Avalanche Forecast

A problematic snowpack structure can be found throughout the zone. 1 to 3 feet of recent snow rests on faceted grains, sometimes over a crust, sometimes not. In any case, its a weak layer that makes me cringe. You won't find me in complex terrain or on steep slopes in the area. Do measure your slope angles. Don't go near steep slopes that may be connected to where you are. These persistent slabs have been breaking very widely across terrain features. The reactivity is has been reported to be decreasing with the calm weather at lower elevations near Washington Pass, though not everywhere, and that doesn't mean they are healed up yet. It appears that the shallower snowpack zones further east, or closer to the Columbia River may have a far worse snowpack structure. A large amount of uncertainty remains for upper elevation terrain, but i'd be mindful of areas that are recently wind loaded.

Professional guides and avalanche workers continue to avoid steep, and unsupported slopes, and sticking to well supported features. Even small, steep slopes below treeline are worth going out of your way to avoid. Why? Observations of:

1. Recent wide crowns from avalanches likely failing on faceted layers.

2. Continued reports of collapsing, which makes a "whumphing" sound as air escapes from the collapsed weak layer, sometimes shaking trees a long distance away. 

3. A layer of weak, sugary facets on, or beneath a crust 2 to 3 feet below the snow surface, and tests that indicate propagation on it.

4. A large amount of uncertainty.

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.

Northerly winds may have formed thin wind slabs recently. Older wind slabs may be reactive in isolated locations, and any avalanche in the upper layers could run on a deeper, faceted layer. Steer around slopes where you see firmer, slab like snow. Any avalanche triggered in the upper layer could result in a wider 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.