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West Slopes North - Canadian Border to Skagit River

Issued: 5:33 AM PST Monday, March 5, 2018
by Dennis D'Amico

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.

A serious involvement occurred near Setting Sun Mountain in the Cascade East-North zone on Sunday.

On Saturday, March 3rd an avalanche occurred in the Long's Pass area,  within the North Fork of the Teanaway River Drainage, resulting in two fatalities and serious and minor injuries to two others . The D2.5 (large to very large) hard slab avalanche released on South and West aspects in complex terrain.  The slab released 3' down on a weak layer believed to be the 2/13 facets. NWAC forecasters visited the location of the avalanche both Saturday and Sunday. We are compiling information for a more detailed report.

Snoquamlie Pass forecast updated to include the Storm Slab avalanche problem for Monday March 5th. 

At today's moderate danger you can trigger large persistent slab avalanches which are taking lives. Persistent Slab avalanches are low likelihood but high consequence, breaking widely across terrain features if you trigger them. Choose conservative terrain, putting a wide buffer between where you travel and open slopes over 35 degrees as well as large avalanche paths. Slopes on the south half of the compass are of particular concern. It may also still be possible to trigger a wind slab at higher elevation.

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Avalanche Problems for Monday

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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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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Forecast for Monday:

An unstable atmosphere will continue to produce decreasing snow showers with some embedded heavier squalls on Monday.

You may be able to trigger Persistent Slab avalanches in the upper snowpack on sun-exposed slopes (generally southerly aspects) greater than 35 degrees. In specific terrain, if you dig about 2-3 feet below the surface you will find a series of thin sun crusts surrounded by very small facets. If you find this layer, avoid steep, open, sunny slopes as well as large avalanche paths to reduce your risk of these difficult to manage avalanches.

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

While it will not be listed as an official problem, small loose dry avalanches are likely to run particularly fast and far on the 3/3 crust on SW-S-SE aspects. 24 hour snow totals of 6-10" inches are expected by Monday morning. 

Expect roller ball activity during the middle of the day and significant shedding of the surface snow with any radiation input.

 

Avalanche Summary:

The Mt. Baker area received 6" of new snow on Sunday which now sits on settled powder or a crust, depending on aspect.

On Saturday morning, sunshine impacted the surface snow on solar aspects creating numerous small Loose Wet avalanches. Snowmachines were able to trigger large wind slabs formed earlier in the week on the south side of Mt. Baker Saturday. 

The last major storm cycle wrapped up Wednesday morning and during this period, new snow combined with periods of moderate to strong winds transported snow in exposed terrain forming wind slabs. About seven inches of snow has accumulated in the last two days and four feet of new snow has fallen in the last week at Mt. Baker ski area.

On E-S-W aspects, a thin breakable sun crust was formed early last week and buried on 2/23. Very small weak facets have been reported surrounding the crust. This layer has not yet had significant time to heal. It is found approximately 3 feet below the surface on steeper slopes that have received direct sun during the past week.

Some observations suggest the presence of 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 in other parts of the Cascades.

Observers continue to report potential for a Deep Persistent Slab avalanche on a layer of weak sugary facets buried on 2/13. This weak layer is generally 3 to 5 feet below the snow surface just above a very firm melt-freeze crust (2/8). It's worth paying attention to this layer if your venturing into shallower snowpack areas. While these avalanches are unlikely, the consequences could be grave.

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

Observations

On Saturday, an NWAC professional observed two large wind slab avalanches several feet deep triggered by riders on the Easton Glacier around 6000'.

On Saturday, an avalanche professional in the Bagley Lakes area noted recent wind transported snow in the near treeline zone, but no skier triggered avalanches on that wind-affected snow. Small loose avalanches were triggered by sunshine and the sun influenced the release of slab avalanches that approached large (D2) in specific terrain features on the south side of Mt. Herman. On east aspects, the 2/8 crust was down 60 inches or more and facet crystals above this layer were rounding. No other significant layers were present on this aspect.

On Wednesday NWAC professional observer Lee Lazzara traveled in the Mt Baker backcountry. Lee reported wind slabs forming on a variety of aspects near treeline. Snowpack observations showed a highly variable snowpack, but the facet/crust combination was found on steep sunny aspects 2 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.