Avalanche Forecast
Issued: Apr 11th, 2015 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Loose Wet and Storm Slabs.
NWAC,
Northwest Avalanche Center
Keep your terrain selection conservative Sunday: Human triggered slides are likely on wind loaded aspects 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.
Summary
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 some areas of shallow 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
Continued winter-like freezing levels and a frontal passage Friday night, followed by showers Saturday have deposited 2-6 inches of new snow as of Saturday afternoon. Moderate winds have likely transported new snow at higher elevations to build both wind slab and new cornices along ridges.
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
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: North, North East, East, South East.
Elevations: Alpine.
Likelihood
Expected Size
Loose Wet
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. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. 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.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: East, South East, South, South West, West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Apr 12th, 2015 11:00AM
