Avalanche Forecast
Issued: Apr 14th, 2018 11:18AM
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
In areas where you find cold dry snow, use visual clues such as fresh cornices and snow drifts to identify and avoid wind loaded slopes greater than 35 degrees. At lower elevations expect wet surface snow conditions due to warm air temperatures and rain. Use caution if you travel on steep open slopes at lower elevation where small avalanche may carry you into terrain with large consequences.
Summary
Detailed Forecast
A relative break in the weather Sunday will allow recently formed avalanche problems time to heal. However, do not let your guard down. This winter storm has added new layers to our snowpack. Springtime weather and snowpacks can be tricky as they go through their transition from winter to spring. Rules-of-thumb we often use during the winter don’t always apply. If the sun comes out or clouds thin, expect snow conditions to changes quickly. Loose avalanche may occur naturally from steep terrain and slab avalanche may become easier to trigger.
New wind slabs have formed at higher elevations. They may be tough to trigger but a resulting avalanche may be large enough to injure or kill you. Use visual clues such as frim surface snow, drifts, and fresh cornices to identify and avoid wind loaded slopes greater than 35 degrees.
Changes in temperature during the storm have likely created shallow storm slabs on sheltered slopes. Use caution if traveling in areas with more than 6” (15cm) of recent snow. Take time to consider the consequences of the terrain. Would a small avalanche take you off of a cliff or into a gully? If so, would another route be better?
At lower elevations, expect wet surface snow conditions. New rollerballs, fan shaped avalanche debris, and periods of rain on snow all suggest that you are likely to trigger a loose wet avalanche on similar slopes. Don’t underestimate the power of even small loose wet avalanches. They may carry you into or over slopes where even small avalanches can have large consequences.
Other springtime hazards are present in the mountains. Cornices have begun to droop. Glide cracks have formed on steep smooth slopes. Creeks are opening at lower elevations. Use caution if you travel near or below any of these hazards.
Snowpack Discussion
Around 6-8” (15-20cm) of new snow has fallen near Washington Pass over the last few days. Air temperatures reached above freezing at Pass level for much of the day Saturday. Moderate southwest winds likely transported snow near ridgelines forming new wind slabs on lee slopes
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
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: Treeline, Below 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
Valid until: Apr 15th, 2018 11:18AM
