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East Slopes North - Canadian Border to Lake Chelan

Issued: 8:45 PM PST Sunday, March 4, 2018
by Robert Hahn

At today's moderate danger you can trigger large persistent slab avalanches and these layers are taking lives. Many experienced people may be fooled by the lack of observable avalanche activity in any given area, but make no mistake, this is a very tricky snowpack right now. Persistent Slab avalanches are low likelihood but high consequence, breaking widely across terrain features if you trigger them. Choose conservative terrain, putting a wide buffer between where you travel and open slopes over 35 degrees as well as large avalanche paths. 

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

Avalanche Problems for Monday

Persistent Slabi

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Wind Slabi

  • Avalanche Problemi
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Forecast for Monday:

On Sunday, March 4th, a serious involvement occurred near Setting Sun Mountain in the Cascade East-North zone.

On Saturday, March 3rd an avalanche occurred in the Long's Pass area,  within the North Fork of the Teanaway River Drainage, resulting in two fatalities and serious and minor injuries to two others. . The D2.5 (large to very large) hard slab avalanche released on South and West aspects in complex terrain.  The slab released 3' down on a weak layer believed to be the 2/13 facets. NWAC forecasters visited the location of the avalanche both Saturday and Sunday. We are compiling information for a more detailed report.

The snowpack on the Cascades East slopes remains complex and scary. A moderate avalanche hazard tells you that large avalanches may be triggered in isolated areas on Monday and you will not be able to identify these layers just by looking at the surface snow.

It's currently much easier to trigger dangerous Persistent Slab avalanches on the East side of the Cascades compared to the West Slopes and there are multiple weak layers to avoid. Several layers of persistent grain types that have been reactive in tests are likely to remain reactive on Sunday. 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.

You are most likely to trigger Wind Slab avalanches above treeline on Monday. You can avoid these avalanches by staying off of recent snow drifts, deeply pillowed features, and fresh cornices on slopes 35 degrees and steeper. These areas may exist far below ridge-lines and on mid-slope cross-loaded features. In some locations soft non-wind-effected snow may cover new wind slabs making them harder to identify.

Avalanche Summary:

Up to 4" of new snow fell during the day Sunday along the east slopes with gusty winds reported transporting snow. On SW-S-SE aspects this new snow sits on a melt-freeze crust from Saturday morning melt which refroze Saturday night into a breakable crust on SW-S-SE aspects in many areas.

The last major snow storm Wednesday through Thursday morning brought up to 1.5' of snow with significant accumulations as far east as Mission Ridge. Moderate to strong southerly flow during the storm building wind slabs. Four days of settlement is allowing these wind slabs to become stubborn to trigger. The Wednesday-Thursday snow fell on a variety of snow surfaces including thin sun crust, uneven wind surfaces, and soft unconsolidated snow. 

Several potential persistent weak layers exist in the snowpack along the East Slopes of the Cascades and they remained dangerous and deadly the weekend of March 3-4. 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 2/13 facet layer is most widespread and most concerning as it cost two lives in an avalanche in the Teanaway on Saturday. The exact layer and depth depend on aspect, elevation, and proximity to the Cascade crest. A high level of uncertainty remains surrounding these layers. 

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 formed early last week and buried during last weekend’s storms.

The deeper (2/13) facet/crust combination is typically found 2-3 feet below the snow surface. This layer has been around for two weeks with two confirmed skier triggered avalanches and more recent collapsing and whumphing. With significant new snow added to the snowpack last weekend, this layer may be reactive in areas where we have not seen previous avalanche activity nor snowpack test results. These weak sugary facets are located above a firm wide spread crust buried on Feb 5th.

While several layers exist in the snowpack, there are no significant layers of concern below the 2/5 crust.

Observations

North

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 with reactivity in deep tap tests, but not compression. 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  River 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 strong, breakable 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