Avalanche Forecast

These deadly avalanches can surprise even the most experienced backcountry travelers. Put a large buffer of terrain between you and any slope that is steep enough to slide. Be diligent to not stray too close to roll-overs and changes in slope angle. Do not travel immediately below slopes 35 degrees or steeper. Some of the most dangerous slopes will be where wind has drifted thick slabs over deeply buried weak layers.

While natural avalanche activity will be ending, it may still be easy to trigger persistent slab avalanches from far away, on adjacent slopes, nearby ridges, and from low angle terrain. In the past week, observers have reported natural and triggered avalanches along with whumpfing collapses and cracks shooting through the snow. The avalanches have been large and surprising. Some have been triggered remotely. On January 2nd skiers remotely triggered an avalanche on an east aspect of Portley Bowl, near Cutthroat Peak, at 6,700ft. On the 4th, observers reported another remotely triggered avalanche on a north aspect at 6,200ft in the Pine Creek drainage. Crown profiles from both locations found a persistent weak layer, likely buried on December 27th. The same weak layer is suspected in other recently reported avalanches. Buried 2-5 feet below the surface, you can find feather-like surface hoar or possibly facets with an associated crust. The layer is preserved above 6,000ft, where these avalanches are most likely. Suspect avalanche terrain at mid and upper elevations for this weak layer until proven innocent.

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.

You may be able to trigger avalanches in the recent snow, where it has accumulated in the past 24 hours. Watch for stiffer snow near the surface, overlying softer snow. Avoid avalanche terrain on all slopes where you suspect underlying persistent weak layers. Strong and gusty winds will drift snow onto leeward sides of ridges at upper elevations.

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.