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
This weather pattern isn’t letting up. Expect dangerous avalanche conditions in the backcountry Sunday. Avoid open slopes greater than 35 degrees at all elevations and limit your exposure to areas where avalanches may run and stop. If the sun comes out or the clouds thin, expect conditions to change quickly causing natural avalanches to occur.
Summary
Detailed Forecast
You will be able to trigger avalanches on slopes greater than 35 degrees Sunday. Avoid steep open slopes and limit your exposure to areas where avalanches may run and stop. Significant precipitation and wind continues to impact the area maintaining dangerous avalanche conditions. Weather and snowpacks can be tricky as they transition from winter to spring. Some of the rules-of-thumb we use during the winter may not apply. This is the time to dial it back and be cautious.
If the sun comes out or the clouds thin, conditions will change quickly. Expect natural avalanches to occur from steep terrain. These avalanches may entrain significant amounts of snow, grow large, and travel long distances downslope.
You can use simple observations Sunday to identify the avalanche problems as you travel. Snow drifts, uneven surfaces, and fresh cornices all indicate wind slabs exist in nearby terrain. New rollerballs, fresh fan-shaped debris, or rain on snow mean you can trigger loose wet avalanches on similar slopes. Strong surface snow over weaker snow (up-side-down) signifies storm slabs may be triggered on convex rollovers or unsupported slopes. Use conservative decisions as you travel.
In locations receiving less precipitation, avalanche danger will be slightly lower. However, do not underestimate these avalanche problems. Although potentially smaller you will be able to trigger wind slabs at higher elevations and loose wet avalanches at lower elevations Sunday.
Other springtime hazards exist in the mountains. Cornices have grown large all winter and are beginning to sag with the recent warming. Glide cracks have been observed on steep smooth slopes. Creeks have are opening as snow-bridges become thin and fail. Use appropriate travel routes and techniques to limit your exposure to these potential hazards.
Snowpack Discussion
Significant precipitation has impacted the area over the last several days. Water amounts of 2 to 4” have been seen at many Cascade West weather stations since Wednesday. At higher elevations, this has translated to substantial snow totals. Below the rain line, this has created a snowpack of weak wet grains.
Several natural avalanche cycles have occurred. Some loose wet avalanches have gouged down into older snow and run further downslope than normal. Observations at higher elevations have been very limited due to poor visibility and difficult conditions.
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
North
On Saturday, NWAC staff traveled in the Mt Baker backcountry. They found 2 feet (60cm) or more of recent storm snow over the 4/11 melt-freeze crust. Below 5000' moist to wet surface snow produced numerous rollerballs suggesting the potential for loose wet avalanches to occur.
On Friday Mt Baker Ski Patrol reported wind slabs up to 16” (40cm) deep near treeline. Loose wet avalanches were observed below 4500 feet despite lower snow levels.
Stevens
NWAC forecaster Dallas Glass traveled in the Stevens Pass area Wednesday. Dallas found the upper 5 feet (1.5m) of the snowpack to be generally strong. He did not find any significant layers of concern below the wet surface snow.
Snoqualmie
On Saturday NWAC staff traveled in the Alpental Valley. Numerous natural loose wet avalanches had occurred at lower elevations. Some of these avalanches were large enough to bury, injure, and kill a person. Around 2 feet (60cm) of recent storm snow was observed over the 4/11 crust. Light rain was observed up to over 4000 feet and the freezing level was rising as the team exited the field.
Alpental Ski Patrol, Saturday, reported large loose wet avalanches that gouged into older wet snow producing large avalanches. Many of these avalanches were running far and fast.
South
On Saturday Crystal ski patrol reported no appreciable precipitation. Warm air temperatures had created loose wet avalanche conditions in steep terrain.
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
