Avalanche Forecast

It is possible to trigger wind slabs on lee slopes and terrain features. Wind slabs are gaining strength but still need additional time to heal in specific areas near and above treeline. Be especially careful of convex rollovers, cross-loaded gullies, and wind loaded slopes just below ridgelines. In some areas, wind drifted snow may sit over a layer of buried surface hoar making a recipe for a surprising avalanche that could propagate widely.

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.

A layer of buried surface hoar has been the culprit in many recent avalanches throughout the region. In the Mt. Baker area, this layer exists 1-2ft below the surface between 4500-5500ft primarily on north and east aspects. Recent observations from this area indicate the surface hoar has a spotty distribution and can be hard to find. Regardless, impressive and unusual recent avalanches still have us concerned. Be especially mindful of steep slopes near treeline that are sheltered from the wind and that stayed above the rain during the recent storm. This is a good time to break out the shovel and dig. Look for a thin grey line and/or snowpack tests that fail suddenly in the upper snowpack. If you find the buried surface hoar intact, experience collapses, shooting cracks, or see recent avalanches, stick to lower angled terrain.

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.