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Avalanche Forecast

Archived

Mar 22nd, 2018–Mar 23rd, 2018

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

Regions

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You will be able to trigger newly formed wind slab avalanches on steep slopes near and above treeline. Use visual clues such as fresh cornices, snow drifts, and uneven snow surfaces to identify and avoid wind loaded slopes. Very large triggers or small avalanches may step down into deeper older layers in the snowpack. You can avoid being involved in a deep persistent slab by staying out of large steep open slopes where these bigger avalanches may occur.

Detailed Forecast

New wind slabs have formed on a variety of aspects near and above treeline. You can avoid these avalanches by staying off of steep wind loaded slopes. Use visual clues to assess and avoid wind loaded features.  Snow drifts, fresh cornices, and uneven snow surfaces with cracking can all indicate wind slabs exist on nearby slopes.

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. While we have not seen any recent avalanches on this layer, professionals in the field continue to find this layer 4-5 feet below the snow surface. This is a low likelihood high consequence scenario. While it may be difficult for a traveler to trigger a deep slab, smaller avalanches or other large triggers 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 where more than 6-8 inches of new snow has fallen. 

Snowpack Discussion

Around 6 inches of new snow had fallen as of Thursday afternoon in the Cascade West –South region with generally cooling temperatures. At lower elevations rain turned to snow as freezing levels lowered throughout the day. This likely created a favorable bond between the new and old snow surfaces and generally right-side-up storm snow.

Above the rain line, the new snow fell 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 soft settled snow were observed earlier in the week. This may form a new buried weak layer within the snowpack.

Moderate to strong winds occurring with the bulk of the precipitation likely transported new snow forming wind slabs near and above treeline Thursday afternoon and evening. Wind deposition of snow may be found near ridgelines and mid-slope cross-loaded terrain features.

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.