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Avalanche Forecast

Archived

Dec 11th, 2021–Dec 12th, 2021

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

Regions

North Rockies.

Recently formed storm slabs are expected to remain reactive to human triggers on Sunday and conservative terrain selection remains essential. Use extra caution around steep south-facing slopes if the sun is shining bright in the afternoon. 

Confidence

Moderate - Uncertainty is due to field data and reports showing a wide variation in conditions throughout the region.

Weather Forecast

A ridge of high pressure is expected to build on Sunday bringing cold and dry conditions to the region for the next few days.

Saturday Overnight: Partly cloudy, patchy snow flurries 1-5 cm, moderate SW winds, treeline temperature around -12 °C.

Sunday: A mix of sun and cloud, light to moderate SW winds, treeline high around -8 °C. 

Monday: A mix of sun and cloud, light E winds, treeline high around -12 °C. 

Tuesday: Mostly sunny, light to moderate SE winds, treeline high around -15 °C. 

Avalanche Summary

On Friday, the North Rockies field team reported shooting cracks and were triggering small slabs with ski cuts near treeline in the Pine Pass area. Explosive control on Wednesday and Thursday produced wind slabs in exposed high elevation terrain up to size 2.5. 

On Sunday, the recent storm snow is expected to remain reactive to human triggering, especially in wind affected areas. If the sun is shining on Sunday afternoon, the reactivity of the slab may increase on south facing slopes. While the basal weakness appears to be gaining strength in the region, it still may be reactive to heavy triggers and a smaller storm slab avalanche still has the potential to step down to a deeper layer resulting in a much larger avalanche. 

Snowpack Summary

On Friday and Saturday, up to 20 cm of new snow was reported bringing the weekly storm snow accumulation to 50-70 cm. This new snow came in with strong winds from the southwest which formed highly reactive slabs in wind exposed terrain. Below the past week's storm snow, some surface hoar had been observed in sheltered areas at and below treeline but this appears to be isolated and has not been reactive in recent days. The November 27th rain crust is now buried 40-100cm and extends as high as 1500m elevation. 

The lower snowpack consists of a series of early season crusts. Cold temperatures will have promoted faceting around these crusts. Shallow alpine slopes along the eastern side of the Rockies towards Jasper may have weaker, faceted snow near the ground. 

Snowpack depths at treeline range from 60cm-200cm, with the shallower value mainly on the eastern side of the range. The alpine snowpack ranges from 150cm-200cm. 

Terrain and Travel

  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
  • Minimize exposure to sun-exposed slopes when the solar radiation is strong.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.