Avalanche Forecast
Regions: Olympics.
You can trigger Wind Slabs near and above treeline on leeward slopes. These could result in dangerous Persistent Slab avalanches that are difficult to manage and can break widely. Avoid wind loaded areas and large open slopes 35 degrees and steeper.
Detailed Forecast
You can trigger Wind Slab avalanches at upper elevations where drifted wind features exist. Avoid these avalanches by staying off of recent snow drifts, deeply pillowed features, and fresh cornices on leeward slopes 35 degrees and steeper. Soft non-wind-effected snow may cover wind slabs making them harder to identify. With fresh snow and sunny skies, you may see some small loose avalanches release on steep slopes, today. If you trigger an avalanche in the upper snowpack, it could dig down and become a dangerously large Persistent Slab avalanche.
You can trigger Persistent Slab avalanches on slopes 35 degrees and steeper. Avoid steep, open slopes and large avalanche paths to reduce your risk of these difficult to manage avalanches. Weak sugar-like facets can be found just above a firm crust layer formed in early February on all but south aspects. This layer is down about 2-4 feet. You may also be able to trigger shallower Persistent Slab avalanches in the upper snowpack on steep sunny slopes.
Snowpack Discussion
Up to 2 feet of snow fell near Hurricane Ridge in the past week. Winds as recent as Thursday formed Wind Slabs on a variety of aspects near and above treeline. In sheltered areas generally soft unconsolidated surface snow exists.
Some observations suggest other persistent grains may exist at the interface of this weeks snow. Facet crust combinations, buried surface hoar, and large preserved stellars have been reported in recent avalanches and snowpack tests at this interface from the West Slopes of the Cascades.
An active storm cycle began a week ago Friday. By Monday morning about 2 ft of new snow had accumulated in the Hurricane Ridge area. Â
About 3 or more ft of of settled snow sits on top of the weak sugary facets that formed on a strong crust earlier in February. Snowpack tests continue to suggest that these facets can fail and produce avalanches. This layer has not been reported on South aspects in the Hurricane Ridge area.
There are no other significant layers of concern in the mid and lower snowpack.
Observations
On Friday, March 2nd, NPS rangers reported a large avalanche on a recently wind loaded slope. Elsewhere, profiles showed a faceted weak layer buried in mid- February was found 2-4 feet below the surface and was showing some signs of rounding.
On Thursday, Feb 22, 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 Thursday 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: Possible
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: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1