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East Slopes North - Canadian Border to Lake Chelan

Issued: 7:01 PM PST Tuesday, March 6, 2018
by Dennis D'Amico

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 Wednesday.  

Danger Scalei
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  • Low (1)
  • Moderate (2)
  • Considerable (3)
  • High (4)
  • Extreme (5)

Avalanche Problems for Wednesday

Persistent Slabi

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Wind Slabi

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Loose Weti

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Forecast for Wednesday:

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 at 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 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. 

Wind Slabs are trending toward unlikely, but may still exist in isolated areas especially above treeline where there have been few recent observations. 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. 

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

Avalanche Summary:

Tuesday was mostly sunny with freezing levels rising to 4000-5000'. The last snowfall was up to 4" of new snow 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 2/28 through Thursday 3/1; 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 above a thin crust.   

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. 

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' everywhere they dug. 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 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 2/28, 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 traveler triggered an avalanche almost 3 feet deep on a small steep slope near McCue Ridge. 

No Corresponding Mountain Weather Forecast Available
This information is provided by the U.S.D.A. Forest Service and describes general backcountry avalanche hazard and conditions. It does not apply to ski areas and highways where avalanche mitigation is conducted. Read more here.