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Avalanche Forecast

Archived

Dec 4th, 2016–Dec 5th, 2016

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

Regions

Stevens Pass.

Cold temperatures will help preserve recent storm snow instabilities found on wind loaded aspects near and above treeline. In steep terrain where the new snow lacks a slab structure, watch for small loose dry avalanches near terrain traps like cliff bands where even small avalanches can have big consequences.  

Detailed Forecast

A few inches of low density snow may accumulate Monday with little impact on the avalanche hazard. Westerly ridgetop winds are forecast to decrease Sunday night, but before then plenty of new low density snow is available for transport to lee aspects. Storm slabs should be less sensitive on Monday, but sustained cold temperatures will help preserve recent storm snow instabilities. In particular watch for lingering storm or wind slab on lee aspects near and above treeline that can still be human triggered. Only wind slab will be listed in the avalanche problems moving forward.    

In steep terrain where the new snow lacks a slab structure, watch for small loose dry avalanches near terrain traps like cliff bands where even small avalanches can have big consequences. 

Even though the lower part of the below treeline band is filling in, watch for early season travel hazards such as barely covered rocks and open creeks. 

Snowpack Discussion

Weather and Snowpack

A front crossed the Cascades Friday with 2-8 inches of new snow reported at NWAC stations through early Saturday morning. The new snow was generally on the heavier side and was accompanied by a slight warming trend Friday.

A much stronger front sagged south across the area Saturday night delivering a welcome 1-2 feet of snow across the west slopes through mid-day Sunday. This system also ushered in the coldest air mass of the season.  

Avalanche problems should be confined to the most recent storm snow and  we are not tracking any weak layers in the lower or middle of the snowpack at this time. 

Recent Observations

Despite the lower density snowfall and a generally right-side up layering of the new snow, sensitive and widespread soft storm slabs were reported by the Baker pro-patrol throughout the terrain near and below treeline in the Mt. Baker area this morning. The slabs were releasing within storm layers and up to 10" deep. Wind transport of the low density new snow continued through along ridges. Crystal patrol reported widespread loose dry avalanches running fast on steep terrain, both natural and human triggered. Wind effects were relegated to immediately below ridgelines but there was plenty of new snow available for transport.  Stevens Pass pro-patrol found sensitive slabs on lee aspects near treeline during morning control. Most slabs released on storm layers averaging 10-12" deep, but a few larger slabs stepped down to most recent old snow/new snow interface from Saturday. Alpental pro-patrol found 12-18" soft slabs on a few wind-loaded slopes near treeline Sunday afternoon. 

Prior to this storm, professional observations from the Stevens Pass area (pro-patrol) and Crystal backcountry (Dallas Glass) described generally stable conditions Saturday outside pockets of shallow wind slab found near-treeline. 

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.

Loose Dry

Loose Dry avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.