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Avalanche Forecast

Archived

Dec 21st, 2020–Dec 22nd, 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 avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Northwest Coastal.

Give the snowpack time to adjust to recent loading, especially in open areas where snow may have been wind stiffened. Avalanche activity, cracking, whumphing and stiff snow are signs of instability. 

Confidence

Moderate - Confidence is due to a stable weather pattern with little change expected.

Weather Forecast

The lull before the next storm. This region can expect some light precipitation as a weak trough crosses the north.  

Tuesday: Mostly cloudy with some flurries. Alpine temperatures near -6 with light wind from the South. Freezing levels valley bottom.

Wednesday: Arrival of next storm. Snow amounts 10-20 cm. Alpine temperatures near -4 and ridgetop winds moderate from the South. Freezing levels 600 m.

Thursday: Snow 20-30 cm with a strong southwest wind. Freezing levels 600 m.

Avalanche Summary

On Monday a few natural avalanches up to size 2 were reported near Terrace. Also, explosives control work triggered a few size 2 avalanches. I suspect natural avalanche activity will taper off tomorrow but human triggered avalanches remain likely.

On Sunday there were reports of shooting cracks in convex (roll over) type features. Explosive triggered storm slabs up to size 2 and a few natural loose dry avalanches up to size 1 from steep and unsupported terrain. 

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

Some areas have seen upwards of a meter of new snow in the past week. Storm slabs may continue to reactive to human triggering especially where the snow has been stiffened by wind.

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. Around this crust are facets and potentially surface hoar in sheltered areas. Storm slabs may step down to this layer.

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. 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.
  • Be especially cautious as you transition into wind affected terrain.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.
  • Don't let the desire for deep powder pull you into high consequence terrain.

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.