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Avalanche Forecast

Archived

Dec 30th, 2016–Dec 31st, 2016

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

Regions

Olympics.

The 12/17 persistent slab remains the main avalanche problem in the Hurricane Ridge area but this layer should be less likely to trigger Saturday except in isolated areas. Continue to avoid steeper slopes of consequence especially in less skied areas at Hurricane Ridge until we can confirm this layer is un-reactive to human triggering. While wind affected snow should be widespread, wind slab will likely be stubborn to trigger Saturday. 

Detailed Forecast

After a mostly clear start to Saturday, a fast moving frontal system will bring light snow to the Olympics by late morning which should continue through the day. New snow amounts through 4 pm will generally be light, but westerly winds, especially near and above treeline, are expected spike up by mid-day. 

The 12/17 persistent slab remains the main avalanche problem in the Hurricane Ridge area but this layer should be less likely to trigger Saturday except in isolated areas. Continue to avoid steeper slopes of consequence especially in less skied areas at Hurricane Ridge until we can confirm this layer is un-reactive to human triggering.

While wind affected snow should be widespread, wind slab will likely be stubborn to trigger Saturday. 

 

 

Snowpack Discussion

Weather and Snowpack

A strong front and strong west flow aloft was seen over the Olympics and Cascades on Monday and Tuesday. The NWAC station indicated strong south to southwest winds Monday and Tuesday with almost 2 feet of new snow for the 48 hours ending Wednesday morning with a cooling trend.

A warm front caused moderate to strong winds and periods of light to moderate snow Thursday with about half an inch of water through Thursday evening at the Hurricane Ridge station. New snow initially fell during a warming trend with moderate S-SW winds but stormy conditions quickly subsided behind the front Thursday evening. 

Recent Observations

NWAC pro-observer, Matt Schonwald was at Hurricane Ridge on Wednesday and reported the 12/17 PWL is still causing collapsing on all aspects especially in less skied areas below ridge lines. Recent natural wind slab releases of 10-12 inches where also seen on N-NE slopes off ridges at about 6000 feet. A 2-4 foot x 150 foot wide wind slab crown on the convex north slope below the visitor overlook was seen which may have released on a buried surface hoar layer from around Christmas Eve. The road to Hurricane was closed on Thursday. 

On Friday, NPS rangers reported wind affected snow surfaces well below treeline with about 4" of dense new storm snow. No natural avalanches from Thursday or Thursday night were observed with good visibility Friday morning.  Little loose surface snow was available for future transport. 

Problems

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.

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.