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East Slopes South - South of I-90 to Columbia River

Issued: 7:59 PM PST Monday, March 5, 2018
by Dennis D'Amico

NWAC avalanche forecasts apply to backcountry avalanche terrain in the Olympics, Washington Cascades and Mt Hood area. These forecasts do not apply to developed ski areas, avalanche terrain affecting highways and higher terrain on the volcanic peaks above the Cascade crest level.

Persistent Slab avalanches claimed lives along the east slopes of the Cascades over the weekend. The snowpack remains complex and scary. Avoid complex terrain and sit out this low likelihood - high consequence problem; ensure a wide buffer between where you travel and open slopes over 35 degrees as well as large avalanche paths. Also watch for lingering wind slabs at higher elevations and small loose wet avalanches on sunny slopes Tuesday.  

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Avalanche Problems for Tuesday

Persistent Slabi

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.

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Wind Slabi

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.

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Loose Weti

Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

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Forecast for Tuesday:

Fatal avalanche incidents along the east slopes of the Cascades occurred this weekend in the Teanaways near Long's Pass (Saturday) and north of the Methow Valley at Setting Sun Mountain (Sunday). Persistent Slab avalanches were reported both the Long's Pass and Setting Sun incidents. 

The snowpack on the Cascades East slopes remains complex and scary. It's currently easier to trigger dangerous Persistent Slab avalanches on the East slopes of the Cascades than the West Slopes of the Cascades. There are multiple weak layers to avoid, complicating the snowpack.  A Moderate avalanche hazard tells you that large avalanches may be triggered in isolated areas. You will not be able to identify deeper instabilities just by looking at the surface snow.

Several layers of persistent grain types that have been reactive in tests are likely to remain reactive over the next week or perhaps longer. Several older weak layers of facets exist in the snowpack, but are still shallow enough that you can trigger them. In many areas with a deeper snowpack, you may get little warning signs of a Persistent Slab avalanche. While these avalanches are difficult to trigger, they are also very difficult to predict. A resulting avalanche will likely be large enough kill you. Put a wide margin of terrain between you and any slopes 35 degrees and steeper where you suspect the Persistent Slab problem. Continue to be cautious and stay away from steep, open slopes connected to large avalanche paths. If you experience collapsing or audible whumphs, avoid any nearby avalanche terrain. Snow profiles and snowpack tests can help confirm the presence of a weak layer but cannot prove its absence.

Wind Slabs are trending toward unlikely, but may still exist in specific areas near and above treeline. Generally avoid areas of recently drifted snow, deeply pillowed features, and fresh cornices on slopes 35 degrees and steeper. These areas may exist well below ridge-lines. Soft non-wind-effected snow may cover new wind slabs making them harder to identify. 

Small loose wet avalanches are likely on Tuesday on steep solar slopes. Avoid steep sunny slopes near terrain traps as the surface snow becomes moist and watch for signs of natural pinwheeling and rollerballing as a clear signal to change aspects. 

Avalanche Summary:

Up to 4" of new snow accumulated through Sunday evening along the east slopes of the Cascades with gusty winds transporting snow. On solar aspects this new snow sits on a melt-freeze crust formed Saturday.

The last major snow storm was Wednesday through Thursday morning; dropping up to 1.5' of snow with significant accumulations as far east as Mission Ridge. Moderate to strong southerly flow during the storm built wind slabs. Several days of settlement has allowed these wind slabs to become stubborn to trigger. 

Several potential persistent weak layers exist in the snowpack along the East Slopes of the Cascades. Two common layers that have been reported in many locations are a facet/crust combination buried on 2/23 and a facet/crust layer buried on 2/13. The Setting Sun avalanche released on faceted grains, but no crust was present. 

The upper (shallower 2/23) layer can be found 1-2 feet below the snow surface on steeper slopes that have received direct sun. Small weak facets have been found in other regions surrounding a thin sun crust.

The 2/13 facet/crust combination is typically found 2-4 feet below the snow surface and above the 2/5 firm crust. This layer has two confirmed skier triggered avalanches and more recent collapsing and whumphing. 

The exact depth of these layers depend on aspect, elevation, and proximity to the Cascade crest. A high level of uncertainty remains surrounding these layers. 

Observations

North

Monday NWAC and NCMG professionals visited the Setting Sun Mt accident site. They found the large avalanche had released on a WNW aspect at 6900 ft. The hard slab avalanche had released on 1.5 mm rounding facets. 

On Sunday, North Cascades Heli observed a recent avalanche (likely from Friday) which released mid-slope and featured a deep crown. They suspected the avalanche to involve the 2/13 layer.

On Sunday, NCMG was in the Washington Pass area near and below treeline where they observed no new avalanches and no results with ski tests.

On Saturday, NCMG traveled in the Cuthroat area and observed small wind slab avalanches in steep terrain that had run naturally on Friday. The 2/23 crust was not observed on north facing terrain above 5600'. The 2/13 layer down 3' (85 cm) at 6100' on a NNW aspect showed mixed results in tests. Widespread collapses and a stubborn small persistent slab release was reported on Vasiliki ridge from a third party. 

On Friday, observers in the Washington Pass area reported continued test results indicating the potential for triggering Persistent Slab avalanches on the 2/13 facets.

Central

On Sunday, NWAC forecasters Dallas Glass and Josh Hirshberg were in the Long's Pass area of the North Fork of the Teanaway drainage where they traveled up to 5700' on S-W-NW aspects. They found the 2/13 persistent layer down 3' and it was everywhere they checked for it. A new breakable surface crust formed from direct sunshine Saturday on S and SW aspects, but due west aspects had settled powder without the crust. Winds continue to transport snow with NW winds loading SE slopes in that location.

On Friday, NWAC observers Matt Primomo and Matt Schonwald traveled in the Bean Creek area north of Cle Elum. On both south and northeast slopes, they reported large and small column tests indicating potential for human triggering on the 2/13 facets. This weak layer was 3-4 feet below the surface. They also found the 2/23 facets about 2 feet below the surface on a south aspect at 5450 ft and several reactive layers of preserved snow crystals within the upper 1.5' of the snowpack.

On Wednesday, an avalanche professional in the Chiwakum Mountains reported collapses and whumps on the 2/13 buried facet layer. Depth to the layer was highly variable (1-3 feet). Another observer triggered an avalanche almost 3 feet deep on a small steep slope near McCue Ridge. 

No Corresponding Mountain Weather Forecast Available

USE AT YOUR OWN RISK

This Backcountry Avalanche Forecast is provided in conjunction with the US Forest Service, and is intended for personal and recreational purposes only. Safe backcountry travel requires preparation and planning, and this information may be used for planning purposes but does not provide all the information necessary for backcountry travel. Advanced avalanche education is strongly encouraged.

The user acknowledges that it is impossible to accurately predict natural events such as avalanches in every instance, and the accuracy or reliability of the data provided here is not guaranteed in any way. This forecast describes general avalanche conditions and local variations will always occur. This forecast expires 24 hours after the posted time unless noted otherwise.