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Avalanche Forecast

Archived

Apr 10th, 2015–Apr 11th, 2015

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

Regions

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Keep your terrain selection conservative Saturday: Human triggered slides are likely on wind loaded aspects near and above tree-line, which also happens to be the elevation bands with solid snowcover. Shower intensity and sunbreaks will locally mix in loose wet and storm slab avalanche potential. Choose more moderate and less wind affected terrain and safely enjoy a taste of winter. 

Detailed Forecast

Cool mid-April storms in a snow starved season can be a recipe for accidents: Enjoy a taste of winter Saturday but keep your terrain selection conservative. Human triggered avalanches are likely on wind loaded aspects near and above tree-line. Shower intensity and sunbreaks will locally mix in shallow loose wet and storm slab avalanche potential.  

A vigorous frontal system passing through Friday night will bring a sharp drop in snow levels by early Saturday morning.  Light and scattered post frontal showers Saturday morning should become more widespread in the afternoon. The heaviest showers are likely near the crest and downwind of a Puget Sound convergence zone. 

Despite the strong cooling trend that will help shallow new storm snow bond well to Friday night's snowfall, moderate and consistent westerly transport winds will continue to load lee aspects near and above treeline throughout the day.

Even with the winter-like temperatures, mid-April sunbreaks in the late morning or mid-day can trigger loose wet avalanches involving the new storm snow on solar aspects.  

Cornices won't be highlighted as an avalanche problem due to the cool weather but use caution on slopes below cornices and on ridges where it can be hard to know if a large cornice is present.

Snowpack Discussion

A broad upper trough moved over the Northwest last weekend through early this week causing periods of very light snow east of the crest at low snow levels. On Wednesday and Thursday, clear skies transitioned to scattered afternoon showers with light new snow accumulations. 

Reports via TAY and from NWAC observer Jeremy Allyn around Washington Pass last weekend indicate shallow powder over crusts on north slopes, corn snow and crusts on solar slopes and no signs of instability. Greater recent storm snow was being preserved west of Rainy Pass, where about 10 inches of unconsolidated powder was seen on shaded terrain at higher elevations, providing a bit of welcome powder skiing!

Snowdepths vary greatly across the east slopes with a regionally healthy snowpack in the northeast Cascades to bare solar and lower elevation slopes in the central and southeast Cascades. Many areas at lower elevations and further away from the Cascade crest do not have enough snow to cause an avalanche danger.

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.

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.