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Avalanche Forecast

Archived

Dec 17th, 2020–Dec 18th, 2020

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

Regions

Northwest Coastal.

Storm slabs continue to build and remain reactive through the forecast period. Stick to non-avalanche terrain and avoid overhead avalanche slopes that have the potential to run into treeline terrain.

Confidence

Low - Forecast snowfall amounts are uncertain. Uncertainty is due to the limited number of field observations.

Weather Forecast

Another strong Pacific frontal system arrives Thursday afternoon bringing moderate to heavy storm snow amounts and strong southwest wind.

Overnight Thursday: Snow 10-20 cm with strong West winds and freezing levels 700 m.

Friday: Snow 20-30 cm with strong southwest wind. Alpine temperatures near -1 and freezing levels 1000-1500 m.

Saturday: Heavy snow up to 30 cm possible. Strong southwest wind and freezing levels near 800 m.

Avalanche Summary

At the time of publishing, there have been no recent avalanche reports for Thursday.

Data in this region is very sparse. 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 region has seen up to 50 cm of recent storm snow by Thursday. Expect reactive storm slabs particularly in wind-affected terrain at and above treeline. The snow will fall onto previous wind-affected snow at higher elevations, potentially a layer of surface hoar in sheltered areas, and a crust at lower elevations. Initially, the new storm snow will likely have a poor bond to these old snow surfaces.

The early-December crust is now down 80-100 cm in the alpine but close to the surface at and below treeline elevations.

The early-November crust is buried around 100 to 200 cm at treeline elevations. The early-November crust may have weak and sugary faceted grains above it in parts of the region, which have produced large, full-depth avalanches in the past weeks. 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

  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Only the most simple non-avalanche terrain free of overhead hazard is appropriate at this time.
  • The more the snow feels like a slurpy, the more likely loose wet avalanches will become.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

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.