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Avalanche Forecast

Archived

Dec 26th, 2016–Dec 27th, 2016

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

Regions

Olympics.

Very dangerous avalanche conditions are expected in much of the terrain Tuesday. Storm or persistent slabs will be sensitive Tuesday in the Olympics. The safest plan is to avoid avalanche terrain of consequence until storm and persistent slabs stabilize. 

Detailed Forecast

Stormy conditions Monday night into Tuesday will cause an increasing avalanche danger through Tuesday.

Storm slabs will continue to build over a variety of weak surface conditions with increased load. Natural or triggered storm slabs may break down to deeper persistent layers, making larger and more dangerous avalanches possible Tuesday.  

The persistent slab problem should become more sensitive to natural or triggered avalanches as the load increases.

Fresh wind slabs should continue to build Monday night and Tuesday near and above treeline. 

The persistent slab problem warrants your attention in the Olympics, especially with significant loading by Tuesday. Remember that persistent weak layers are generally involved in larger avalanches and cautious route-finding and conservative decision making will be essential for safe travel Tuesday.  Err on the side of caution Tuesday by avoiding avalanche terrain of consequence.  

Snowpack Discussion

Weather and Snowpack

A front crossed the Northwest on Thursday, followed by an upper trough on Friday. This produced about 8-10 inches of snow at Hurricane Ridge.

Scattered snow showers, a mix of sun and clouds, and generally light winds summed up the weather on Saturday with fair and cold weather on Christmas Day. 

A quiet Monday morning became a storm as the snow arrived with winds increasing to 25-40 mph at Hurricane Ridge by Monday afternoon. 

Recent Observations

NWAC pro-observer, Matt Schonwald was at Hurricane Ridge on Friday, 12/23, and gave an important report. He was triggering collapsing (whumpfing) in every open area that he visited on Friday. In two test snowpits on slopes less than 30 degrees, Matt found the December 17th persistent weak layer (PWL) consisting of well preserved, buried surface hoar and faceted snow at about 45-50 cm below the surface, propagating in extended column tests. While the ski conditions were excellent, the persistent slab problem prohibited safe access to steeper and more open terrain, limiting further investigation as to the distribution and sensitivity of the persistent slab layer.

Matt also reported that cornices were growing on the lee northeast sides of ridges on Friday.

On Saturday NPS rangers indicated several 30-40 cm slabs had been skier triggered on S-SE aspects above the Hurricane Ridge Road, with one larger slide hitting the road. In the more north facing terrain, backcountry skiers reported no whumpfing, shooting cracks or general signs of instability to NPS rangers on Saturday. The road to Hurricane was closed on Christmas Day.

No additional observations were received Monday. 

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.

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.