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Avalanche Forecast

Archived

Dec 29th, 2014–Dec 30th, 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 possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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Even as storm slab avalanche concerns diminish, old wind slab on easterly aspects and new wind slab on south and westerly aspects will require careful terrain management. The persistent slab varies in depth and likelihood of triggering across the east slopes. Careful snowpack evaluation and conservative decision making will be essential Tuesday. 

Detailed Forecast

Moderate to strong ridgetop east winds should ease Tuesday afternoon under mostly clear skies and cold temperatures. When traveling into the near and above treeline zones, be prepared for new wind slab formation on more southerly and westerly aspects. Older wind slab on more westerly aspects may still be sensitive in certain areas. Treat wind loaded slopes with caution, it is difficult to determine how sensitive they are with snowpack tests.  

Great caution also remains warranted near avalanche terrain in this area due to the persistent slab, this avalanche problem varies across the east slopes. Subsequent snowfalls may make this layer tougher to trigger but the increasing slab depth, especially on wind loaded slopes translates to larger avalanches. Slides beginning in new storm layers may possibly step down to this more deeply buried layer. Professionals are treading carefully in this area with this snowpack structure and enjoying lower angled terrain not connected to large avalanche above.

Snowpack Discussion

A cold front moved over the Northwest on Saturday and Saturday night. West winds changed to northwest, with moderate snow east of the crest and a cooling trend. NWAC and Snotel sites east of the crest picked up about 3-10 inches of new snow by Monday morning.  Sites further from the crest such as Mission Ridge and Blewett Pass picked up most of their snowfall after the front Sunday night/Monday with easterly upslope.  New snowfall generally fell right side up but strong NE-E transport winds likely built new wind slabs on S-W aspects in the near and above treeline zones 

Observations via the NWAC observers and North Cascade Mountain Guides have been describing a persistent slab due to a facet/crust from 50-100 cm below the surface in the Washington Pass area. Remotely triggered avalanches were reported last week, but have been less reactive lately in the below and near treeline elevation bands.

In the central east zone overlapping with Stevens Pass, NWAC observer Tom Curtis was on Nason Ridge Saturday. He also a found a persistent slab with moderate but sudden collapse tests and likely propagation in snowpits due to a similar MF/facet layer at 55 cm on varied aspects.

PSTEnd SC results at Rainy Pass on Nason Ridge on 12/27 by Tom Curtis.

On Monday, Mission Ridge pro patrol had sensitive soft wind slabs release with ski cuts around 5700' as very strong winds distributed the new snow lower down the hill. In areas that had not been controlled this winter, 2-3 foot slabs released with explosives down to facet/crust combination near the ground.  

At this time we believe the persistent weak layer is more dangerous and easier to human trigger in the central east Cascade zone. 

Problems

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.