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Avalanche Forecast

Archived

Dec 22nd, 2021–Dec 23rd, 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 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.

Regions

Purcells.

 New snow accompanied by strong southwest wind will likely build fresh and reactive storm slabs, especially on leeward slopes that are wind-loaded.  

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. Possible heavier snowfall amounts in the Southern part of the region on Thursday with the passing cold front.  

Wednesday Night: 5- 10 cm of new snow accompanied by strong ridgetop wind from the southwest. Alpine temperatures near -7 and freezing levels 900 m.

Thursday: New snow throughout the day, with 10-15 cm accumulation. Strong ridgetop wind from the southwest. Alpine temperatures near -5 and freezing levels 1200 m. 

Friday: Broken skies with flurries 5 cm with moderate to strong southwest wind. Alpine temperatures near -10 and freezing levels valley bottom.

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

Avalanche Summary

On Wednesday, a few natural and human-triggered wind slab avalanches up to size 1 were reported. 

New snow and cranking wind will likely form reactive slab avalanches on Thursday. Loose-dry sluffing may be seen from steep terrain features.

Snowpack Summary

New snow and wind will likely build reactive storm slabs. Older wind slabs at upper elevations below ridgecrest may continue to be touchy with the additional load. Low-density storm snow exists in sheltered terrain. 

Below the new snow exists a well-consolidated upper snowpack which overlies a substantial crust that formed in early December. This crust is approximately 60-120 cm below the surface, 10-30 cm thick on average, and is present across all aspects to at least 2300 m. A layer of weak facets (sugary snow) has been reported above this crust. Snowpack tests are generally showing hard results on this interface.

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 120-250 cm. The deepest snowpack can be found in the southern half of the region. 

Terrain and Travel

  • Watch for fresh storm slabs building throughout the day.
  • Avoid all avalanche terrain during periods of heavy loading from new snow and wind.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.

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.