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Avalanche Forecast

Archived

Dec 18th, 2020–Dec 19th, 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 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 Coastal.

Continued stormy weather with heavy snow and strong winds will keep avalanche danger elevated. Avoid avalanche terrain.

Confidence

Moderate - We are confident the likelihood of avalanche will increase with the arrival of the forecast weather.

Weather Forecast

FRIDAY NIGHT - Mainly cloudy with flurries, 5 cm / southwest wind, 40-60 km/h / alpine low temperature near -4

SATURDAY - Snow, 10-20 cm / southwest wind, 80-100 km/h / alpine high temperature near -2 / freezing level 1000 m

SUNDAY - Flurries, 5-10 cm / southwest wind, 30-60 km/h / alpine high temperature near -3 

MONDAY - Cloudy with sunny periods and isolated flurries / light winds / alpine high temperature near -5 

Avalanche Summary

Widespread natural avalanches are expected with the continuing stormy weather. 

There were a few explosives triggered size 2 storm slab avalanches reported on Thursday.

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 Northwest Coast has been getting hit by wave after wave of intense precipitation. There has been upwards of 80 cm of new snow this week. With another 10-20 cm in the forecast for Saturday, storm slabs will continue to be reactive.

This new snow sits on previously wind-affected snow at higher elevations, potentially a layer of surface hoar in sheltered areas, and a crust at lower elevations. Initially, the storm snow will likely have a poor bond to these old snow surfaces.

A crust that was buried in early December is now down 80-100 cm in the alpine but is closer to the surface at and below treeline elevations.

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. This layer has produced large, full-depth avalanches in the past few 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

  • Only the most simple non-avalanche terrain free of overhead hazard is appropriate at this time.
  • 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.
  • 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.