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Avalanche Forecast

Archived

Dec 16th, 2020–Dec 17th, 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.

Storm slabs are likely reaching threshold and reactive to human triggers. Choose conservative terrain and watch for signs of instability such as whumphing, cracking and recent avalanches.

Confidence

Moderate - Forecast snowfall amounts are uncertain. Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather.

Weather Forecast

Wednesday Night: Snow amounts 10-15 cm with ridgetop wind strong from the southwest. Freezing levels 1000 m.

Thursday: Cloudy with some sunny periods. Snow amounts 5-10 cm through the day. Freezing levels 1200 m and ridgetop wind strong from the southwest.

Friday: Light to moderate snow amounts developing through the day and overnight. Moderate gusting strong southwest wind.

Saturday: Light snow through the day with more snow and rising freezing levels to 1400 m overnight. 

Avalanche Summary

On Wednesday, numerous avalanches were seen in the alpine up to size 2. With continued snow and wind, storm slabs and persistent slabs will likely remain reactive on Wednesday. Loose-dry sluffing may exist from steeper terrain features. 

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 coupled with the new snow will likely build reactive storm slabs. The recent storm snow accumulations bring 40-60 cm above the early December crust. To date, this layer has not been an avalanche concern but by Thursday it may reach its tipping point and that concern and hazard will likely increase with snow/wind load and rising freezing levels. Use caution if you find cohesive snow above the crust, particularly if there are weak, sugary faceted grains immediately above or below the crust. 

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 all avalanche terrain during periods of heavy loading from new snow and wind.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Avoid areas where the snow feels stiff and/or slabby.

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.