Avalanche Forecast

The wind slab forecast is tricky in the West-South zone Sunday. Weather models are showing quite a bit of variability model to model, and place to place. However, we do feel certain that the wind will drive the avalanche danger. In locations where the wind drifted the new snow into firmer and thicker slabs, you may be able to trigger an avalanche. You are most likely to find wind slabs just below ridgelines and on wind exposed features at any elevation. Look for blowing snow, fresh cornices, and new snow drifts to indicate wind slabs may be on nearby slopes. You can use wind stripped areas, ridgelines, and lower angled slopes to avoid triggering wind slabs as you travel.

On the volcanoes, more snow may accumulate than other in areas such as Crystal, Chinook Pass, and White Pass. If you experience more than 8 inches of new snow in these locations, stop and look for strong snow over weak snow. Small slope test and simple hand pits can help you identify up-side-down storm snow that may cause an avalanche.

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.

It’s been several days since we have heard about any avalanches on the most recent layer of buried surface hoar. The lack of avalanches combined with a few limited observations suggest that this layer may be gaining strength. However, it’s not totally gone from our minds. Higher elevation locations near Crystal Mountain and White Pass might still harbor a layer of buried surface hoar around 2 feet below the snow surface. If you are traveling in these or similar areas above 6000 ft, take the time to consider and discuss how this low-likelihood but high-consequence avalanche problem might shape where and how you travel.

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.