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Avalanche Forecast

Archived

Dec 23rd, 2020–Dec 24th, 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

Lizard-Flathead.

The recent storm snow sits on a buried weak layer 100 cm down. This weak layer may be at a tipping point just waiting for a trigger. Conservative terrain selection is crucial until we have good evidence that the layers have stabilized.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

An upper ridge of high pressure will generally keep the region cool and dry through the forecast period. A weak temperature inversion may slightly warm ridgetop temps on Thursday.

Thursday/Friday: Mix of sun and cloud. Alpine temperatures near -4 and freezing levels valley bottom -6. Ridgetop wind light from the southwest.

Saturday: Light snow 5-10 cm. Alpine temperatures -3 and freezing levels valley bottom. Ridgetop wind moderate from the southwest.

Avalanche Summary

On Wednesday numerous storm slabs up to size 1.5 were triggered with explosives. 

On Tuesday, a widespread natural avalanche cycle up to size 3 was reported from the lizard range. 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. Storm slabs will likely remain reactive to human triggers, especially where the wind stiffens the new snow. Deeper slabs will be found on lee slopes due to strong winds.

A persistent slab 80-130 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 is reaching a tipping point. 

Deeper in the snowpack are two hard melt-freeze crusts that formed in November that may have some weak crystals around them. This potential avalanche problem is dormant at this time, however; it remains on our radar.

Terrain and Travel

  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • Shooting cracks, whumphs and recent avalanches are strong indicators of an unstable snowpack.
  • Don't let the desire for deep powder pull you into high consequence terrain.
  • Good day to make conservative terrain choices.

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.