Avalanche Forecast
Issued: Dec 29th, 2015 10:10AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
NWAC,
Northwest Avalanche Center
Wind slabs should be more difficult to trigger, but not impossible Wednesday. Watch for local wind slabs on NW through SE aspects, mainly near and above treeline. Remember MODERATE danger means human triggered avalanches are possible. Recent unconsolidated storm snow is still very deep in places with possible large tree wells lurking, so stick near your partner in the trees.
Summary
Detailed Forecast
Partly cloudy to mostly sunny weather with cool temperatures and light winds are expected Wednesday. This should allow for recent wind slabs to continue to slowly settle and stabilize.
Wind slab will most likely be found near higher ridges and specific terrain features from NW to SE aspects near and above tree line.
Any triggered wind slab avalanches should be relatively shallow and remain within upper surface layers, but colder temperatures along the east slopes of the Cascades will help slow bonding of newly formed wind slabs to the weaker layers underneath. Test for inverted strong over weak storm snow and give cornices a wide margin.
Dry loose avalanches have been removed as an avalanche problem from the forecast as recent deep snow continues to settle making these less likely.
There have been snow immersion fatalities in tree wells already this season at Snoqualmie and in Canada. Recent unconsolidated storm snow is still very deep in places with possible large tree wells lurking, so stick near your partner in the trees and maintain visual contact.
Snowpack Discussion
For the week ending on Dec. 24th, a snowy and cool storm cycle brought 2-4 feet of snowfall to the east slopes. Only light additional accumulations over the last five days have allowed the snowpack to gradually settle, but it remains deep out there with Washington Pass area snowdepths over 2 meters and fine riding conditions reported! The upper snowpack has been reported to be generally right side up with increasing hardness with depth. Moderate southerly transport winds above treeline Sunday may have built shallow new wind slab on NW through E aspects near high ridges.
An observation posted to the NWAC site Sunday described a shallow triggered wind slab on an SSE facing slope at 4500 feet, well below treeline, on a wind loaded feature near Merritt Lake off Hwy 2.
Storm slab instabilities reported last week have been reported to have settled as of Monday near Washington Pass.
The southeast zone has a relatively shallower snowpack versus areas further north, but after a snowy few weeks, has more than enough snow to warrant an avalanche risk. No snowpack observations have been received from this 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
Valid until: Dec 30th, 2015 10:10AM
