Avalanche Forecast

Issued: Feb 4th, 2021 4:00PM

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

Avalanche Canada swerner, Avalanche Canada

Recent snow and strong wind may add cohesion to the upper snowpack creating a stiffer and reactive slab 30-70 cm thick. Obvious clues of instability may not exist, making this problem harder to predict. Conservative terrain choices are the best way to manage it.

 

Summary

Confidence

Moderate - Uncertainty is due to whether buried persistent weak layers become active, triggering avalanches, with the arrival of the forecast weather.

Weather Forecast

Friday: Cloudy with isolated flurries. Alpine temperatures near -6 and light wind from the west-northwest. Freezing levels at valley bottom.

Saturday: Snow 10 cm. Alpine temperatures near -10 and ridgetop wind moderate to strong from the northeast. 

Sunday: Mix of sun and cloud. Alpine temperatures falling to a low of -21 and a high of -14. Ridgetop wind light from the northeast. 

 

Avalanche Summary

On Thursday, reports indicated several small features were reactive up to size 1 on the late-January buried surface hoar interface. I suspect that once the persistent slab gains more cohesion and stiffens, this problem may be more widespread. Even a small avalanche can catch you by surprise and have enough mass to push you into a terrain trap below. 

Fresh wind slabs and storm slabs may be touchy on Friday, especially where they sit above surface hoar, crust, or old surface facets. 

Snowpack Summary

Up to 20 cm of recent snow fell by Thursday bringing 30-70 cm of accumulative storm snow from the past week over a variety of older snow surfaces. These old surfaces, at upper elevations (upper treeline and the alpine), include surface hoar in locations sheltered from the wind, surface facets, and stiff wind affected snow. At lower elevations (lower treeline and below treeline) 10-20 cm of snow sits above isolated pockets of surface hoar and a crust that is more predominant on solar aspects. Strong winds have stiffened the surface snow and have formed reactive wind slabs, especially in areas where they sit above the buried surface hoar. This recent MIN report is a great example of that. 

Additional snow and wind combined with cohesion may stress these potentially weak interfaces, creating the persistent slab problem. 

The mid-pack seems to be well settled. Deep persistent layers appear to have mostly gone inactive with the exception of the Bear Pass area and the far reaches south of Kitimat that have seen some sporadic avalanche activity on weak snow near the base of the snowpack and triggered by large loads such as explosives, icefalls or cornice collapses.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Watch your sluff: it may run faster and further than you expect.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.

Problems

Wind Slabs

An icon showing Wind Slabs

Moderate southwesterly wind continues to redistribute the new snow and build fresh wind slab in lee terrain features in the alpine and exposed treeline. Wind slabs will be particularly reactive anywhere snow overlies buried surface hoar or crust. The surface hoar layer is likely preserved in sheltered areas at and below treeline. Loose-dry sluffing may be seen from steep terrain features.

Large, looming cornices exist along ridgelines. Give them a wide berth from above and below.

Aspects: North, North East, East, South East, South.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 2

Persistent Slabs

An icon showing Persistent Slabs

30-70 cm sits above a buried weak interface that consists of surface hoar, facets, and/ or a crust. Forecast snow and strong wind will likely add cohesion and load to these weak layers resulting in a reactive persistent slab problem.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1.5 - 2.5

Valid until: Feb 5th, 2021 4:00PM