Avalanche Forecast
Issued: Nov 29th, 2016 11:10AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
NWAC,
Northwest Avalanche Center
Wind slab found in the near and above treeline bands will be the primary avalanche problem on Wednesday. Look for signs of recent wind loading on a variety of aspects.
Summary
Detailed Forecast
A weak frontal system passing through the Cascades early Wednesday morning should bring light amounts of new snow above 3000 feet.
Previously formed and fresh wind slabs found in the near and above treeline bands and on a variety of aspects will be the main avalanche problem Wednesday. Watch for signs of firmer wind transported snow that indicate wind slab.
Be aware of a buried surface hoar layer in the Washington Pass zone (see discussion). Reports are few early this season and it's not known how reactive or widespread this layer might be along the east slopes.
Snowpack Discussion
Weather Discussion for the Cascade East Slopes
A frontal system passed through the Cascades Sunday night and was followed by NW flow aloft on Monday. This pattern favored the west slopes for significant snow accumulations. Near the Cascade crest along the east slopes, snow amounts were in the 3-5 inch range with lesser amounts further east. Moderate WNW winds continued through the day Monday as recorded at the Mission Ridge and Dirty Face weather stations. Tuesday was a fair day with light winds along the east slopes.
Recent Reports for the Cascade East Slopes
Last Friday a skier triggered a deep wind slab on a 40 degree northwest slope at about 7500 feet on Slate Peak near Harts Pass. The crown varied from about 2 to 7 feet and ran about 2-300 vertical feet with car sized chunks of debris. Fortunately the skier was not caught. He noted that windward slopes across the valley were scoured indicating recent wind transport.
On Tuesday, NWAC professional observer Jeff Ward was in the Washington Pass area. He found generally good ski conditions and a stable snowpack. Previous wind loading was obvious but varied by aspect throughout the terrain. However, wind slabs were stubborn and unlikely to trigger in the drainages he traveled through.
One layer of interest was buried surface hoar at 65 cm down on a north aspect at 7000 feet. Older avalanche crowns did not seem to step down to this deeper layer. It's not known how widespread this buried weak layer might be but the upcoming weather over the weekend should give it a good load.
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Nov 30th, 2016 11:10AM
