Avalanche Forecast

Numerous triggered avalanches and field investigation Monday and Tuesday confirm that a layer of buried surface hoar survived the most recent storms. Surface hoar can cause avalanches to act in unusual ways. Remote triggering, wide propagation, and failure on low angle slopes are all characteristics of surface hoar avalanches. When we see signs like this, it's time to step way back. Even 30-degree slopes can avalanche and require conservative terrain choices in these conditions. You are most likely to find a layer of buried surface hoar 1-2 feet below the snow surface on slopes above 6000 ft.

It is hard to say where exactly the buried surface is and is not. Snow profiles can demonstrate its presence, but they cannot prove its absence. The growing list of avalanches suggests this layer is more widespread than previously thought. We will continue to monitor these conditions and keep you informed.

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.

Older wind slabs formed over the weekend are gaining strength, but have not fully healed. Strengthening winds were seen forming new wind slabs on New Year's Day below ridges. Backcountry travelers and local professionals reported finding wind slabs in unusual locations such as low on the slope and in areas often wind-stripped. You may still trigger wind slabs on convex rollovers, near the sides of cross-loaded gullies, and on wind drifted snow well below ridgeline. In some locations, wind drifted snow may sit over a layer of buried surface hoar. In these locations, an avalanche may propagate widely and surprise you.

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.