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Snoqualmie Pass

Issued: 6:05 PM PST Thursday, March 8, 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.

Watch for Wind and Storm Slabs forming in the new snow as the Avalanche Danger increases throughout the day. Persistent Slab avalanches claimed lives along the east slopes of the Cascades in the past 2 weeks. Avoid steep, complex terrain and sit out this low likelihood - high consequence problem; ensure a wide buffer between where you travel and open slopes over 35 degrees as well as large avalanche paths.

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Avalanche Problems for Thursday

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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Storm Slabsi

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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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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Persistent Deep Slabi

Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.

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Forecast for Thursday:

Expect the avalanche danger to increase throughout the day, peaking around dark. Today, you can easily trigger avalanches in the upper layers of the snow due to new snow, wind, and light rain at low elevations. New snow may fall on a variety of surfaces and some may be slick. Watch for cracking, wind stiffened snow, and freshly formed drifts. Steer around fresh wind features, convex rolls, and slopes holding a foot or more of new, cohesive snow that are 35 degrees and steeper. Rain will fall later today bringing a chance for wet avalanches up to 4500ft. Avoid high consequence terrain such as cliffs, rocks, and gullies where even a small avalanche could be dangerous.

Avalanches in the upper snowpack and heavy snowfall are making it easier to trigger deeper and dangerously wide avalanches. Persistent weak layers lurk deeper in the snowpack. The signs of Persistent Slab avalanches may not be obvious. These low likelihood, high consequence avalanches are very difficult to manage. The best way to stay safe is to avoid the slopes where you can trigger them. Take a day or two to choose more cautious terrain before returning to the kinds of slopes you traveled on prior to this storm. Avoid large avalanche paths, start zones, and unsupported slopes steeper than 35 degrees. Tracks on a slope don’t mean that the slope is safe. In one recent fatality in the Cascades, the slope had numerous tracks on it before the avalanche was triggered. While these avalanches may give little warning, the consequences could be un-survivable.

Avalanche Summary:

Fatal avalanche incidents along the east slopes of the Cascades occurred this weekend in the Teanaways near Long's Pass (Saturday) and north of the Methow Valley at Setting Sun Mountain (Sunday). Persistent Slab avalanches were reported both at the Long's Pass and Setting Sun incidents. Recent Persistent Slab avalanche activity has been confined to the Stevens Pass area along the west slopes of the Cascades, but a similar snowpack structure exists in other areas along the west slopes.  

On Wednesday, a small triggered slab avalanches were reported from wind loaded high elevation terrain near Snow Lake on Snoqualmie Pass, the Crystal backcountry,  and just out of bounds from Mt Baker on Shuksan Arm.

On E-S-W aspects, a thin breakable sun crust 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. While it has been difficult to trigger in the past week, it's worth paying attention to it through the current storm cycle. It is found 1-3 feet below the surface on steeper slopes that have received direct sun during the past week. Snow profiles and snowpack tests can confirm the presence of this layer; however they are not good for proving its absence. That said, the crust is more likely to be found at lower elevations in the below treeline band. Several other crusts exist within the upper snowpack on slopes that received direct sunshine.

Some observations from last week suggest persistent grains at this same 2/23 interface on shaded slopes. Buried surface hoar and large preserved stellars were reported in avalanches and snowpack tests at this interface about one week ago.

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 6 feet below the snow surface and just above a very firm melt-freeze crust (2/8). In the southern Cascades, recent observations suggest it may be easier to trigger avalanches on the 2/13 facets near the Crystal Mountain area compared to terrain near the Paradise side of Mount Rainier where the layer is considerably deeper.

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

Observations

North 

On Saturday, an NWAC professional observed two large wind slab avalanches several feet deep triggered by snowmobilers 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. 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.

Central 

On Wednesday, NWAC observers reported surface hoar sitting on all shaded slopes near Steven's Pass.

On Wednesday, NWAC observer Ian Nicholson reported sudden test results on a crust buried over the weekend, about 1 foot below the snow surface on a sun exposed slope near Snoqualmie Pass.

On Tuesday, Forecaster Josh Hirshberg found sudden test results on the 2/23 facets on a West aspect at 5,800ft just east of Steven's Pass. Large column tests did not indicate propagation.

An avalanche professional in the Skyline area of Stevens Pass Saturday through Monday found the 2/23 facet/crust interface on south aspects becoming less reactive in snowpack tests. However, the 2/13 layer continued to show the potential for an avalanche to fail and propagate on this layer on most aspects.   

Several pertinent observations were reported Saturday on our observations page, including an avalanche professional that observed a loose wet avalanche step down and trigger a deeper avalanche on an east aspect of Jim Hill near Stevens Pass that potentially ran on the 2/23 facet/crust interface.

On Saturday, NWAC professional Observer Jeremy Allyn was in the Mt. Snoqualmie area where he observed a right-side-up density profile to the 2/8 crust down 5' (150 cm) on a SW aspect at 5100 feet. No 2/23 crust was observed at this location. No new or recent avalanche activity was observed. 

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

Stevens DOT reported two avalanches Wednesday morning 2/28. One failed on the 2/23 interface on a NE aspect. The weak layer appeared to be buried surface hoar. An avalanche from earlier in the week was larger and suspected of failing on the 2/13 facet/crust combination. This is the most recent avalanche report we have on the 2/13 PWL from the West Slopes of the Cascades.

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.