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Avalanche Forecast

Archived

Dec 29th, 2016–Dec 30th, 2016

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

Regions

Olympics.

The 12/17 persistent slab remains the main avalanche problem in the Hurricane Ridge area. Continue to avoid steeper slopes of consequence especially in less skied areas at Hurricane Ridge. Also, treat wind loaded slopes in all elevation bands with caution on Friday.

Detailed Forecast

A strong but quick moving front will bring a period of strong winds along with precipitation Thursday afternoon and evening.  Winds and shower intensity should quickly taper down on Thursday night with a clearing trend expected along with cool temperatures on Friday. 

The 12/17 persistent slab remains the main avalanche problem in the Hurricane Ridge area. Recent loading may make this layer more sensitive to triggering. Remember that persistent weak layers are generally involved in larger avalanches. Continue to avoid steeper slopes of consequence especially in less skied areas at Hurricane Ridge. 

Wind slab should be the secondary avalanche problem due to a period of moderate to strong westerly winds Thursday that may have loaded leeward aspects further downslope than usual. Treat wind loaded slopes in all elevation bands with caution on Friday.

Storm slabs are expected to be the most reactive and likely to trigger Thursday night during peak warming and storm intensity. However, sensitive storm slabs may linger and be found in wind sheltered terrain Friday. More snow is expected from Snoqualmie Pass and north to Mt. Baker with this storm and the avalanche danger will be rated higher to account for the additional expected snowfall.  

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 6 pm Thursday at the Hurricane Ridge station. New snow initially fell during a warming trend with moderate S-SW winds but stormy conditions were subsiding 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.

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.

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.