Avalanche Forecast
Issued: Dec 21st, 2015 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
New or existing storm slab and wind slab are likely Tuesday, with new wind slab mainly forming on lee slopes ranging from N-SE facing, especially by afternoon as the next storm increases.
Summary
Detailed Forecast
Continued cool weather with snow showers and moderate winds overnight should build further storm slabs. A brief decrease is expected early Tuesday ahead of another approaching Pacific frontal system. The next system moves into the Cascades early Tuesday afternoon with renewed moderate snow and moderate westerly winds. This should begin to build new areas of wind slab on lee slopes below ridges and create more extensive storm slabs.
Dangerous avalanche conditions are expected in the above and near treeline bands Tuesday, especially later in the day.
Remember to give cornices a wide margin. Also above tree line strong winds recently have scoured snow from exposed slopes, leaving exposed rock and vegetation. Terrain anchors are still causing anchoring at the lowest elevations, but less and less with every storm passage!
Snowpack Discussion
A series of cool storms has deposited increasing storm snow at relatively low freezing levels since early December. There has been about 1-2.5 feet of snowfall over the past five days.
NWAC pro observer Tom Curtis was at Rainy Pass near Stevens Pass on Wednesday and noted extensive surface hoar. This layer may have been buried intact Thursday though no evidence of avalanches due to the layer has been reported.
Tom visited Mt Cashmere via the Icicle Creek drainage Thursday and did not find the persistent layer of buried surface hoar seen prior to the rain event December 8-9. There has been good evidence that the rain event destroyed this layer along the east slopes so the PWL has been removed as an avalanche problem along the east slopes. Tom did note small storm and wind slab up to near treeline, but did not venture higher.
The Mission Ridge pro patrol on Friday reported widespread sensitive 4-8 inch storm slab avalanches but then reported no significant avalanche activity on Saturday.
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
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
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.
Aspects: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 22nd, 2015 10:00AM
