Avalanche Forecast

Archived

Dec 25th, 2020–Dec 26th, 2020

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 unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

North Columbia.

Deeply buried weak layers continue to be a concern. If triggered, avalanches running on these weak layers will likely be large and destructive. Choose conservative terrain.

Confidence

Low - Uncertainty is due to the limited number of field observations. Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

FRIDAY NIGHT: Cloudy with isolated flurries, light south wind, alpine temperature near -10 C.

SATURDAY: Scattered flurries, accumulation 5-10 cm, light to moderate southwest wind, alpine temperature near -8 C.

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

MONDAY: Mix of sun and cloud, light west wind, alpine temperature near -10 C.

Avalanche Summary

Thursday's reporting showed a naturally triggered size 3.5 persistent slab avalanche running in the alpine on a southeast aspect. There was also a report of size 2.5 naturally triggered windslab in the alpine on a south aspect.

On Wednesday there was a report of several size 3 to 4 explosives controlled persistent slab avalanches in the south 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.

Snowpack Summary

Snow falling last week has been redistributed by winds blowing from a veriety of directions at upper elevations. Underneath this storm snow there is a sandwich of weak layers which are widespread throughout the region.

About one meter below the surface is the mid December surface hoar. This layer has recently been sensitive to human triggering.

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

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.
If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

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.