Avalanche Forecast
Issued: Feb 21st, 2017 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
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.
Summary
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
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
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.
Aspects: North, North East, East, South East, South.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Feb 22nd, 2017 10:00AM
