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East Slopes North - Canadian Border to Lake Chelan

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

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.

Significant snow and wind have brought a return to dangerous avalanche conditions. Watch out for Storm and Wind Slabs in the upper snowpack. 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 Persistent Slabs; ensure a wide buffer between where you travel any large avalanche terrain.

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Avalanche Problems for Friday

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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Storm Slabsi

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

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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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Forecast for Friday:

Today, you can easily trigger avalanches in the upper layers of the snow due to new snow, wind, and light rain at low elevations. 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:

As of Thursday night almost a foot of snow has fallen on the east side of the Cascade crest. Expect less snow as you move further east. 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. In areas of higher snow totals both Storm and Wind Slabs are adding to the underlying weak layers in the North East zone. 

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 2/13 (or 16) 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 triggered avalanches and more recent collapsing and whumphing. The exact depth of these layers depend on aspect, elevation, and proximity to the Cascade crest.  

Observations

North

Mid-week, observers on Washington Pass reported a mix of sun crusts and surface hoar/near surface facets on snow surfaces 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. 

Last 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.

Last Saturday, NCMG traveled in the Cuthroat area and observed the 2/13 layer down 3' (85 cm) at 6100' on a NNW aspect. 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

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.