Avalanche Forecast

Watch for recently drifted stiffer snow at upper elevations and near ridges. Steer around pillowed features. You can trigger wind slabs more easily on slopes 40 degrees and steeper, on convexities, and unsupported slopes. Slabs likely sit on hard surfaces or even a thin layer of buried surface hoar. Check for slab structure and strong snow over weaker snow. Look for signs of instabilities such as shooting cracks under the weight of a person. If finding these signs, be careful to avoid any steep slopes nearby.

4" to 8" of very low density snow fell on the 4th, and the winds picked up that evening, redistributing much of the upper elevation snow into thicker drifts and slabs. These recently formed wind slabs may still be sensitive to human triggering, as the cold temperatures tend to preserve these instabilities. 

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.

With recently formed wind slabs, we have to ask ourselves, what could be underneath? Watch for cracking, listen for collapses, and dig down to identify these layers of weak, sugary snow with snowpack tests. Use the terrain to your advantage, avoiding areas with thick slabs adjacent to shallow, rocky slopes. Use extra caution around large terrain features, and unsupported convex rolls.

In the Wenatchee Mountains and east of Highway 97, the snowpack is often less than 4 feet deep and highly variable. The most recent layer of buried surface hoar (January 22) may be found about a foot beneath the surface. A pair of thin freezing rain crusts associated with the early January and late December weak layers are still a concern in this area as well. You may find these to be best preserved on shaded, and open slopes above 5,500ft. Further west, the snowpack is generally strong and well settled.

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.