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Avalanche Forecast

Archived

Dec 21st, 2014–Dec 22nd, 2014

Alpine
Natural avalanches possible, human triggered probable.
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.

Regions

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Unstable storm and wind slabs should persist above treeline Monday, but begin gradually settling.  Wet snow should slowly refreeze below treeline with shallow new snow forming a good bond. 

Detailed Forecast

The storm over the weekend should be in the rear view mirror by Monday, as high pressure rebuilds.  This should cause light to moderate northwest crest level winds and little if any accumulation of new snow.  Temperatures should remain cool Monday.

Recent storm and wind slabs should persist at higher elevations, especially above treeline and near ridgetops. Best to perform numerous quick test pits or pole tests to determine recent snow conditions. The cooler temperatures Monday should continue to slowly refreeze the old rain soaked snow surface to limit avalanche potential at lower elevations. 

Snowpack Discussion

A very strong storm began affecting the region Friday.  This system deposited varying amounts of new snow ranging mainly from 5-10 inches below treeline to 1-2 ft or more near and above treeline by Saturday afternoon.  Freezing levels rose significantly Saturday night and early Sunday changing snow to rain in most areas, at least to the near treeline zone and in many areas above that.

The rain has changed the landscape of the snow conditions by Sunday. The rain likely caused some natural avalanches Saturday night in areas with significant recent snow.  Little field information has been available Sunday, however expect lower to mid elevations to begin slowly refreezing and forming a new crust with shallow amounts of new snow being well bonded to the old surface.

The overall still shallow snowdepths are maintaining many terrain anchors (trees, rocks, etc.), preventing large avalanches.  

Above 5000' to the top of our forecast range (7000-8000 ft)... snowdepths are still seasonally well below average, but are deep enough to provide a bed surface for new snow avalanches from the current storm snow.  Generally, there is 3-5 foot base above 5000', deepest in the North Cascades and at Mt. Rainier.  Expect a shallower snowpack in the Stevens-Snoqualmie zones and south of Mt. Rainier.

In the upper elevations above treeline, expect some deep storm or wind slabs, especially in areas that have received the greatest precipitation over the past few days.

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.