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Avalanche Forecast

Archived

Dec 16th, 2021–Dec 17th, 2021

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

Purcells.

As the sun comes out today, solar input has the potential to substantially increase the reactivity of a buried persistent weak layer. This will demand conservative terrain travel and diligent decision making. Read more about this problem in our forecaster's blog here.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast. Uncertainty is due to the timing or intensity of solar radiation and its effect on the snowpack.

Weather Forecast

Thursday Overnight: A cloudy evening with flurries, trace to 5cm of accumulation. Clearing overnight with temperatures plummeting into the -15C to -20C range in the alpine. Light to moderate northwest winds at ridgetop.

Friday: Clear skies and cold temperatures. Alpine temperatures in the -15C to -20C range, a slight temperature inversion is possible in the morning. Light to moderate northwest winds at ridgetop. Cloud cover and winds increasing overnight, trace to 10 cm of new snow accumulation

Saturday: A mainly cloudy day with flurries, trace to 10cm of accumulation. Strong to extreme southwest winds at ridgetop and alpine temperatures around -12 C. 

Sunday: Partially cloudy with isolated flurries. Light to moderate westerly winds at ridgetop with alpine temperatures around -13 C. 

Avalanche Summary

On Wednesday, avalanche control triggered a very large (size 3) avalanche on a northwest aspect around 2050 m that released on the early December weak layer. 

On Tuesday, a large (size 2) avalanche was remotely-triggered on this same layer by a skier. This occurred on a west aspect near 2100 m and broke 100 cm deep. 

On Sunday, a small, skier remote triggered avalanche was reported that failed on a reloaded bed surface near the base of the snowpack on a north aspect in the alpine. There were other deep failures reported in this region about a week ago. This information is spotty, but it is consistent with a deep persistent problem that may be difficult to trigger. We're not done worrying about this one yet.

Snowpack Summary

Between 20-40 cm of snow has accumulated this week. Periods of moderate wind from varying directions this week have created wind slabs in the alpine and open treeline . With snow available for transport, be aware of the potential for today's northwest wind to form new reactive wind slabs in lee terrain features.

Below this layer, consolidated snow from the previous weekend storm sits over a substantial crust that formed in early December. This crust is 10-40 cm thick and can be found down 40-80 cm. It is present across aspects below 2300m. A thin layer of weak facets (sugary snow) can found above this crust that has demonstrated reactivity in snowpack tests and recent avalanche activity. 

A late October facet/crust layer of concern sits at the bottom of the snowpack above 1900m. Although this layer has been trending less reactive, it remains on our radar.

The snowpack depth at treeline is around 115-200 cm. The deepest snowpack can be found near the Bugaboos. 

Terrain and Travel

  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • In times of uncertainty conservative terrain choices are our best defense.

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.