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West Slopes North - Canadian Border to Skagit River

Issued: 7:31 PM PST Thursday, February 23, 2017
by Garth Ferber

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.

Isolated lingering wind slab in the near and above treeline will continue to heal and become less sensitive to human triggering on Friday. You will need to watch for a wider variety of avalanche problems as we transition to spring. Continue to evaluate snow and terrain carefully on Friday.

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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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Snowpack Analysis:

Weather and Snowpack

The latest of several warm, wet SW streams of moisture this season arrived on Valentines Day 2/14 bringing another round of heavy rain, avalanches, crusts and consolidation through Thursday 2/16 along the Cascade west slope zones. Minor new snow amounts were deposited at the tail end of the storm.

A short period of fair weather on Friday, 2/17 caused another surface crust, especially on solar aspects.

Light amounts of new snow accumulated over the weekend, except at Mt. Baker where about a foot of snow fell by early Monday morning, along with moderate SW winds Sunday night.

A storm that tracked across the southern Washington Cascades Monday, deposited up to a foot of snow in the Paradise/Crystal/White Pass areas with lesser amounts further north. Winds that were easterly initially, switched to westerly Monday afternoon and were the strongest in the south Washington Cascades.

A mix of sun and light snow showers Tuesday to Thursday freshened the surface with a few inches of snow with cool temperatures and generally light winds.

Recent Observations

North

The Baker area had an eventful Monday 2/20 with the pro-patrol reporting a natural avalanche cycle on Shuskan Arm Sunday night. Widespread and sensitive storm slabs 6-10" deep released during control work Monday morning.  A 6" storm slab was skier triggered on a steep north aspect of Table mountain, carrying the skier 100 or more feet. The skier was partially buried and suffered a knee injury.

NWAC observer Lee Lazzara was also in the Mt. Herman-Artist Point area Monday 2/20 and found 15 cm (6") of denser new snow poorly bonded to less dense snow from earlier in storm cycle. In wind loaded areas, the slab was up to 60 cm (2 ft) deep.

Lee was back out in the Heliotrope ridge area on Thursday 2/23 on mostly N-E aspects and no results from ski tests. He found some reactivity in ECT tests due to wind slab from Sunday 20-60 cm below the surface. He also reported some small triggered loose dry avalanches.

Central

On Tuesday 2/21, the Alpental patrol reported widespread 6" storm slabs, occasionally up to 10" in isolated locations during control work with explosives. The slabs were soft and generally didn't run far. Shallow loose avalanches were becoming touchy by midday with a slight rise in temperatures even with cloudy skies.

A report for the Snoqualmie area for Wednesday 2/22 via the NWAC Observations page indicates pockets of 20 cm likely storm or wind slab on a steep S-NE slope above 4500 ft.

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.

South

Conditions were quite touchy in the Crystal backcountry Monday 2/20. Several reports from East Peak indicated shallow natural and skier triggered, loose and storm slab avalanches in the near treeline and the upper portion of the below treeline band. One report indicated a slab near the ridgeline of East Peak on a Westerly aspect had released down to the Valentine's Day crust. 

Small natural slab avalanche Monday 2/20, East Peak, Crystal Mountain backcountry. Photo by Shane Robinson.

NWAC observer Ian Nicholson was at Paradise on Monday 2/20 and observed only minor wind transport of new snow. Ian also reported storm slab instabilities were a bit less touchy than in the Crystal area. 

Backcountry reports from professionals in the Crystal area on Tuesday 2/21 indicated less sensitive avalanche conditions overall, but storm instabilities were still reactive to ski triggering about 20 cm (8") down on steep slopes.

NWAC forecasters Garth and Dennis were in the East Peak area Wednesday 2/22 finding improved mainly right side up conditions. Storm layers were still producing moderate test results, but not indicating propagation. Cornice drops onto previously loaded east facing slopes did not give significant results. Wind slabs seemed confined to isolated nearby ridges.

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

Detailed Forecast for Friday:

A weak low pressure system should move north to south over the Northwest coastal waters on Friday. This should trigger light occasional moderate snow showers over the Cascade west slopes by Friday afternoon with light winds and low snow levels.

Lingering wind slabs should continue to heal, become isolated to specific terrain features, and become less sensitive to human triggering Friday. The most significant recent winds along the Cascade crest were generally S-W so NW-SE aspects in the near and especially above treeline will be indicated. Watch for signs of firmer wind transported snow.

Storm slabs are expected to have stabilized by this time.

Cloudier weather should limit the potential for loose wet avalanches on Friday and this will not be indicated as an avalanche problem. Watch for wet surface snow deeper than a few inches if you find yourself on sun exposed slopes during any extended sun breaks.

Loose dry avalanches will also not be indicated as an avalanche problem. You can do tests for loose dry avalanches by pushing snow onto small safe test slopes.

Avoid areas along ridges where there may be a cornice and slopes below cornices, since cornices can fail at any time.