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Avalanche Forecast

Archived

Dec 19th, 2020–Dec 20th, 2020

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

Regions

Lizard-Flathead.

New snow, strong wind and rising freezing levels will combine to make dangerous avalanche conditions. Staying out of avalanche terrain and away from overhead hazards is recommended.

Confidence

Moderate - Uncertainties in both the snowpack structure and the weather forecast limit our confidence.

Weather Forecast

SATURDAY NIGHT - Snow, heavy at times 12-25 cm / southwest wind moderate, gusting to strong / alpine low temperature near -2 / freezing level 1200-1500 m

SUNDAY - Chance of flurries with sunny breaks / gusty southwest winds / alpine high temperature near 0 / freezing level 2000 m

MONDAY - Snow/Rain 10-20 cm / southwest wind, 30-40 km/h gusting to 85 km/h / alpine high temperature near 0 / freezing level up to 1700 m 

TUESDAY - Periods of of snow, possibly heavy at times / moderate east winds / freezing levels initially high, but dropping at tail end of storm

Avalanche Summary

On Saturday explosives produced results up to size 2 on the early December crust. On Friday there were a few natural size 2 wind slab avalanches reported in the alpine.

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 fresh snow has likely formed new reactive storm slabs. Depending on aspect and elevation 60-100 cm now sits on the early December crust. This crust may have persistent weak, and facetted crystals above and below it. 

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. 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.
  • Don't let the desire for deep powder pull you into high consequence terrain.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.

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.