Avalanche Forecast
Regions: Olympics.
Significant uncertainty exists in the Olympics right now so it is a good time to make conservative terrain choices. Identify and avoid steep wind-loaded slopes Friday where you will be able to trigger a wind slab avalanche. A Persistent Slab avalanche problem likely persists in the Hurricane Ridge area, but a lack of recent observations requires travelers to avoid open slopes where this difficult to manage problem may exist. Watch for extended direct sunshine on steep south facing slopes that may cause Loose avalanches.
Detailed Forecast
You can trigger Wind Slab avalanches at upper elevations. Today, you are most likely to trigger them above treeline. You can avoid these avalanches by staying off of recent snow drifts, deeply pillowed features, and fresh cornices on slopes 35 degrees and steeper. In some locations soft non-wind-effected snow may cover new wind slabs making them harder to identify. These areas may exist far below ridge-lines and on mid-slope cross-loaded features.
You can trigger Persistent Slab avalanches in the upper snowpack on shaded slopes greater than 35 degrees. 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 2-3 feet, shallow enough to be human triggered or triggered by a wind slab stepping down to the weak layer.
A series of thin sun crusts surrounded by very small facets has been observed in the Cascade Passes. These layers have been the source of several avalanches earlier this week. Snow profiles and snowpack test can confirm the presence of this layer; however they cannot prove its absences. It is uncertain if this persistent layer exists in the Hurricane Ridge area.Â
Due to a lack of recent observations and a complicated snowpack, this is a good time to make conservative terrain choices.
Snowpack Discussion
Winds Wednesday night formed new wind slabs on a variety of aspects near and above treeline. In sheltered areas generally soft unconsolidated surface snow exists.
In the Cascades on E-S-W aspects, a thin breakable sun crust formed early last week and was buried on 2/23. Very small weak facets have been reported surrounding the crust. This was the weak layer found or suspected in several avalanches 1.5-3 feet deep. This layer has not yet had significant time to heal. It is found 2-3 feet below the surface on steeper slopes that have received direct sun during the past week. It is uncertain if this layer exists in the Hurricane Ridge area, but it seems consistent with the recent weather.
Some observations suggest other persistent grains at this same interface on shaded slopes. Buried surface hoar and large preserved stellars have been reported in recent avalanches and snowpack tests at this interface.
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
No recent observations since Monday.Â
NPS ranger Monday morning reported the most recent 9 inches of new snow at Hurricane Ridge had surprisingly little wind effects and was low cohesion.
Sunday, NPS rangers indicated the additional new 8 inches of snow was stiffer and more cohesive, being deposited over the previous days weak snow.
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: 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: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1