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Avalanche Forecast

Archived

Feb 21st, 2017–Feb 22nd, 2017

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Snoqualmie Pass.

Fresh or lingering wind slab may still be sensitive to skier triggering Wednesday. Wind slab will mostly likely be found on N-E-S aspects near and above treeline. Evaluate recent snow carefully before committing to steeper terrain in all elevation bands. 

Detailed Forecast

Wednesday should be cool with scattered showers increasing in coverage and intensity in the afternoon. A few sunbreaks are possible in the afternoon but generally cloudy conditions are expected along the west slopes of the Cascades.  

Light to moderate amounts of new snow received over the last day or so should have been transported to lee easterly slopes mainly near and above treeline in the central-west and southwest Cascades. The avalanche danger will taper from south to north along the west slopes of the Cascades Wednesday to reflect the recent snowfall and wind patterns.

Fresh or lingering wind slab may still be sensitive to skier triggering near and above treeline Wednesday. Wind slab will mostly likely be found on N-E-S aspects near and above treeline. Shallow storm slabs should continue to settle and be less sensitive on Wednesday. Evaluate recent snow carefully before committing to steeper terrain in all elevation bands. 

Loose wet avalanches will not be indicated as an avalanche problem but watch for loose wet surface snow if you find yourself on solar slopes during sunbreaks.

Snowpack Discussion

Weather and Snowpack

Hopefully the last atmospheric river of the season arrived on Valentine's Day 2/14, causing heavy rain, avalanches, crusts and significant snowpack consolidation through Thursday 2/16 along the Cascade west slopes. Freezing rain also occurred along the lower slopes of the Cascade Passes. A little snow was seen at the tail end of the storm. A short period of fair weather on Friday 2/17 formed another surface crust in many areas. Light amounts of new snow accumulated over the weekend except at Mt. Baker who received a foot of new snow by early Monday morning along with moderate SW winds Sunday night.

A low pressure system tracking across the south Washington Cascades during the day on Monday brought up to a foot of snow in the Paradise/Crystal/White Pass areas with lesser amounts further north. Winds that were easterly in the morning switched to westerly Monday afternoon and were the strongest in the south Washington Cascades. A slight warming trend was observed during the storm Monday.   

Light snowfall seen mainly in the southwest Cascades Tuesday was transitioning to light to occasionally moderate showers in the afternoon. Gusty westerly winds began to increase Tuesday afternoon in the central-west and southwest Cascades. 

Recent Observations

North

The Baker area had an eventful Monday with 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 in the Mt. Herman-Artist Point area Monday 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') deep.  

Central

On Tuesday, Alpental pro-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 mid-day with a slight rise in temperatures even with cloudy skies.   

South

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

NWAC pro-observer Ian Nicholson was at Paradise on Monday 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 indicated less sensitive avalanche conditions overall, but storm instabilities were still reactive to ski triggering about 20 cm (8") down on steep slopes. 

Small natural slab avalanche, East Peak, Crystal Mountain backcountry. Shane Robinson, 2-20-17

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.