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East Slopes Central - Lake Chelan to South of I-90

Issued: 7:48 PM PST Thursday, March 8, 2018
by Josh Hirshberg

Watch for Wind Slabs forming in the new snow and dangerous avalanche conditions above treeline. Persistent Slab avalanches claimed lives along the east slopes of the Cascades in the past 2 weeks. Avoid steep, complex terrain and large avalanche paths to reduce the threat of the low likelihood - high consequences of Deep Persistent Slabs; ensure a wide buffer between where you travel any large avalanche terrain.

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

Avalanche Problems for Friday

Wind Slabi

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Persistent Slabi

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Forecast for Friday:

Today, you can trigger avalanches in the upper layers of the snow due to new snow, wind. New snow may have fallen on a variety of surfaces that could form a poor bond with the new snow. Watch for cracking, wind stiffened snow, freshly formed drifts, and small test slopes where the new snow easily slides as clues that you could trigger an avalanche. Steer around fresh wind features, convex rolls, and slopes holding a foot or more of newly fallen, cohesive snow that are 35 degrees and steeper.

Avalanches in the upper snowpack and heavy snowfall are making it easier to trigger deeper and dangerously wide avalanches. Persistent weak layers lurk deeper in the snowpack. The signs of Persistent Slab avalanches may not be obvious. These low likelihood, high consequence avalanches are very difficult to manage. The best way to stay safe is to avoid the slopes where you can trigger them. Take a day or two to choose more cautious terrain before returning to the kinds of slopes you traveled on prior to this storm. Avoid large avalanche paths, start zones, and unsupported slopes steeper than 35 degrees. Tracks on a slope don’t mean that the slope is safe.  In two recent fatalities in the East Central Cascades, the slopes had numerous tracks on it before the avalanches were triggered. While these avalanches may give little warning, the consequences could be un-survivable.

Avalanche Summary:

After prolonged calm and sunny conditions, Gusty winds and snow are forming our next layer of the snowpack. Crusts, surface hoar, and small facets could all create a poor bond for the new snow. Asses this new interface with small test slopes and quick tests. 

The past two weeks brought three fatal avalanche accidents across the East Slopes of the Cascades. All of these were triggered on persistent grain types. Several potential persistent weak layers exist. Two common layers that have been reported 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

Mid-week, observers reported a mix of sun crusts and surface hoar/near surface facets prior to snow on 3/8.

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 Wednesday, an observer in the Icicle drainage identified the mid-February facets over a crust on a northwest aspect.

Observer Matt Primomo reported a large slab avalanche on a southeast aspect at 7,500ft in Icicle creek that may have occurred last weekend.

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.

Last 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.

No Corresponding Mountain Weather Forecast Available