Avalanche Forecast

Issued: Dec 2nd, 2016 11:00AM

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is low. Known problems include Wind Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Watch for wind slab that may linger on Saturday which may get shallowly covered by less cohesive snow Friday night.

Summary

Detailed Forecast

West winds will slightly decrease but not let up on Saturday with mostly light snow showers much of the day expected at the higher elevations along the east slopes and cooler temperatures. However alpine west winds and snow should begin to increase in the afternoon as the next system approaches.

Any shallow wind slab from Friday seems likely to linger on Saturday. This is mostly likely on northwest to southeast slopes near ridges in the above treeline. Watch for firm wind transported or hollow sounding snow. Wind slab from Friday may get shallowly covered by less cohesive snow Friday night making it harder to detect the underlying wind slab layer.

Be prepared to change your plans if the next storm begins to arrive and significant snow begins to accumulate sooner than expected on Saturday afternoon.

In most of the lower part of the below treeline band there is still insufficient snow for avalanches but watch for early season hazards such as terrain traps, rocks and creeks. The shallowest snow is at pass level at Stevens, Snoqualmie and White Passes.

Snowpack Discussion

Weather Discussion for the Cascade East Slopes

A bit of a break has been seen along the east slopes on Thursday with 1 or 2 inches of new snow except for 7 inches indicated at the Lyman Snotel.

A front is crossing the area on Friday causing moderate to strong south to west alpine winds in the Olympics and Washington Cascades, hopefully with several inches of new snow at higher elevations along the east slopes and a warming trend.

Recent Reports for the Cascade East Slopes

Last Friday November 25th 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 pro-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. Wind slabs were stubborn and unlikely to trigger in the drainages he traveled through.

Jeff relayed another report to the NWAC from the Washington Pass area for Wednesday. On east slopes near the pass no signs of instability were seen with compression tests giving inconsistent hard or negative results at a layer at about 45 cm. This coverage or open creeks were still an issue at the pass elevation. Stubborn wind slab was the only noted problem.

Jeff was out again on Jove Peak east of Stevens Pass on Friday which might be somewhat representative of the east slopes. He saw no signs of instability and compression tests gave no results on a south slope at 4800 feet. He noted some minor wind transport along ridge and felt small wind slab might be possible there.

Problems

Wind Slabs

An icon showing 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

Likely

Expected Size

1 - 1

Valid until: Dec 3rd, 2016 11:00AM