Avalanche Forecast

Archived

Dec 20th, 2020–Dec 21st, 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 and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural and human triggered avalanches likely.

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 new storm with intense loading rates is set to hit the region by Monday afternoon and continue overnight. Danger ratings may rise to HIGH by late afternoon. Avoid all avalanche terrain during periods of heavy loading from new snow and wind.

Confidence

Moderate - Uncertainty is due to the timing, track, & intensity of the incoming weather system.

Weather Forecast

SUNDAY NIGHT - Early evening flurries, minor accumulations / light to moderate south west wind / low -4

MONDAY DAY - Moderate to heavy accumulations 15-25 cm / moderate south west wind / high of -1

MONDAY NIGHT - 20-30 cm / gusty south west winds / low of -4

TUESDAY - Cloudy with lingering flurries, light accumulations possible / windy at times / high of -3

WEDNESDAY- Sunny with cloudy periods, high of -5

Avalanche Summary

On Sunday there was extensive natural and explosives triggered avalanche activity up to size 3.

There have also been a few accidentally skier triggered avalanches involving both storm snow and the December crust layer such as this one from the MIN.

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

After a break Sunday night, snow will pick up again on Monday. This continued loading will mean more new reactive storm slabs.

Depending on aspect and elevation 60-100 cm 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 are 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

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.