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Snoqualmie Pass

Issued: 9:05 PM PST Sunday, March 4, 2018
by Robert Hahn

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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  • Extreme (5)

Avalanche Problems for Monday

Persistent Slabi

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Wind Slabi

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Forecast for Monday:

Persistent weak layers are costing lives this week across the Cascades.

You may be able to trigger Persistent Slab avalanches in the upper snowpack on sun-exposed slopes (generally southerly aspects) greater than 35 degrees. These avalanches are difficult to manage. To reduce your risk of being caught, avoid steep, open, sunny slopes, and large avalanche paths. Also watch out for unsupported slopes that end in rocks, cliffs or steep rolls.

Deep Persistent Slab avalanches are unlikely and difficult to trigger, but are also very difficult to predict. You would likely not survive a Deep Persistent Slab avalanche. Stay safe by avoiding triggering smaller avalanches in the surface snow and staying off the previously mentioned steep slopes.

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 in the Snoqualmie Pass area by Monday morning with smaller amounts elsewhere.

Solar energy has also given the upper snowpack slab-like properties and if the daytime heating influences the snowpack (even through a layer of clouds), there is the possibility to trigger small avalanches within the top 12" of the snowpack.

Avalanche Summary:

6-10" inches of snow are expected in the Snoqualmie Pass area by Monday morning with smaller amounts elsewhere. This snow now sits on settled powder or a crust, depending on aspect.

Sunshine on Saturday morning moistened surface snow and created numerous small loose wet avalanches on steep solar aspects. The moist . Winds as recent as Thursday formed recent Wind Slabs on a variety of aspects near and above treeline. In sheltered areas generally soft unconsolidated surface snow exists.

The central Cascades picked up 2-3.5 feet of snow during the past 10 days, with the most snow since Wednesday in the Mt. Rainier area where 1-1.5 feet of snow fell. Strong S and SE winds accompanied much of the snow.

On E-S-W aspects, a thin breakable sun crust formed early last week and 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. This layer has not yet had significant time to heal. It is found 2-3 feet below the surface on steeper slopes that have received direct sun during the past week. Snow profiles and snowpack test can confirm the presence of this layer; however they are not good for proving its absences.  That said, the crust has not been found in near treeline terrain and is more likely to be found at lower elevations in the below treeline band. Several other crustss exist within the upper snowpack on slopes that received direct sunshine.

Some observations suggest 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.

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 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.

Central Cascades Observations

Several pertinent observations were reported Saturday on our public observations page, including a party that saw a loose wet avalanche step down and trigger a deeper weakness to create a large avalanche.

An avalanche professional was in the Rampart Ridge area on N-E facing slopes below treeline and reported slab formation with the warmer temperatures in the middle of the day. Slabs were 1' deep and breaking 15-20' across and running to the bottom of the limited terrain features.

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. He found stubborn results in tests and no propagation on a layer 28" down in the snowpack on NE and SW aspects in near treeline terrain where a density change or very small rounded facets were observed. 

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

NWAC forecaster Josh Hirshberg was on Nason Ridge Wednesday. He found the 2/23 crust 1.5 feet below the snow surface. While snowpack tests were variable, he did report several avalanches on sunny aspects which occurred earlier this week.

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

NWAC observer Jeremy Allyn was in the Snoquamlie Pass area Wednesday. He reported wind transported snow near treeline. He found the 2/23 interface about 2 feet below the snow surface.

Avalanche and snowpack observations from around the western regions of the Cascades and Passes continue to indicate that propagation of an avalanche on the 2/13 facets is possible. This layer has been found 2.5 to 4 feet below the snow surface.

South Cascades Observations

On Thursday, Forecaster Dallas Glass reported natural slab avalanches in wind-loaded terrain near treeline in the Crystal Backcountry. Dallas reported test 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. 

 

No Corresponding Mountain Weather Forecast Available