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Avalanche Forecast

Archived

Nov 29th, 2011–Nov 30th, 2011

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.
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

A ridge of high pressure should result in mainly dry conditions and sunny skies throughout the forecast period. The freezing level could rise as high as 2400m on Wednesday and Thursday during the day, and drop back to 1600m on Friday. Winds should ease off by Wednesday morning and remain light and variable for the next few days.

Avalanche Summary

Explosives control work on Monday produced several large avalanches including one very impressive Size 4 avalanche in the Blackcomb backcountry. Many of these avalanches were from paths that recently released and were reloaded during the storms on the weekend. Several released in relatively low angle terrain and ran very far. 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 may be 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 above 1900m. A solid rain crust has formed at or near the surface in most areas below treeline. This crust has provided good bridging strength and greatly reduced the likelihood of avalanches at lower elevations. 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.