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
Expect dangerous avalanche conditions at all elevation as significant precipitation continues to impact the area. Avoid all slopes greater than 35 degrees Saturday and limit your exposure to large overhead avalanche terrain.
Summary
Detailed Forecast
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 good time to be cautious. Avoid all slopes greater than 35 degrees and limit your exposure to large overhead avalanche terrain.
You will be able to trigger large wind slab avalanches Saturday on steep wind loaded slopes near and above treeline. Use visual clues such as blowing snow, new drifts, or fresh cornices to identify and avoid wind loaded features.
Fluctuating temperatures 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 sheltered avalanche startzones above the rain line. Storm slabs will be easiest to trigger in the morning as strong overnight precipitation tapers off. 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.
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 as of Friday afternoon with more on the way overnight. Moderate to strong winds during the storm transported snow forming wind slabs near and above treeline. 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
North
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.
NWAC professional observer Lee Lazzara traveled in the Baker backcountry Thursday. Lee found 8 inches (20cm) of new snow well bonded to the old snow surface. While Lee did see wind slabs, no wind slab activity was reported. Warming air temperatures and thin clouds resulted in several small (D1) loose wet avalanches in the afternoon.
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.
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: All elevations.
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
