Avalanche Forecast

Issued: Dec 23rd, 2017 11:04AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Recent storm snow should continue to stabilize. Avoid steeper terrain features with firmer wind transported snow. Be prepared to step back from your plans if the next system arrives stronger and sooner than expected. 

Summary

Detailed Forecast

The next system should begin to bring light snow mainly the south Cascades Sunday afternoon. Alpine winds should stay fairly light, but continue to blow east in the Cascade passes, with no great change in snow levels or cool temperatures.

This should mainly continue to allow recent storm snow to stabilize.

Keep an eye out for previous or newly forming wind slabs on Sunday. Winds will have potentially load non-traditional aspects. If you see signs of winds transporting snow including plumes, drifts, and uneven snow surfaces, then avoid slopes where the wind loading is occurring.

Be prepared to step back from your plans if the next system arrives stronger and sooner than expected.

There is a lot of deep soft snow out there from the past week. Unconsolidated snow, particularly around small trees, can present a non-avalanche snow immersion hazard. Keep visual and verbal communication with your travel partners when traveling in treed terrain.

Also despite all this new snow, early season hazards still exist at lower elevations and especially around creek beds that are not filled in.

Snowpack Discussion

About 1-11 inches of light snow fell across the west slopes of the Washington Cascades and Passes in the 24 hours ending Saturday morning with the most in the southwest Cascades. This added to the very impressive 1-4 feet that fell across the same area early in the week. Recent deep storm snow has been settling and stabilizing over the last several days. Numerous field observations from throughout the west slopes of the Washington Cascades indicate that storm layers continue to stabilize.

Moderate to strong crest level winds transported snow in exposed terrain Tuesday to Thursday forming wind slabs on a variety of aspects. Limited information about the distribution and sensitivity of these wind slabs has been reported.

In general, 2.5 to 3.5 feet of increasingly firm snow now sits above the 12/15 layer. Observations have not demonstrated consistent layers of concern within the upper snowpack. 

Observations

North

No avalanche activity was observed by Mt. Baker pro-patrol Thursday in-area or in nearby backcountry terrain. 

On Wednesday, the Mt Baker Pro Patrol observed significant wind transport on exposed terrain features mainly above treeline as ridge top winds increased. There was a 2 foot human triggered slab avalanche that propagated about 45-75 feet wide near ridgeline.

Central 

NWAC pro observer Jeremy Allyn was in the Chair Peak - Mt Roosevelt area on Saturday and reported windy conditions on the shoulder of Chair Peak. His party triggered 2 small wind slabs on the north slope of Chair Peak at about 4800 feet. Storm layers are healing slowly due to cold temperatures.

NWAC forecaster Robert Hahn was on Jove Peak on Friday and found deep trail breaking, small subtle wind slab on the ridge, storm slab no longer an issue, no signs of instability and good skiing.

A public observation from Stevens Pass Thursday reported triggering very small isolated wind slabs in wind loaded terrain. The party also noted a mostly right-side-up snowpack that was gaining strength.

South

Professional observations from the Crystal Mountain backcountry Wednesday and Thursday are consistently demonstrating a well bonded and strengthening snowpack. Many newly formed cornices along the ridges failed easily under a travelers weight.  No recent observations are available from the Mt Rainier area.

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: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Dec 24th, 2017 11:04AM