Avalanche Forecast

Archived

Dec 24th, 2016–Dec 25th, 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.

Persistent slabs still warrant your attention in the Olympics. Remember that persistent weak layers are generally involved in larger avalanches and above normal caution is still advised. Identify the snowpack structure in the area you want to ski or ride before committing to avalanche terrain and err on the side of caution. Wind slab may be still be sensitive on lee slopes mainly near and above treeline.

Detailed Forecast

Christmas should be cold and mostly sunny with generally light winds.

Shallow wind slab formed Thursday and Friday has become less likely to trigger.

Persistent slabs still warrant your attention in the Olympics. Remember that persistent weak layers are generally involved in larger avalanches and above normal caution is still advised. Identify the snowpack structure in the area you want to ski or ride before committing to avalanche terrain and err on the side of caution. Wind slab may be still be sensitive on lee slopes mainly near and above treeline.

Snowpack Discussion

Weather and Snowpack

A front crossed the Northwest on Thursday followed by an upper trough on Friday. This has caused about 8-10 inches of snow at Hurricane. South winds Thursday became light on Friday a cooling trend Friday.

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

Recent Observations

NWAC pro-observer Matt Schonwald was at Hurricane Ridge on Friday 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 PWL and preserved surface hoar and preserved faceted snow at about 46-50 cm below the surface with moderate Extended Column tests indicating propagation. While the ski conditions were excellent he was unable to access steeper higher 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 slide hitting the road. However, outside of this slab avalanche activity, only small loose natural and skier triggered slides were noted in steep terrain. In more north facing terrain ski tourers ventured out of Hurricane Ridge, no whumpfing, shooting cracks or general signs of instability were noted.

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.