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Stevens Pass

Issued: 7:27 PM PST Tuesday, February 20, 2018
by Kenny Kramer

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. 

You can still trigger dangerous Persistent Slab avalanches that are big enough to kill you on slopes steeper than about 35 degrees. Dangerous avalanche conditions, including a combination of Wind Slabs and uncommon Persistent Slabs require terrain choices with a wide margin of safety. 

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Avalanche Problems for Wednesday

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 Wednesday:

Continued cold temperatures with light winds Wednesday will not significantly improve the current avalanche danger, allowing recent slabs and the known weak persistent layer to persist longer than usual. 

Even a small triggered Loose-Dry avalanche or a small Wind Slab may be enough to trigger a Persistent Slab. Snowpack observations and one serious skier triggered avalanche from along the west slopes of the Cascades Sunday show you can still trigger dangerous Persistent Slab avalanches that are big enough to kill you on slopes steeper than about 35 degrees. Persistent Slabs can surprise even very experienced travelers and can break over terrain features. Avoid large terrain, especially areas exposed to avalanches that could come from above. Avalanches may be surprisingly large and run farther than you expect. Make conservative terrain choices until we know more about these avalanches. A small triggered Loose-Dry avalanche or a small Wind Slab may be enough to trigger a Persistent Slab.

The persistent slab problem is likely more sensitive on shaded, non-solar aspects, but can not be ruled out on any aspect. 

The combination of Wind Slabs and the more uncommon Persistent Slabs require terrain choices with additional safety margins

Avalanche Summary:

An additional 4-6" of cold low density snow accumulated from about Mt Rainier southward by Tuesday evening. The newest snowfall is not increasing the danger as it has no slab properties. However, the new powder snow may be masking more dangerous snow structures below.  

The storm snow late last week fell on weak, near surface facets or surface hoar that developed during a fair weather period on a rain crust. These weak sugary facets over a hard crust, created both a smooth sliding surface and a very weak layer above, now loaded with a slab, a dangerous combination!

Since February 15th, up to 3.5 feet of snow has fallen with extreme winds on Saturday. Rain was reported up to 4000 feet from Stevens Pass south Saturday. If you dig 2.5-4 feet below the surface, you will find recent, firmer snow sitting on a thin layer of weak, sugar-like facets adjacent to a crust. This strong slab over weak facets is the key recipe for Persistent Slab avalanches.

Persistent Slab avalanches were reported in Snoqualmie Pass area Saturday and Sunday with a snowboarder caught, carried and injured near the Alpental ski area Sunday. The culprit,  weak sugar-like snow has been observed on a hard crust 2-4 feet below the snow surface in almost all of the forecast zones. Professionals from around the region Sunday and Monday found this layering reactive in snowpack tests. You will still be able to trigger these large and dangerous avalanches Wednesday.  

An extended cycle of natural and triggered avalanches occurred Wednesday through Saturday on the west slopes of the Cascades. Peak avalanche activity occurred late Friday to Saturday. 

Observations

Mt Baker

NWAC observer, Lee Lazzara was back in the Baker BC Monday. Lee reported a high degree of spatial variability both of the persistent slab depths and also the crust/facet layering structure. Wind slabs were seen on a variety of aspects but were stubborn to trigger. Snowpack tests still indicated the persistent slab was likely to trigger and propagate. Terrain choices were limited due to the observed conditions. 

On Sunday, Mt. Baker ski patrol reported 3 separate skier triggered wind slab avalanches up to 2 feet deep outside the ski area. One wind slab avalanche on Hemispheres broke 180 ft wide and caught, carried and partially buried a skier. The skier was able to self-extricate.

Details are sparse but two snowmobile triggered avalanches were reported Sunday, one on a WNW aspect at 5800 feet. One was in the Canyon Creek area, the other in the Glacier Creek area with a crown depth of 3-4 ft.  

NWAC pro-observer Lee Lazzrra on Sunday found NE winds significantly redistributing snow near Artist Point with scoured surfaces near ridges. In this area the 2/8 crust was 3+ feet down in non-wind scoured areas.  A crust facet sandwich is widespread about 3-4 feet below the surface on all but steep solar slopes. Snowpack tests were limited and inconclusive regarding the potential for a persistent slab avalanche in this area. 

Stevens and Snoqualmie Pass

A snowboarder triggered a large persistent slab avalanche Sunday outside the Alpental ski area near Powder Bowl on a NE aspect near treeline. The persistent slab avalanche released on the smooth crust buried 2/13. The rider was carried 1000 feet and suffered serious but non-life threatening injuries.    

On Sunday, NWAC staff performed snowpack tests indicating propagation (Propagation Saw Test) on the facets buried February 13th. These tests were located on W and SW aspects at Snoqualmie below treeline in the Kendall Peak and Rampart Ridge areas. A ski-supportable re-freezing rain crust was noted up to 4200 ft. The depth of the 2/8 crust varied depending on elevation, between 15-30" (40-80 cm) down.

In the Yodelin area near Stevens Pass in snowpack tests Sunday, persistent slabs were deemed likely to propagate on the facets above the 2/8 crust which was 2.5-3 ft (70-90 cm) deep. A slight rain crust was noted 6" below recent low density snow. 

On Saturday, Alpental Ski Patrol reported ski cuts triggering large avalanches that were 2 feet deep and ran on facets. These avalanches were surprising, with widely propagating crowns, and involved audible collapsing of the snowpack. One slide was roughly 300 feet wide. They were also able to trigger a small storm slabs on steep slopes within the storm snow.

Mt Rainier

Monday, NWAC forecaster, Dallas Glass traveled above Paradise on Mt Rainier reporting consistent 2 ft of low density snow over 2 ft of firm slabs formed Saturday. The slabs are sitting over a very weak structure of facets on the rain crust from Feb 8th. In similar terrain Tuesday, Dallas heard an audible, very loud whumpf in the area and also triggered another whumpf indicating a collapse of a layer below, often a weak layer.

On Sunday, an avalanche professional at Mt. Rainier reported 4 feet of snow above the 2/8 crust. Facets were found above the crust but were rounding. Recent wind distribution was significant from Saturday and debris from several from large avalanches that ran on Saturday were noted. Due to continued shower activity Sunday, new storm instabilities were present in the storm snow with natural small storm slab avalanches observed. 

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.