Avalanche Forecast
Issued: Dec 3rd, 2016 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
NWAC,
Northwest Avalanche Center
Moderate to locally high westerly winds will build shallow wind slab on lee aspects near and above treeline for terrain adjacent to the Cascade crest. Look for wind slab development further downslope than you might expect due to locally strong winds and persistent loading.
Summary
Detailed Forecast
A frontal system will quickly sweep through the Cascades Saturday night and Sunday morning. However this system will not favor the east slopes and should only deliver light amounts of new snow along with a significant cooling trend.
Moderate to locally high westerly winds will build shallow wind slab on lee aspects near and above treeline for terrain adjacent to the Cascade crest. Look for wind slab development further downslope than you might expect due to locally strong winds and persistent loading.
In much of the lower part of the below treeline band there is still insufficient snow for avalanches, so watch for early season travel hazards such as terrain traps, rocks and open creeks.
Snowpack Discussion
Weather and Snowpack
A front crossed the Cascades Friday but likely only produced an inch or two of new snow near the Cascade crest with no accumulations further east. Moderate westerly ridgetop winds continued on Saturday. With the mild snow levels seen so far this season, most avalanche problems have been confined to the most recent storm layers found in the upper snowpack.
Recent Observations
During midweek NWAC pro-observer Jeff Ward was in the Washington Pass area and found generally good ski conditions and a stable snowpack. Stubborn and shallow wind slab existed near and above treeline. Thin coverage and open creeks continued to be a travel hazard at the pass elevation.
On Friday Jeff was out again on Jove Peak east of Stevens Pass which can be representative of the east slopes closer to the Cascade crest. He saw no signs of instability but noted minor wind transport along the ridge.
In general more snow is needed in the Mission Ridge area to present an avalanche hazard. In wind-loaded terrain near and above treeline, no weak layers of note have been reported by the pro-patrol through Saturday.
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, South.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 4th, 2016 10:00AM
