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Avalanche Forecast

Archived

Dec 20th, 2012–Dec 21st, 2012

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

Regions

Northwest Inland.

Confidence

Fair - Due to limited field observations

Weather Forecast

Overnight Thursday and Friday: Very light snowfall / Light to moderate southeast winds / Temperature inversion with ridgetop temperatures of -18.0Saturday: Mostly clear with trace amounts of new snow / Light southeast winds / Temperature inversion with ridgetop temperatures of -12.0Sunday: Trace amounts of snowfall / Light east winds / Ridgetop temperatures of -17.0

Avalanche Summary

No recent avalanches have been reported. On Wednesday last week, a snowmobiler triggered a hard wind slab at the Sinclair riding area. It failed on the November crust/facet layer 30 cm above the ground while side-hilling. Check out this incident report for more details. This deep crust/facet layer has been the failure plane for a number of large avalanches further to the west and was suspected to have failed at the Hankin-Evelyn area.

Snowpack Summary

Light amounts of new snow overlie older storm and windslab instabilities which are likely settling and gaining strength.  Spotty surface hoar was buried in the upper snowpack, particularly in sheltered treeline areas. Of key concern is a November facet/crust layer which can be found near the base of the snowpack. This layer, which is widespread, can be triggered from thin-spot trigger points, or with a heavy load, such as storm slabs stepping down, a cornice fall or a snowmobile track digging a trench. It has the potential for large, destructive avalanches. In general the snowpack depths, and therefore strength is highly variable due to windy conditions this season.

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.

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.