Avalanche Forecast

Moderate to strong Southwesterly winds are expected to accompany the cold front as it passes overnight into the morning of the 12th. These will create thick, dense slabs on leeward slopes and features. Some of these slabs are sitting on old weak snow, and may surprise you by breaking well upslope. As a result, it is best to avoid steep, wind drifted terrain today.  

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.

Storm totals from the 11th and 12th are expected to be substantial. The deep new snow is not bonding well to the old snow surface. We don’t have observations from the 11th in this area, however in adjacent regions numerous avalanches and signs of instability were observed. Near Washington Pass, observers reported triggering avalanches remotely (from a distance) and widespread collapses on the old snow layer near the hairpin on Hwy 20. Test results indicated the potential for wide propagation on this layer of buried surface hoar and near surface facets. On Snoqualmie Pass an observer reported test columns failing upon isolation. On Stevens Pass numerous natural avalanches were reported on a variety of aspects down to 4700ft in elevation. You are likely to trigger an avalanche that will break on the weak, old snow interface which can be found one to two feet down, and could surprise you. Look for signs of instability such as whumphing, shooting cracks through the snow, and recent avalanches. Travel in avalanche terrain will require a very cautious approach.

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.