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Avalanche Forecast

Archived

Dec 11th, 2020–Dec 12th, 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 - Uncertainty is due to the fact that deep persistent slabs are particularly difficult to forecast.

Weather Forecast

Cold arctic air pushes up along the eastern slopes of the Rockies this weekend.

FRIDAY NIGHT: Mostly cloudy, moderate northwest wind, temperatures drop to -15 C.

SATURDAY: Mix of sun and cloud with some isolated flurries along the eastern slopes, moderate northwest wind, steady temperatures around -12 C.

SUNDAY: Mix of sun and cloud, light southwest wind, high temperatures around -12 C.

MONDAY: Mostly cloudy, strong west wind, high temperatures around -10 C.

Avalanche Summary

In the wake of the storm earlier this week there were a few reports of wind slab avalanches and cornice failures in northeast facing terrain (see this MIN report from Wednesday). These wind slabs have likely gained strength since then.

While there have not been any recent reports of deep persistent slab avalanches, we can not rule out the possibility this problem still lurks in parts of the region. The crust near the bottom of the snowpack was the likely failure layer in a fatal avalanche near Pine Pass on Nov 28. With little recent information about this layer it is best to remain cautious around large high consequence slopes, especially if they are thin and rocky.

Snowpack Summary

The alpine is heavily wind-affected and lower elevations are capped by a hard rain crust (perhaps up to 1300 m). Sheltered areas at treeline may still have 5-20 cm of low density snow from earlier this week.

Snow depths are approximately 150 cm around Pine Pass, Torpy and McBride and 50-100 cm in the northeast around Tumbler Ridge. A widespread crust layer can be found at the bottom of the snowpack, and there remains to be uncertainty about if and where this layer is a problem. Recent observations from the McBride area suggest the snow is well bonded to this crust, however there is no recent information from northern parts of the region, such as Pine Pass and Tumbler Ridge, where this layer was a problem earlier this season.

Terrain and Travel

  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.
  • Be careful with wind slabs, especially in steep, unsupported and/or convex terrain features.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

Problems

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.

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.