Avalanche Forecast

Lots of snow was redistributed near Washington Pass, and even in some places nearer to the Columbia on Saturday, with moderate sustained winds. This likely created thick slabs in unusual locations, as strong Frasier River Outflow winds came from the north. Pockets of wind slab may be found in steep gullies and ravines even in sagebrush country above Twisp. While the winds have tapered off, the wind slab instability may linger enough for a person to easily trigger. You may be able to trigger avalanches from a distance as well. If the snow feels firm, or hollow, that slab structure is likely in place. Stay off of these recent wind loaded areas if the slopes are steeper than 30 degrees. 

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.

Near Washington Pass, facets may be found underneath the new snow on a variety of aspects. If the facets lay over a firm wind board, or stout melt freeze crust that may be a lasting weak layer. Look out for slab structure, such as recently wind loaded areas and check for weak snow underneath the slab. This may be a good time to step it back, and give the snowpack some time to adjust while we collectively discover more about this recently buried layer. 

We have less information about areas near, east, and south of Mazama, but persistent slab avalanches may be more likely there, where more new snow may have fallen on a weaker snowpack. In these areas they could break deep, or all the way to the ground. Watch for  cracking, listen for collapses, or dig down to look for these layers of concern. If you can find layers of facets of surface hoar, all you need is a slab on top (which we may or may not have with the recent storm), and you’ve got a recipe for dangerous avalanches. This structure may be most dangerous in the near treeline band, and on west to north to east aspects where weak snow is well preserved.

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.