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Avalanche Forecast

Archived

Dec 6th, 2021–Dec 7th, 2021

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

Regions

Yukon.

Avoid all avalanche terrain during periods of heavy loading from new snow and wind. This storm will be a good test for the deeply buried persistent weak layer and very large natural avalanches are possible Tuesday.

Confidence

Low - Uncertainty is due to the limited number of field observations. Uncertainty is due to the track & intensity of the incoming weather system.

Weather Forecast

A storm begins to bear down on the region Monday night and is expected to deliver a significant amount of new snow and heavy wind through Tuesday evening.

MONDAY NIGHT: Snow, temperature around -15 C, strong to extreme southwest wind, 5 to potentially 15 cm of snow.

TUESDAY: Snow, heavy at times, 10 to 15 cm of snow expected during the day, temperature around -12 strong to extreme southwest wind. Potential for another 5 to 10 cm of snow Tuesday night.

WEDNESDAY: Broken cloud cover, daytime high temp around -10 C, light wind generally out of the south, trace of snow possible.

THURSDAY: Scattered cloud cover, daytime high temp around -12 C, light wind generally out of the west, trace of snow possible.

Avalanche Summary

Tuesday's storm adds a significant new load to our juvinile snowpack in the form of new snow and wind. A natural avalanche cycle is expected and the deeper weak layers in the snowpack are guilty until proven innocent. We may see some deep natural avalanche activity Tuesday when the storm is in full force.

Avalanche control work on Saturday produced many large wind slab avalanches on NE facing features around 1700 m. Most of these avalanches stepped down to weak facets and rocks.

On Nov 30, 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

As of Monday afternoon, the White Pass has picked up 5 new cm of snow. 5 to 15 additional cm are expected tonight and then as much as 15 cm is incoming tomorrow with strong to extreme southwest wind. This combo will form a fresh round of storm slabs that could be quite touchy, especially during the storm.  

Snow depth at treeline is approximately 80-120 cm, depths in the alpine range from 100-200 cm.

A facet/crust layer can be found near the bottom of the snowpack. This layer was reactive to human triggering last week with a notable skier-remote avalanche on November 30th. Tuesday's storm will be a good test for this interface.

Terrain and Travel

  • Avoid all avalanche terrain during periods of heavy loading from new snow and wind.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • Stick to simple terrain features and be certain your location isn't threatened by overhead hazard.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.