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Avalanche Forecast

Archived

Dec 20th, 2015–Dec 21st, 2015

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.

New storm slab and wind slab are likely depending a lot on the changeable weather pattern on Monday.

Detailed Forecast

A strong low pressure system will move west to east across Washington on Monday. There should be a big difference in the weather and avalanche conditions between the Olympics and Cascades with the strongest winds and heaviest snowfall in the south Cascades. Right now it looks like the main effects of the low may reach as far north as the central Cascades. But a different track of the low could bring much different than expected weather and avalanche conditions on Monday so the forecast is somewhat uncertain.

Light to moderate winds and snow are expected in the Olympics on Monday. New storm slab and wind slab are likely depending a lot on the changeable weather pattern. Dangerous avalanche conditions are expected in the above and near treeline on Monday.

Remember to give cornices a wide margin. Also above tree line strong winds recently have scoured snow from exposed slopes leaving exposed rock and vegetation. Terrain anchors are still causing significant anchoring at the lowest elevations.

Snowpack Discussion

A series of cool storms has deposited increasing storm snow at relatively low freezing levels since early December. Some rain probably made it up to Hurricane on December 17th. There has been about 1-1.5 feet of snowfall at Hurricane the past 2-4 days.

NWAC pro observer Matt Schonwald and a NPS ranger travelled extensively Friday around Hurricane Ridge testing the recent storm snow. At around 5500 feet the snow depth is about 4-5 feet with the December 9 crust buried about 1.5-2 feet from the recent storm snow. The snowpack in the Hurricane Ridge area was mostly unconsolidated low density surface snow with a gradually increasing density down to the well bonded crust, a very good profile. No slab character was noted in the upper snowpack during testing. Only isolated wind slabs were noted with no releases. Extensive cornices were noted on north to northeast sides of ridges. Skiing was very good.

The general break in the weather should have brought some more stabilizing on Saturday. The ranger reported 8 inches of new snow on Sunday.

Problems

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.

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.