Avalanche Forecast

Issued: Dec 11th, 2018 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

We have received significant new snow, strong winds, and have a weak snowpack. This is not the time to travel into avalanche terrain.

Summary

Snowpack Discussion

Coming soon.

Problems

Persistent Slabs

An icon showing Persistent Slabs

Storm totals from the 11th and 12th are expected to be substantial. The deep new snow is not bonding well to the old snow surface. During the day on the 11th a professional observer reported a large natural avalanche in the Spire Gully on Washington Pass with an impressive powder cloud. He triggered avalanches remotely (from a distance) with long propagation, and reported widespread collapsing near the hairpin on Hwy 20. Test results confirmed the potential for propagation on buried surface hoar and near surface facets. These slabs are sitting on old weak snow two to three feet down, and may be surprising as they could break well up slope of a person.

We have limited observations of the snowpack at upper elevations due to difficult access, though the day of the road closure-November 28, observers found faceted snow near the ground near and above treeline on shaded aspects. This layer is likely still there, still weak, and could fail with this heavy loading event. If this layer is triggered, it could create very large and destructive avalanches.

Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.

 

The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.

 

This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.

Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 2

Wind Slabs

An icon showing Wind Slabs

Moderate to strong Southwesterly winds are expected to accompany the cold front as it passes overnight into the morning of the 12th. These will create thick, dense slabs on leeward slopes and features. Some of these slabs are sitting on old weak snow, and may surprise you by breaking well upslope. As a result, it is best to avoid steep, wind drifted terrain today.

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

Likely

Expected Size

1 - 1

Valid until: Dec 12th, 2018 10:00AM