Avalanche Forecast
Issued: Dec 8th, 2016 11:55AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs, Wind Slabs and Loose Dry.
NWAC,
Northwest Avalanche Center
Relatively lighter snowfall amounts and a more subdued warming trend along the east slopes Thursday night and Friday should dampen the rising avalanche danger trend seen elsewhere in the Cascades. Heavier than expected snowfall or a quicker warming trend would rapidly increase the danger Friday.
Summary
Detailed Forecast
Relatively lighter snowfall amounts and a more subdued warming trend along the east slopes Thursday night and Friday should dampen the rising avalanche danger trend seen elsewhere in the Cascades. Heavier than expected snowfall or a quicker warming trend would rapidly increase the danger Friday.
Look for shallow new wind slab on a variety of aspects mainly above treeline. If new storm slab does build Friday, it should quickly become sensitive and poorly bonded to the existing weak snow surface. Watch for shooting cracks and signs of increasing instability Friday on the new snow/old snow interface especially near and below treeline.
In steep sheltered terrain continue to watch for small loose dry avalanches near terrain traps like cliff bands where even small avalanches can have big consequences.
Even though the lower part of the below treeline band is filling in, you will need to watch for early season travel hazards such as barely covered rocks and open creeks.
Snowpack Discussion
Weather and Snowpack
An active and cool weather pattern has resulted in a snowy December thus far with NWAC stations along the east slopes recording about 1-1.5 feet of snow.
Cold and fair weather has taken hold since Monday. There have been many reports of right side up, favorable density profiles with lower density snow nearer the surface. We've been hearing the term "as good as it gets" quite a lot lately!
Given the favorable snowpack profile, good bonds between older snow layers and a lack of deeper layer concerns, avalanche problems should be confined to the upcoming old snow/new snow interface or within new storm layering.
Cold fair weather like this usually causes some new surface hoar layer and near surface faceted snow. This snow can make for great skiing or riding. But these layers will need to be watched if loaded by wind transported snow or new snow.
Recent Observations
The most recent observations from near Washington Pass Monday and Tuesday indicate great skiing, favorable snowpack profiles and good bonds of older snow layers.
While coverage in most below treeline remains thin, the upper elevations of this lowest band are filling in quite impressively and we are bumping the overall danger to moderate in this band as a result. But the lower elevations in many areas lack good coverage and open creeks and other early season hazards remain.
In general, more snow is needed in the Mission Ridge area and southeast Cascades to present an avalanche danger. In wind-loaded terrain near and above treeline, no weak layers of note have been reported lately by the Mission pro-patrol.
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: All elevations.
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Loose Dry
Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.
Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.
Loose Dry avalanche with the characteristic point initiation and fan shape.
Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 9th, 2016 11:55AM
