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Avalanche Forecast

Archived

Apr 11th, 2015–Apr 12th, 2015

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

Regions

Stevens Pass.

Human triggered slides are likely on wind loaded aspects above tree-line. Increasing sun or filtered sun may quickly destabilize recent snow on sun exposed slopes. Choose more moderate and less wind affected terrain and safely enjoy a taste of winter. 

Detailed Forecast

Showers should continue overnight Saturday, then end early Sunday with partial clearing expected. Freezing levels should remain low with moderate westerly ridgetop winds. This should continue to build areas of wind slab on lee slopes in the upper elevations as well as build cornices along ridges.  

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 winter-Iike frontal passage Friday night was followed by cool showers in moderate to strong westerly winds Saturday. This has deposited from about 6-12 inches of new snow as of Saturday afternoon. Reports Saturday from the Chinook Pass area identified extensive new cornice formations along ridges, but no new avalanches were seen with several ski tour parties in the area. The new snow was reportedly fairly well bonded to the old snow surface.

Backcountry reports from across the Cascades earlier this week near and above treeline indicate cool, preserved snow on non-solar aspects with limited wind effects (see exception below), gradual settlement and generally good bonds to the previous crust.  On solar aspects, sun breaks have caused only small loose wet avalanches and crusts. Some large cornices continue to be reported along ridges, especially in the north Cascades. 

Snowpack problems west of the crest should remain in the most recent storm snow or near surface layers. The mid and lower snowpack west of the crest consists of layers of stable consolidated rounded grains or melt forms and crusts from multiple warm periods this season. Many areas at lower elevations 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.

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.