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Avalanche Forecast

Archived

Dec 20th, 2014–Dec 21st, 2014

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

Regions

Stevens Pass.

Unstable storm and wind slabs should persist and build near and above treeline Sunday.  Wet snow and rain below treeline should maintain areas of unstable wet snow. 

Detailed Forecast

Following a warm frontal passage Saturday night, strong moist westerly flow and generally mild temperatures should maintain moderate rain and snow through Sunday afternoon, tapering late Sunday. A likely convergence zone may form and affect the areas from Stevens to Snoqualmie Passes with locally heavier precipitation.

Rain should reach at least to near tree line elevations Saturday night and Sunday to cause a chance of natural avalanches or wet snow avalanches.

Above tree line, wet, heavy snow and very strong crest level winds should build further unstable storm and wind slabs. Dangerous avalanche conditions should persist Sunday with triggered avalanches remaining likely.

Snowpack Discussion

Although the strong storm that arrived early Saturday has deposited 10-15" as of Saturday evening, the overall snowpack remains limited below treeline. In general, there now is about 20-40" (50-110cm) from 3000-5000 feet along the west slopes. Periods of mild temperatures and several rain events have formed a very firm base layer below tree-line.  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.

Professional NWAC observers, out Saturday 12-20 from the Mt Baker backcountry, to the central Cascades near Stevens Pass and just north on Jove Peak, all report that storm slabs are the primary avalanche problem with storm snow amounts ranging from 14-24" (35-60cm). In all areas, warming Saturday afternoon created an upside down new snow layering with denser snow overlying weaker colder snow below. This was causing easy new snow failures from a variety of stability tests. The recent snow overlies shallow firm old snow near the ground. Bonding of new snow to the old snow surface above ground varied, but was mostly weakly bonded.    

In the Mt. Baker area, new snowfall received Thursday and Thursday 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 where the new snow bonded poorly to the old crust.

Similar results were reported from the limited avalanche control at the Mt Baker ski area Saturday, with new storm snow reactive and releasing as soft slabs from ski cut and explosive means.   

50 cm storm slab on W-SW aspect at Mt. Baker ski area Friday, 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.