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Stevens Pass

Issued: 6:36 PM PST Saturday, February 25, 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.

Watch for generally shallow new wind slab on lee easterly aspects. Fresh wind slab should mainly be found near and above treeline but may also form in the upper portion of the below treeline band. Fast moving loose dry avalanches are possible on steeper slopes in non-wind affected terrain.

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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 Dryi

Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.

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Snowpack Analysis:

Weather and Snowpack

The most recent atmospheric river arrived on Valentines Day 2/14 forming the uppermost significant rain crust in our snowpack. A short period of fair weather on Friday 2/17 caused another surface crust, especially on solar aspects.

A pair of storms last Sunday and Monday deposited about a foot of snow at most areas by Monday 2/20. This was combined with periods of moderate to strong W-SW winds, forming wind slabs at the time.

A slightly unstable weather pattern this week caused a mix of sun and light snow showers with cold temperatures and light winds from Tuesday through Friday. The weather this week helped to freshen the surface with up to several inches of snow while sun crusts formed on solar aspects.

Saturday was mostly cloudy, with light snow showers seen mainly in the north and central Cascades. Westerly ridgetop winds began to increase Saturday afternoon ahead of a frontal system. 

Recent Observations

North

NWAC observer Lee Lazarra was out in the Mt. Baker backcountry on Saturday and found lingering wind slab on lee slopes above treeline but that the older wind slab was unreactive in snowpit tests and unlikely to trigger. Loose dry avalanches were a concern in steep terrain.   

Central

NWAC observer Jeremy Allyn was in the Snoqualmie Pass area on Thursday 2/23 and found old wind slab weaknesses at 30-40 cm but little in the way of avalanche problems.

NWAC observer Ian Nicholson was in the Stevens Pass area on Saturday and found sun crusts affecting the snow surface on an increasing range of solar aspects with shallow amounts of new snow below treeline above the stout 2/14 crust. Weak surface snow near treeline was beginning to be transported by westerly winds beginning around mid-day.

South

Ian was in the East Peak area on Thursday 2/23 and found recent snow was generally right side up with good conditions and good bonding to the 2/14 crust. He noted small pockets of shallow potential 10-15 cm 1F wind slab.

NPS rangers at Mt. Rainier National Park on Saturday relayed a report that six people were hit by a large loose dry avalanche that naturally released in the Fly Colouir on Lane Peak in the Tatoosh Range. One or more of the individuals were partially buried. Details are limited but at this time there were no known injuries. 

Detailed Forecast for Sunday:

A low pressure system moving south along the Washington Coast on Sunday should produce light amounts of low density snow along the west slopes of the Cascades in the north with light to moderate amounts of new snow for the central and southern portions. 

While older wind slab continues to exist on isolated lee slopes mainly above treeline, we will shift our focus to new wind slab formed Saturday afternoon through Sunday morning. Existing weak surface snow in addition to new low density snowfall will be transported to lee slopes mainly near and above treeline. Watch for generally shallow new wind slab on lee easterly aspects. Fresh wind slab should mainly be found near and above treeline but may also form in the upper portion of the below treeline band. Be aware that lower density snowfall received as the winds ease during the day on Sunday may obscure recently wind loaded slopes.  

Watch for loose dry avalanches on steeper slopes in non-wind affected terrain. Be especially wary of fast running loose dry avalanches near terrain traps. 

Storm slabs will not be listed as a primary avalanche problem on Sunday but watch for shallow storm slabs particularly from Snoqualmie Pass and south if new storm layers form a cohesive slab above weaker snow in specific terrain on Sunday. 

Give cornices a wide berth when traveling along ridgelines and avoid lingering on slopes below cornices as they may fail at any time.