Avalanche Forecast
Issued: Mar 14th, 2018 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet, Wind Slabs and Deep Persistent Slabs.
NWAC,
Northwest Avalanche Center
You may be able to trigger avalanches Thursday in specific locations such as steep rock sunny slopes or wind deposited features. Take time to identify and avoid areas in the terrain where these avalanches may occur. A low likelihood-high consequence avalanche problems continues to be found in the snowpack. Smaller avalanches or other large triggers may step down into these older deep weak layers creating destructive avalanches. You can avoid this difficult to predict problem by avoiding steep large open terrain.
Summary
Detailed Forecast
Expect wet unstable surface snow to occur as the sun comes out Thursday. This will result in new roller balls, pin wheels, and loose wet avalanches. These conditions will occur first on steep sunny slopes and move from eastern to southern to western aspects as the day continues. Stay off of slopes greater than 35 degrees where you see signs of wet surface snow. Even though loose wet avalanches can seem common and are often small, they may carry you into terrain with high consequences such as over cliffs or into gullies.
You may be able to trigger wind slabs above treeline. Use simple visual clues to locate and avoid slopes where winds have recently deposited snow. Snow drifts, wind textured snow, and firm snow surfaces with cracking all indicate that wind loading has occurred on nearby slopes. You will be most likely to trigger a wind slab on steep convex rollovers and on unsupported slopes.
A low likelihood-high consequence avalanche problem continues to exist in the snowpack. This weak layer resulted in a fatality on March 10th. While it may be difficult for you to trigger this deep persistent layer, small avalanches or other large triggers may step down and trigger a very large avalanche. Remember tracks on a slope do not mean it is safe. You can avoid this difficult to predict weak layer by simply staying out of large avalanche startzones.
Cornices may grow weak and fail during day time warming and periods of direct sunshine. Select travel routes and techniques that limit your exposure to cornices above you.
Snowpack Discussion
Generally 4-6 inches of snow fell in most locations in the western Cascades Tuesday night and Wednesday. At higher elevations southerly winds transported the snow forming new wind slabs on lee slopes. In areas where the sun came out Wednesday, moist to wet surface snow quickly formed. As a result numerous loose wet avalanches were seen in the Snoqualmie Pass area. Om other slopes cold dry surface snow exists.
Observations Wednesday showed the new snow bonded well to the old snow surfaces. Below the most recent snow a new crust layer is gaining strength as it freezes. As of Wednesday this new crust was not fully supportable.Current reports indicate that no new persistent layers were buried Tuesday night.
Below the most recent crust, a variety of melt-freeze layers and dry snow have been seen.
A layer of weak sugary facets have been observed just above a very firm and wide spread layer (2/8). This layer is typically 3-5 feet below the snow surface. These facets were buried along the western region on February 13th and have been the source our deep persistent slab problem. This layer resulted in a fata avalanche on Park Butte near Mt Baker last weekend.
No significant layers of concern exist below the 2/8 crust.
Observations
Baker
On Monday and Tuesday, Mt Baker ski patrol reported a natural large slab releases on a variety of aspects from SW-W-
On Sunday, NWAC professionals near the Park Butte accident site observed the 2/8 crust i down 4 ft (1.2 m) with a layer of facets above the crust. Recent wind effect was noted throughout the immediate terrain.
Snoqualmie
NWAC professional observer Ian Nicholson traveled in the Snoqualmie area Wednesday. Ian found 6 inches of new snow with no evidence of wind transport belwo 5200'. He reported several loose wet avalanches up to size D2 from steep rock terrain. These avalanches occurred during periods of sunshine.
South
An avalanche professional in the Paradise area Monday found the 2/8 crust 5 feet below the snow surface.
Problems
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.
Aspects: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood
Expected Size
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: North, North East, East, West, North West.
Elevations: Alpine.
Likelihood
Expected Size
Deep Persistent Slabs
Release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage. They commonly develop when Persistent Slabs become more deeply buried over time.
Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.
A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.
Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Mar 15th, 2018 11:00AM
