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Avalanche Forecast

Archived

Apr 11th, 2017–Apr 12th, 2017

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

Regions

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Watch for fresh wind slabs on lee slopes in steep terrain mainly above treeline. Sun breaks can quickly create loose wet avalanche conditions on steep slopes facing the sun. Give cornices a wide safety margin.

Detailed Forecast

An approaching front will move across the Cascades late Tuesday night and early Wednesday with SE-SW winds.

Light to briefly moderate snow and strong winds will likely build fresh wind slabs on lee slopes below ridges, near and especially above treeline by Wednesday. 

Watch for new wind slabs, mainly on NW-SE slopes above treeline. Keep an eye out for firmer wind transported snow that is a sign of wind slab.

Strong winds and new snow will continue to build and grow recent large cornices. There have been numerous recent cornice failures with some being very large in the WA Cascades. 

Recent cornices are very large and resulting slab avalanches are dangerous and unpredictable. Five people were tragically killed by a cornice release in BC on Saturday. Give cornices a wide berth if traveling along ridge-lines and avoid slopes below large cornices. See a blog post regarding cornices here.

Snowpack Discussion

Weather and Snowpack

Warming and rain in mid March has left behind a well consolidated old snowpack with one or more strong melt freeze crusts in the upper snowpack.

A series of strong spring storms was seen last week. For the 4 days ending Saturday morning NWAC stations along the east slopes had about 0.5-1.5 inches of water equivalent. Most or all of this fell as snow near and above treeline.

A low pressure system moved north along the coast on Friday. Very strong winds built large wind slabs and fresh cornices during sustained SE-SW winds averaging over 35 mph with gusts over 80 mph! These winds were so strong that wind slabs were less widespread and formed lower on leeward slopes than typical.

A calmer, cooler pattern was seen Saturday and Sunday with possible some light amounts of new snow at higher elevations east of the crest. A period of fair weather and sunshine or filtered sun Sunday allowed for more consolidation and some surface snow melt on all but steep shaded slopes in higher terrain.  

A front and upper trough crossed the Northwest on Sunday night and Monday, depositing up to a few inches of snow. Moderate winds built fresh wind slabs above treeline on steep lee terrain features.

Sunny weather Tuesday helped storm snow to begin to settle, however, winds remained strong enough above treeline Tuesday to continue to transport surface snow, maintaining areas of wind slabs. 

Recent Observations

North

No recent observations.

Central

NWAC pro-observer Tom Curtis was at Blewett Pass on Friday and above about 5000 feet reported a thin crust over moist to wet rounded poly crystals with increasing density with depth and no current avalanche problems. Most SW-W slopes had little snow.

South

No recent observations.

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.

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.