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Avalanche Forecast

Archived

Dec 20th, 2014–Dec 21st, 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

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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. Storm conditions should persist Sunday; don't be fooled by the low snow cover at the trailhead and be aware of what terrain you are connected to above.

Detailed Forecast

The biggest avalanche concern for Sunday will be direct-action wind and storm slab avalanches.  The warm temperatures should cause an upside down upper snowpack with dense wet snow over weaker colder snow.  At lower and mid elevations rain will load snowfall received earlier. Strong W-WSW winds near treeline should scour exposed ridgelines and quickly load north through southeast aspects.   

New snowfall received Saturday and Saturday night will test recently buried weak layers where some natural avalanches may have occurred by early Sunday. Warmer air should mix into the northeast Cascades, by Sunday morning.  

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

Snowpack Discussion

The northeast Cascades is the one forecast area where the snowpack is at least a meter deep or more in the tree-line elevation band and has already experienced several natural avalanche cycles during this season that has begun so slowly elsewhere.  

Pro-observer Jeff Ward and the North Cascade Mountain Guides have been tracking several layers of concern... either buried surface hoar or facet/crust combos formed earlier this month during fair high pressure periods. These layers are likely to be found in the upper 40-60 cm of the snowpack between 5500-6500 feet in the Washington Pass area.  While these layers have been reactive in snowpack tests... they have not been observed in any recent avalanche activity and need more load to stress and test these layers. 

We do not have any current or recent snowpack information for our Cascade East - Central zone.  Snowpack depth away from the crest (Blewett Pass, Mission Ridge) is marginal for avalanche activity even at higher elevations. The avalanche rating for this area corresponds best to areas near the Cascade crest where a deeper snowpack exists.

The strong storm Saturday, caused moderate to heavy snowfall at rising temperatures. This should have built unstable storm and wind slabs in areas with a sufficient existing snow cover.  

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.