Northwest Avalanche Center NWAC, Northwest Avalanche Center

Cascades - North West Avalanche Forecast

Jan 1st, 2018 10:00AM

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

Shallow stubborn wind slabs continue to linger in wind exposed terrain near and above treeline. Identify and avoid areas of recently wind loaded snow below ridge line and cross loaded terrain features. In some areas, firm surface crusts will make for difficult travel conditions so be prepared to self-arrest and think about the sliding hazard.  

Summary

Detailed Forecast

Generally mild and cloudy weather should be seen Tuesday with the exception of cooler temperatures in the Cascade Passes due to increasing offshore flow. 

Wind slabs should continue to heal but look for areas of recently wind transported snow such as fresh cornices, snow drifts, and uneven snow surfaces. Identify and avoid locations where recent wind loading has occurred.

Wind slabs can be deceptively difficult to manage in the terrain. Take a moment and read our recent blog post by NWAC Pro Observer Jeremy Allyn on wind slabs.

Cornices along ridge crest in the west slopes of the Cascades and Passes have grown quite large especially at higher elevations. Remember to give these features a wide berth. Cornices can break much farther back from the ridge than expected.

Photo: NWAC Forecaster Robert Hahn: A large cornice on Table Mountain in the Mt Baker backcountry. 12-31-17

In some areas, firm surface crusts will make for difficult travel conditions so be prepared to self-arrest and think about the sliding hazard.  

Despite all this new snow, early season hazards still exist. Many creek beds have still not filled in for the winter.

 

Snowpack Discussion

Happy New Year from your friends at the Northwest Avalanche Center!

Mild weather seen on Sunday and Monday allowed lingering wind slabs to gain strength. Cool easterly flow brought clouds and lower temperatures to the Passes, initially slowing the pace of stabilization in these areas. On Monday, easterly flow briefly abated allowing higher elevation stations to bump above freezing (see graph below). 

Winds Saturday formed shallow slabs on a variety of aspects near and above treeline. Depending on elevation these wind slabs sit on soft snow storm snow or firm rain crust. Shallow wind slabs were triggered by recreational skiers on Sunday in the Snoqualmie and Mt. Baker backcountry. 

The Dec 28th/29th storm cycle formed a rain and/or freezing rain crust throughout the west slopes of the Cascades including the Passes that reached at least into the near treeline elevation band. A few inches to a foot of snow fell at the tail end, with the most snow at Mt. Baker. 

These firm crusts capped storm snow which fell earlier during the storm cycle. Below the 12/30 crust (date the crust was buried), a generally strengthening snowpack can be found. Weather stations from across the region confirm the upper snowpack is settling and observations demonstrate that it is gaining strength.

Observations

North

On Saturday and Sunday Mt Baker Pro Patrol reported wind transportation of the new snow at the upper elevations forming small shallow slabs. Evidence of rain was found to the top of the ski area.

NWAC pro observer Lee Lazzara was on Bear Paw Mountain Saturday. Lee reported a variety of snow surfaces conditions due to recent wind transportation of snow. He observed wind slabs up to 2 ft thick sitting on a firm rain crust formed during the overnight rain event.

Central

On Sunday, Stevens Pass Patrol described a freezing rain event to the top of the ski area that occurred Friday night. Winds Saturday formed shallow wind slabs within the area. Approximately 4” of soft snow sits above the freezing rain crust in wind sheltered areas.

Also on Sunday, Alpental Patrol reported a ½” freezing rain crust to the top of the ski area.

South

On Sunday, Crystal Mt Ski Patrol saw pockets of wind slabs at higher elevations. Friday’s rain crust extended to 6500’ in the ski area.

On Saturday NWAC Pro Observer Jeremy Allyn was in the Crystal backcountry. He observed evidence of rain in the form of a breakable rain crust all the way to ridge crest (6500’). Recent winds had redistributed the overnight snow forming pockets of wind slabs on lee slopes. Jeremy noted that variable snow surface conditions exist due to the recent wind event.

Problems

Wind Slabs

An icon showing Wind Slabs

Likelihood

possible

Expected Size

1 - 1

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.

Valid until: Jan 2nd, 2018 10:00AM