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Avalanche Forecast

Archived

Dec 9th, 2020–Dec 11th, 2020

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

Regions

North Rockies.

Remain cautious in steep terrain at higher elevations. Weather trends suggest the snowpack has gained strength, but there is uncertainty about the likelihood of triggering large avalanches.

Confidence

Moderate - Confidence is due to a stable weather pattern; little change is expected for several days. Uncertainty is due to limitations in the field data.

Weather Forecast

A building ridge will bring cool and clear conditions for the next few days. 

WEDNESDAY NIGHT: Partly cloudy, moderate northwest wind, alpine temperatures drop to -10 C.

THURSDAY: Mix of sun and cloud, light wind, high temperatures around -5 C.

FRIDAY: Mix of sun and cloud, moderate west wind, high temperatures around -7 C.

SATURDAY: Mix of sun and cloud, moderate northwest wind, high temperatures around -10 C.

Avalanche Summary

Reports from Tuesday suggest there were some cornices failures that triggered loose avalanches on the slopes below (see this MIN report). There were also some wet loose avalanches near valley bottoms. While the warm wet storm earlier this week likely caused some natural storm and wind slab avalanches, these problems have become less likely.

There was an avalanche fatality near Pine Pass on Saturday, November 28. Reports indicate a size 3 deep persistent slab avalanche at approximately 1600 m elevation on a northeast aspect. The avalanche may have run on a weak layer of faceted crystals sitting on a crust that was buried in early November.

Snowpack Summary

The warm storm earlier this week has reshaped the snowpack. The alpine is heavily wind-affected with quickly strengthening wind slabs. Below 1500 m the snowpack is rain soaked and likely forming a thick surface crust. At elevations in between there is 10-30 cm of recent snow sitting above old interfaces. There is a chance the snowpack at these intermediate elevations could be bonding poorly, but weather trends suggest they should be strengthening.

The main uncertainty right now is whether there is weak snow around the crust that formed in early November. This layer is now buried 50-150 below the surface, and with little data is is best to remain cautious around large high consequence slopes, especially if they are thin and rocky. 

Snow depths are approximately 150 cm in the northwest of the region (Pine Pass), 180 cm in the south (Renshaw, Kakwa) and 100 cm in the the northeast (Tumbler Ridge). 

Terrain and Travel

  • Be careful with wind slabs, especially in steep, unsupported and/or convex terrain features.
  • Avoid steep convexities or areas with a thin, rocky, or variable snowpack.
  • 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.

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.