Avalanche Forecast

A persistent weak layer within the snowpack in this zone was the cause of several very large (D3) natural and explosive triggered avalanches in the Crystal region over the last few days. The danger rating in our zone has been on a downward trend because naturally triggered avalanches have slowed down or stopped, but the threat of avalanches due to this persistent weak layer remains, specifically in the Crystal area. 

The nature and existence of the persistent weak layer differs across the zone, but the effect is the same. Where present, a large load now sits atop this weak layer, which resides about a foot off the ground and is generally found on West, North, and East aspects near and above treeline. In some areas, slopes have not yet reached their tipping point.  Determining which slopes are about to reach their tipping point is a very difficult task. If you trigger an avalanche, it could be 3-6ft deep, break on slopes as low as 30 degrees, and wrap around features in the terrain. While snowpack tests can confirm this layer’s presence, they can not prove its absence. Don’t rely on a snowpit to interpret stability. Avoid traveling on steep slopes above 5800’ on W-N-E aspects. We don’t have confirmation this layer exists in other areas in the zone, please submit your observations to our website. 

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.