Avalanche Forecast
Regions: Cascades - South West.
While the Avalanche Danger is slowly decreasing, dangerous conditions remain at upper elevations. You can trigger Wind Slabs near and above treeline and Persistent Slab avalanches on sunny slopes in the upper snowpack. Avoid wind loaded areas and large open sun-exposed slopes 35 degrees and steeper.
Detailed Forecast
You can easily trigger Wind Slab avalanches near and above treeline, today. You can avoid these avalanches by staying off of recent snow drifts, deeply pillowed features, and fresh cornices on slopes 35 degrees and steeper. These areas may exist far below ridge-lines and on mid-slope cross-loaded features. In some location soft non-wind-effected snow may cover new wind slabs making them harder to identify. You may encounter some small loose avalanches releasing on the surface of steep slopes today.Â
You can trigger Persistent Slab avalanches in the upper snowpack on sun-exposed slopes greater than 35 degrees. Avoid steep, open, sunny slopes as well as large avalanche paths to reduce your risk of these difficult to manage avalanches. If you dig about 2 feet below the surface you will find a series of thin sun crusts surrounded by very small facets. These layers have been the source of several avalanches in the Cascades. Snow profiles and snowpack test can confirm the presence of this layer; however they cannot prove its absences.
Observers continue to report potential for a Deep Persistent Slab. Avalanches triggered in the surface snow could step down to release a large and dangerous avalanche in deeper layers in the snowpack. These avalanches are becoming increasingly difficult to trigger. The best way to avoid this low likelihood-high consequence problem is by avoiding triggering smaller avalanches in the surface snow and staying out of large avalanche paths.
Snowpack Discussion
The Rainier and Crystal Mountain area picked up over a foot of snow since Wednesday and about 3 feet in the past week. Strong west and southwest wind accompanied much of the snow. Observers in the Crystal Mountain backcountry reported natural and triggered avalanche activity of many flavors since Sunday. Significantly more avalanche observations have come from this area in the past three days than other parts of the West Slopes.
On Thursday, Forecaster Dallas Glass reported natural slab avalanches in wind loaded terrain near treeline. Dallas reported test showing potential for triggering avalanches on crust/facet combinations in the upper snowpack as well as up to 3 feet down on the older faceted 2/13 layer.
NWAC professional observers in the Crystal area Tuesday and Wednesday observed the 2/23 interface, involved in two previous avalanches, still reactive in snowpack tests. In this area the 2/23 layer was found 1-2 feet below the snow surface.
Avalanche Problems
Wind Slabs
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.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Persistent Slabs
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.
Elevations: Treeline, Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1