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Avalanche Forecast

Archived

Dec 21st, 2021–Dec 22nd, 2021

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Lizard-Flathead.

Another winter storm with strong wind and heavy snowfall will build fresh and reactive storm slabs by Wednesday afternoon and Thursday.

Confidence

Moderate - Uncertainty is due to the timing, track, & intensity of the incoming weather system.

Weather Forecast

Expect some enhanced snowfall amounts Wednesday through Thursday as a warm and wet air mass rises from the South and collides with the predominant colder air mass sliding down from the North.

Tuesday Night: 10 cm of new snow accompanied by strong ridgetop wind from the South West. Alpine temperatures near -8 and freezing levels at the valley bottom.

Wednesday: New snow throughout the day, heavy at times with 15-20 cm accumulation. Strong to extreme ridgetop wind and alpine temperatures near -5. Freezing levels 900 m.

Thursday: Heavy snowfall amounts, possibly up to 30 cm accompanied by strong SW winds. Alpine temperatures near -5 and freezing levels rising to 1000 m.

Friday: Mostly cloudy with some flurries. Gusty ridgetop winds from the South and freezing levels dropping back to the valley bottom. 

Avalanche Summary

On Monday, operators ski cut storm slab and loose dry avalanches up to size 1. 

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.

Natural avalanche activity will be likely through the forecast period with new storm snow and wind loading. . 

Snowpack Summary

New snow and wind will likely build reactive storm slabs through Friday. This new snow will bury older wind slabs from last weekend's storm. 

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 all aspects below 2400m. A layer of weak facets (sugary snow) can be 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

  • Avoid all avalanche terrain during periods of heavy loading from new snow and wind.
  • Watch for fresh storm slabs building throughout the day.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

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.

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.