Avalanche Forecast

It will be easy to trigger large to very large avalanches above 6,000ft on Sunday. New snow and wind continue to load deeper buried weak layers. The recent natural, and 4ft deep remotely triggered slides in Portley Bowl may be precursors to more activity. A thin grey stripe indicating buried surface hoar, or weak snow above or below a crust may be the culprit. Look for this layer 2-4ft below the snow surface. If you observe whumps and or shooting cracks, these are obvious signs that you can trigger a persistent slab. They can break surprisingly wide, connecting slope features. Places that may be more likely to avalanche are large alpine bowls, steep convex rollovers, and heavily wind loaded slopes that face the eastern half of the compass. Choose conservative terrain options that keep you off of and well away from slopes steeper than 30 degrees. 

Below treeline and outside of the wind, loose dry avalanches may become possible. Watch for a spike in danger if snowfall rates increase during the day. 

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.

Light snowfall and winds strong enough to transport it will continue on Sunday. These drifts are significant at higher elevations, and closer to the crest where it has been snowing more. Pay attention to how the snow changes as you enter higher elevation terrain. Look for recent avalanches, and use small test slopes to check how well the slabs are bonded to layers below. If you find shooting cracks or recent avalanches, best to steer around heavily loaded slopes. Remember to always consider the consequences before entering slopes steeper than 35 degrees. Be extra cautious beneath steep open slopes with cornices looming above, near the edges of gullies, and near terrain traps and strainers such as stands of trees.

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.