Avalanche Forecast
Issued: Feb 22nd, 2018 11:32AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
New shallow wind slabs will form during the day Friday around the East Slopes of the Cascades. Identify and avoid steep slopes where the wind is actively depositing snow such as near blowing snow, on snow drifts, and below fresh cornices. A weak buried facet layer exists in many locations. Stay away from large open slopes where large persistent slab avalanches may be triggered.
Summary
Detailed Forecast
New shallow wind slabs will form during the day Friday as the next wave of snow and wind impact the Cascades. These avalanches will become larger and easier to trigger as the day goes on. Identify and avoid slopes where the wind is actively depositing snow. Use visual observations such as blowing snow, plumes, fresh cornices, and drifting snow to locate wind loaded slopes as you travel.
A weak facet layer can be found in many locations in the Cascade-East zones. This layer is typically down 1.5-2.5 feet and sits on top of a firm (2/5) crust. This layer has been reported in at least one skier triggered avalanche. You will be more likely to trigger this persistent weak layer in areas where the surface snow is firmer. Information about the distribution and sensitivity of this layer can be difficult to gather. Avoid steep large open slopes where large persistent slab avalanches may occur.
Excellent travel conditions exist on lower angled slopes away from overhead hazard.
Snowpack Discussion
Loose unconsolidated surface snow conditions have been report on sheltered slopes from many locations. In general 18-30 inches of snow now sits over the weak sugary facets buried on 2/13. These facets can be easily located above the most recent (2/5) crust layer.
In exposed terrain near and above treeline, winds over the past week have redistributed snow in some locations. Old wind slabs have been limited to isolated features along ridgelines.
While there are numerous layers in the snowpack, there are currently no significant layers of concern in the mid and lower snowpack.
Observations
North
On Wednesday a skier triggered a size 1.5 slab avalanche near Harts Pass on NE aspect at 6000’. The avalanche was 2 feet deep and failed on weak facets above the 2/5 crust.
Avalanche professional around the Northeast zone Sunday through Tuesday observed snowpack tests indicating that the persistent weak layer could fail and produce avalanches.
Central
NWAC field staff traveled in the Icicle Creek area Wednesday on NE facing slopes up to about 6000 feet. The persistent layer we are tracking was found consistently buried about 2 ft from the surface, though the distribution of this layer was intermittent. In many areas the snow above the facet/crust layer was low cohesion powder, lacking the ability to propagate a slab avalanche. Shallow fresh wind slabs were noted near and above treeline.
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
Expected Size
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
Expected Size
Valid until: Feb 23rd, 2018 11:32AM
