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Avalanche Forecast

Archived

Nov 30th, 2020–Dec 1st, 2020

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

Regions

South Columbia.

10 cm of new snow and moderate northwest winds on Monday night may create fresh wind slabs on lee features at treeline and above. Lingering wind slabs may remain surprisingly reactive to human triggers in areas where they are sitting on a layer of small surface hoar.  

Confidence

Low - Uncertainty is due to the limited number of field observations.

Weather Forecast

  

MONDAY NIGHT: Clearing skies / Moderate to strong, northwest ridgetop wind / alpine low temperature -12 / Freezing level valley bottom.

TUESDAY: Sunny / Light to moderate, northwest ridgetop wind / alpine high temperature -3 / Above freezing layer of air moving in late afternoon and creating a strong temperature inversion with warm air above roughly 2000 m.

WEDNESDAY: Sunny / Light, south ridgetop wind / alpine high temperature +1 / Strong inversion with an above freezing layer of air above 2000 m.

THURSDAY: Mix of sun and cloud / Light, southwest ridgetop wind / alpine high temperature +1 / Strong inversion with an above freezing layer of air above 2000 m.

Avalanche Summary

There have been very few reports of avalanches in the region, however, observations are very limited at this time. 

Several recent MIN reports from neighboring Glacier National Park have reported human triggered avalanches on small surface hoar below the recent storm snow.

If you get out into the mountains, please consider sharing your observations on the Mountain Information Network!

Snowpack Summary

10 cm of new snow and moderate northwest winds on Monday night may create fresh wind slabs on lee features at treeline and above. Recent strong southwest winds created wind slabs that are still lingering on lee features at treeline and above. These wind slabs may be surprisingly reactive in areas where they are sitting on a layer of small surface hoar. There are a couple of crusts in the mid snowpack, from mid and early November. The most prominent of these is from early November, and it has been reported that there are weak, sugary facets above and below this crust. It is down approximately 70-100 cm, and is most likely to be reactive to human triggers on south facing slopes and in shallow, rocky, snowpack areas in the alpine.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Avoid lee and cross-loaded terrain.

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.