Avalanche Forecast

In many locations, the winds built new and reactive wind slabs, even into the below treeline areas. Keep your eyes open for clues that the wind transported recent storm snow. Textured snow surfaces, drifted snow, and fresh cornices all indicate that wind loaded slopes are nearby. Use these visual clues to identify and avoid wind slabs on open slopes greater than 35 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.

We don’t have direct confirmation of a persistent slab in the West-South zone. That may simply be due to lack of observations, and not the absence of a persistent weak layer. Nearby zones, including Mt Hood, reported avalanches on a persistent weak layer. This high degree of uncertainty and our high degree of suspicion is why we are forecasting persistent slabs in this region. We are particularly leery of areas around the Rainier, Adams, St. Helens, and the Columbia River. The only way to know if a persistent slab exists is to dig in the snow. You may find a layer of weak sugar facets or feathery surface hoar above a prominent crust 3-5 feet below the snow surface. Snow profiles and snowpack test can confirm the presence of a persistent weak layer, but cannot prove it's absence. If you suspect a persistent slab exists in the area you are traveling, put plenty of distance between you and any startzone. Consider simply avoiding areas where avalanches can run or stop.

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.