Avalanche Forecast

Avalanche conditions will get more dangerous as you climb higher in the mountains. Both snow amounts and wind drifting will be more significant at upper elevations. Gusty winds will form deeper drifts on leeward sides of ridges and terrain features at upper elevations. Check the interface between the new and old snow and within storm layers. Use small test slopes to check how easily the snow can slide. Watch for recent avalanches, cracking in the snow, and blowing snow as signs of instability. 

Use caution on slopes 35 degrees and steeper where you find more than 6 inches of new and drifted snow. Be extra cautious around terrain traps such as gullies, rocks, and cliffs where even a small avalanche could have serious consequences. Avalanche conditions may be dynamic, changing throughout the day with the storm. You may be able to trigger avalanches in the storm snow in wind-sheltered areas, as well.

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.

This storm could be enough to activate some lingering weak layers. Persistent slab avalanches can be surprising and can break widely across terrain. The best way to deal with these avalanches is to pick conservative terrain options.  Specific features to avoid include steep convex rollovers, areas of variable height of snow, shallow and rocky features, and unsupported slopes. 

There are a few weak layers to watch. Just under the old snow surface is a layer of surface hoar, buried on Dec 27th, that has been preserved on some shaded northerly slopes. A thin layer of facets buried in early December can be found just below the middle of the snowpack. Facets formed in late November can be found near the bottom of the snowpack, often resting on a stout crust. These last two weak layers were suspected as the cause of numerous large avalanches and signs of instability during the avalanche cycle around December 20th. While avalanches haven't been triggered on them more recently, the snowpack structure suggests the weak layers could be a problem with enough load.

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.