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Avalanche Forecast

Archived

Mar 10th, 2016–Mar 11th, 2016

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
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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Wind slabs and cornices along ridges should be less sensitive Friday but still capable of producing large avalanches near and above treeline. The loose wet potential should be relegated to steeper slopes primarily below treeline but will have the potential to entrain heavy wet snow. Avoid terrain traps where even a small yet heavy loose wet avalanche would have serious consequences. You may not see natural avalanche activity Friday, but the human triggered potential remains elevated. 

Detailed Forecast

Clouds should spread over the Cascades from south to north on Friday as moisture rotates northward and a weak frontal band offshore approaches. Light precipitation should develop during the afternoon hours, specifically in the south Cascades. Snow levels should remain moderate Friday, rising along with daytime warming. 

All the travel advice associated with Considerable danger will be pertinent on Friday. Strong winds and the cross-loading of slopes Wednesday night and Thursday will require cautious route-finding and conservative terrain selection Friday. Wind slabs and cornices along ridges should be less sensitive Friday but still capable of producing large avalanches in specific areas near and above treeline.

The loose wet potential should be relegated to steeper slopes primarily below treeline Friday but will have the potential to entrain heavy wet snow. Avoid terrain traps where even a small yet heavy loose wet avalanche would have serious consequences. There continues to be a lot of water in our snowpack, so don't underestimate loose wet avalanches in the wrong terrain. 

Powerful glide avalanches remain possible in isolated terrain features such as on steep slopes with smooth bed surfaces and especially where the slabs are unsupported from below.

 

 

Snowpack Discussion

Weather and Snowpack

The nonstop active weather pattern continues to push a storm system through the PNW almost every day or two with fluctuating but generally moderate snow levels. The last storm impacted the area Wednesday through Thursday with a warming trend that peaked Wednesday night with the frontal passage in the early morning hours and also featured very strong winds seen throughout the range. Winds decreased Thursday afternoon along with decreasing shower activity. Generally 1.5 to 3 inches of water accumulated along the west slopes in the 36 hours ending 4 pm Thursday. Outside the Cascade Passes, rain reached up to 5000' feet in the north and 6500 feet in the south with snow levels only rising to about 4000 feet at Stevens and 4500 feet at Snoqualmie Pass.  About 4-14 inches of snow accumulated at NWAC stations through Thursday morning. 

There remains a significant amount of water in the snowpack. There are several crusts in the upper snowpack interspersed with moist or wet grains. The mid and lower snow pack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.

Recent Observations

Stevens Pass and Alpental Pro Patrol reported an extensive natural cycle Wednesday night. Control results were sensitive and widespread Thursday AM involving new storm snow. As the sun poked out and temperatures rose Thursday, large loose wet avalanches, both natural and skier triggered were reported by the pro patrol in the Stevens area with smaller loose wet noted in the Alpental area. Crystal patrol generally had 1-2 ft slab releases during control work Thursday with large and sensitive cornices along ridgelines. 

In areas that experienced rain Wednesday night, mainly outside the Passes and in the below and near treeline bands, a breakable crust made for tough skiing Thursday. 

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.

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.

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.