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

Issued: 8:12 PM PST Friday, March 2, 2018
by Josh Hirshberg

You can trigger Persistent Slab avalanches that could break widely over terrain features. Put a wide buffer of terrain between where you travel and open slopes over 35 degrees as well as large avalanche paths. Reduce your risk of triggering a Wind Slab avalanche by avoiding fresh wind drifts and cross loaded features on steep slopes at upper elevations. 

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

Avalanche Problems for Saturday

Wind Slabi

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Persistent Slabi

  • Avalanche Problemi
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Forecast for Saturday:

You are most likely to trigger Wind Slab avalanches above treeline, today. 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 location soft non-wind-effected snow may cover new wind slabs making them harder to identify.

It's currently much easier to trigger dangerous Persistent Slab avalanches on the East side of the Cascades compared to the West Slopes. Several old weak layers exist in the snowpack. In many areas with 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. They have a low likelihood of triggering but high consequences. 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 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.

Avalanche Summary:

Up to a foot of snow has fallen with variable snow totals around the East Slopes since Wednesday. This fresh snow has fallen on a variety of snow surfaces including thin sun crust, uneven wind surfaces, and soft unconsolidated snow. Moist snow surfaces and roller balls may be found at lower elevations.

A number of potential persistent weak layers exist in the snowpack around the eastern 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 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 Friday, observers in the Washington Pass area reported continued test results indicating the potential for triggering Persistent Slab avalanches on the 2/13 facets.

On Tuesday and Wednesday, avalanche professionals near Washington Pass observed small wind slabs near ridgeline and localized cracking. Moderate, gusty winds were transporting snow.

On Monday, the crown of a large avalanche was visible on Scaffold Ridge (Twisp River) in the North Cascades. Interestingly the red line in the photo marked the initial crown width that released Sunday and the remainder of the slab released sometime later Monday morning.

Avalanche professionals in the Washington Pass area reported a cycle of large avalanches on the 25th. This is consistent with avalanche cycles that occured throughout much of the Cascades.

Large natural slab avalanche visible on Scaffold Ridge in the North Cascades, starting zone about 7300'. Image, Matt Firth  

Central

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 5450ft.

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. 

Another avalanche professional in the Chiwaukum mountains traveled in a low-elevation terrain on a N-NW aspect and found the 2" thick 2/13 buried facet layer well preserved and 16" down. He also observed moist surface snow conditions up to 3400 feet with lots of roller balls.

 

No Corresponding Mountain Weather Forecast Available