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East Slopes South - South of I-90 to Columbia River

Issued: 8:10 PM PST Tuesday, February 6, 2018
by Robert Hahn

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. 

Wind slabs will actively form in the terrain on Wednesday, near and above treeline. Watch for signs of wind transport and avoid steep, wind-loaded slopes on any aspect and continue to avoid travel under or near corniced slopes.

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Avalanche Problems for Wednesday

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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Avalanche Summary:

This has been a long and windy period for the east slopes of the Cascades where gusty westerly winds blew snow around from Sunday night through Tuesday, continuing to creating wind slabs on all aspects near treeline and above in the northeast Cascades. Winds were generally decreasing to the light to moderate range by Tuesday. Up to 8" of snow now sits and is well bonded to a crust formed by a rain event on February 4th up to 5800-6800 feet in the northeast Cascades, extending to slightly higher elevations further south. Significantly less snow sits on the crust further east of the crest and also to the south, decreasing wind slab potential in these areas.

Wind slabs were identified throughout the near and above treeline terrain, but they were pretty stiff and well bonded in the alpine, trending the hazard from stubborn toward unreactive on Tuesday.

A natural avalanche cycle has been reported in many areas from Monday’s (1/29) warm and wet weather. Rain occurred along the east slopes as far north as Holden, WA.

Scattered observations from the last week of January found buried surface hoar on top of the 1/16 crust. This persistent weak layer was found or thought to be the cause of several avalanches. Extra caution should be taken when traveling in areas further east of the crest where this layer may survive. Snow profiles and snowpack tests are the only means to identify and locate this layer.

Observations

North

On Tuesday, snow professionals in the Washington Pass area reported lots of wind-effected snow, which was pretty stiff, but well-bonded, producing very resistant results in shear tests. Closer to the Cascade crest there is 8" of snow available for wind transport. Also of note, the party witnessed evidence of produced a very large avalanche (D2.5) in Spire Gulley, which was likely produce by cornice fall triggering a wind slab, which then entrained a lot of loose and wet snow during the slide cycle on Sunday. 

On Monday, snow professionals in the Washington Pass area reported a recent maximum rain/snow line of 6,800 ft. Subsequent snowfall prevented much refreezing of the wet snow. Winds were unusually gusty and variable with no dominant wind direction and some wind slab was present into the below treeline zone. The party triggered a small wind slab quite low in the terrain. They messaged a high level of uncertainty regarding reactivity and location of wind slabs in this zone. Glide activity was also evident to 6000'.

On Sunday, snow professionals at the Barron Yurt near Hart's Pass reported stormy conditions with denser snowfall increasing the likelihood for storm slabs during the day due to rising temperatures in and around the 6000' level. Moderate winds were continuing to transport snow onto lee slopes near and above treeline. No deeper instabilities were noted in snowpack tests. Observations were limited in this terrain due to poor visibility. On Sunday in terrain further to the east, a wet loose cycle was noted below 6000'. 

On Saturday, snow professionals at the Barron Yurt near Hart's Pass reported about 6" of new snowfall over the preceding 24 hours. Touchy wind slabs were present near and especially above treeline with moderate to strong W-SW winds transporting new and recent snow onto lee slopes. Storm slabs were generally unlikely to trigger. The 1/16 crust was 4 to 5 feet down. Similar conditions were reported Friday in this area. 

Central

An avalanche professional traveling in Icicle Creek Thursday found 6-8” of settled snow well bonded to the 1/29 crust. Observations demonstrated a strong upper snowpack. No buried surface hoar was found in this location.

On Tuesday, Mission Ridge Pro Patrol identified several layers of concern in this regionally shallower snowpack. These layers should be watched during future loading events. Similar basal weak layers were found in the nearby shallow snowpack of Blewett Pass.

South

No recent observations

Forecast for Wednesday:

Warming conditions, light precipitation, and moderate to strong crest-level winds are expected on Wednesday. Winds will be particularly strong east of the Cascade crest from the central Cascades northward.

Moderate to strong westerly winds will create small to occasionally large fresh wind slabs, transporting the 4" or less of recent snow still available for transport on Wednesday. Expect these slabs to be reactive near and above treeline with easterly aspects favored for wind-loading.

Old wind slabs were small to large and found on a variety of aspects. They will continue trending from stubborn toward unreactive on Wednesday as warming helps these already stiff and well bonded slabs to heal more rapidly.

Large cornices exist along ridgelines. Warming temperatures will make cornice failure more likely on Wednesday, so continue to give cornices a wide berth and avoid travel directly below corniced slopes.

The avalanche hazard will be lower in areas receiving less precipitation further east of the Cascade Crest. 

Although significant warming will occur in the southern parts of this forecast region, crusts near the surface of the snowpack at lower elevations should mitigate the impact of the warming and loose wet avalanches are unlikely to be a significant concern on Wednesday.

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.