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Avalanche Forecast

Archived

Dec 21st, 2013–Dec 22nd, 2013

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

South Coast.

Confidence

Fair - Intensity of incoming weather systems is uncertain on Monday

Weather Forecast

Sunday: Light snowfall / Moderate southwesterly winds / Freezing level at 1200mMonday: Moderate to (locally heavy in the south) snowfall / Moderate to strong westerly winds / Freezing level at 1200m Tuesday: Broken cloud with no precipitation / Light westerly wind / Freezing level at 900m

Avalanche Summary

Several remotely triggered size 1-1.5 soft slab avalanches were observed in the North of the region on Friday. They occurred on a north/northeast aspect at treeline. The December 12th surface hoar layer was the culprit in these events.Several size 1-1.5 natural loose dry avalanches also occurred in response to recent snowfall in north facing alpine terrain.

Snowpack Summary

Snowpack depths vary greatly across the region with 130cm reported at treeline in the Northern Cascades but only 50 cm along the Duffy. In general, the snowpack across the region is significantly thinner than average for this time of year. Terrain below treeline is still mostly below threshold for avalanche activity.15-20 cm new snow now sits on a generally well consolidated snowpack. Previous weak interfaces within the snowpack appear generally well bonded and have been unreactive to recent snowpack tests. The exception to this seems to be in the north of the region where very touchy surface hoar buried on December 12th sits below about 30cm of recent snowfall. This will be a weakness to watch as the overlying slab develops. Early season riding hazards such as rocks, stumps and logs are lurking below the surface at treeline elevations and below.

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.