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Avalanche Forecast

Archived

Dec 18th, 2020–Dec 19th, 2020

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

Regions

Lizard-Flathead.

The recent snow has added load to buried weak layers and may still be reactive to human triggers, especially in wind loaded areas. Conservative terrain selection is recommended.

Confidence

Moderate - Uncertainty is due to the limited number of field observations. Forecast snowfall amounts are uncertain.

Weather Forecast

FRIDAY NIGHT - Mainly cloudy with flurries, 5-15 cm / southwest wind, 50-80 km/h, gusting to 100 km/h / alpine low temperature near -4 / freezing level 1100 m

SATURDAY - Mainly cloudy with scattered flurries, and 10-20 cm of snow on Saturday night / southwest wind, 40-80 km/h / alpine high temperature near -2 / freezing level 1500 m

SUNDAY - A mix of sun and cloud / southwest wind, 30-60 km/h / alpine high temperature near 0 / freezing level 2000 m

MONDAY - Flurries, 10-15 cm / southwest wind, 30-40 km/h gusting to 85 km/h / alpine high temperature near 0 / freezing level 2000 m 

Avalanche Summary

On Friday there were a few natural size 2 wind slab avalanches reported in the alpine.

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 recent fresh snow has likely formed reactive storm slabs. The recent storm snow accumulations mean that 40-60 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. 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.