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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

South Rockies.

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: Cloudy with flurries in the evening, 5-10cm of new snow accumulation. Clearing overnight with temperatures plummeting in the -20s in the alpine. Moderate to strong northwest winds at ridgetop. 

Friday: Clear skies and cold temperatures. Alpine temperatures between -15C to -20C with chilling moderate to strong northwest winds. Increasing cloud cover and winds in the evening with a low pressure system moving across BC.

Saturday: Mainly cloudy with flurries, 5-10cm of accumulation. Strong to extreme westerly winds at ridgetop with freezing levels rising to 1000m in the afternoon. 

Sunday: Winds and snow easing overnight, another 5cm of accumulation in the evening. A partially cloudy day with flurries. Strong southwest winds easing to light in the afternoon. 

Avalanche Summary

Over the last two day, operators have reported several large (size 2) explosive-triggered avalanche releasing on the early December crust layer. 

Observations from last weekend indicated that the snowpack was quite touchy. Avalanche control with explosives and ski cuts produced several size 2 avalanches, and one size 3. Most failed on top of the rain crust from the late November/early December rain events.

Snowpack Summary

Up to 30 cm of snow has accumulated since Monday. This unconsolidated new snow will be redistributed by northwest winds into fresh, reactive wind slabs in the alpine and treeline.

Below this layer, more consolidated snow from the previous weekend storm sits over a substantial crust that formed in early December. This crust is likely 20 cm thick (or more) and is present across aspects below 2400m. A thin layer of weak facets (sugary snow) can found above this crust that has demonstrated reactivity in snowpack tests and avalanche activity. 

Snowpack depths vary due to strong to extreme southwest winds from early December that stripped snow off of exposed areas and deposited it onto lee slopes. Below 2300m, several early season crusts are breaking down and forming a cohesive lower snowpack. Snowpack depths range from 60-110 cm at treeline elevations and taper quickly below 1900m.

Terrain and Travel

  • Pay attention to the wind, once it starts to blow fresh sensitive wind slabs are likely to form.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.

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.