Avalanche Forecast

Issued: Dec 9th, 2020 4:00PM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Storm Slabs and Deep Persistent Slabs.

Avalanche Canada shorton, Avalanche Canada

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Dangerous avalanche conditions may exist above at elevations above the rain crust. Make careful observations of in your local area and make conservative decisions.

Summary

Confidence

Low - Uncertainty is due to the limited number of field observations.

Weather Forecast

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

WEDNESDAY NIGHT: Partly cloudy, light southwest wind, alpine temperatures drop to -10 C.

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

FRIDAY: Mostly sunny, light wind, high temperatures around -7 C.

SATURDAY: Mostly sunny, moderate west wind, high temperatures around -10 C.

Avalanche Summary

As the weather clears we are uncertain about how quickly the recent snow will gain strength and bond to old interfaces. It likely sits atop a weak layer of surface hoar in wind-sheltered areas at treeline and at upper below treeline elevations (above the extent of the rain crust). Wind slabs likely linger in higher wind-affected terrain.

With little in terms of field observations, a cautious and conservative approach to terrain is needed. Be aware of these new avalanche problems by looking for signs of instability like shooting cracks and whumpfs. Furthermore, an avalanche triggered in the upper snowpack could cause a larger deep persistent slab avalanche.

Our eyes and ears in the mountains are limited this winter due to fewer professional observations. If you go out in the mountains, please share your observations on the Mountain Information Network (MIN).

Snowpack Summary

A warm storm earlier this week has likely reshaped the snowpack. Heavy rain was reported up to 1500 m, so lower elevations will now have a thick surface crust. 15-40 cm of snow fell at higher elevations, with the largest accumulations around Valemount and Blue River. In the alpine this snow has formed thick wind slabs, which should generally be strengthening quickly. Intermediate elevations (below alpine terrain and above the rain line) could have highly variable conditions and perhaps a dangerous problem developing. The most concerning thing at this elevation is if a slab of dry snow has formed above widespread surface hoar layer (or above sun crusts). Carefully assess open slopes at intermediate elevations if you see recent accumulations of dry snow.

The lower snowpack is characterized by a couple of crusts, the most notable of which is sitting near the base of the snowpack, surrounded by a weak layer of sugary facets. It is most likely to be reactive to human triggers on south facing slopes and in shallow, rocky areas in the alpine.

Terrain and Travel

  • Choose conservative terrain and watch for clues of instability.
  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Be aware of the potential for large avalanches due to the presence of a deep persistent slab.

Problems

Storm Slabs

An icon showing Storm Slabs

Storm and wind slab problems likely exist at elevations above the rain crust. They are most concerning around Blue River and Valemount where 30+ cm of snow may sit above crusts and a widespread layer of large surface hoar. There was less snow in other areas, but strong wind would have left reactive slabs on wind-loaded slopes and on steep convexities.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 2.5

Deep Persistent Slabs

An icon showing Deep Persistent Slabs

A facet/crust layer sits near the base of the snowpack. For weeks it has sat dormant, showing no avalanche activity but producing the occasional snowpack test result. We are curious to see whether the weight of the recent snow has awakened this problem. Keep in mind that if a storm slab avalanche triggers/steps down to this layer, the resulting avalanche would be large and destructive.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

2 - 3

Valid until: Dec 10th, 2020 4:00PM

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