Avalanche Forecast
Issued: Dec 22nd, 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
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.
Summary
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.
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
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 23rd, 2015 10:00AM
