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Avalanche Forecast

Archived

Dec 22nd, 2021–Dec 23rd, 2021

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.
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.

 New snow accompanied by strong southwest wind will lead to a widespread avalanche cycle on Thursday.

Confidence

Moderate - Uncertainty is due to the timing, track, & intensity of the incoming weather system. Forecast snowfall amounts are uncertain.

Weather Forecast

Continued snow through the Christmas holidays. Expect some heavier snowfall amounts Thursday with the passing cold front.  

Wednesday Night: 5- 20 cm of new snow accompanied by strong ridgetop wind from the southwest. Treeline temperatures near -5 and freezing levels 900 m.

Thursday: New snow throughout the day, with 15-25 cm accumulation. Strong ridgetop wind from the southwest. Treeline temperatures near -2 and freezing levels 1100 m. Potential for heavy amounts again Thursday night.

Friday: Flurries with new snow 5-10 cm with moderate to strong southwest wind. Treeline temperatures near -5 and freezing levels valley bottom.

Saturday: Mostly cloudy with some flurries 5-10 cm. Gusty ridgetop winds from the South and temperatures dropping to -12 with freezing levels valley bottom. 

Avalanche Summary

On Wednesday, avalanche control using explosives showed a few slab avalanches up to size 2 failing in the recent storm snow. 

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 the new snow exists a well-consolidated upper snowpack which overlies a substantial crust that formed in early December. This crust is approximately 90-150 cm below the surface, 20 cm thick on average, and is present across all aspects to at least 2400 m. A layer of weak facets (sugary snow) has been reported above this crust. Snowpack tests are generally showing hard results on this interface.

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.