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Avalanche Forecast

Archived

Dec 30th, 2014–Dec 31st, 2014

Alpine
Natural avalanches unlikely, human triggered possible.
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, human triggered possible.

Regions

Stevens Pass.

As the weekend storm slabs become less of a problem, older wind slab on easterly aspects and more recent wind slab on south and westerly aspects will require travelers to evaluate snow and terrain carefully. The persistent slab in this area should be unlikely to trigger, but has bigger consequences.

Detailed Forecast

Previously moderate east winds should have diminished by Wednesday under mostly clear skies and gradually warming temperatures. When traveling into the near and above treeline zones, be prepared for recent 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.

We have a fair amount of uncertainty in the coverage and the likelihood of triggering a persistent slab in the Stevens Pass area.  Look for weak layers in the lower and mid snowpack near treeline.  Recent snowfall has made this layer tougher to trigger, but increased the overlying slab depth, especially on wind loaded slopes which of course translates to larger avalanches. Choose your terrain cautiously and wisely!  

Snowpack Discussion

A storm system moved through the Northwest on Saturday and Saturday night delivering some much needed snowfall. West winds changed to northwest, with moderate to heavy snow west of the crest and a cooling trend. A convergence zone contributed to snowfall in the central Cascades on Sunday. NWAC sites west of the crest picked up about 12-20 inches of storm snow over the weekend except lower amounts at Snoqualmie Pass due to initially very wet snowfall. 

Professional observers west of the crest reported storm snow instabilities quickly settling out and not exceptionally reactive over the weekend. The snow layering ended right side up with low density snowfall at the end of the storm.   

East winds both through the Cascade passes and along ridgetop began to increase Monday. This has begun to redistribute the recent snow on more S-W aspects. While the most recent storm snow avalanche problem should be decreasing, the new wind slab will be an issue with certain aspects generally near and above treeline.  

One notable exception for the west side zones is the presence of a spatially variable persistent weak layer in the Stevens Pass area. 2 out of 4 pro-observers have found PWLs over the last few days in this zone. Tom Curtis found a weak faceted layer 2-3mm thick in the mid-pack just east of Stevens Pass in the Nason Ridge area Saturday that propagated in snowpack tests. (See photo).

Another observation to NWAC by John Race Monday in the Stevens Pass area, did not find a PWL. 

The video below is from Jeff Ward, who found a facet/crust combo at the bottom of the snowpack reactive in extended column tests on Cowboy Ridge near treeline almost 1 meter down on a SW aspect.

 

With the amount of ski traffic in this area over the weekend and new snowfall making these layers harder to affect, we do not believe this problem is very sensitive to human triggering. However, the persistent slab problem will be added to the Stevens Pass zone with a low likelihood of triggering. 

Photo by T. Curtis 12-27-14 in the Stevens Pass area.  Location Nason Ridge/Rainy Pass 4900 ft

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.