Avalanche Forecast

Moderate to strong winds have created stiffer slabs at upper elevations and in wind-exposed areas. At upper elevations, a layer of surface hoar formed around Christmas may have persisted through the storm. Use extra caution above treeline, as slabs may break surprisingly wide and wrap around terrain features. 

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.

Very large avalanches remain possible within old, faceted layers near the ground. Persistent slabs often become easier to trigger after a significant loading event such as this one. 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.