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Snoqualmie Pass

Avalanche Warning i

Issued: Thu, March 9, 2017 at 8:35 PM PST
Expires: Fri, March 10, 2017 at 6:00 PM PST

Issued: 8:35 PM PST Thursday, March 9, 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.

Numerous serious avalanche involvements occurred in the Olympics and Washington Cascades over the March 4th-5th weekend. NWAC has posted brief summaries of the incidents here: http://www.nwac.us/accidents/accident-reports/. Full reports will be posted when finished. 

Due to the potential for very large avalanches in specific areas following Thursday's complex storm, travel in avalanche terrain is not recommended on Friday. Allow the recently stressed snowpack time to settle and stay out of consequential terrain. 

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Avalanche Problems for Friday

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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Storm Slabsi

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

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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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Snowpack Analysis:

Weather and Snowpack 

The most recent warm, wet storm arrived on Valentines Day forming a strong rain crust, now buried several feet or more in most areas along the west slope Cascade zones. This crust may be a bed surface for very large avalanches during the current storm cycle. 

Strong southwest flow carried a strong front across the Northwest on Friday evening March 3rd. Along the Cascade west slopes this caused strong southwest alpine winds, heavy, moist, dense new snow above about 3-4000 ft and wet snow or rain below about 3-4000 ft. A region wide avalanche cycle was seen late Friday 3/3 and in certain areas on Saturday 3/4. 

The first week or so of March has been remarkably snowy; NWAC stations along the Cascade west slopes have had about 3 to 8 ft of snow with the most at Mt Baker!  A weak low pressure system brought another 6-10 inches of snow to the west slopes Wednesday afternoon and night. 

A strong frontal system brought increasing precipitation and winds along with a warming trend to the Cascades on Thursday. As of 6 pm Thursday, rain had pushed up to at least 6000 feet in the southwest Cascades. Easterly flow was still allowing wet and heavy snowfall at Stevens and Snoqualmie Passes, where 4-6 inches had accumulated since late morning. Mt. Baker's snow level had risen to about 4000 feet at the end of the day Thursday with another 6 inches of snow accumulating along with occasionally gusty winds. 

Recent Observations

North

On Wednesday morning, Mt. Baker pro-patrol saw mostly loose dry avalanches running within the new 8" they had picked up Tuesday night. These results represent the ski area's mitigated terrain only.  

Lee Lazzara traveled in non-consequential terrain in the Mt. Baker area on Thursday. He found dense wind slab quickly building and becoming sensitive to skier triggering on small test slopes. He also found several lingering storm slab instabilities within the past week's storm snow. While no one particular deeper layer was likely to be skier triggered, they represent the ability for initially shallow slab avalanches to step down and become deeper avalanches. 

Central

WSDOT, Alpental ski area and NWAC Pro-observers in the Snoqualmie Pass area reported signs of a widespread natural cycle that had occurred Tuesday afternoon and evening, with D1-1.5 storm slabs featured on all aspects. NWAC pro-observers Ian and Matt found storm layers beginning to settle and not indicating likely propagation in column tests. The March 3rd melt-freeze crust was thin and down 70-85 cm in the below treeline band. Matt observed shallow wind slab on NW-N aspects near treeline. Small loose dry avalanches ran fast on steep rollovers below treeline but didn't entrain much recent snow. 

Early Wednesday morning, WSDOT Stevens Pass easily released wind and storm slabs during control work involving Tuesday/Tuesday night's storm snow. 

WSDOT Snoqualmie DOT reported a widespread natural cycle that occurred Wednesday night as denser snow layered on top of lighter snow received earlier in the storm. Additional heavy wet snow accumulating during the day on Thursday increased the avalanche hazard for both Snoqualmie Pass and Stevens Pass and resulted in Pass closures.  

South

Crystal pro-patrol experienced touchy avalanche conditions during Tuesday afternoon's increased snowfall and warming but only limited results during control work Wednesday morning. These results represent the ski area's mitigated terrain only.

Crystal pro-patrol reported little avalanche activity mid-day but that winds and dense snowfall were beginning to increase.  

Detailed Forecast for Friday:

Dense snow received at higher elevations in the southwest Cascades and in the Cascade Passes should be followed by a period of rain Thursday evening and then by light showers with a slow cooling trend during the day on Friday. Dense snowfall followed by moderate showers with the same slow cooling trend is expected in the northwest Cascades through mid-day Friday. W-SW winds will increase Thursday night and stay strong through Friday morning.

With a complex weather pattern afflicting the Cascades Thursday and Thursday night and stressing our deep storm snow received over the last week, travel in avalanche terrain is not recommended on Friday. Allow the recently stressed snowpack time to settle and avoid consequential terrain. Realize that very large avalanches are possible if initially small avalanches step down to deep and lingering storm instabilities or if a large natural trigger like a cornice collapse occurs. 

Deep wind slab should be suspected on all aspects near and above treeline but most likely found on NW to SE aspects due to recent SW to W winds. Watch for firmer wind transported snow on varied aspects especially in areas of complex terrain.

Sensitive storm slab is mostly likely in the northwest zone and in the Cascades Passes for elevations and areas that do not see rain Thursday night. Very large storm slabs are not likely to be skier triggered but are listed in the problem set due to the step down potential.  

Loose wet avalanches are likeliest in the northwest Cascade zone due to periods of moderate rain forecast to affect the below treeline band on Friday. However, large wet slab avalanches will be possible in isolated areas throughout the west slopes of the Cascades that received significant rainfall Thursday night.  

Cornices won't be listed as an avalanche problem but avoid travel on ridges near where cornices may have formed and avoid steep slopes below cornices that may fail at any time. Cornices have been reported as large and in charge in many areas. They will have likely been weakened during the recent storm cycle becoming more likely to fail. 

No Corresponding Mountain Weather Forecast Available