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Avalanche Forecast

Archived

Dec 19th, 2020–Dec 20th, 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 unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Northwest Inland.

The snowpack needs a bit of time to adjust to the new snow and wind. Avoid steeper rolls and areas with deeper pillows of wind affected snow. 

Confidence

Moderate - Uncertainty is due to how the snowpack will react to the forecast weather.

Weather Forecast

SATURDAY NIGHT - Flurries, 5 cm possible / west wind, 35-75 km/h / alpine low temperature near -5 

SUNDAY - Sunny with cloudy periods and isolated flurries / west wind, 25-65 km/h / alpine high temperature near -5 

MONDAY - Mainly sunny / light winds / alpine high temperature near -7 

TUESDAY - Cloudy with sunny periods / light winds / freezing levels at valley bottom, but temperature inversion possible

Avalanche Summary

Friday's reports suggest continued snow accumulations are keeping the snowpack near the tipping point. Check out Aaron's MIN here, and this one reporting whumfing (a really important obs about deeper layers). 

Thursday was also an active day. Check out Bryan's great MIN report from Hankin that outlines a few avalanche observations.

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

Snowpack Summary

20-40 cm of recent fresh snow and strong southwest wind has formed reactive storm slabs. This new snow sits on an early-December melt-freeze crust with weak and sugary faceted grains around it.

 

A crust that was buried in early November is near the base of the snowpack. This crust has weak facets associated with it. These facets have produced large avalanches in both the south and north of the region. Friday's reports of whumpfs suggest this layer is still a player.

Snowpack depths vary substantially with aspect, elevation, and wind exposure. 

Terrain and Travel

  • Watch for fresh storm slabs building throughout the day.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.