Avalanche Forecast

Observations from around the West South Saturday reported stubborn wind slabs near ridge crest and on mid-slope gullies. These potential avalanches are a few days old and becoming harder to trigger. When conditions are like this, you can turn your attention to specific features where you are more like to trigger an avalanche. Convex rollovers, steep slopes below cornices, and unsupported terrain are all locations you could trigger a lingering wind slab. Keep your eyes open for clues such as wind drifts and textured snow surfaces to indicate where wind loading occurred. 

At lower elevations, you could encounter wet snow avalanches. If you see roller balls or experience sticky surface snow, steer around slopes greater than 35 degrees.

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.

Snowpack observations and explosive testing by Cyrstal Mountain ski patrol continue to help us gain confidence that this weak layer is gaining strength. Conditions seem to indicate that it would take a large trigger such as a cornice fall or other avalanche to cause this layer to fail. An NWAC forecaster at White Pass Saturday identified this persistent weak layer in two profiles but did not see any associated avalanches. Remember, White Pass did not receive the same substantial loading during last week’s storm as other locations. If you are in the Crystal or White Pass areas, you could find a layer of facets and/or buried surface hoar associated with a crust about a foot above the ground on W-N-E aspects above 5800’. We’ll continue to monitor this layer going forward.

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.