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Avalanche Forecast

Archived

Jan 11th, 2018–Jan 12th, 2018

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
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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Dangerous avalanche conditions will persist on Friday. Time and patience is required to let recent storm snow instabilities heal. Recent and fresh wind slab, potentially deep, will exist near and above treeline and may build onto open slopes below treeline. Loose wet avalanches are possible below treeline. All types of avalanches may become large by entraining recent storm snow. Travel conservatively Friday and avoid consequential terrain. 

Detailed Forecast

Snow levels should eventually rise to around 4000-4500 feet later Thursday night and Friday across the east slopes of the Cascades. Light to occasionally moderate showers seen through Friday morning should taper down quickly in the afternoon. Moderate post-frontal W-SW winds should taper down by Friday afternoon but continue to transport new and recent snow onto lee slopes near and above treeline. 

Dangerous avalanche conditions will persist on Friday. Time and patience is required to let recent storm snow instabilities heal. Recent and fresh wind slab, potentially deep, will exist near and above treeline and may build onto open slopes below treeline. Loose wet avalanches are possible below treeline. All types of avalanches may become large by entraining recent storm snow. Travel conservatively Friday and avoid consequential terrain. 

Despite the recent snow, early season hazards still exist. Many creek beds have still not filled in for the winter.

Snowpack Discussion

A strong storm system Wednesday night and Thursday generally brought 8"-14" of new snow to the east slopes of the Cascades with the lower amounts at Mission Ridge and Mazama, and the higher amounts at Holden, Lake Wentachee, Tumwater and with 20" at Berne through 6 pm Thursday. Moderate westerly winds persisted through the storm in the Mission Ridge area. Storm snow piled up quickly and many areas likely experienced natural storm slab avalanches primarily releasing within the new storm snow. Closer to the Cascade crest, natural avalanche cycles were observed at both Stevens and Snoqualmie Passes. 

This system follows on the heels of several inches of snow received Tuesday night/Wednesday near the Cascade crest. Moderate westerly winds late Tuesday through Wednesday re-distributed snow below treeline and exposed areas and particularly near and above treeline. In the central-east zone downwind of Stevens Pass, it exposed the most recent crust in some locations or formed pockets of wind slab nearby.

The supportive 1/5 crust formed from a widespread freezing rain event in the central-east zone, extending to the Stevens and Snoqualmie Pass areas. In the northeast Cascades stretching from Washington Pass to Holden, no freezing rain crust exists.

Snowdepth still decreases substantially the further east of the Cascade crest one travels. In many areas below treeline, there has not been enough snow to present an avalanche danger but Thursday's storm may have expanded potential avalanche terrain to lower elevations.  

Observations

North

No recent observations

Central

Older but still relevant observations:

A public observation from 1/5 in the Mission Ridge backcountry noted a 1/4" thick freezing rain crust up to 5000'. 

Basal facets (2-3mm) observed on 1/2 above 5000' on NW-N-E aspects and failing in snowpit tests in the Blewett Pass area (and likely the Mission Ridge area) will need to be watched when enough snow returns to this area to create a potential avalanche problem.  

South

No recent observations

Problems

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.

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.