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Avalanche Forecast

Archived

Dec 5th, 2021–Dec 6th, 2021

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

Regions

Purcells.

Several weak layers exist in the Purcell snowpack. Stick to simple terrain and exercise careful route-finding in avalanche terrain. 

Confidence

Moderate - Recent weather patterns have resulted in a high degree of snowpack variability within the region.

Weather Forecast

SUNDAY NIGHT: Partially cloudy. Moderate westerly winds. Freezing level valley bottom. 

MONDAY: Partly cloudy with isolated flurries. Strong westerly winds. Alpine high temperature -12 C. Freezing level valley bottom. 

TUESDAY: Increasing cloud. Moderate to strong southwest winds. Alpine high temperature -12 C. Freezing level valley bottom.

WEDNESDAY: Cloudy with strong westerly winds and light snow. Freezing level below 1000m. 

Avalanche Summary

The Purcells currently has a complex snowpack - with several reactive persistent weak layers in the mid and lower snowpack.

Two Mountain Information Network (MIN) reports (MIN 1 and MIN 2) on Friday, Dec 3 from Quartz Creek report deep persistent avalanches failing at the base of the snowpack. While its unknown if the images MIN 2 cover the same rider-triggered avalanche avalanche reported in MIN 1, they certainly highlight weak and reactive snow at the base of the snowpack.

A MIN report submitted yesterday from the Brewer Creek area reported a column test failure on the Nov 21 SH down 60cm. Several avalanches that occurred in the Toby Creek drainage last week 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.

The recent pounding from the atmospheric river delivered heavy precipitation, strong winds, and warm weather. Widespread natural avalanches were observed following each wave of the storm, up to size 3. Explosive control work throughout also produced storm slab avalanches to size 2.5.

Snowpack Summary

10-40cm of low density snow sits over a supportive melt freeze crust, observed up to 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 60 cm in sheltered features at treeline and below. Recent snowpack testing has 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 1900-2400 m.  

A late October facet/crust layer of concern sits at the bottom of the snowpack above 1900m. This layer has been recently reactive to human and explosive triggers, and is reactive to snowpack tests. 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

  • In times of uncertainty conservative terrain choices are our best defense.
  • Be alert to conditions that change with elevation and wind exposure.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.

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.