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West Slopes North - Canadian Border to Skagit River

Issued: 6:00 PM PST Wednesday, December 7, 2016
by Garth Ferber

NWAC avalanche forecasts apply to backcountry avalanche terrain in the Olympics, Washington Cascades and Mt Hood area. These forecasts do not apply to developed ski areas, avalanche terrain affecting highways and higher terrain on the volcanic peaks above the Cascade crest level.

 

A lot is going to start happening on Thursday and regional timing of weather and avalanche conditions will be tricky.

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Avalanche Problems for Thursday

Wind Slabi

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

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Loose Dryi

Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.

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Snowpack Analysis:

Weather and Snowpack

Active snowy weather has been seen so far in December. NWAC stations along the west slopes had about 2-4 feet of snow so far in December through Tuesday morning with an overall cooling trend.

Cold fair weather has been seen the past couple days. There have been many reports of right side up, favorable density profiles with lower density snow nearer the surface. We've been hearing the term "as good as it gets" quite a lot lately!

Given the favorable snowpack profile, good bonds of older snow layers and a lack of deeper layers of concern, avalanche problems should be confined to the most recent storm snow at this time.

Cold fair weather like this usually causes some new surface hoar layer and near surface faceted snow. This snow can make for great skiing or riding. But these layers will need to be watched if loaded by wind transported snow or new snow.

Recent Observations

Despite the lower density snowfall and a generally right-side up layering of the new snow, sensitive and widespread soft storm slabs were reported Sunday by the Baker pro-patrol throughout the terrain near and below treeline in the Mt. Baker area. Crystal patrol reported widespread loose dry avalanches Sunday running fast on steep terrain, both natural and human triggered. Wind effects were relegated to immediately below ridgelines but there was plenty of new snow available for transport.  Stevens Pass pro-patrol found sensitive slabs on lee aspects near treeline during morning control Sunday. Most slabs released on storm layers averaging 10-12" deep.  Alpental pro-patrol found 12-18" soft slabs on a few wind-loaded slopes near treeline Sunday afternoon.

Multiple professional reports Monday and again Tuesday from the Stevens and Snoqualmie Pass areas, and above Paradise on Mt Rainier, indicating that the storm related sensitivity experienced Sunday had improved greatly by Monday and followed that trend again Tuesday. No slab like character was noted with a favorable density profile providing excellent surface snow conditions. These were areas where sensitive human triggered soft slab conditions were reported Sunday, indicating an improving trend in conditions.

A report from Tuesday via the NWAC Observations tab for the east part of Snoqualmie Pass indicated widespread new surface hoar.

NWAC pro-observer Ian Nicholson was on East Peak near Crystal Mountain on Wednesday and reported great conditions and an overall right side up snowpack. Minor earlier wind transport did not make slab layers. New surface hoar was not widespread and limited to sheltered areas. A strong temperature gradient and some faceted snow was seen in the top of the snowpack.

 

Detailed Forecast for Thursday:

A lot is going to start happening on Thursday and regional timing of weather and avalanche conditions will be tricky.

Southeast winds at exposed locations in the Cascades and east winds in the Cascade passes should continue to increase Thursday in advance of an incoming front.

Watch for wind transport and new wind slab on unusual southwest to northeast aspects on Thursday due to southeast and east winds. Regionally it doesn't sound like there is much if any old wind slab out there so the southwest to northeast  aspects along the west slopes will be highlighted for new wind slab on Thursday. Classic places for new wind slab like this are on Granite Mountain near Snoqualmie, around Crystal Mountain and above Paradise. But watch for firmer wind transported snow on all aspects since confidence is not high on which aspects to highlight. The slight warming trend could enhance the formation of new wind slab on Thursday.

The front with increasing snow and a slight warming trend should begin to lift south to north over the Northwest starting in the south at Mt Hood by Thursday afternoon and reaching the Olympics and Washington Cascades by Thursday night. Be prepared to change your plans if the front and significantly increasing snow arrives earlier than expected along the west slopes during the day on Thursday.

In steep sheltered terrain Thursday continue to watch for small loose dry avalanches near terrain traps like cliff bands where even small avalanches can have big consequences.

With deep unconsolidated snow in most areas, there is an increased risk for tree well and snow immersion suffocation at this time. Ride or ski with a partner and keep them in sight at all times!

Even though the lower part of the below treeline band is filling in, watch for early season travel hazards such as barely covered rocks and open creeks.

Further new wind and storm slab likely Thursday night. The warming trend should enhance the formation of these layers.

Observations