Avalanche Forecast

Issued: Dec 26th, 2020 4:00PM

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Wind Slabs and Persistent Slabs.

Avalanche Canada cmortenson, Avalanche Canada

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Windslabs & buried weak layers mean that we need to keep playing it cautious a bit longer in wind affected alpine terrain & exposed treeline features. Remember that a small windslab can trigger a deeper layer in a step-down. Enjoy your holiday riding & avoid the steep alpine.

Summary

Confidence

Moderate - Uncertainty is due to the limited number of field observations. Confidence is due to a stable weather pattern; little change is expected for several days.

Weather Forecast

A building high pressure system will bring a mix of sun and clouds, cool temperatures and light to moderate winds.  

Saturday night: Clear with cloudy periods. Alpine low temperatures near -7C and freezing levels valley bottom. Ridgetop wind light gusting moderate from the west.

Sunday: Mix of sun and cloud. Alpine high temperatures near -7C and freezing levels valley bottom. Ridgetop wind light gusting moderate from the west/southwest.

Monday: Mix of sun and cloud. Alpine high temperatures near -9C and freezing levels valley bottom. Ridgtop winds light and variable. A mild temperature inversion may bring valley clouds and warmer clear temperatures above.

Tuesday: A mix of sun and clouds. Alpine low temperatures near -14C and freezing levels dropping to valley bottom. Ridgetop wind moderate gusting strong from the southwest.

Avalanche Summary

No new avalanches were reported within the forecast region. However, every region surrounding the south rockies reports a similar story: wind slabs are reactive (most are size 1 but a few size 2) and sometimes step-down to trigger deeper layers (size 2-3). Wind slabs are reactive on fans, cross-loaded features and in reverse loaded terrain. Sluffing of loose dry snow is building mass to size 1 from steep wind sheltered terrain. A nearby neighbour reported a large avalanche (size 2) that initated as a windslab or cornice failure and stepped down to the December crust on Friday. Another neighbour reported a large (size 2) windslab from a west aspect from cross-loading from recent variable east winds.  

On Thursday there were reports of small (size 1) and large (size 2) windslabs from explosive control within the region and explosive controlled very large (size 3) avalanches and cornice triggered very large (size 2.5-3) avalanches from adjacent regions. The previous day, numerous natural wind slab avalanches up to size 2 were reported on Wednesday from our field team in the Crowsnest. These were mostly seen from West aspects in the alpine. On Tuesday widespread natural avalanche activity up to size 3 was reported through the region. Many of these avalanches ran deep into their runout zones.

Please consider sharing your observations on the Mountain Information Network. Thank you to those that have already submitted this winter!

Snowpack Summary

30-50 cm of recent storm snow blanketed the region by Tuesday morning from a series of recent storms. Recent strong to extreme winds have redistributed some of this new snow building wind slabs on leeward slopes and scouring windward slopes. Much of this wind was west / southwest but variable winds have been reported across the region - including northerly winds.

Below 1800m a rain crust has about 10-20cm of recent snow on top of it. Above this elevation, a deeper crust remains a concern. A deeper crust from early December is down 70-100 cm. This persistent weak layer, with facetted crystals and surface hoar crystals above and/or below it may be reaching a tipping point. Smaller avalanches may step-down to this weak layer. 

The base of the snowpack consists of a hard melt-freeze crust from early-November that may also have weak crystals around it. This potential avalanche problem is dormant at this time, however; it remains on our radar. The most likely spot to trigger it would be on thin and rocky slopes or, like above, from step down avalanches in the recent storm snow.

Terrain and Travel

  • Be alert to conditions that change with elevation and wind exposure.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Shooting cracks, whumphs and recent avalanches are strong indicators of an unstable snowpack.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Avoid exposure to slopes that have cornices overhead.

Problems

Wind Slabs

An icon showing Wind Slabs

Wind slabs remain reactive to human triggering on most aspects due to strong, changing winds and recent new snow being available for transport. Predominately west / southwest winds have loaded northerly and easterly aspects. However, assess each slope carefully for wind loading. Observe wind effect like scouring, sastrugi, cross-loading and lee deposition fro clues to how recent winds have reworked the terrain. Natural avalanches, whumphing, and shooting cracks in the snow are all indicators of a instability. 

Be on the look-out for cornice formation: these are a hazard underfoot as well as being an overhead hazard that is capable of triggering large avalanches should they fail.

Loose-dry sluffing from sheltered steep terrain is likely, especially if the sun comes out.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

1 - 2

Persistent Slabs

An icon showing Persistent Slabs

60-100 cm of snow currently sits above a weak layer from early December. Combinations of sugary facets , surface hoar and/or a crust mean persistent slabs can produce large destructive avalanches. Although naturally triggered avalanches have tapered, it remains possible to tigger this layer in thin to fat zones, with a cornice collapse, with a step-down avalanche or on slopes that have received significant wind transport. This layer appears to be more concerning above 1700m and on northerly aspects.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

1.5 - 3

Valid until: Dec 27th, 2020 4:00PM