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Stevens Pass

Issued: 7:09 PM PST Wednesday, March 29, 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.

Fresh storm and wind slab formed Wednesday night should generally be shallow and found near and above treeline on Thursday. If previously wet surface snow has not refrozen, be suspicious of loose wet avalanches that may begin small but entrain older snow and become dangerous and difficult to manage. Sunbreaks Thursday afternoon will likely activate loose wet slides on solar slopes.

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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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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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Cornicesi

Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.

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Snowpack Analysis:

Special Note: For more information on the massive natural cornice triggered avalanche on the north side of Ruby Mountain on Sunday 3/19 and general thoughts about low-likelihood/high consequence avalanches, please see NWAC's blog post issued, Sunday, March 26. 

Weather and Snowpack

Let's just say it's been a wet and wild few weeks regarding weather and avalanches in the Cascades. 

This past week has also been active weather-wise, but water amounts/snowfall totals have been slightly lower relative to the extreme wetness of the past few weeks. In the last 5 days ending Monday morning NWAC stations near and west of the Cascade crest have picked up 1.5-4 ft of snow with the most at Mt. Baker and above the Pass levels.

A strong low pressure system brought rising snow levels and locally heavy precipitation Tuesday night through Wednesday along the west slopes of the Cascades. Most ski areas and DOT programs checked in reporting natural and explosive controlled avalanches in their area of responsibility. The most snow was received at Mt. Baker (2 feet) and Paradise (14 inches) with more moderate amounts elsewhere before changing to rain on Wednesday. 

Recent Observations

North

On Wednesday Mt. Baker pro-patrol reported widespread storm slab and loose wet avalanches below 4500 feet during morning control work. 1 large natural avalanche was observed off Shuskan Arm. Start zones above 5000' were stubborn to trigger during early morning control work but likely became more sensitive as warming pushed the snow-line higher during the day. 

Central

On Wednesday Alpental pro-patrol reported widespread natural and explosive triggered storm slab avalanches late morning. Loose wet avalanches on the lower half of the mountain were beginning to entrain deeper layers. Snoqualmie DOT reported large natural avalanches (up to size D2.5) running in start zones above 4500 feet late Wednesday morning. Stevens Pass DOT reported large slides during control work Tuesday night with avalanches gouging down to deeper layers. 

South

Crystal pro-patrol reported sensitive 4-8" storm slab on the upper half of the mountain Wednesday morning. A widespread but shallow natural loose wet cycle was observed in the surrounding backcountry later in the day with continued warming. 

Detailed Forecast for Thursday:

Fresh storm and wind slab formed Wednesday night should generally be shallow and found near and above treeline. Due to the cooling trend, only wind slab will be listed in the avalanche problem set and emphasized above treeline. W-SW winds should have primarily built fresh wind slab on NW-SE aspects.

The danger will be rated higher in the Mt. Baker area where once again, more snow is expected with the cooling trend Wednesday afternoon and night. As well as new snow hazards, snow levels were regionally lower in the northwest zone during this event, continuing the low probability/high consequence potential for very large avalanches above treeline and large wet snow avalanches below treeline. 

Despite the cooler snow levels forecast for Thursday, if previously wet surface snow has not refrozen, be suspicious of loose wet avalanches that may begin small but entrain older snow and become dangerous and difficult to manage. Sunbreaks Thursday afternoon will likely activate loose wet slides on solar slopes. Stay off steeper slopes with terrain traps. Wet slab avalanches won't be listed but are possible in isolated locations at lower elevations that received ample rainfall on Wednesday. Moderate avalanche danger allows for large avalanches in isolated areas.

Recent cornices are very large. Natural cornice releases and resulting slab avalanches are dangerous and unpredictable. Give cornices a wide berth if traveling along ridge-lines and avoid slopes directly below large cornices. See a blog post regarding cornices here.