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Avalanche Forecast

Archived

Nov 28th, 2011–Nov 29th, 2011

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

Regions

Sea To Sky.

Confidence

Fair - Due to limited field observations

Weather Forecast

Light to moderate precipitation is expected for tuesday with light southwesterly winds and freezing levels sitting at about 700m.A ridge is meant to dominate the region for wednesday and thursday bringing moderate northerly winds, clear skies and freezing levels at surface.

Avalanche Summary

There have been few observers in the field during the storm but it is likely that a very widespread and large avalanche cycle occurred throughout the region on Saturday. A size 3 natural avalanche was observed on sunday, while another report mentioned hearing about large avalanches at all elevations, several of which ran full path to valley bottom runouts at 200-300m elevation. Widespread size 1 natural activity was observed in the Whistler area on sunday running on recent storm snow/ wind slabs.Natural avalanche activity should ease off early this week, but the potential for human triggered avalanches remains.This is not the classic coastal instability where you can charge hard a few days after the storm. The slope you're thinking about riding maybe waiting for a bit more load, or just the right trigger. Be patient and realize that a large avalanche may become more difficult to trigger over time, but the consequences of a deep release could be devastating. Check out the Nov. 23rd post in the Forecaster's Blog for tips on how to manage risk immediately after a storm.

Snowpack Summary

Recent strong and shifting winds have created a variable snow surface and pockets of deep and dense wind slabs in exposed areas. Fluctuating freezing levels and heavy precipitation will likely produce various storm snow weaknesses. There is a persistent weakness of facets associated with a crust around 40-70cm above the ground. This weakness exists at treeline and alpine elevations and has been sensitive to human triggers. Several recent avalanches on this interface have shown very wide propagation and there have been cases of triggering from a distance. In some areas there is also a factet/crust layer just above the ground. This layer may still be a concern in areas with smooth underlying ground.

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.

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.