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Avalanche Forecast

Archived

Feb 27th, 2013–Feb 28th, 2013

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

Regions

Sea To Sky.

Danger ratings are based on a very intense storm hitting the coast. There is still some uncertainty around the timing and intensity of this system.

Confidence

Fair - Timing of incoming weather is uncertain on Thursday

Weather Forecast

A very wet flow (Pineapple Express) is forecast, but exactly when and where the intense precipitation will occur is a matter of uncertainty.  Thursday-Saturday: Moderate to very heavy precipitation. Strong to extreme SW winds. Freezing level around 1400 m, rising for a time to 1800 m.

Avalanche Summary

Recent reports include several human-triggered slab avalanches to size 2, involving wind slabs, recent storm snow weaknesses and the persistent weakness buried early last week. Some of the avalanches involving the persistent weakness were remotely triggered from as far as 50 m away. A size 3 slab failed naturally on the persistent weakness, wrapping around an entire north-facing bowl and running across a flat bench. A size 2.5 avalanche was triggered by a party of sledders on Sunday in the Brohm Ridge area, and left a man buried 1.8 metres below the surface. The man was successfully rescued.  Check out the Forecaster blog for the full story.

Snowpack Summary

Incoming snow and wind will add to existing storm slab and wind slab problems, making them deeper and more destructive. A touchy interface of buried surface hoar, facets, and/or a crust, in the upper metre of the snowpack, is a key concern. It appears to be most touchy at treeline and above, but below treeline is not ruled out. It's been triggered on most slope aspects. Remote triggers and wide propagations have already demonstrated the touchiness of this interface. Avalanches triggered in storm snow layers may step down to this weakness, or a deeper interface buried earlier in the month. Cornices are also large and unstable.

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.