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Avalanche Forecast

Archived

Mar 23rd, 2018–Mar 24th, 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 unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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Wind slabs formed over the past few days will slowly gain strength Saturday. You will be able to trigger a wind slab avalanche steep wind loaded slopes with convex rollovers, unsupported features, or below fresh cornices. Use visual clues such as snow drifts, cornices, and firm snow with cracking to identify and avoid this wind loaded terrain. Older deep persistent slabs can still be found. You can avoid triggering a deep persistent slab by staying off of steep large open shaded slopes where these bigger avalanches may occur.

Detailed Forecast

Avalanche danger will slowly decrease Saturday as new wind slabs begin to gain strength. Even though the hazard is decreasing, you can still trigger a wind slab on steep slopes near ridgeline and on cross-loaded mid-slope features. You are more likely to trigger a wind slab on convex rollovers, steep unsupported slopes, or below fresh cornices. You can use visual clues to identify and avoid wind loaded slopes greater than 35 degrees. Snow drifts, cornices, and firm snow surfaces with cracking all indicate that nearby slopes received wind deposited snow.

The strong March sun can quickly change conditions on slopes receiving direct sunshine. If the sun comes out, expect loose wet avalanches, rollerballs, and pinwheels. Be ready to move off of and avoid steep sunny slopes during periods of intense sunshine. With higher snowfall totals near in the Cascade West South region, these loose avalanches may become large.

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 4-6 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.

Snowpack Discussion

Ten to fifteen inches of new snow fell in the Cascade West South region with up to twenty or more inches near Chinnook Pass as of Friday afternoon. Winds during the storm transported snow forming wind slabs near ridgelines and on cross-loaded features.

Rain on Thursday reach around 5500’ before transitioning to snow. This formed a new rain crust and created a generally good bond between the old and new snow. Below this rain line a wet and refreezing snowpack can be found. 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.

An old weak layer of sugary facets (2/13) can still be found in some locations just above a very firm and wide spread crust (2/8). This pronounced crust has typically been found about 4-6 feet below the snow surface in the Crystal and Paradise areas You are most likely to find this layer still intact on shaded aspects near and above treeline.

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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.