Avalanche Forecast

Use extra caution at upper elevations on leeward, wind-loaded slopes steeper than 35 degrees. Watch for fresh drifts and areas of variable height storm snow as indicators that you could trigger a wind slab avalanche. Steer around thickly pillowed areas and convex rolls where the terrain gets steeper. Use small, inconsequential test slopes to clue you in to how well the new snow is bonded. In some areas of the zone, wind slabs will be your main concern. In areas where you find underlying weak layers, triggering an avalanche in the new snow could result in a much bigger and more dangerous persistent slab avalanche.

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.

Snow, rain, and wind are putting a new load on an already weak snowpack. While you can trigger avalanches at all elevations, the most dangerous slopes will be near and above treeline where the wind is loading leeward slopes. You can trigger dangerously large and surprising avalanches on slopes over 30 degrees. Avalanches may be more reactive and break wider than you can expect. You can trigger persistent slabs from a significant distance. Put plenty of space between you and any suspect avalanche terrain. Make sure you are well out from under any areas where avalanches could start. Two weak layers of concern are a thin layer of surface hoar buried 1-2 feet below the surface and sugar-like facets near the bottom of the snowpack.

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.