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Avalanche Forecast

Archived

Dec 4th, 2020–Dec 5th, 2020

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
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.

Rain, snow, and wind will further load an already stressed snowpack. Stick to simple terrain and maintain conservative margins from overhead hazard.

Confidence

Moderate - Forecast precipitation (either snow or rain) amounts are uncertain. Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather.

Weather Forecast

The storm continues through the weekend.   

Friday night: Overcast, 20-30 cm of snow above 1700 m, heavy rain below, strong southwest winds with extreme gusts. 

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

Sunday: Mostly cloudy, 10-20 cm of snow above 1400 m, strong southwest winds, freezing level dropping to 1200 m.

Monday: Mostly cloudy, 20-30 cm of snow above 1200 m, moderate southwest winds freezing level dropping to 1000 m.

Avalanche Summary

On Friday, observers reported several small (size 1) wet loose and storm slab avalanches releasing in the recent storm snow above the Dec 3 crust. 

In the aftermath of last weekend's storm, a series of notable avalanches have been reported further north in the region. These were very large deep persistent slab avalanches (size 3-3.5) that were both explosive-triggered and natural releases, running on an early November crust and weak snow at the ground. The extent of this snowpack structure in the region is not well-known.

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

Snowpack Summary

The storm will continue overnight and is expected to taper into Saturday morning. An additional 25-40 mm of water is forecast to saturate the snowpack below 1700 m, which is likely to trigger widespread wet loose avalanches. The precipitation may fall as snow on the high peaks. Strong southwest winds are expected to rapidly load areas receiving new snow into reactive slabs. Temperatures are rising during the storm event, increasing instability.

The significant load from intense periods of precipitation has the potential to bring buried weak layers to their breaking point. A layer of buried surface hoar and facets that formed in late November may still linger in isolated areas. A widespread layer of facets on a crust from early November warrants concern. Further north in the region, we're learning that weak snow developing near the ground has produced very large deep persistent slab avalanches. As the snowpack becomes more saturated, smaller wet loose or storm slab avalanches have the potential to step-down to these deeper layers and produce large, destructive avalanches.

Snowpack depths range from roughly 150 to 200 cm at upper treeline and alpine elevations. 

Terrain and Travel

  • Watch for rapidly changing conditions during periods of heavy loading from new snow, wind, or rain.
  • If triggered loose wet avalanches may step down to deeper layers resulting in larger avalanches.
  • The more the snow feels like a slurpy, the more likely loose wet avalanches will become.
  • Avoid areas with overhead hazard.

Problems

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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.