Avalanche Forecast
Issued: Dec 20th, 2015 10:25AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
There should be a big difference in the weather and avalanche conditions between the north and south Cascades on Monday. The most dangerous conditions should be seen in the central to south Cascades.
Summary
Detailed Forecast
A strong low pressure system will move west to east across Washington on Monday. There should be a big difference in the weather and avalanche conditions between the north and south Cascades with the strongest winds and heaviest snowfall in the south Cascades. Right now it looks like the main effects of the low may reach as far north as the central Cascades. But a different track of the low could bring much different than expected weather and avalanche conditions on Monday so the forecast is somewhat uncertain.
Light to moderate winds and snow are expected in the north Cascades on Monday. Stronger shifting winds, moderate to heavy snow and rapid loading are expected in the central to south Cascades. New storm slab and wind slab potential will be greatest in the central to south Cascades. Back country travel is not recommended above treeline in the central and south Cascades on Monday. Dangerous avalanche conditions are expected in the other areas along the east slopes.
Terrain anchors are still causing significant anchoring at the lowest elevations and barely hidden hazards like rocks, streams or snags remain.
Snowpack Discussion
A series of cool storms has deposited increasing storm snow at relatively low freezing levels since early December. There has been about .5-2 feet of snowfall at sites along the east slopes the past 2-4 days. The skiing in non-wind affected areas with enough snow cover has been good!
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 on a surface hoar 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.
A guide report from Thursday at Washington Pass zone indicated good snow and good stability at the elevation of the highway hairpin with blowing snow along the ridge tops.
The Mission Ridge pro patrol on Friday reported widespread sensitive 4-8 inch storm slab avalanches but then reported no signficant 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.
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
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: Alpine, Treeline.
Likelihood
Expected Size
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.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 21st, 2015 10:25AM
