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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 unlikely, human triggered possible.
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 Coastal.

If your area receives more than 15-20 cm of new snow through Sunday bump the danger rating by one step. In other areas the snowpack will need time to adjust to recent storm snow and wind loading so conservative terrain selection is still recommended.

Confidence

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

Weather Forecast

SATURDAY NIGHT - Flurries 5-10 cm / light southwest wind but strong gusts possible / alpine high temperature near -2 / freezing level 700 m

SUNDAY - Flurries, sunny periods possible, 5-10 cm / light southwest wind gusting to strong / alpine high temperature near -2 / freezing level up to 600 m

MONDAY - Cloudy with sunny periods and isolated flurries / light winds / alpine high temperature near -5 

TUESDAY - Cloudy with sunny breaks, no new precipitation / light winds / alpine high-4 /temperature inversion possible

Avalanche Summary

On Sunday there were reports of shooting cracks in convex (roll over) type features. Avalanches up to size 2 out of steep, wind loaded features were reported on Saturday.

On Wednesday and Thursday there were many MIN reports suggesting the ever increasing storm snow amounts were reactive to human triggering.

Please consider sharing your observations with the Mountain Information Network; even just a photo of your day helps. Thank you to those that have already submitted this winter!

Snowpack Summary

Precipitation will start to ease in most areas on Sunday but storm slabs will likely continue to be reactive to human triggering. Some areas have seen upwards of a meter of new snow in the past week.

This new snow sits on previously wind-affected snow at higher elevations, potentially a layer of surface hoar in sheltered areas, and a crust at lower elevations. Initially, the storm snow will likely have a poor bond to these old snow surfaces.

A crust that was buried in early December is now down 80-120 cm in the alpine but is closer to the surface at and below treeline elevations.

The early-November crust is buried around 100-200 cm at treeline. This crust may have weak and sugary facets above it in parts of the region. This layer has produced large, full-depth avalanches in the past few weeks. Uncertainty remains about where this layer remains a problem in the region, but it may be localized to the northern half of the region.

Terrain and Travel

  • Don't be too cavalier with decision making, storm slabs may remain sensitive to human triggering.
  • 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.

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.