Avalanche Forecast

You can use visual clues such as snowdrifts behind trees and rocks, fresh cornices, and uneven snow surfaces to identify and avoid wind loaded slopes. We aren’t expecting significant snow accumulation with this weather system, but the winds should effectively drift snow into firmer and deeper slabs. On Thursday, numerous observations of weak snow surfaces including facets and surface hoar were received from the west south zone. If fresh wind slabs form on this weak snow, they could be very easy to trigger and surprise you. 

In sheltered areas away from the wind, loose snow avalanches have been reported. Watch for sluffs to entrain additional snow as they travel and grow larger. 

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 persistent slab problem has been quiet for a few days and we have not received any reports of new avalanches. Thursday night’s wind event, forming fresh wind slabs, could continue to load the snowpack on certain aspects. Will this make these very large avalanches easier to trigger? That’s very hard to say, but we do know that the slab over the weak layer will have changed. You’re most likely to find a  layer of weak sugary facets and/or buried surface hoar a foot above the ground in terrain above 5800’, on W-N-E aspects, near Crystal Mountain. If you trigger an avalanche on this layer it could be very large and destructive, and fail on slopes as low as 30 degrees.

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.