Avalanche Forecast

The recent, remotely triggered slide is the canary in the coal mine. With more snow and warming temperatures, expect to be able to trigger very large avalanches on a deeply buried layer of weak snow over a crust. Watch for whumpfing collapses and cracks shooting through the snow as signs that you can trigger a persistent slab. They can be surprising and can break widely, connecting slope features. The best way to deal with these avalanches is to pick conservative terrain options, and put a wide berth of distance between any steep slopes and where you choose to travel. Features that may be more likely to slide are alpine bowls, steep convex rollovers, and heavily wind loaded slopes even in sparse trees. 

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.

Strong winds continue to drift snow both new and old snow into firm, cohesive slabs. These drifts will be deeper and more significant at higher elevations. Check to see how these slabs are bonding with the snow below. Are you experiencing shooting cracks through the snow or hear whumphing collapses? Is the wind actively transporting snow? Look for recent avalanches, and use small test slopes to check how easily the snow can slide.

Some of these slabs have grown large and may not exhibit direct signs of instability before releasing, so think twice before entering any features that appear heavily duned by the wind. Consider the consequences before entering slopes steeper than 35 degrees. Be extra cautious around convex rolls, below ridgelines, and near terrain traps and strainers, such as stands of trees.

With daytime warming, wet loose avalanches may become a concern at lower elevations. Check for roller balls and recent fan shaped avalanches for signs of these. Avoid wet and gloppy steep slopes with daytime warming if you see this happening. 

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.