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Avalanche Forecast

Archived

Dec 28th, 2014–Dec 29th, 2014

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

Regions

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New wind and storm slab should be the main avalanche problems on Sunday. A surface hoar layer from Thursday night will increase instability where it gets buried intact.

Detailed Forecast

Decreasing northwest winds, decreasing light snow showers and a cooling trend should be the main story near and west of the crest on Sunday. New snow should run in the 7-15 inch range near and west of the crest.

The main avalanche problems Sunday should be new storm and wind slab. Storm slab will occur in areas that receive more than an inch of snow an hour for several or more hours. For wind slab watch for firmer wind transported snow on lee slopes. 

The surface hoar will increase instability where it gets buried intact. You can check for this layer using hand shear tests or shovel tilt tests.

A trend of decreasing danger should be seen on Sunday as new layers begin to stabilize. The trend of decreasing winds, decreasing light showers and cooling should also cause some nice right side up conditions on some sheltered slopes.

Snowpack Discussion

A warm storm moved over the Northwest Tuesday and Wednesday with strong west winds followed by lowering snow levels. About 3-12 inches of snow was seen at NWAC sites near and west of the crest at the tail end of the storm.

A cooler period with little precipitation was seen about Thursday to Friday. Some skiers were reporting better conditions on lower angle slopes with skis scraping crusts on some steeper slopes.

NWAC observer Dallas Glass was touring above Paradise Friday and found about 10 inches of recent storm snow well bonded to a firm rain crust. The lower snowpack was generally composed of stable crusts and polycrystals. He saw widespread 5 mm surface hoar which had formed Thursday night.

 

Sparkly surface hoar on a SW aspect at 6000 feet at Paradise Friday by Dallas Glass.

 A cold front is moving over the Northwest on Saturday and Saturday night causing west winds and moderate to heavy snow and snow showers near and west of the crest and a cooling trend. This should build some new wind and storm slab by 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.