Avalanche Forecast

Issued: Dec 12th, 2018 10:00AM

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

NWAC, Northwest Avalanche Center

Cascades - East.

Recent new snow and wind have covered a pronounced weak layer, creating a recipe for dangerous avalanches conditions. Safe travel in avalanche terrain will require a cautious approach and conservative decision making.

Summary

Discussion

Avalanche Summary

The first 9 days in December were clear and cold. Surface hoar grew large during this time, and facets developed near the surface of the cold, low density snow. On December 9th this layer was covered up and preserved on many aspects and elevations. On December 12 an observer reported shooting cracks at Mission Ridge. In adjacent regions numerous avalanches and signs of instability were observed on December 11th. A professional observer reported a large natural avalanche in the Spire Gully on Washington Pass with an impressive powder cloud. Near here, the observer reported triggering avalanches remotely (from a distance) and widespread collapses on the old snow layer near the hairpin on Hwy 20. Test results indicated the potential for wide propagation on this layer of buried surface hoar and near surface facets.

You are likely to trigger an avalanche that will break on the weak, old snow interface which can be found one to two feet down, and could surprise you. Look for signs of instability such as whumphing, shooting cracks through the snow, and recent avalanches.

Problems

Persistent Slabs

Slabs from one to two feet thick are sitting on old weak snow. Look for signs of instability such as whumphing, shooting cracks through the snow, and recent avalanches. Any failure on this weak layer would be surprising, as avalanches could wrap widely around terrain features. Travel in avalanche terrain will require a very cautious approach.

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: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 1

Wind Slabs

Moderate to strong winds are likely to create wind slabs during the day. Look for visual clues such as thick, hard wind drifts. Avoid terrain features such as gullies and the leeward sides of ridges that may become loaded with wind driven snow.

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.