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Stevens Pass

Issued: 7:48 PM PST Thursday, March 8, 2018
by Josh Hirshberg

Significant snow and wind have brought a return to dangerous avalanche conditions. Watch out for Storm and Wind Slabs in the upper snowpack. Persistent Slab avalanches claimed lives along the east slopes of the Cascades in the past 2 weeks. Avoid steep, complex terrain and large avalanche paths to reduce the threat of the low likelihood - high consequences of Deep Persistent Slabs; ensure a wide buffer between where you travel any large avalanche terrain.

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Avalanche Problems for Friday

Wind Slabi

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Storm Slabi

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Persistent Slabi

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Persistent Deep Slabi

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Forecast for Friday:

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 have fallen on a variety of surfaces that could form a poor bond with the new snow. Watch for cracking, wind stiffened snow, freshly formed drifts, and small test slopes where the new snow easily slides as clues that you could trigger an avalanche. Steer around fresh wind features, convex rolls, and slopes holding a foot or more of newly fallen, cohesive snow that are 35 degrees and steeper.

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 and Deep 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 two recent avalanche fatalities in the Cascades, the slopes had numerous tracks on it before the avalanches were 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 last weekend in the Teanaways near Long's Pass and north of the Methow Valley at Setting Sun Mountain. Persistent Slab avalanches were reported at both of these locations. Recent Persistent Slab avalanche activity has been reported from the Stevens Pass area, but a similar snowpack structure exists in other areas along the west slopes of the Cascades.  

1-2 feet of snow, gusty winds, and rain at low elevation slopes have raised the Avalanche Danger. Snow lines generally stayed below 3,000ft near the passes and around 4,000ft at Mt Baker. The storm will taper off by Friday, but avalanche conditions will remain dangerous for another day.

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. Pay attention to the old surfaces that are now at the interface of the most recent snow that fell late Wednesday into Thursday. They range from wind affected snow, crusts to near surface facets and surface hoar. 

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. 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.   

Last 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.

South 

On Wednesday, Forecaster Dallas Glass traveled in the Crystal backcountry and reported snowpack tests showing potential for triggering avalanches on crust/facet combinations in the upper snowpack as well as up to 3 feet down on the older faceted 2/13 layer. On Wednesday, he reported small triggered slab avalanches in wind-loaded terrain near treeline.

On Wednesday, Jeremy Allen traveled in the Tatoosh Range. Jeremy reported sudden test results on the 2/23 facets about 2 feet below the surface on a south slope at 6,200ft. He also found crusts on all aspects below 5,000ft.

No Corresponding Mountain Weather Forecast Available
This information is provided by the U.S.D.A. Forest Service and describes general backcountry avalanche hazard and conditions. It does not apply to ski areas and highways where avalanche mitigation is conducted. Read more here.