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Avalanche Forecast

Archived

Mar 3rd, 2014–Mar 4th, 2014

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

Regions

Stevens Pass.

Avoid avalanche terrain especially at higher elevations on Tuesday.

Detailed Forecast

Three day snowfall ending Tuesday morning should range from 1-4 feet near and west of the crest. West southwest flow aloft should continue to carry warm front moisture to the Northwest. This should cause rain or snow showers with rising snow levels on Tuesday.

Wind slab will be a likely concern mainly on lee slopes at higher elevations. This should be mainly north to southeast aspects. Watch for signs of firmer wind transported snow.

Storm slab should also remain likely above the snow line on Tuesday on a wider variety of aspects. Wetter denser new snow accumulating over lower density snow will contribute to this concern.

Below the snow line wet loose avalanches will be the primary concern. Steep slopes that received heavy snowfall and then receive rain will be primed for wet-loose avalanches. Natural snowballing and smaller natural releases are usually precursors to this type of avalanche.

Deep persistent slab will remain an unpredictable concern and snowpit tests are not practical. Triggering such an avalanche is unlikely but the consequences could be catastrophic. The best way to deal with this concern for the time being would be to avoid large open avalanche terrain. 

Snowpack Discussion

Recent Northwest Weather

An extended mild and dry period occurred the latter half of January generally forming a crust in all areas.

Very cold weather followed in early February which produced light low density snowfall and facets just above the late January crust.

A 2 week storm cycle ended a week ago causing 7-12 feet of snowfall and avalanche cycles near and west of the crest.

Warm dry weather was seen late last week Wednesday to Saturday causing wet snow avalanches and melt/freeze crusts throughout the region. Surface cold air also redeveloped east of the crest last weekend.

Washington Cascades near and west of the crest

Another storm cycle began on Sunday. An avalanche cycle was seen on Sunday night and Monday morning. DOT crews at Stevens and Snoqualmie reported widespread natural 1-2 foot avalanches Sunday night and Monday morning which closed Snoqualmie Pass. Poor bonds to the melf-freeze crust from late last week have been reported with the crust probably providing bed surfaces. Our NWAC observer Jeff Hambelton reported less active recent snow layers from Mt Baker Monday afternoon.

The late January crust layer and overlying facets are now very deeply buried and have been getting tested by the recent very heavy snowfall. Large avalanches in the Stevens and Snoqualmie areas have been seen over the past week. The Stevens Pass ski patrol produced this impressive video of explosive control producing a 10-15 foot avalanche, you might want to turn down your speaker volume first. A large natural hard slab avalanche with a 10 foot crown released off of steep north facing terrain of Chair Peak near Alpental Saturday and caught 3 skiers. Here is a link to more information about deep persistent slabs which are unusual in our area and not predictable.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.