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Avalanche Forecast

Archived

Dec 5th, 2016–Dec 6th, 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

Olympics.

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

Detailed Forecast

Showers should early Tuesday with light winds and cold temperatures. This weather should allow for continued very slow settlement of the recent storm snow, allowing for a slow decreasing trend in the avalanche danger overall.

In particular, watch for any lingering wind slab on lee aspects near and above treeline that could still be human triggered. 

In some steep terrain, 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 Olympics Friday, causing a warming trend and created a crust layer up to about 5600 ft on all aspects reported Saturday in the Hurricane area.  A much stronger front Saturday night delivering a welcome 1-2 feet of snow across the west slopes of the Cascades and Olympics through mid-day Sunday, with additional showers Monday. New storm snow from Saturday night through Monday afternoon at the snowdepth plot at Hurricane Ridge has been about 16 inches. This pattern has 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

The new snowfall received Saturday night through Monday was very low density, but wind transport of the new snow helped form some cornices along ridgelines. NPS rangers reported up to 2 natural slab avalanches in the Hurricane Ridge area that likely occurred Saturday night during the storm. One was in Maggie's Bowl, on an east or southeast aspect and ran at least 100 ft. The slab depth was estimated around 10".  Later Sunday, NPS rangers reported most of the avalanche activity revolved around loose dry avalanches in steep less wind affected areas below ridges. There were isolated reports of human triggered soft slab releases in wind loaded terrain that released down to the old snow surface. 

NWAC pro observer, Matt Schonwald visited Hurricane on Thursday and reported widespread wind effects, but a generally well bonded snowpack and snow profiles in pits that lacked a slab structure. The snow was found to be unconsolidated at lower elevations making terrain traps and creek crossings a hazard.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

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.