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Avalanche Forecast

Archived

Mar 20th, 2015–Mar 21st, 2015

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.

Regions

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A spring mix of avalanche problems will be possible Saturday with shallow new wind slab concerns near and above treeline and loose wet possibilities on steeper solar aspects in the afternoon. 

Detailed Forecast

A frontal system passing through Friday night should bring light rain and snow along with a cooling trend by Saturday morning. Post frontal showers could locally be intense as the air mass destabilizes Saturday morning. Moderate W-SW winds will transport snow onto lee easterly aspects near and above treeline. 

Despite the cooling trend, there should be some shallow storm and wind slab near treeline by Saturday morning. New snow that does accumulate will be susceptible to potential sunbreaks later Saturday afternoon, so use extra caution on steeper solar slopes above treeline. Locally intense showers Saturday could contain graupel and quickly lead to sensitive shallow storm slabs. 

Less recent snow and ample terrain anchors should greatly limit the avalanche danger below treeline especially in the south and central east Cascades. 

Due to the low snowpack, especially below treeline, watch for terrain hazards such as open creeks, partially covered rocks and vegetation. Many areas below treeline do not have enough snow to cause an avalanche danger.

Snowpack Discussion

During the storm cycle the weekend of March 14-15th, 1 to 2 inches of water fell along the east slopes, with the snow level lowering to around 4500-5000 feet in the north and about 6000 feet in the south last Sunday. 15-18 inches of snow fell at the NWAC Washington Pass station and the Hart's Pass Snotel by Monday morning.

North Cascades Heli operations in the Washington Pass area reported small some loose dry avalanches near and above treeline earlier in the week along with some sensitive but shallow wind slab on lee aspects. NWAC observer Tom Curtis was on Nason Ridge near Stevens Pass on Wednesday and found ongoing possible loose wet avalanche conditions on solar slopes and lingering shallow wind slab on northwest to northeast slopes.

Benign weather and milder temperatures have aided storm snow settlement and promoted a general stabilizing trend this week.  

Snowdepths vary greatly across the east slopes with a regionally healthy snowpack in the northeast Cascades to bare solar and low elevation slopes for the central and southeast Cascades.

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.

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.