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Avalanche Forecast

Archived

Dec 23rd, 2020–Dec 24th, 2020

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

Regions

South Columbia.

The load sitting on a variety of buried weak layers has reached a tipping point. If triggered avalanches running on these weak layers will likely be large and destructive. Choose conservative terrain in the coming days.

Confidence

High - Confidence is due to a stable weather pattern with little change expected. The number, quality, or consistency of field observations is good, and supports our confidence.

Weather Forecast

WEDNESDAY NIGHT: Mostly clear skies, light to moderate southwest wind, alpine temperature near -6 C. 

THURSDAY: Mainly sunny, light southerly wind, alpine temperature near -3 C with a chance of a weak temperature inversion.

FRIDAY: Mix of sun and cloud, light to moderate southwest wind, alpine temperature near -5 C. 

SATURDAY: Mainly cloudy with scattered flurries, light west wind, alpine temperature near -7 C.

Avalanche Summary

Reports on Tuesday show a widespread storm slab and persistent slab avalanche cycle in the alpine and at treeline. Avalanches sizes ranged from size 1.5 to 3 on all aspects. Additionally reports from Tuesday in Rogers Pass indicate several natural and explosives controlled persistent slab avalanches to size 3.5-4 running full path from the alpine to the valley floor Monday night. 

On Monday there were several reports of persistent slab avalanches running size 2.5 to 3 at treeline and in the alpine on east and west aspects.

Snowpack Summary

An active storm cycle has produced 60 to 100 cm of storm snow over the last week. Underneath this storm snow there is a sandwich of weak layers which are widespread throughout the region.

Just under the new snow, down about one meter below the surface is the mid December surface hoar. This layer has been failing naturally and has been quite sensitive to human triggering throughout the storm cycle. Large avalanches have been running on this interface connecting big terrain features. 

Below that lies the early December persistent weak layer, it's about 80 to 140 cm below the surface. This weak layer is a mixed bag that often presents as surface hoar and facets sitting on top of a supportive crust. This is a nasty weak layer, when avalanches fail on it, they have been large and destructive. Smaller avalanches in motion can step down to this one too. This weak layer is going to haunt us for the foreseeable future. 

Terrain and Travel

  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Remote triggering is a concern, watch out for adjacent and overhead slopes.
  • Uncertainty is best managed through conservative terrain choices at this time.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.