Avalanche Forecast

Recent strong winds have drifted both new and old snow into firm, cohesive slabs. The drifting will be more significant at higher elevations, where more snow fell, and the winds were stronger. Check to see how these slabs are bonding with the snow below. Are you experiencing shooting cracks through the snow? Is the wind actively transporting snow? Look for recent avalanches, and use small test slopes to check how easily the snow can slide. 

If you find more than 6 inches of new and drifted snow, use extra caution. Consider the consequences before entering slopes steeper than 35 degrees. Be extra cautious around terrain traps such as gullies, rocks, and cliffs where even a small avalanche could have serious consequences. If the sun comes out and the new snow warms up, you may see roller balls start to occur on southerly facing slopes. These are signs that you may be able to trigger wet loose avalanches. 

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 previous snowpack structure suggests that weak layers could be a problem with new snow and wind loading. Triggering a deeper buried layer is more likely above 6,000ft and on the north facing half of the compass.  Watch for whumpfing collapses and cracks shooting through the snow as signs that you can trigger a persistent slab avalanche. They 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. 

In general, the question is less about which weak layer to be watching, and more about if recently formed slabs are enough of a load to activate any of the numerous weak layers. If you find recent slabs to be thicker than 1ft, expect that you could. Near the Cascade Crest, you may find roughly three distinct weak layers, as shown in this observation. 

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.