Avalanche Forecast

Plenty of recent low-density snow is available for transport. Another round of light snowfall on Tuesday will easily be redistributed by the wind. New snow and wind will form fresh wind slabs near and above treeline. Previously formed slabs still exist, and will be masked by the new snow. The most reactive slabs will exist above treeline. The snow will also bury weak old snow surfaces, making fresh slabs especially easy to trigger. Look for signs of wind transported snow such as textured snow surfaces, drifting, or fresh cornices. If you see these features, expect wind slabs on nearby slopes. You can avoid triggering a wind slab by steering around wind loaded slopes greater than 35 degrees.

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 slab/weak layer combination required for these persistent slabs does not exist everywhere in the West-North zone. It is this spotty distribution that makes our current situation tricky. We have reports of avalanches and/or snow profiles highlighting this problematic layer on all aspects and in each elevation band. However, recent observations targeting our buried persistent weak layer have been highly variable, and evidence of unstable old snow layers is becoming harder to find.

This layer is buried 2-3 feet below the snow surface. On colder shaded slopes, the culprit seems to be buried surface hoar. On sunnier aspects, we are finding small facets over a crust. Obvious signs of instability such as collapses or shooting cracks have been uncommon. Only digging in the snow can help you define where this layer may be problematic. This could be a good time to simply stay out of large, open slopes greater than 35 degrees where you could trigger one of these lingering persistent slabs.

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.