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Avalanche Forecast

Archived

Mar 4th, 2014–Mar 5th, 2014

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

Regions

Stevens Pass.

An avalanche warning means avoid avalanche terrain at all elevations on Wednesday.

Detailed Forecast

A warm front should cause more moderate to heavy rain or snow at higher snow levels Tuesday night and Wednesday morning. Four day WE and snowfall ending Wednesday morning should range from 4-6 inches and 2-5 feet respectively near and west of the crest. This is generally heavy snow and lots of water for 4 days.

Due to rain reaching higher elevations and further warming the main concern should further increasing wet loose avalanches on Wednesday. The rain and warming should further load and weaken recent upper snowpack layers. This should be at least up to the near treeline zones. Steep slopes that recently received heavy snowfall should be primed for wet loose avalanches. Natural snowballing and natural releases are usually precursors to this type of avalanche.

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

Storm slab will also be listed as a concern above treeline on a wider variety of aspects. Wetter denser new snow accumulating over lower density snow in the above treeline zone will contribute to this concern.

The continued loading by snow and rain will further test the deep persistent slab from late January and make it an unpredictable concern for which 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 should be to avoid large open avalanche terrain. 

Snowpack Discussion

Recent 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 avalanches 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

The latest 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 and triggered 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.

Snow levels have been rising and temperatures warming with increasing instability and avalanches on Tuesday afternoon. The Alpental ski patrol reports widespread easy to trigger 8 inch storm slab avalanches in the upper part of the area and widespread easy to trigger wet loose avalanches in the lower part of the area.

The late January crust and facet layers remain a concern and are now very deeply buried and have been getting tested by the recent very heavy snowfall. Large avalanches have been seen at Stevens and Snoqualmie the past week. The Stevens Pass ski patrol produced this 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 on a steep north slope on Chair Peak near Alpental Saturday 22 February 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

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.

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.

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.