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Avalanche Forecast

Archived

Dec 19th, 2014–Dec 20th, 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 possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Stevens Pass.

Due to a shallow snowpack at low elevations... the greatest avalanche danger should be present in the near and above treeline zones within the new storm snow. Poor visibility is expected Saturday; don't be fooled by low snow cover at the trailhead and be aware of what terrain you are connected to above. 

Detailed Forecast

A strong plume of moisture headed toward the Pacific Northwest should bring copious amounts of precipitation and wind to the mountains this weekend.  

On Saturday, snow levels should start low and rise throughout the day as precipitation intensity increases during the daylight hours. The biggest avalanche concern for Saturday will be direct-action wind and storm slab avalanches.  Rising temperatures should cause an upside down snowfall... and at lower and mid elevations a changeover to rain will load snowfall received earlier in the day. Heavy periods of snowfall will lead to storm snow instabilities. Also, increasing SW winds should scour exposed ridgelines and quickly load north through southeast aspects.   

Precipitation/snowfall amounts are expected to be somewhat lower in the central Cascades due to shadowing from the Olympics... and the avalanche danger in the alpine is lower for these zones. 

This is not the weekend to be exploring new areas. If you do venture out Saturday, know your route and avoid areas where you are connected to slopes with higher avalanche potential above. 

Snowpack Discussion

We have a limited snowpack below treeline. In very broad terms, there is generally 10-30" (20-40 cm) from 3000-5000 feet along the west slopes. Periods of mild temperatures and several rain events have formed a stiff brick of a base below tree-line.  Due to the paltry snowdepths...many terrain anchors (trees, rocks, etc) are still present, preventing large avalanches.  

Above 5000' to the top of our forecast range (7000-8000 ft)... snowdepths are still seasonally meager but are deep enough to provide a bed surface for new snow avalanches over the upcoming weekend.  Generally, there is 2-4 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.

Due to slow start to winter...we do not have many observations to share with you.  Professional NWAC observers who have been out, have not found layers of concern within the snowpack. 

One area of avalanche interest is around Mt. Baker. New snowfall received Thursday and Thurdsay night in the Mt. Baker area led to a human triggered slab avalanche that released to the surface on a lee W-SW aspect near treeline. The new snow bonded poorly to the old crust.   

50 cm storm slab on W-SW aspect at Mt. Baker ski area, near tree-line. Photo by Garrett Elwood.

 

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.