Avalanche Forecast
Issued: Mar 21st, 2018 11:10AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Deep Persistent Slabs.
NWAC,
Northwest Avalanche Center
Avalanche conditions will change Thursday as new wind slabs develop and grow throughout the day. Stay off of wind loaded slopes where you will be able to trigger these developing wind slabs. A low likelihood-high consequence deep persistent slab still exists in the snowpack. You can avoid this difficult to assess problem by staying off of large steep open slopes.
Summary
Detailed Forecast
Expect winds to transport new snow throughout the day Thursday forming new and reactive wind slabs especially at higher elevations. You can use simple observations to identify where these new wind slabs are forming. Blowing snow, fresh cornices, and plumes all indicate winds slabs are forming on slopes below. Stay off of wind loaded features where you will be able to trigger new wind slab avalanches.
Older weak snow still exists deep within the snowpack. Deep persistent slab avalanches have been the culprit in several accidents and fatalities over the last month. This is a low likelihood high consequence scenario. While it may be difficult for a traveler to trigger a deep slab, smaller avalanches may step down into this deep layer creating large and destructive avalanches. You can avoid being involved with a deep slab avalanche by staying off large steep open slopes on W-N-E aspects.
The new snow is falling on a variety of snow surfaces including firm surface crust and near surface facets. Be ready to dial back your terrain if you experience any signs of unstable snow such as observing new avalanches, hearing whumphs, or seeing shooting cracks. This is most likely to occur in areas receiving the most precipitation such as around Paradise.
Snowpack Discussion
New snow will exist in most locations by early Thursday morning. The highest snow amounts are likely to be around Paradise. This new snow will be falling on a variety of old snow surfaces. On sunny aspects, firm melt-freeze crusts have formed during the recent clear weather. In shaded locations, near surface facets and settled soft snow was observed over the past several days. Rain turning to snow at lower elevations will help create a more favorable bond between the new and old snow. However, above the rain line a new buried weak layer may develop.
The mid-snowpack generally consists of several well bonded layers of melt-freeze and strong rounded grains. In isolated locations, thin buried persistent weak layers have been reported. You are most likely to find a buried persistent weak layer in shaded locations (see a recent public observation), at higher elevations, and/or in terrain slightly east of the Cascade Crest.
An old weak layer of sugary facets can still be found around the 2/8 crust. Very firm and thick surface crust on sun exposed slopes will greatly reduce the ability for a deep slab avalanche to occur on SE-SW aspects. However, where this protective layer does not exist, smaller avalanches may step down to this deep layer. This very distinct crust can generally be found 4-6 feet below the snow surface. While snowpack tests are good for demonstrating the presence of a weak layer, they are not a decision-making tool to determining whether or not a slope is safe.
South
On Wednesday NWAC professional observer Jeremy Allyn traveled in the Crystal backcountry. Jeremy found a firm and supportable surface crust on S-facing slopes. On shaded slopes, he reported soft snow. The 2/8 crust was down about 4 feet and still reactive in snowpack tests.
NWAC Forecaster Dallas Glass traveled in the Paradise area Saturday and Sunday. Dallas found a very firm and thick (12”) surface crust on slopes receiving sun shine. The 2/8 crust layer was down 4 feet and still reactive in snowpack tests.
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
Deep Persistent Slabs
Release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage. They commonly develop when Persistent Slabs become more deeply buried over time.
Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.
A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.
Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.
Aspects: North, North East, East, West, North West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Mar 22nd, 2018 11:10AM
