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Avalanche Forecast

Archived

Mar 21st, 2018–Mar 22nd, 2018

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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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.

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

Wind Slab avalanches are the 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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the 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. 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.