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Avalanche Forecast

Archived

Dec 21st, 2020–Dec 22nd, 2020

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
Alpine
Natural and human triggered avalanches likely.
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.

A significant winter storm will lambaste the region Monday night further destabilizing the smorgasbord of weak layers in our upper snowpack. Avoid all avalanche terrain at this time, large to very large natural avalanches are expected as the snowpack adjusts to this new load.

Confidence

High - We are confident the likelihood of avalanche will increase with the arrival of the forecast weather. We have a good understanding of the snowpack structure and confidence in the weather forecast

Weather Forecast

One last pulse tonight, and then it looks like we’re moving into a bit of a clearing trend this week.

MONDAY NIGHT: Freezing level around 1000 m, strong northwest wind, 15 to 40 cm of snow expected.

TUESDAY: Broken cloud cover, freezing level near valley bottom, moderate northwest wind, 1 to 5 cm of snow possible.

WEDNESDAY: Scattered cloud cover, freezing level at valley bottom, light southwest wind, no new snow expected.

THURSDAY: A few clouds, freezing level at valley bottom with potential for a light alpine temperature inversion, light southerly wind, no new snow expected.

Avalanche Summary

Over the weekend a prolific natural avalanche cycle took place with avalanches averaging size 1.5 to 2.5, but there were a few size 3 and even size 3.5's in the mix as avalanches ran on the early December crust. Avalanches were running on both the mid December surface hoar and the early December crust/surface hoar/facet combo.

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 stout supportive crust. There are facets below the crust too. 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

  • Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.
  • Only the most simple non-avalanche terrain free of overhead hazard is appropriate at this time.
  • Remote triggering is a concern, watch out for adjacent and overhead slopes.
  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent 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.