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Avalanche Forecast

Archived

Dec 6th, 2020–Dec 7th, 2020

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

Regions

Northwest Coastal.

Another day of snow, wind, and rain continues to load an already stressed snowpack. Stick to simple terrain and keep your distance from overhead hazards. 

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

Sunday night: Overcast, 15-25 cm of snow above 1300 m, moderate southwest winds, freezing level dropping to 1300 m.

Monday: Mostly cloudy, 5-10 cm of snow above 1200 m, moderate southwest winds, freezing level dropping below 1000 m by end of day.

Tuesday: Mostly cloudy, 5-15 cm of snow above 1000 m, moderate southwest winds, freezing level near 1000 m. 

Wednesday: Partly cloudy, scattered flurries with up to 5 cm of snow, light winds shifting to the northwest, alpine high temperature near -2 C, freezing level near 500 m.

Avalanche Summary

Over the weekend, observers reported a natural storm slab avalanche cycle (size 2-3). One storm slab reportedly released from a corniced alpine ridge feature. Additionally, numerous small to large (size 1-2) wet loose avalanches were observed on all aspects and up to alpine elevations during peak warming.

Areas near Ningunsaw continue to see large to very large deep persistent slab avalanches (size 2-3.5) releasing on weak snow at the ground. Last week, operators near Bear Pass reported natural and explosive triggered avalanches releasing on the early November facet/crust later. Easy-to-trigger storm slab avalanches have the potential to step-down to these deeper layers and produce large, destructive avalanches.

Our eyes and ears in the mountains are limited this winter due to fewer professional observations. If you go out in the mountains, please share your observations on the Mountain Information Network (MIN). Photos are especially helpful! 

Snowpack Summary

As of Sunday morning, weekend storm totals ranged from 35-60 mm of water in 48 hours and above freezing temperatures up to 1900 m. At summit elevations and in areas further north in the region, precipitation fell as snow. Strong winds from the south have rapidly loaded areas receiving snow with touchy storm slabs. Ongoing snow (another 20-30 cm) and strong winds overnight and into Monday will continue to build these touchy storm slabs and overload cornices. Below the freezing level, the snowpack is saturated. Freezing level is forecast to drop below 1000 m by the end of day Monday.  

The significant load from intense periods of precipitation has the potential to bring buried weak layers to their breaking point. Two early season weak layers are still on our radar: a facet/crust combination that formed in early November and a layer of facets near the ground. In the Ningunsaw area, basal facets have recently produced very large deep persistent slab avalanches. The early November crust also demonstrated reactivity in the Bear Pass area within the last week. The extent of this problematic snowpack structure in the region is not well-known, but it likely exists in colder, shallower areas in the region. 

Snowpack depths are highly variable across aspects and elevations as a product of wind scouring, above-freezing temperatures, and rain. Snow has melted fast at lower elevations, and snowpack depths have seen rapid settlement.

Terrain and Travel

  • Storm snow and wind is forming touchy slabs. Use caution in lee areas in the alpine and treeline.
  • Avoid exposure to overhead avalanche terrain during periods of heavy loading from new snow, wind or rain.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • A moist or wet snow surface, pinwheeling and natural avalanches are all indicators of a weakening snowpack.

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.

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.