Avalanche Forecast

Moderate to strong winds drifted the new snow into firmer and thicker slabs. As you go higher in elevation wind slabs will become larger and easier to trigger. You can use visual clues to identify and avoid wind loaded slopes over 35 degrees. Fresh cornices, blowing snow, snow drifts, and uneven snow surfaces are all signs that wind slabs are in the nearby terrain.

If you travel near Mt Rainier or in other locations where more storm snow accumulated. You may find lingering storm slabs. If you see more than 8 inches of new snow, stay away from convex rollovers, unsupported slopes, and slopes over 40 degrees where you may trigger a lingering storm snow instability.

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.

A layer of buried surface hoar was observed in the rash of natural and human triggered avalanches last Sunday and Monday in the Crystal backcountry. We did not receive any information about this persistent weak layer from other locations. That does not mean it isn’t there. How did this layer fair in the most recent storm? It's hard to say. In locations like Crystal and White Pass the limited precipitation may not have stressed the weaklayer. Observations from the Crystal area Friday continued to find the buried surface hoar above 6000 ft. You can avoid this layer by stay off of open slopes greater than 35 degrees above 6000 ft.

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.