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Avalanche Forecast

Archived

Apr 7th, 2018–Apr 8th, 2018

Alpine
Natural avalanches possible, human triggered probable.
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

Olympics.

Winter-like stormy weather will create dangerous avalanche conditions at higher elevations by Sunday. Wind Slabs will grow on lee slopes, especially below ridges where you will be able to trigger a large avalanche. Use visual clues such as blowing snow, fresh cornices, and cracks in the snow all indicating that you could trigger a Wind Slab avalanche.  Give fresh Cornices a wide margin of safety. 

Detailed Forecast

Heavy rain Saturday morning transitioned to snow by late Saturday afternoon following a strong frontal passage. Very strong winds and further snowfall in moderate showers will continue Saturday night and Sunday.

This continued winter-like stormy weather will create dangerous avalanche conditions at higher elevations by Sunday.

Wind Slabs will build on lee slopes, mainly below ridges from NW-E facing where you will be able to trigger a large avalanche. Use visual clues such as blowing snow, fresh Cornices, and cracks in the snow all indicating that you could trigger a Wind Slab avalanche.  

While the bonding of new snow to the old wet snow surface should be favorable, unstable Storm Slabs may form in areas receiving the greatest new snow deposits. Watch for deeper stiffened slabs developing Sunday on a variety of aspects.

Fresh Cornices are likely to build along ridges, so give Cornices a wide berth if traveling along ridgelines as they often break back further than expected. Be aware of overhead hazard if traveling on slopes with Cornices above as they may fail naturally or be human-triggered. 

Snowpack Discussion

Periods of heavy rain and warm temperatures early Saturday transitioned to snow or wet heavy snow by Saturday afternoon with shallow new snow beginning to accumulate by Saturday afternoon.  The old rain soaked snowpack has begun to refreeze and form a crust under continued cooling. 

Older weak snow has been observed on N-E aspects near and above treeline above a crust over the last 2 weeks. This layer is generally 1 to 1.5 feet (30-45 cm) below the snow surface. Rainfall and continued above freezing temperatures Friday night and Saturday may allow Wet Slabs to fail at this interface.

There are no other significant layers of concern in the snowpack at this time.

Observations

On Friday 4/6, NWAC professional observer Matt Schonwald found the facets above a crust on NE aspects between 5200-5400 ft healing.  While this interface may come into play with Wet Slabs Saturday, the Persistent Slab problem has ended. Matt also found glide cracks opening on the 20th of June path. Weak snow at the ground could allow the 85 cm of firmer snow above to fail as a Glide avalanche if thoroughly wetted. While he found other areas with glide cracks, they were not widespread. The snowpack on solar aspects is becoming patchy, especially below treeline. 

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.