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Avalanche Forecast

Archived

Dec 24th, 2020–Dec 25th, 2020

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

Regions

South Rockies.

Warming temperatures and sunshine may be enough to stiffen the slab and increase the reactivity for natural and human triggered avalanches. Loose dry avalanches from steep terrain may also occur and run far and fast downslope. 

Confidence

Moderate - Uncertainty is due to the extreme variability of wind effect on the snowpack.

Weather Forecast

Friday: Mix of sun and cloud. Alpine temperatures near -1 and freezing levels 1700 m. Ridgetop wind moderate to strong from the southwest.

Saturday: Mix of sun and cloud with a trace of new snow. Alpine temperatures near -5 and freezing levels valley bottom. 

Sunday: Cloudy with isolated flurries. Alpine temperatures near -6 and freezing levels 1100 m and dropping to valley bottom overnight. 

Avalanche Summary

No new reports on Thursday at the time of publishing.

Numerous natural wind slab avalanches up to size 2 were reported today from our field team. These were mostly seen from West aspects in the alpine. 

On Tuesday widespread natural avalanche activity up to size 3 was reported through the region.

Natural avalanche activity may taper off a bit but the snowpack remains primed for human triggering. 

Please consider sharing your observations on the Mountain Information Network. Thank you to those that have already submitted this winter!

Snowpack Summary

30-50 cm of recent storm snow blanketed the region by Tuesday morning. Recent strong to extreme southwest wind has redistributed some of this new snow building touchy wind slabs on leeward slopes. A persistent slab 70-100 cm thick now sits on the early December crust. This persistent weak layer, with facetted crystals and surface hoar crystals above and/or below it may be reaching a tipping point. Smaller avalanches may step-down to this weak layer. 

The base of the snowpack consists of a hard melt-freeze crust from early-November that may also have weak crystals around it. This potential avalanche problem is dormant at this time, however; it remains on our radar. The most likely spot to trigger it would be on thin and rocky slopes or, like above, from step down avalanches in the recent storm snow.

Terrain and Travel

  • Shooting cracks, whumphs and recent avalanches are strong indicators of an unstable snowpack.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Uncertainty is best managed through conservative terrain choices at this time.

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.