Avalanche Forecast

Issued: Dec 2nd, 2017 11:42PM

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

NWAC, Northwest Avalanche Center

Cascades - West.

Don't overlook pockets of wind-loading on steeper terrain, particularly in the near and above treeline in the northwest zone, where avalanches will be dangerous. More recent snowfall and deeper storm and wind slab layers require more careful snowpack evaluation in the Mt. Baker area on Sunday. Evaluate snow and terrain carefully in the other areas along the Cascade West slopes.

Summary

Detailed Forecast

Snow showers Saturday night should decrease from the north on Sunday with light NNW winds. See the NWAC mountain weather forecast tab for more details.

Mt. Baker has received a lot more snow and has had a lot more storm and wind slab avalanche activity than the rest west slopes the past few days. NWAC forecasters remain highly uncertain about the degree of the avalanche danger in the Mt. Baker Area, particularly in the near tree-line zone. Firmer wind slab layers over lower density snow is suspected to be lingering into the weekend and these layers are gradually becoming less sensitive, but in some steeper and more wind-affected locations there will continue to be a good chance of triggering. The wind slab from Friday has been buried by about 2' of newer snow. It will be critical to dig and examine for these layers in specific wind-affected locations or avoid steeper wind-effected terrain. High uncertainty suggests extra caution in avalanche terrain.

Along the rest of the west slopes observations reveal a mostly well-bonded snowpack without significant layers of concern. Lesser amounts of recent snowfall or wind transport and smaller lingering potential storm wind slabs are the primary concerns in these areas. Carefully evaluate the character of new storm snow and approach ridges cautiously and watch for firmer wind-transported snow.

Remember that early season terrain hazards still exist, such as poorly covered rocks, vegetation and creeks, particularly at lower elevations.

Snowpack Discussion

Warm, wet weather before Thanksgiving caused wet snow and glide avalanches as well as significant snowpack consolidation. There are no snowpack concerns below the Thanksgiving crust and in general new snow received post-Thanksgiving has reportedly bonded well to the Thanksgiving crust.

During the week, a series of frontal systems have generally produced 1-2' of new snow for the west slopes of the Cascades. The exception is in the Mt. Baker area which received about 4' of snow this week, including the 24 snowfall of 19" ending Saturday morning. No significant avalanche activity has been observed outside of the Mt. Baker zone.

Observations

North

On Friday, steady snowfall led to widespread and very touchy wind and storm slabs on all aspects and elevation bands on terrain ~35 degrees or steeper. NWAC observers in the Mt. Baker backcountry reported average slab depths of 30 cm in sheltered areas and about 1 meter of snow above the Thanksgiving Day crust. Winds were gusty and distributing snowfall onto all aspects. They reported 1F wind slab layers over 4F snow that should linger into the weekend.

On Saturday morning, the Mt. Baker Ski Area reported widespread 10-14" touchy, large ski-triggered storm slab on all aspects along with a couple similarly sized natural avalanches. During the day, the resort also reported 3 close calls due to tree-well incidents.

By Saturday afternoon, NWAC pro observer Lee Lazara found that slab layers were becoming more stubborn to trigger.

Central

On Thursday, NWAC observer Jeff Ward found about 40 cm of settled snow well bonded to the Thanksgiving Day crust in the Stevens Pass backcountry. With good visibility, no avalanche activity was noted in the surrounding terrain.

On Saturday, the Alpental Ski Patrol reported generally well-bonded settled snow.

South

On Thursday in the Crystal backcountry, NWAC pro-observer Dallas Glass found 25 cm (10") of this week's storm snow bonding well to the stout and fully supportable Thanksgiving rain crust. Shallow wind slab averaging 10-15 cm (4-6 cm) thick was found below ridges on W-N-E aspects, but was isolated in distribution. Total snow depth in this area peaked at about 1 meter at 6000' and above.

NWAC pro-observer Jeremy Allyn found similar conditions on Friday. Jeremy also reported 2-3 mm surface hoar from below tree line to ridge crest. It was not widespread, but present through much of the terrain and even on wind exposed features in high elevations.

NWAC Pro Observer Dallas Glass was out on Saturday in the Paradise area. On W-S-E aspects he found 8” new of new snow well-bonded to 2 feet of previous snow on the Thanksgiving crust. He found 5400-6000’ W-SW-S-SE-E. He did not find evidence of storm or wind slab.

Problems

Storm Slabs

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

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.