Continue to monitor for the presence of weak snow near a crust.

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack. Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

A short burst of southeast wind on Monday

Sunday night: Mainly cloudy, up to 5 cm of snow, winds shifting to southeast and increasing to moderate, treeline temperatures near -9 C, freezing level near valley bottom. 

Monday: Mainly cloudy, up to 5 cm of snow, moderate southeast easing to light, treeline temperatures near -8 C, freezing level around 500 m.

Tuesday: Mix of sun and cloud, light variable winds, treeline temperatures near -10 C, freezing level at valley bottom.

Wednesday: Cloudy, isolated flurries with trace accumulations, light southeast winds, treeline temperatures near -7 C.

Avalanche Summary

Early Saturday morning, operators in the north of the region reported a natural avalanche cycle releasing several large (up to size 2.5) and one very large (size 3-3.5) avalanches breaking within the storm snow.

In the neighboring Sea to Sky region, we received reports of four large (size 2-2.5) explosive-triggered persistent slab avalanches failing on the early December crust/facet layer. These occurred on northwest and northeast aspects around 2000 m. 

Snowpack Summary

An impressive storm over the weekend brought 35-45 cm of new snow to northern parts of the region and 60-90 cm to areas in the south. Strong southwest winds had an ample supply of snow to drift into wind slabs at upper elevations. It remains possible to trigger these wind slabs in lee terrain features, such as down-wind of ridges and roll-overs. A shift in wind direction from southwest to southeast on Monday may form fresh reactive wind slabs in a tricky cross-loading pattern. Monitor for changing conditions as you move into wind-exposed terrain and pay attention to cracking or hollow sounds.

In the north of the region, weak faceted snow can be found 50-70 cm deep on a crust that formed in early December. This layer has shown reactivity in the neighboring Sea to Sky region. This persistent slab problem warrants careful evaluation and conservative route-finding as the recent snow consolidates into a more cohesive slab over the weak layer.

The south of the region has seen substantially higher snow totals over the past week, with areas near the Coquilhalla seeing over a meter of accumulation since Monday. Expect to find deeper snow accumulations above the December crust and anticipate larger slab avalanches. Having endured warmer temperatures and more rainfall, the southern part of the region is just reaching the threshold for avalanches at most elevations. 

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Be aware of highly variable recent wind loading patterns.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Uncertainty is best managed through conservative terrain choices at this time.

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.