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Avalanche Forecast

Archived

Mar 28th, 2015–Mar 29th, 2015

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

Regions

Stevens Pass.

Use caution in higher elevations where possibly greater new snow may accumulate, especially lee slopes near ridges. Watch for areas of wet snow at mid elevations, and during daytime warming. 

Detailed Forecast

A frontal passage Friday night should cause some light to moderate new snow, but initially rain changing to snow should leave good bonding of new snow to the old wet snow surface.

Update Saturday morning: Wind slab has been added to the above treeline forecast for the Cascade west slopes and Mt Hood due to stronger than expected winds and greater than expected snow. But the cooling trend seen Friday night should also help limit new instability in many areas.

Watch for areas where new snow has been transported by wind to lee slopes at higher elevations along ridges. Also, watch for cornice development that may be soft and sensitive to trigger or release from daytime warming.  

Less recent snow and ample terrain anchors may limit the avalanche danger below treeline. Many areas at the lowest elevations do not have enough snow to cause an avalanche danger.

Snowpack Discussion

Winter has been making a comeback in the PNW...now that it's technically spring. Though Thursday and Friday were definitely springlike with plenty of sunshine and temperatures climbing into the 50's and even 60's in most areas. Several systems moved through the region beginning last weekend, bringing 1-3 feet of storm snow and produced a few avalanche involvements. 

A low pressure system moved east over the Mt Hood area Monday night/Tuesday morning and deposited new snow amounts west of the crest ranging from 3-5 inches from Mt Baker to Snoqualmie and from 8-12 inches from Crystal to Mt Hood. Small to large natural and skier triggered, loose wet and dry avalanches were reported at Alpental (TAY report), Crystal and Chinook backcountry Tuesday, the type depending on the time of day and aspect. Larger loose avalanches entrained snowfall from earlier in the week, and carried one skier in Powder Bowl at (closed) Crystal Mt.  NWAC observer Dallas Glass observed a remotely triggered storm slab near treeline above Paradise releasing within sensitive storm layers.

Steady warming, along with moderate precipitation brought more sensitive conditions to the Paradise backcountry again late Wednesday afternoon. Dallas reported small natural wind slabs on lee NE aspects near treeline in the morning transitioning to wet avalanche concerns with a switch to rain mid-day. Rain extended above 7000 feet late Wednesday and by Thursday morning above Paradise on Mt Rainier, there was evidence of a widespread large natural avalanche cycle.

Large natural slab release from 3/25 pm, Mt Rainier, Wilson Chute, east aspect ~ 7000 feet. Crown extended over 1 km along ridge! photo: Dallas Glass 

More natural avalanche evidence from rain event 3/25 Mt Rainier, east of McClure Rock, E aspect. Photo: Gary Vogt

Crystal and Chinook received far less loading, and professionals using explosives observed only localized loose and generally small avalanches. 

Snowpack problems west of the crest for the time being should be in the upper or surface layers. The mid and lower snowpack west of the crest consists of layers of stable consolidated rounded grains or melt forms and crusts from multiple warm periods this winter.

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.