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Avalanche Forecast

Archived

Mar 25th, 2015–Mar 26th, 2015

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
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

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Dangerous avalanche conditions are expected Thursday near and above treeline mainly due to loose wet avalanches that will have the ability to start in the surface layers and entrain significant amounts of snow received earlier this week. Stay conservative with your terrain selection Thursday. 

Detailed Forecast

Partly sunny skies Thursday along with high freezing levels will maintain the threat of loose wet avalanches Thursday, especially on solar slopes above treeline that did not naturally avalanche Wednesday.

Loose wet avalanches Thursday will have the ability to start in the surface layers and entrain significant amounts of snow received earlier this week. Larger avalanches will be possible above treeline where more recent snowfall has been received.  Avoid steeper slopes if you are sinking in more than a few inches, and be aware the even small loose wet avalanches are powerful and can push you into unintended terrain features.  

Fresh cornices along ridgelines have likely been weakened and should become susceptible to failure Thursday with warming and potential sunshine. Make sure to give them a wide berth, and be aware of corniced slopes above you. 

Lingering wind slab is possible above treeline on lee aspects especially above Wednesday's snow line.    

High avalanche danger is forecast for the southwest Cascades were more recent snow/water has been received and sunnier skies are expected. 

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. The storm last weekend caused 1-2 feet of snow above treeline in the Mt Baker area. At the NWAC station at the base of the Mt Baker Ski Area there was about 8 inches of snowfall and there was up to a few inches elsewhere along the west slopes. On Sunday high on Heliotrope Ridge, a skier triggered and was partially buried by a 1-1.5 foot storm or wind slab avalanche.

A low pressure system moved east over the Mt Hood area Monday night/Tuesday morning and brought a taste of winter to the Cascades. New snow amounts west of the crest generally ranged from 3-5 inches from 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 in the 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 Thursday. 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. Crystal and Chinook received far less loading, and professionals using explosives observed only localized loose and generally small avalanches. No reports were received from areas further north Wednesday.  

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

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

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.