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Avalanche Forecast

Archived

Dec 22nd, 2020–Dec 23rd, 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 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 possible, human triggered probable.

Regions

Northwest Coastal.

This forecast is based on 15-25 cm of new snow accumulating throughout the day on Wednesday. If you see higher amounts, especially earlier in the day consider bumping the hazard up to HIGH. Fresh storm slabs will likely increase in reactivity in the afternoon.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

Heavy snow and strong wind will arrive with the strong Pacific frontal system.

Wednesday: Snow heavy at times 15-25 cm accumulating through the day. Alpine temperatures near -3 and freezing levels 600 m. Ridgetop wind strong from the South.

Thursday: Heavy snow with 20-30 cm accumulating. Alpine temperatures -3 and freezing levels 600 m. Ridgetop wind switching to the East.

Friday: Cloudy with isolated flurries. Alpine temperatures near -1 and freezing levels 1000 m. 

Avalanche Summary

On Tuesday at the time of publishing only a few size 1 avalanches were reported from steep West aspects. 

On Monday a few natural avalanches up to size 2.5 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.

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

The incoming storm will likely build new and reactive storm slabs, especially where the snow has been stiffened by the wind and/ or sit above a recent surface hoar layer buried 40 cm down. I'm uncertain with widespread this buried interface is throughout the region besides the Nass Valley. Check out this awesome Facebook Post showing its sensitivity to human triggers. 

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

  • Watch for changing conditions today, storm slabs may become increasingly reactive.
  • 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.

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.