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Avalanche Forecast

Archived

Dec 25th, 2016–Dec 26th, 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 and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Olympics.

Pay attention to changing weather and avalanche conditions! New wind and storm slab will develop Monday. The avalanche danger will ramp up quickly Monday afternoon and evening, so plan accordingly for deteriorating conditions. Persistent slabs still warrant your attention in the Olympics. Err on the side of caution Monday and dial back your objectives by choosing more conservative terrain. 

Detailed Forecast

After a benign start to Monday, an incoming frontal system will quickly spread light to moderate rain and snow over the Olympics with precipitation beginning late Monday morning. Near and above treeline winds should quickly ramp up in the afternoon. Expect stormy conditions to develop Monday evening with increasing avalanche danger late in the day and overnight. 

Fresh wind slab should begin to build Monday afternoon near and above treeline. 

Shallow storm slabs may also develop in the Hurricane Ridge area by Monday afternoon as the storm intensifies. 

Persistent slabs still warrant your attention in the Olympics especially with new snow arriving Monday. Remember that persistent weak layers are generally involved in larger avalanches and above normal caution is still advised. Careful snowpack evaluation will be essential for safe travel Monday.  Err on the side of caution Monday and dial back your objectives by choosing more conservative terrain. 

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 seen on Christmas Day.

Recent Observations

NWAC pro-observer Matt Schonwald was at Hurricane Ridge on Friday 12/23, and gave an important report. He was triggering collapsing and whumpfing in every open area that he visited on Friday. In 2 snowpits on slopes less than 30 degrees he found the December 17th persistent weak layer (PWL) consisting of preserved surface hoar and faceted snow at about 46-50 cm below the surface propagating in extended column tests. While the ski conditions were excellent he was unable to access steeper and more open terrain safely.

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. However, outside of this slab avalanche activity, only small loose slides were noted in steep terrain. In 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.  

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.

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.