Avalanche Forecast

Moderate to strong winds have created stiffer slabs at upper elevations and in wind-exposed terrain. On the eastern edge of the zone, very weak snow has been recently observed. Any fresh wind slabs that lay over these sugary facets may easily be triggered and may break widely across terrain features. 

You can trigger storm slab avalanches where about a foot of fresh snow accumulates such as in the upper Salmon la Sac and Teanaway drainages. Use small, inconsequential test slopes to check and see how this new snow is bonding. Use extra caution on slopes over 35 degrees if finding instabilities within the storm snow. A general cooling trend will help these bond, but this will take time. 

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.

The snowpack on the eastern slope is highly variable. In some places it is fairly strong and supportable, but has a layer of facets near the ground that warrants patience and respect. In others, it is shallow, weak, and may not support the weight of a snowmobile or a person very well. An observer reported "facet sluffs" on steep terrain near Mission Ridge on the 28th, an indicator to very weak snow. 

Very large avalanches remain possible within old, faceted layers near the ground. Persistent slabs are often very difficult to predict because you may not get direct signs of instability with these deeper layers. What we do know is that after a significant loading event such as this one, persistent slabs often become easier to trigger. The snowpack needs time to adjust to the new load.

Reduce the risk of this high consequence situation by avoiding particular types of steep slopes. Things to look out for are wind stiffened slabs, and where they may overly shallow, rocky areas near and above treeline. Avoid steep, unsupported slopes with recent wind loading. Don’t underestimate how far and wide a slab failing in old snow could run when identifying safer areas to stop and regroup. Be intentional about putting a significant distance in between yourself, and where avalanches start, run, and stop.

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.