Avalanche Forecast
Issued: Apr 13th, 2018 12:07PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Additional precipitation in the forecast will maintain dangerous avalanche conditions in the Cascade East North zone Saturday. Avoid open slopes greater than 35 degrees near and above treeline where you will be most likely to trigger a slab avalanche. At lower elevations stay off of steep slopes during periods rain or if you see signs of other loose wet avalanches.
Summary
Detailed Forecast
Significant precipitation and wind Friday night into early Saturday will continue to build wind and storm slabs at higher elevation. You will be able to trigger wind or storm slabs on open slopes greater than 35 degrees. Avalanches will likely be deeper in areas closer to the Cascade crest. Use visual clues such as blowing snow, fresh cornices, and uneven snow surfaces to identify and avoid wind loaded terrain. Fluctuating temperatures during the storm have created an “up-side-down” snowpack where heavy stronger snow sits on top of lighter weaker snow. This is a classic strong over weak slab scenario. This snowpack layering will exist in all avalanche startzones that are not stripped by the wind.
At lower elevations rain showers will create wet surface snow conditions. If you see new rollerballs, observe fan shaped avalanche debris, or experience prolonged rain, expect loose wet avalanches on slopes greater than 35 degrees. Some recent loose wet avalanches have run significant distances and entrained large amounts of snow. Even small loose wet avalanches can harm you if they carry you into terrain with higher consequences.
If the sun comes out or the clouds thin, snow conditions may change very quickly making it easier to trigger avalanches.
Several spring hazards exists in the mountains. Limit your exposure to large overhead cornices which may fail during periods of warming and rain. Creeks around the area are beginning to open as snow bridges thin and weaken. Choose travel routes carefully if you plan on traveling over larger creeks.
Snowpack Discussion
New snow began to accumulate in the Washington Pass area Friday afternoon. Light winds likely limited the initial development of new wind slabs in all but the most exposed locations. Washington Pass stayed just below freezing Friday afternoon maintaining snow at Pass level.
Generally warm weather over the earlier in the week allowed the upper snowpack to gain strength. Several freeze and thaw cycles likely created a strong surface crust in many locations.
We have very limited information about older persistent weak layers in the Washington Pass area. You are most likely to find a reactive persistent weak layer on shaded aspects or at higher elevations above recent rain events. If you have any information on snowpack conditions consider sharing your observations with NWAC on our public observations page.
Two common persistent weak layers seen in the Northeast Cascades are:
- 3/25 and 3/22: Buried surface hoar layers found on shaded aspects near and above treeline.
- 2/13 Facets above a firm and thick melt-freeze crust can be found in many locations near and above treeline. This layer is thought to be dormant at this time but may re-awaken with a substantial weather input.
Lower elevations further east of the crest have melted back for the season and no longer present an avalanche risk.
Observations
On Friday, WA DOT reported several avalanches 3.5 feet deep near Cutthroat Peak as they cleared Highway 20. Although the exact weaklayer was not confirmed, the depth suggests an older persistent weak layer from mid-february.
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: All aspects.
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: Alpine, Treeline.
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.
Elevations: Below Treeline.
Likelihood
Expected Size
Valid until: Apr 14th, 2018 12:07PM
