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West Slopes South - South of I-90 to Columbia River

Issued: 7:48 PM PST Saturday, March 18, 2017
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.

The avalanche danger should decrease Sunday as a wet snowpack refreezes. Fresh but shallow wind slab will mostly likely be found above treeline. Shallow loose wet slides are likely on steeper solar aspects in the afternoon. Continue to avoid unsupported slopes due to the low likelihood - high consequence threat of wet slab or glide avalanches for one more day. 

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Avalanche Problems for Sunday

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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Loose Weti

Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

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Wet Slabsi

Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.

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Snowpack Analysis:

Weather and Snowpack 

The first week or so of March was very cool and snowy. NWAC stations along the west slopes of the Cascades piled up about 3 to 8 ft of snow with the most at Mt Baker.

The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately these systems delivered far more rain than snow. At least two regional avalanche cycles occurred during the stretch. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures. 

After a short respite from the active weather pattern on Thursday, another strong low pressure system brought several inches of rain to the west slopes of the Cascades outside the Cascade Passes Friday night. 2.5 to 4 inches of water has been seen at Paradise, Crystal and Mt. Baker respectively over the last 24 hours ending at 5 PM PDT Saturday, mainly in the form of rain. The Cascade Passes picked up 6-8 inches of new snow Friday night before changing to rain later Saturday morning. Rapid cooling with the frontal passage produced about 8 inches of snow to accumulate at Mt. Baker with lesser amounts seen elsewhere along the west slopes of the Cascades. 

Recent Observations

North

NWAC observer Lee Lazarra was in the Mt Baker backcountry prior to the storm on Friday and reported a well settled snowpack with a stout crust. The recent snow ranged from 4 to 8 inches of recent dry snow depending upon wind distribution. There were some shallow wind slabs noted on lee slopes in higher terrain. 

Early Saturday morning, Mt. Baker pro-patrol reported widespread natural loose wet activity that had occurred Friday night off of Table Mt, Mt. Herman and Shuskan Arm. Very heavy rain overnight had increased the potential for wet slab/glide avalanches in the Mt. Baker area. 

Central

Alpental pro-patrol reported easy and widespread ski triggered 6-8" storm slabs on the upper half of Alpental Saturday morning with rain reaching the summit. Stevens Pass pro-patrol had similar results Saturday morning with widespread sensitive loose wet slides, up to size 2, natural and human triggered during the warming and switch to rain.  

South

Prior to the storm, NWAC pro-observer Jeremy Allyn was out in Bullion Basin in the Crystal area Friday and found little to no avalanche problems due to the re-freeze and stout crust. Very thin wind slabs were possible on steep lee or cross-loaded features above treeline.

Crystal patrol reported a natural cycle Friday night, presumably loose wet, but little to no results during avalanche control work Saturday morning. 

Detailed Forecast for Sunday:

Decreasing snow showers along with rapid cooling Saturday night through Sunday morning should transition to mostly sunny skies by Sunday afternoon. Freezing levels will be on the cool side Sunday but late March sunshine will help bump up temperatures to near or above freezing at lower and mid-elevations. Winds are forecast to be fairly light on Sunday.

The avalanche danger should decrease Sunday as a wet snowpack refreezes.

Fresh but shallow wind slabs have likely built on NW to SE aspects, mainly above treeline. Watch for firmer wind transported snow on other aspects, especially in areas of complex terrain. All aspects will be listed until more information is received on recent general loading patterns. 

Generally small loose wet avalanches are likely at lower elevations and on solar slopes. Watch for surface wet snow deeper than a few inches, rollerballs or increasing small natural releases.

Although the likelihood of wet slab or glides avalanches has greatly decreased with the cooling trend, these avalanche problems may still occur 24 to 48 hours following a heavy rain event. Continue to avoid unsupported slopes, especially where you know there is a smooth underlying surface or slopes with glide cracks.

New snow accumulated with a cooling trend and diminishing winds, but small storm slabs may remain in isolated areas on Sunday. 

It is always a good plan to travel well back from ridges, suspected of cornice formation, or on steep slopes below cornices.

No Corresponding Mountain Weather Forecast Available