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Avalanche Forecast

Archived

Dec 23rd, 2015–Dec 24th, 2015

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
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

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Another round of storm slab and wind slab are likely Wednesday, with new wind slab mainly forming on lee slopes ranging from NW-SE facing, especially by afternoon as wind speeds increase. Loose dry avalanches in non-wind affected terrain will be difficult to manage on steeper slopes, so use extra caution around terrain traps. 

Detailed Forecast

Another Pacific frontal system, in the long series of recent storms, is moving into the Cascades Tuesday evening. This next system will cause renewed light to moderate snow and light to moderate westerly winds Tuesday night. This should begin to build new areas of wind slab on lee slopes below ridges and create some isolated storm slabs. Light to moderate showers should persist with increasing westerly ridgetop winds through the day Wednesday, but be mainly confined to areas nearer the crest. 

Dangerous avalanche conditions should persist Wednesday in all elevations bands and especially in the near and above treeline zones. 

Expected avalanches should remain within the near surface storm related weak layers. Watch for evidence of wind deposited snow or pillows. Test for inverted of strong over weak storm snow and give cornices a wide margin.

Loose dry avalanches in non-wind affected terrain will be difficult to manage on steeper slopes. Even a small loose dry avalanche could have unintended consequences such as knocking you off your feet and into a terrain trap. 

Snowpack Discussion

The Christmas snow globe keeps giving here in the PNW! The east slopes are getting in on the action as well! It's already well beyond anything we achieved last year and getting deeper by the day. Most east slope areas received 1.5-3 feet of new snow over the past several days.

NWAC pro observer Tom Curtis was in the Blewett Pass area Tuesday and found great snow conditions, but also recent wind and storm slab conditions on lee slopes and exposed terrain features. Wind slabs were still forming on W-NW-N-NE aspects below treeline with exposed features, NTL, and ATL and in many areas were well developed, sensitive, and large enough to kill you in the right terrain feature! 

A back country skier on Mt Cashmere on Saturday found about 5 inches of new snow and a right side up upper snow pack with no results from snow pit tests and ski cuts.

A guide report from Sunday in the Washington Pass zone, indicated excellent snow and good stability with about a foot of unconsolidated storm snow. No slab character was noted in the storm snow in travel to about 7000 feet, though safe travel and good route finding techniques may be just as evident in that report! 

The southeast zone should have a much shallower snowpack, more affected by recent rain and warm temperatures. However, we have no recent observations from the southeast 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.

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.

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.