Avalanche Forecast
Regions: Olympics.
New shallow and wind slabs will form in the Olympics on Friday. Identify and avoid areas where the wind is actively depositing snow. Persistent slabs are still present in the Olympics. Avoid steep open slopes greater than 35 degrees where you are most likely to trigger this very difficult to manage avalanche problem.
Detailed Forecast
New snow and moderate winds will cause the avalanche danger to increase slightly Friday, as slabs grow larger and become easier to trigger.
Fresh shallow wind slabs will form on lee slopes as winds increase. You will most likely trigger these avalanches on steep slopes below blowing snow, plumes, and fresh cornices. Identify and avoid steep terrain where winds are actively loading new snow.
In many locations a persistent weak layer exists within the snowpack. Around 2 feet of settled snow sits on top of weak sugary facets buried on 2/13. At least 2 avalanches have been triggered by backcountry travelers on this layer during the past week in the Olympics. Persistent slabs can be difficult to manage. They do not heal as quickly as other avalanche problems and can remain active for weeks at a time. Observations on where these weak layers are present and how easy they are to trigger is hard to obtain. Give youself a wide safety buffer to handle the uncertainty around persistent slabs. Avoid slopes greater than 35 degrees where this problematic avalanche concern is thought to exist.
Excellent travel conditions can still be found in lower angle sheltered terrain not exposed to steep overhead slopes.
Snowpack Discussion
Two feet of settled snow exist around the Hurricane Ridge area over the 2/13 weak layer in sheltered areas. In exposed terrain, winds have redistributed snow near and above treeline. Wind deposited snow can be found on a variety of aspects along ridgelines.
Weak old snow (facets) can be found above the most recent crust layer. In most locations this is 1.5-2.5 feet below the snow surface. Observations continue to indicate this layer is reactive and capable of producing large avalanches. This layer has been reported on all but due South aspects.
There are no other significant layers of concern in the mid and lower snowpack.
Observations
On Friday Matt Schonwald and NPS rangers observed 20-30 inches of generally right-side-up surface snow over weaker snow (facets). Snowpack tests indicated the weak facet layer could still fail and produce avalanches. Northeast winds during the day Friday redistributed snow onto SW-W-NW aspects.
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.
Aspects: North, North East, East, South East, South West, West, North West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1