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Avalanche Forecast

Archived

Dec 26th, 2015–Dec 27th, 2015

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

Regions

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Lingering wind slabs instabilities will continue to slowly settle on Sunday. Watch for sensitive wind-loaded slopes especially near and above treeline, and be wary of terrain traps where even a small loose dry avalanche could have unintended consquences. 

Detailed Forecast

A frontal system on Sunday should drop a few inches of new snow with light to moderate southerly transport winds. This system will stack up short compared to some of the burly snow totals received over the last week. 

However, recent cold temperatures should continue to slow the otherwise stabilizing trend in the upper snowpack. 

Recent wind slab will most likely linger on north to east slopes in the near and above tree line. Wind and storm slab avalanches should stay within recent storm layers but could still move fast. Test for inverted strong over weak storm snow and give cornices a wide margin.

Beware of loose dry avalanches in steep non-wind affected terrain that could have unintended consequences such as knocking you off your feet and into a terrain trap.  Loose dry avalanches are listed above storm slab along the east slopes because of the colder temperatures, loose dry avalanches are more likely to be found and triggered than storm slabs. 

Remember to discuss plans and maintain visual contact with your partners. Recent unconsolidated storm snow is very deep in many places with possible large tree wells. There have been snow immersion fatalities in tree wells already this season at Snoqualmie and in Canada.

Snowpack Discussion

We have had about a week of heavy snow and cool temperatures in the Olympics and Cascades. Sites east of the crest have had 2-4 feet of snowfall during this time. There has been slow settlement over the last few days, but the main message is that it is still deep out there; Washington Pass area snowdepths are over 2 meters!

A snow pit report from 5700-6500 foot range in Stemilt Basin near Mission Ridge also on Tuesday indicated a buried surface hoar at 45 cm. This layer gave some cracking and whumping. 

NWAC pro observer Tom Curtis was at Iron Mountain a bit west of Blewett Pass on Wednesday and found some wind slab on steep open slopes even to below treeline and often above terrain traps like creeks or trees. A storm layer was seen at 35 cm with wind and storm slabs reacting to tests to this interface. 

Recent reports from the North Cascade Mountain guides near Washington Pass:

Wednesday indicated a storm layer at 15-25 cm but ski cuts on test slopes did not give any results. On Thursday a report indicated a storm layer at 30 cm with isolated small ski triggered storm slab and some loose dry releases and good stability. Quite a bit of wind effect was seen on some nearby slopes. On Christmas Day, a buried surface hoar layer was found in the Washington Pass area around 40 cm down and gave sudden planar (pop!) results in compression tests but was not found to be involved in any natural or skier triggered avalanches. This layer probably needs more of a cohesive slab above it before it becomes apparent whether this is an PWL issue or not. 

The southeast zone has a relatively shallower snow pack versus areas further north, but after the snowy December has more than enough to warrant an avalanche risk. No snowpack observations have been received from this zone. 

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.

Loose Dry

Loose Dry avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.

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.