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Avalanche Forecast

Archived

Mar 3rd, 2016–Mar 4th, 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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Conservative terrain selection will be essential near and above treeline as human triggered avalanches are still likely. Choose moderately angled terrain and avoid wind loaded slopes. Avoid travel on or below overhanging cornices. Watch for wet snow conditions below treeline. 

Detailed Forecast

A mostly dry but cloudy day is in store Friday with warming and increasing winds. A period of light rain and snow at rising freezing levels is expected again late Friday along with strong S-SW  ridgetop winds. New storm related danger will continue Friday, especially near and above treeline as strong southwest wind loads lee slopes building new wind slab layers. Wind slabs will build on lee slopes, facing generally NW-NE.  Earlier formed wind slabs may still be reactive Friday and may add to the increasingly complex snow structure, by Thursday. Large cornices should form near ridges as well, requiring travelers to avoid overhead hazard.

Conservative terrain selection will be essential near and above treeline to avoid wind loaded terrain and features. Choose moderately angled terrain and avoid wind loaded slopes, choose the terrain void of overhead hazard such as wind slabs or cornices.

Non-avalanche hazard: Despite our seasonally, healthy snowpack, many creeks are open and difficult to cross due to the periodic warm temperatures and rain events.  

Snowpack Discussion

Weather and Snowpack

Very mild weather a week ago allowed for the formation of melt-freeze crusts, especially on solar slopes in most areas by Friday. Also during this period, wide-spread surface hoar formed in the NE Cascades, mainly surviving outside of steeper solar aspects and wind affected terrain above treeline. 

A weak front brought light amounts of rain and snow Friday night to Saturday morning, burying the surface hoar layer in the Washington Pass area by 2/27. 

A Pacific frontal system passed the Cascades midday Sunday. A few inches of snow accumulated through the early afternoon above 3-4000 feet near the Cascade crest with rapid drying and very little accumulation further east. Alpine winds were strong with significant W-SW transport winds. By Monday morning, 2-10 inches of new snow had accumulated along the east slopes accompanied by a cooling trend. 

Another strong frontal system brought 8-20 inches of snow along the east slopes Tuesday through early Wednesday along with a slow warming trend. Rain may have pushed as high as 4500-5000 feet along the central and northeast Cascades Tuesday afternoon before cooling Wednesday morning.  

The active weather continued Wednesday depositing another 3-6 inches of snow by Thursday. 

In the NE zone near treeline there are about 2 feet of snow now over the 2/27 interface as of Thursday afternoon, providing some fantastic ski and riding conditions in areas away from wind effects.

We are no longer tracking any layers of concern formed earlier this winter in the mid or lower snowpack, due to lack of recent activity and confirming field observations. 

Recent Observations

NWAC pro-observer Jeff Ward was out near Washington Pass a week ago Thursday and found surface hoar up to 15 mm on non-solar slopes.  Jeff was out again last Friday and noted widespread 6-8 mm surface hoar on non-solar slopes. By Saturday, 2/27 the surface hoar was noted as buried intact in the Washington Pass area, specifically the Cedar Creek drainage. There has been daily travel in this zone this week with a few slides noted, involving this 2/27 layer, however, it remains difficult to tell how active or widespread this layer is at present, but important to be thinking about and looking for.

Observations from Thursday 3/3 in the NE zone indicated very strong winds near and above treeline were rapidly building wind slab with abundant cold snow available for transport. Several natural wind slab releases were observed Thursday on N-E aspects, producing impressive powder clouds, one being triggered by a cornice failure. It was not felt they stepped to a deeper layer, namely the 2/27 layer, as the largest crown was about 30 cm or 1 ft. The slides ran good distances and indicate how quickly conditions can change! 

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.