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Avalanche Forecast

Archived

Dec 22nd, 2020–Dec 23rd, 2020

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 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.

A widespread avalanche cycle occurred in response to the recent storm. Large storm slabs remain primed for human triggering on Wednesday. Conservative terrain selection is crucial as the snowpack slowly settles and stabilizes. 

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast. Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

A high-pressure system off the coast will bring generally cooler, more seasonal-like temperatures and clearing to the region.

Wednesday/ Thursday: Mainly sunny and alpine temperatures near -7. Ridgetop wind 25km/hr from the West. Slight alpine temperature inversion. Freezing levels valley bottom.

Friday: Cloudy with sunny periods. Alpine temperatures near -5 and freezing levels 1000 m. Ridgetop wind light from the southwest.

Avalanche Summary

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 be reactive, especially where the wind stiffens the new snow. Deeper slabs will be found on lee slopes.

A persistent slab 80-130 cm thick now sits on the early December crust. There is increasing evidence that 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. There is uncertainty if and where in the region this may be a problem. 

Terrain and Travel

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

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.