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Avalanche Forecast

Archived

Dec 18th, 2021–Dec 19th, 2021

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

Regions

Yukon.

The White Pass is heavily wind effected and conditions don't look that inviting. The 5 cm of snow that fell Friday is likely being formed into small wind slabs that sit on a mix of sastrugi and old wind slab. Remain mindful that a deep persistent slab is present.

Confidence

Low - Uncertainty is due to the limited number of field observations. Uncertainty is due to the extreme variability of wind effect on the snowpack.

Weather Forecast

SATURDAY NIGHT: Light northeast wind, overnight low temperature around -22 C, no snow expected.

SUNDAY: A few clouds, daytime high temperature around -18, moderate north wind, no snow expected.

MONDAY: A few clouds, daytime high temperature around -16, moderate to strong northwest wind, no snow expected during the day, a few cm possible Monday night.

TUESDAY: Broken cloud cover, daytime high temperature around -12, strong north wind, a few cm of snow possible.

Avalanche Summary

No recent avalanche observations have been reported.

On December 1, a size 2.5 avalanche was remotely triggered by a recreational group on Log Cabin. This avalanche likely failed on the weak, sugary snow at the bottom of the snowpack. 

Triggering the deep layer (see the December 1 avalanche described above) is most likely to occur in shallow areas or where the snowpack variable in depth with a mix of shallow and deep zones. 

Snowpack Summary

The white pass is looking heavily wind effected as evidenced in this MIN. Recently winds have been out of the southwest and northwest, there may be small wind slabs in areas where there remains snow available for transport. 

The middle snowpack is supportive to travel but snow depth still varies greatly with aspect and elevation. 

A layer of weak, sugary snow at the base of the snowpack has been observed in most areas. This layer was reactive earlier in December, and it might still be possible to trigger this deeply buried layer in shallow snowpack areas. A cornice fall or surface avalanche impacting a slope could also trigger it. If triggered, this weak layer at the bottom of the snowpack could produce large avalanches. 

Terrain and Travel

  • Be aware of the potential for large avalanches due to the presence of a deep persistent slab.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.
  • This avalanche problem is difficult to trigger but if so, consequences are serious.
  • Watch for areas of hard wind slab on alpine features.

Problems

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.

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.