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

Issued: 7:27 AM PST Friday, December 29, 2017
by Garth Ferber

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.

Hurricane forecast adjusted 730 am Friday.

Travel in above treeline terrain is not recommended along the east slopes of the Washington Cascades on Friday. If you choose to brave the elements, watch for building denser storm snow building storm slab near and above treeline. Wind slab should develop on a variety of aspects near and above treeline where there is plenty of low density snow available to transport onto lee slopes. Loose wet avalanches will develop as snow changes to rain at increasingly high elevations as the day progresses. Human triggered avalanches will become more likely as the day progresses. Watch for changing conditions, travel conservatively and rein in your plans if conditions deteriorate faster than expected and pan your exit accordingly.

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Avalanche Problems for Friday

Storm Slabi

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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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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Loose Weti

Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

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Forecast for Friday:

The active changeable pattern will continue on Friday. Another moist system arrives on a SW flow. A surface low will track across across NW Washington and a cold front will move across the Cascades Friday evening. Crest level winds ahead of the low will be moderate to strong in the central Cascades, with strong winds in the south Washington Cascades and at Mt Hood. Snow and rain amounts should be moderate to heavy along both the east and west slopes of the Washington Cascades on Friday as SE crest-level flow will contribute to enhancing rain and snow along the east slopes. 

As warmer air moves in aloft on Friday and moves northward during the daylight hours, the cold air in place along the east slopes of the Cascades will create a wintery mix at lower elevations, potentially with a band of rain at mid-elevations, and snow at higher elevations before mixing out the cold air pool in the southern part of the Cascades East slopes with a changeover to mostly rain there.

The complexity of the pattern makes it difficult to predict exactly which problems one will find in any given location because rapid changes in freezing levels, intense precipitation, surface inversions, and strong/shifting crest-level winds will create storm slab, wind slab, and loose wet avalanche problems with loose wet problems replacing growing storm and wind slab concerns as freezing levels rise.

Travel in avalanche terrain is not recommended in above treeline terrain along the east slopes. But, monitor snow accumulation carefully and consider avoiding avalanche terrain altogether if denser storm snow accumulates rapidly in your area.

Storm slab should be the most widespread avalanche problem near and above treeline as the day progresses on Friday due to rapidly accumulating snow and the warming trend. 

Watch for building wind slab on a variety of aspects Thursday, especially near and above treeline where there is plenty of low density snow available to transport onto lee slopes.  

Loose wet snow avalanches will also become a problem in the lower terrain bands Friday due to snow followed by moderate to heavy rain in those areas.

There remains significant deep, soft snow out there from the past week. Unconsolidated snow, particularly around small trees, can present a non-avalanche, snow immersion hazard. Keep visual and verbal communication with your travel partners when traveling in treed terrain.

Keep the 12/15 crust/PWL interface in the back of your mind as we progress through this storm cycle and determine whether loading awakens any pockets of deeper instabilities. 

Also despite all this new snow, early season hazards still exist at some lower elevation locales and especially around creek beds that are not filled in.

Avalanche Summary:

In the Cascade East - North zone, light to moderate snow fell on Thursday with continued cold temperatures. In the Cascade East - Central zone, temperatures from the 10's on Wednesday morning rose into the 20's from late Wednesday and Thursday along with light SW or W-SW winds.

The new snow added to recent snow of up to 3-5" which fell during the previous 48-hours at locations east of the Cascade crest in the East - North and East - Central zones and several prior small events over the past week on the east slopes of the Cascades, each delivering 1-3" of low density snow. Periods of moderate and shifting alpine winds have caused both wind stiffened surfaces and built shallow wind slabs in exposed terrain above treeline. 

1.5 to 3 feet of recent snow sits atop the 12/15 crust/persistent weak layer interface in the Washington Pass area and in the central-east Cascades near the Cascade crest. Overall recent snow has been gaining strength leading to a recent stabilizing trend.

Comments from professional guides in the Washington Pass area are included below that help reduce uncertainty surrounding the 12/15 crust/persistent weak layer interface. 

Snowdepth decreases substantially the further east of the Cascade crest one travels. In many areas below treeline, there is not enough snow to present an avalanche danger. 

Observations

North

On Thursday, NCMG traveled near Washington Pass and observed few signs of instability below treeline, but found some tiny pockets of loose dry snow.

A summary of professional guide reports regarding persistent weak layers for the Northeast zone including Washington Pass:

  • In the above treeline zone, small faceted grains are rounding and gaining strength, down 70 cm and below the storm snow that accumulated starting 12/15. They have not been reactive in snowpack tests.
  • The upper snowpack on solar aspects near treeline are bonding well to a strong solar crust buried on 12/15. However, they cannot rule out buried surface hoar in some sheltered larch glades around 6000 feet.
  • Below treeline, surface hoar that was buried on the 12/15 crust is down 70 cm in flat meadows and creek valleys, but has yet to be found in avalanche terrain. 
  • Aggressive terrain has been skied without direct signs of instability over the last several days.

Central

No recent observations

South

No recent observations

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.