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Avalanche Forecast

Archived

Dec 3rd, 2020–Dec 4th, 2020

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
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 possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Northwest Inland.

New snow and wind are expected to build increasingly reactive wind slabs. Weak layers near the bottom of the snowpack remain possible to trigger.

Confidence

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

Weather Forecast

Thursday night: Mostly cloudy, up to 5 cm of snow, moderate southwest wind, freezing level around 700 m.

Friday: Overcast, 5-15 cm of snow, moderate southwest wind with strong gusts, freezing level rising to 1200 m

Saturday: Overcast, 10-20 cm of snow above 1500 m, moderate southwest wind with strong gusts. 

Sunday: Mostly cloudy, 5-10 cm of snow above 1500 m, moderate southwest winds.

Avalanche Summary

A series of notable avalanches have been reported in the northern half of the region. These were both explosive-triggered and natural releases that resulted in very large deep persistent slab avalanches (size 3-3.5), failing on weak snow near the ground. The extent of this snowpack structure in the region is not well-known. 

If you're out in the field, please consider sharing a photo or description via the Mountain Information Network (MIN). 

Snowpack Summary

Up to 20 cm of new snow is forecast to accumulate by late Friday in favored areas. Moderate to strong southwest winds may redistribute the new snow into reactive slabs on lee features. The new snow is accumulating above a melt freeze crust and hard, wind-affected snow and may not bond well. 

Reports suggest the bottom half of the snowpack consists of weak snow interrupted by a crust from early November. Further north in the region, we're learning that weak snow developing near the ground has produced very large deep persistent slab avalanches. Although we lack data on the distribution of this problem, the bottom line is that easy-to-trigger wind slabs have the potential to step-down to deeper layers and trigger destructive avalanches. 

Snowpack depths are variable across the region, ranging from 60 cm in eastern parts of the region up to over 100 cm in western parts of the region. Lower elevation terrain may still be below the threshold for avalanches. 

Terrain and Travel

  • Fresh wind slabs will likely form throughout the day, diligently watch for changing conditions.
  • Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.