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Avalanche Forecast

Archived

Feb 4th, 2016–Feb 5th, 2016

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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A mixed bag of ugly snow conditions and a high avalanche danger in the near and above treeline elevation bands is expected along the west slopes on Friday. Backcountry travel in avalanche terrain especially in the near and above treeline bands is not recommended on Friday.

Detailed Forecast

The next weather system will move strongly to the Olympics and north Cascades Friday late morning and spread to the rest of the Cascades by Friday evening with fairly high snow levels.

A broad brush will be applied to the avalanche forecast for Friday. Expect a mixed bag of ugly snow conditions and a high avalanche danger in the near and above treeline elevation bands along the west slopes.

Wet snow and rain or warm temperatures should further build or activate wind and storm slab mainly in the above tree line band.

Recent snow will be available for activation by rain in the north Cascades and by warm temperatures elsewhere for potentially large loose wet avalanches. Watch for surface wet snow deeper than a few inches and pinwheels or small natural loose wet avalanches that usually precede large loose wet avalanches.

Wet slab avalanches can't be ruled out in the north Cascades due to rain but this won't be added to the growing list problems for Friday.

But new snow and cooler temperatures should be seen by Saturday.

 

Snowpack Discussion

Weather and Snowpack

Two heavy rain events, one about January 21st and another January 27-28th stabilized the mid and lower snowpack and formed a strong rain crust that is being loaded by recent storms.

A strong occluded front with strong winds crossed the Northwest last Friday and Saturday. NWAC stations along the west slopes received about 1-2 feet of snowfall. The strength of bonds to the rain crust varied depending on location along the west slopes.

Cool, benign weather followed Sunday through Tuesday. A sun crust formed on many solar slopes and surface hoar was seen on many non-solar slopes.

A cold front crossed the Northwest on Wednesday. A warm front is causing more snow mainly in the south Cascades today. NWAC stations along the west slopes for the 2 days ending Friday morning will have about 6-12 inches of increasing damp or wet upside down snow with nearly 2 feet at Paradise!

Recent Observations

The Alpental pro-patrol on Thursday reported some natural and ski triggered 6-8" storm slab on this morning. By this afternoon some triggered small loose wet avalanches were being seen below about 4000 feet.

Deeper, larger avalanches were reported from Paradise this morning. The ranger reported shooting cracks and certain, sensitive, reactive human triggered 15 inch storm slabs on 40-50 degree south facing test slopes that ran onto 35 degree slopes.

 

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.