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Avalanche Forecast

Archived

Dec 19th, 2021–Dec 20th, 2021

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

Regions

Lizard-Flathead.

Avoid wind loaded areas where human triggering remains a concern. If winds pick up today, there is a lot of snow available for transport to create fresh, reactive slabs. 

A buried persistent weak layer continues to warrant careful terrain selection and diligent decision making.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Sunday Overnight: Light northerly winds and partially cloudy skies. Temperatures dropping to -15 C in the alpine. 

Monday: Partially cloudy with a chance of light flurries. Alpine temperatures around -12 C. Light to moderate westerly winds at ridgetop.

Tuesday: Overnight temperatures dropping to around -15 C. A mix of sun and cloud with moderate to strong southwest winds at ridgetop. Temperatures rising with an alpine high of -8 C. Cloud cover increasing into the evening.

Wednesday: Mainly cloudy and snowing, 5-20 cm accumulation possible. Moderate to strong southwest winds at ridgetop. Alpine temperatures rising to around -4 C by the afternoon.

Avalanche Summary

On Sunday, operators reported a natural avalanche cycle from overnight, with avalanches up to size 3 running almost full path. Explosive control work produced size 1-2.5 storm slabs in the alpine and treeline. 

On Sunday, one notable ski cutting result produced a size 1.5 persistent slab avalanche at treeline on the early December crust.

On Saturday, operators reported numerous size 1-2 loose dry and storm slab avalanches.

Snowpack Summary

An intense storm came through the region on Saturday, bringing 40-70 cm of new snow. This new snow has started to settle out, with cold temperatures decreasing slab properties. In wind loaded areas, wind slabs continue to be a concern and remain reactive to human triggering. 

Below this new snow, 50-100 cm of more consolidated snow sits over a substantial crust that formed in early December. This crust is up to 20 cm thick and is present across aspects below 2400m. A layer of weak facets (sugary snow) can found above this crust. This problem is particularly hard to predict and tricky to manage. For this reason wide, conservative terrain margins and disciplined backcountry travel techniques will be very important. Get more details and photos in our forecaster blog. 

The lower snowpack consists of a variety of early season crusts and mainly moist snow. Snowpack depths range from 100-200 cm at treeline elevations and taper quickly below 1800 m.

Terrain and Travel

  • Pay attention to the wind, once it starts to blow fresh sensitive wind slabs are likely to form.
  • Be careful as you transition into wind affected terrain.
  • Fresh snow rests on a problematic persistent slab, don't let good riding lure you into complacency.

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.