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Avalanche Forecast

Archived

Dec 6th, 2021–Dec 7th, 2021

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

Regions

Purcells.

Watch for wind slabs at ridge crests and in cross loaded features. Deeply buried weak layers also remain a concern as conditions improve. Play it safe in simple terrain with reduced exposure. Avoid thin and rocky start zones where deep layers are closer to the surface.

Confidence

Moderate - Uncertainty is due to the complexity of the snowpack’s structure. Recent weather patterns have resulted in a high degree of snowpack variability within the region.

Weather Forecast

Snow returns to the interior on Tuesday evening as cold temperatures continue. 

MONDAY NIGHT: Partly cloudy with no snowfall expected. Moderate westerly winds. Freezing level valley bottom. 

TUESDAY: Increasing cloud with isolated flurries. Moderate to strong southwest winds. Alpine high temperature -10 C. Freezing level valley bottom.

WEDNESDAY: Snowfall begins Tuesday evening, with light accumulations building over Wednesday (up to 5cm), with strong westerly winds. Freezing level below 1000m, alpine highs of -10.

THURSDAY: Cloudy with flurries throughout the day. Winds ease to moderate westerlies. Freezing levels at valley bottom. Alpine highs of -8. 

Avalanche Summary

The snowpack is complex with several reactive persistent weak layers in the mid and lower snowpack.

Explosive control work near Golden yesterday produced a size 2 slab avalanche that failed to ground in a steep and unsupported terrain feature. A naturally triggered size 3 was also observed on a west facing slope in the Northern zone of the Purcells, believed to have occurred around the 2nd of December. 

Two Mountain Information Network (MIN) reports (MIN 1 and MIN 2) from Friday, Dec 3 in Quartz Creek report deep persistent avalanches failing at the base of the snowpack. These observations highlight the weak and reactive snow at the base of the snowpack.

A MIN report submitted on Sunday Dec 5 from the Brewer Creek area reported a column test failure on the Nov 21 SH down 60cm. Several recent avalanches that occurred in the Toby Creek drainage are thought to have failed on the same layer. This is believed to be lurking around the Toby Creek drainage area, however data is sparse this early in the season and this layer is believed to extend to other areas.

Snowpack Summary

10-40cm of low density snow sits over a supportive crust from the recent rain event on all aspects below 2400m. Westerly winds have redistributed new snow into deeper deposits in wind loaded features at higher elevations.

A surface hoar layer extends throughout the Toby Creek drainage and surrounding areas - down 30 to 70 cm in sheltered features at treeline and below. Recent snowpack tests have shown this layer to produce sudden propagating fractures. We have limited knowledge of the distribution of this layer, but suspect is its most active between 1900-2400m.  

A late October facet/crust layer of concern sits at the bottom of the snowpack above 1900m. This layer has been reactive to human and explosive triggers, producing large avalanches. This layer is widespread throughout the region and will likely continue to be a layer of concern.

Average snowpack depth at treeline is 100-180cm, with the deepest snowpack found near the Bugaboos. 

Terrain and Travel

  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Avalanche hazard may have improved, but be mindful that deep instabilities are still present.
  • Avoid shallow snowpack areas, rock outcroppings and steep convex terrain where triggering is most likely.

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.