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Snoqualmie Pass

Issued: 8:12 PM PST Friday, March 2, 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. 

You can trigger Wind Slabs near and above treeline and dangerous Persistent Slab avalanches on steep sunny slopes. Persistent Slab avalanches are difficult to manage and can break widely. Avoid wind loaded areas and large open sun-exposed slopes 35 degrees and steeper. 

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Avalanche Problems for Saturday

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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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 Saturday:

You can trigger Wind Slab avalanches at upper elevations where drifted wind features exist. Avoid these avalanches by staying off of recent snow drifts, deeply pillowed features, and fresh cornices on leeward slopes 35 degrees and steeper. Soft non-wind-effected snow may cover wind slabs making them harder to identify. With fresh snow and sunny skies, you may see some small loose avalanches release on steep slopes, today. If you trigger an avalanche in the upper snowpack, it could dig down and become a dangerously large Persistent Slab avalanche.

You can trigger Persistent Slab avalanches in the upper snowpack on sun-exposed slopes steeper than 35 degrees. These avalanches are difficult to manage. To reduce your risk of being caught, avoid steep, open, sunny slopes, and large avalanche paths. Also watch out for unsupported slopes that end in rocks, cliffs or steep rolls. There is still a chance that you could trigger a Deep Persistent Slab avalanche on many slopes. While these avalanches are difficult to trigger, they are also very difficult to predict. They have a low likelihood of triggering but high consequences. You may not survive a Deep Persistent Slab avalanche. Stay safe by avoiding triggering smaller avalanches in the surface snow and staying off the previously mentioned steep slopes.

Avalanche Summary:

Winds as recent as Thursday formed recent Wind Slabs on a variety of aspects near and above treeline. In sheltered areas generally soft unconsolidated surface snow exists.

A series of thin sun crusts surrounded by very small facets has been observed in the Cascade Passes. These layers have been the source of several avalanches earlier this week. Snow profiles and snowpack test can confirm the presence of this layer; however they are not good for proving its absences. On E-S-W aspects, a thin breakable sun crust formed early last week and was buried on 2/23. Very small weak facets have been reported surrounding the crust. This was the weak layer found or suspected in several avalanches 1.5-3 feet deep. This layer has not yet had significant time to heal. It is found 2-3 feet below the surface on steeper slopes that have received direct sun during the past week.

Some observations suggest other persistent grains at this same interface on shaded slopes. Buried surface hoar and large preserved stellars have been reported in recent avalanches and snowpack tests at this interface.

Avalanche and snowpack observations continue to indicate that avalanches are possible on a layer of weak sugary facets buried on 2/13. This weak layer is generally 3 to five feet below the snow surface just above a very firm melt-freeze crust (2/8).

There are no significant layers of concern below the 2/8 crust.

Observations

On Friday, a guide and avalanche professional reported a large avalanche near Highlands Bowl on a SSE aspect near treeline on Stevens Pass. This slope had seen recent wind loading and likely ran on the 2/23 facets about 2 feet below the surface.

NWAC forecaster Josh Hirshberg was on Nason Ridge Wednesday. He found the 2/23 crust 1.5 feet below the snow surface. While snowpack tests were variable, he did report several avalanches on sunny aspects which occurred earlier this week.

Stevens DOT reported two avalanches Wednesday morning. One failed on the 2/23 interface on a NE aspect. The weak layer appeared to be buried surface hoar. The other was a larger avalanche failing earlier in the weak on the 2/13 facet-crust combination. This is the most recent avalanche report we have on this layer from the West Slopes.

NWAC observer Jeremy Allyn was in the Snoquamlie Pass area Wednesday. He reported wind transported snow near treeline. He found the 2/23 interface about 2 feet below the snow surface.

Avalanche and snowpack observations from around the western regions of the Cascades and Passes continue to indicate that propagation of an avalanche on the 2/13 facets is possible. This layer has been found 2.5 to 4 feet below the snow surface.

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.