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Avalanche Forecast

Archived

Mar 3rd, 2016–Mar 4th, 2016

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

Regions

Olympics.

Conservative terrain selection will be essential near and above treeline as human triggered avalanches are still likely. Choose moderately angled terrain and avoid wind loaded slopes. Avoid travel on or below overhanging cornices. Watch for wet snow conditions below treeline. 

Detailed Forecast

A period of moderate rain and snow at rising freezing levels is expected again Friday along with strong S-SW  ridgetop winds. New storm related danger will continue Friday, especially near and above treeline as strong southwest wind loads lee slopes building new wind slab layers. Wind slabs will build on lee slopes, facing generally NW-NE.  Earlier formed wind slabs may still be reactive Friday and may add to the increasingly complex snow structure, by Thursday. Large cornices should form near ridges as well, requiring travelers to avoid overhead hazard.

Conservative terrain selection will be essential near and above treeline to avoid wind loaded terrain and features. Choose moderately angled terrain and avoid wind loaded slopes, choose terrain void of overhead hazard such as wind slabs or cornices.

Snowpack Discussion

Weather and Snowpack

Very mild weather a week ago allowed for the formation of a strong melt-freeze crust.

The last significant snowfall in the Olympics occurred Sunday, 2/28 through early Monday when about 12 inches of snow accumulated at Hurricane Ridge accompanied by a cooling trend. Moderate S-SW winds transported snow to lee slopes Sunday and Sunday night. 

Fluctuating freezing levels and periods of rain or snow Tuesday through Thursday have maintained about the same total snow depth at Hurricane Ridge with a few inches of new snow received each day. There have been periods of strong, mostly southerly, winds during frontal passages each day this week. Moderate rain fell Tuesday, 3/1 with rain reaching to about the near treeline band or at least 6000 feet.  

The mid and lower snow pack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.

Recent Observations

NWAC pro-observer Matt Schonwald was at Hurricane Ridge Friday and found mostly consolidated stable snow. N to E slopes had areas of 25-30 cm of wind transported 4F snow well bonded to 1F melt form grains in the upper snow pack. South through west slopes were melted out in some places with shallower dense stable snow in others.

No recent observations are available for the Olympics, but rainfall on Tuesday likely caused a natural avalanche cycle.

Generally shallow new snow has accumulated following the rain event Tuesday. Significant wind slab or new snow likely exists above treeline, where conditions should be significantly different than mid and lower elevations.  

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.

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.