Avalanche Forecast

Issued: Dec 22nd, 2017 11:26AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Deep storm snow received earlier this week continues to stabilize with no recent natural or human triggered avalanches reported. Watch for recently wind transported snow as winds increase and change direction heading into Saturday afternoon, avoiding steeper slopes where snow is being deposited. 

Summary

Detailed Forecast

Cool temperatures and mostly clear skies will continue to allow storm snow accumulated from the past week to stabilize.

Keep an eye out for newly forming wind slabs as winds increase and shift direction Saturday. Winds will potentially load non-traditional aspects. If you see signs of winds transporting snow including plumes, drifts, and uneven snow surfaces, avoid slopes where wind loading is occurring. 

There is a lot of deep soft snow out there from the past week; 2-4 feet across the west slopes of the Cascades! 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. Despite all this new snow, early season hazards still exist mainly at lower elevations and especially around creek beds that are not filled in.  

Snowpack Discussion

Several inches of light snow fell across the west slopes of the Cascades and Passes during the day Friday, adding to the very impressive snow totals from the past week. Recent deep storm snow has been settling and stabilizing over the last several days under fair and cool weather. Numerous field observations from throughout the west slopes of the Cascades indicate that storm layers continue to stabilize.

Moderate to strong crest level winds transported snow in exposed terrain Wednesday and 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, 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 

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. This supports professional observations from around the Stevens and Snoqualmie Passes Wednesday where storm slabs were observed to be stabilizing.

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 23rd, 2017 11:26AM