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Avalanche Forecast

Archived

Dec 14th, 2015–Dec 15th, 2015

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

Regions

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Pay extra attention if you see winds transporting snow and find more than a few inches of snowfall on Tuesday.

Detailed Forecast

A weak cold front will cross the Northwest on Tuesday. This should bring northwest winds and mostly light snow with a brief, slight rise in the low snow levels.

New small shallow areas of wind and storm slab are possible mainly where the main snowfall is expected in the central Cascades. Wind slab might be found on more south aspects due to northwest winds. Pay extra attention if you see winds transporting snow and find more than a few inches of snowfall.

Previous wind slab layers from strong winds Saturday should further slightly stabilize but could still be triggered on previous lee slopes.

Snowpack Discussion

An atmospheric river and warm very wet weather was seen last week. About 5-8.5 inches of mostly rain was seen along the west slopes. This caused an avalanche cycle, consolidation and stabilizing of the older part of the snowpack along the west slopes and is expected to have eliminated persistent weak layers at Stevens and Snoqualmie.

A stormy pattern with cooling was seen late last week and over the weekend with about 1.5-4 feet of snowfall along the west slopes.

Reports varied on Sunday. The Mt Baker and Alpental ski areas reported widespread avalanches released from explosives ranging from 1-2 feet and running good distances, releasing on storm snow interfaces. The Stevens Pass ski area reported morning control produced widespread soft slab results from both explosive and ski cuts with best results on NW-SE facing terrain with slides releasing on the most recent storm interface with Friday's storm snow.

Across the street at Skyline Ridge at Stevens Pass NWAC observers found about 30-60 cm of mostly stable, mostly right side up storm snow, well bonded to Tuesday's rain crust. One 30-40 cm storm slab was triggered on a 40 degree slope which ran on the crust from early last week.

A cool day with little if any new snowfall on Monday will have brought some stabilizing.

Terrain anchors have been somewhat buried by recent snowfall but will still add significant anchoring below about 3500-4000 feet.

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.

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.