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Avalanche Forecast

Archived

Dec 17th, 2020–Dec 18th, 2020

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

Regions

Lizard-Flathead.

Slab avalanches may be primed for human triggers. Choose conservative terrain and back off slopes when you see signs of instability such as cracking, whumphing a fresh avalanche activity.

Confidence

Moderate - Uncertainty is due to whether buried persistent weak layers become active, triggering avalanches, with the arrival of the forecast weather. Uncertainty is due to the limited number of field observations.

Weather Forecast

Friday: Snow amounts 5 cm with alpine temperatures near -5. Ridgetop wind strong from the southwest and freezing levels valley bottom.

Saturday: Snow 5-10 cm with alpine temperatures near -7. Strong wind from the West and freezing levels 900 m.

Sunday: Snow amounts 10-20 cm. Alpine temperatures -1 and freezing levels 2000 m. Ridgetop wind 20-85Km/hr wind from the southwest. 

Avalanche Summary

On Thursday, a natural size 2 storm slab from NE alpine terrain was reported. As per the report, this slab likely failed on Wednesday. On Thursday, explosives were able to trigger slabs up to size 2 in the alpine and a size 1.5 at treeline.

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

Snowpack Summary

Strong wind from the southwest and up to 30 cm of new snow will likely build reactive storm slabs. The recent storm snow accumulations bring 40-60 cm above the early December crust which may have persistent weak, and facetted crystals above and below it. With slab cohesion adding stress and load on that interface, we may see this persistent slab avalanche problem come to life.

Deeper in the snowpack are two hard melt-freeze crusts that formed in November. The concern would be if there were weak faceted grains around the crust, but there is uncertainty if and where in the region this may be a problem. Without any recent avalanche activity, it appears that this layer is dormant for the time being.

Terrain and Travel

  • Avoid areas where the snow feels stiff and/or slabby.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Choose relatively conservative lines and watch for clues of instability.

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.