Avalanche Forecast

The recent storm snow landed on a variety of surfaces from surface facets and surface hoar on shaded aspects to melt freeze crusts on southerly aspects. The strong winds built slabs on exposed open slopes. Winds transported both the recent storm snow, and snow that was previously on the surface. Even locations that didn't receive much new snow may have recently formed wind slabs. The snow may fracture quite wide on shaded aspects where the recently buried layer is weak and faceted. Any avalanches on the old surface may step down a foot or so to layers from early March. Check for thick, cohesive layer of snow sitting over a weaker snow layer. If you see obvious clues to instability such as shooting cracks and wind loaded texture, it's a good bet to stay off of steep wind loaded slopes. 

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.

At lower elevations, where the snowpack is shallow, a poor structure can be found with facets underneath a crust from early February This shallow snowpack is weak, especially on shaded aspects. In the foothills, facets can be found near the ground beneath February's storm snow. These deeper slabs are most concerning on isolated, shaded slopes where stronger snow may rest over facets. At upper elevations, these layers are generally not a concern anymore. 

If you find yourself in an area that appears to have a shallow snowpack, stick to well supported slopes and avoid steep convex rolls. Travel one at a time from well defined safe zones when moving through steeper terrain. If you find signs of instability such as cracking and collapsing, avoid steep slopes nearby. 

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.