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Avalanche Forecast

Archived

Dec 24th, 2020–Dec 25th, 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 unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

South Columbia.

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

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

THURSDAY NIGHT: Clear periods, light southwest wind, alpine temperature near -8 C.

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

SATURDAY: Scattered flurries, accumulation 5-10 cm. light southwest wind, alpine temperature near -7 C.

SUNDAY: Mainly cloudy with sunny breaks, light west wind, alpine temperature near -8 C.

Avalanche Summary

On Wednesday there was a report of several size 3 to 4 explosives controlled deep persistent and persistent slab avalanches in the north of the region along the Highway 1 corridor. These ranged from south and east aspects to northeast running from the alpine to the valley floor.

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

60 to 100 cm of snow fell 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 had 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.

Terrain and Travel

  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Remote triggering is a concern, watch out for adjacent and overhead slopes.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Uncertainty is best managed through conservative terrain choices at this time.

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.

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.