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Avalanche Forecast

Archived

Mar 19th, 2015–Mar 20th, 2015

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

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Loose wet avalanches are unlikely but still possible in isolated terrain features, especially on steeper solar slopes above treeline that have not already released. Also watch for isolated pockets of shallow wind slab on lee slopes above treeline.

Detailed Forecast

An approaching frontal system should cause a general increase in clouds and winds throughout the day, but significant precipitation is unlikely until Friday night for the central and south Washington Cascades. 

Loose wet avalanches are unlikely but still possible in isolated terrain features, especially on steeper solar slopes above treeline that have not already released. Also watch for isolated pockets of shallow wind slab on lee slopes above treeline. 

Less recent snow and ample terrain anchors should greatly limit the avalanche danger below treeline. 

Remember the NWAC forecast applies to elevations up to the Cascade crest (~7000 to 8000 ft). Significant new snowfall was received above these elevations on the volcanoes over the weekend and this likely poses a higher avalanche danger.  

Due to the low snowpack, especially below treeline, watch for terrain hazards such as open creeks, partially covered rocks and vegetation. Many areas below treeline do not have enough snow to cause an avalanche danger.

Snowpack Discussion

Last weekend another warm and wet system brought a one-two punch to the Pacific Northwest. The initial system Saturday brought 1.5 - 2.5 inches of rain with the snow line around 6000-7000 ft. On Sunday a second low pressure system deposited about 8-16 inches of snow above 4500 ft in the north Cascades and above about 6000 ft in the south Cascades. Significantly less new snow was received below these elevations. On Tuesday, a weak front dropped 1-3 inches of snow above 5-6000 ft in the south Cascades. Light to occasionally moderate snowfall fell in the north Cascades above 5000 ft with moderate southwest transport winds. 

Natural or skier triggered slides that occurred Sunday or Monday were generally loose wet size D1-D2, and to a lesser extend storm slabs, and reported throughout the west slopes of the Cascades relative to last weekend's snowline. The few inches of new snow from Tuesday were reactive to daytime warming/sunbreaks on Wednesday allowing for easy ski-triggering or natural point releases on steeper solar slopes.  

The Chinook DOT crew on Monday found debris from an natural cycle of loose or storm slab avalanches from Sunday.

 

Evidence Monday, March 16th of storm avalanches on Sunday on Naches Peak. Photo J. Stimberis.

The snowpack at low elevations remains meager to non-existent with the average snow-line around 4500 feet along the west slopes. 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.

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.