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
A major storm has created very dangerous avalanche conditions. Avoid all areas where avalanches may start, run, and stop. We are expecting large natural avalanches to occur Saturday as additional precipitation impacts the area. Some of these avalanches may run downslope and cross common travel routes.
Summary
Detailed Forecast
Very dangerous avalanche conditions are expected Saturday. Stay out of all avalanche terrain, including areas where avalanches may start, run, and stop. Avalanches may impact common travel routes that cross avalanche tracks and runouts. Spring weather and snowpacks can be tricky. General rules-of-thumb used during the winter often do not apply. With the amount of recent new snow and more on the way, this is a time to step way back.
Fluctuating temperatures and strong wind during the storm created an “up-side-down” snowpack with heavier strong snow over lighter weaker snow. This is a classic strong over weak slab avalanche scenario. You are likely to find this snowpack structure in all avalanche startzones above the rain line. Storm slabs may extend into the upper below treeline zone depending on air temperatures and precipitation type.
At lower elevations, expect wet surface snow conditions. New rollerballs, fresh fan shaped avalanche debris, and rain on snow are all key signs that loose wet avalanches are likely. Some loose wet avalanches this week have entrained large amounts of old wet snow and traveled father than normal.
In locations receiving less snow, such as in the Crystal backcountry, avalanche danger will be slightly lower. However, do not underestimate these avalanche problems. Although potentially smaller you will be able to trigger wind and storm slabs at higher elevations and loose wet avalanches at lower elevations Saturday.
Other spring time avalanche hazards exists. Glide cracks have formed on some slopes. Glide avalanches are difficult to predict and can release at any time. Limit your exposure to any slope with glide cracks. Creeks are beginning to open up at lower elevations. Choose your travel routes wisely if you are planning to over larger creeks
Snowpack Discussion
Over a foot (30cm) of new snow has fallen in most locations as of Friday afternoon and more is expected overnight. Wind redistributed the new snow at higher elevations created obvious new snow drifts in many areas. At lower elevations, snow transitioned to rain late Friday afternoon. This new snow is falling on a melt-freeze crust formed and buried on 4/11. The strength and thickness of this crust is largely dependent on elevation.
Mild weather earlier this week allowed the upper snowpack to gain strength. Observations show the snowpack is comprised of several melt-freeze crusts, refrozen snow, and strong rounded grains. This has created a strong spring-like snowpack. Avalanches should be limited to the snow above the most recent (4/11) crust layer
Observations
Snoqualmie
On Friday, Alpental ski patrol reported several feet of new snow at the top of Alpental since Sunday afternoon. Winds had transported the recent snow forming uneven snow depths in exposed areas.
NWAC staff in the Alpental Valley Friday experienced the snow transition to rain mid-afternoon and observed several new rollerballs as the new snow became loose and wet.
South
No recent observations.
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
