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Avalanche Forecast

Archived

Dec 4th, 2021–Dec 5th, 2021

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

Regions

Yukon.

Stiff wind slabs formed by cold temperatures should be approached with caution. Although wind slabs may be stubborn to trigger, they do have the potential to step down to deeper layers.

Confidence

Moderate - Uncertainty is due to the limited number of field observations. Uncertainty is due to the fact that deep persistent slabs are particularly difficult to forecast.

Weather Forecast

Saturday Overnight: Partially cloudy with alpine temperatures reaching -30 C overnight. Light northwest winds shift west in the early morning and increase to strong at ridgetop. 

Sunday: Increasing cloud throughout the day with up to 2 cm of new snow accumulation. Moderate to strong westerly winds. Alpine temperatures rise to -22 C. 

Monday: An incoming frontal system will bring a cloudy day with strong southwest winds. Up to 10 cm of new snow throughout the day, heaviest in the afternoon. Temperatures rise to -10 C by the afternoon.

Tuesday: A cloudy day with continuous snowfall. Up to 30 cm of accumulation may occur by the end of Tuesday night accompanied by strong southwest winds. Alpine temperatures around -10 C. 

Avalanche Summary

Lots of whumping and cracking have been reported across the region, which is most likely associated with an early-season shallow snowpack, wind slabs, and a weak facet/crust layer at the base of the snowpack.

On Tuesday, a size 2.5 skier-remote persistent slab was reported on the MIN (see report here). The avalanche was triggered from around 100m away, by a group of 5 riders on an east aspect at 1400m. The avalanche was suspected to have failed on an early-season facet/crust layer.  

Snowpack Summary

Varied wind directions in the past week will create a mix of old and new wind slabs on all aspects of the alpine and treeline. Expect these wind slabs to be quite stiff from the drop in temperatures on Saturday. 

Snow depth at treeline is approximately 80-120 cm, depths in the alpine range from 100-160 cm.

A facet/crust layer can be found near the bottom of the snowpack. This layer has been reported as being reactive to human triggering in the past week, with a notable skier-remote avalanche on Tuesday.

Terrain and Travel

  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.
  • Back off if you encounter whumpfing, hollow sounds, or shooting cracks.

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.

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.