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Avalanche Forecast

Archived

Dec 4th, 2015–Dec 5th, 2015

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
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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Lingering wind slab on lee NW through SE aspects and shallow new storm slabs are possible on Saturday mainly in the near and above treeline elevation bands.    

Detailed Forecast

A frontal system should take aim mainly at the north Cascades Saturday, with only light precipitation for the south-central Cascades through Saturday afternoon. 

Shallow storm and wind slabs are possible near and above treeline. Watch for new or previously wind loaded NW-SE aspects and be prepared to curtail your plans if conditions deteriorate faster than expected.    

Early season hazards exist for much of the below tree-line band throughout the Northwest, so ski and ride with caution. 

Snowpack Discussion

Note: The snowpack discussion is for the entire west slopes of the Cascades, but the avalanche danger forecast varies by zone.  

 

We had a wild and wet month of November, with 12- 25 inches of water at west side NWAC stations only amounting to about a 1 meter (~3 ft) snowpack in the north Cascades above 4000 ft and above 6000 feet elsewhere. Strong high pressure led to steep temperature inversions and brought very cold air through the Cascade Passes over Thanksgiving weekend. The weather pattern has become more active for the first few days of December with a few inches of new snow over the west slopes paired with moderating temperatures in the Passes. 

Avalanche and snowpack observations: Recent activity along the west slopes away from the Cascade Passes has been confined to new storm snow. New NWAC pro-observer Lee Lazzara noted a natural cycle of loose wet avalanches on Wednesday as temperatures warmed. This natural cycle likely extended to higher elevations in the north Cascades on Thursday as rain pushed above 5000'. Mt. Baker and Crystal patrols reported small avalanches involving new storm snow during control work Fri AM.   

The west slopes do not have a uniform maritime snowpack! Due to cold temperatures and clear skies near the end of November, buried surface hoar can be found in the Stevens Pass area. NWAC pro-observer Dallas Glass found this layer around 5000' in the Skyline area on E-NE aspects around 45 cm down. Faceting may also be found on solar aspects around a buried sun crust. Stevens pass pro patrol reported a 12-14" (30-35 cm) natural persistent slab avalanche in closed Corona Bowl of the ski area Friday on a N-NW aspect.  Professionals also reported shooting cracks and whumpfing in the area as temperatures warmed and the slab above the PWL became cohesive.

We still have many gaps in our observation network as it's early season, so make your own observations and use them in co-junction to our regional avalanche forecast. Thanks for reading the first NWAC avalanche forecast for 2015-16 season, stay safe and have fun! 

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.