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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Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.

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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. The NWAC station at Hurricane indicates about 3 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 in the Cascades 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

The new snowfall received Saturday night through Monday was very low density but wind transport helped form some cornices along ridgelines. NPS rangers reported up to 2 natural slab avalanches in the Hurricane Ridge area that likely occurred Saturday night during the storm. One was in Maggie's Bowl, on an east or southeast aspect and ran at least 100 feet. The slab depth was estimated around 10 inches.  Later Sunday NPS rangers reported most of the avalanche activity revolved around loose dry avalanches in steep less wind affected areas below ridges. There were isolated reports of human triggered soft slab releases in wind loaded terrain that released down to the old snow surface. 

Wind slabs formed during recent storms have now had a few days to slowly settle and stabilize. We have been seeing these isolated wind slabs become less reactive to trigger in the Cascades over the past few days and we expect similar recent improvement in the Olympics.

Detailed Forecast for Thursday:

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

South to southeast winds over the Olympics should continue to increase Thursday in advance of an incoming front.

Watch for wind transport and new wind slab on southwest to northeast aspects Thursday due to south to southeast winds. Regionally it doesn't sound like there is much if any old wind slab out there so just the southwest to northeast aspects in the Olympics will be highlighted for new wind slab. 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 during the day on Thursday.

Avoid ridges that may have a cornices and avoid slopes below cornices seen lately in the vicinity of Hurricane Ridge.

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 formation is likely Thursday night. The warming trend should enhance the formation of these layers.