Avalanche Forecast
Issued: Mar 26th, 2018 12:03PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
The Avalanche Danger will rise to High by Monday afternoon. Be prepared to avoid avalanche terrain near and above treeline as the danger increases. Watch for natural avalanche activity as a sign that the danger is on the rise. You can stay safe by avoiding avalanche terrain with recent snow drifts, cornices, and firm snow that produces cracking. Stay away from areas where large, steep slopes could be above you.
Summary
Detailed Forecast
The avalanche danger will increase to High by Tuesday afternoon. Expect very dangerous conditions due to new snow, wind, and rain at low elevations. Large natural avalanches are likely at upper elevations. Be aware of overhead hazard from steep slopes above you. Avoid terrain where avalanches can start or run from above. It will be easy to trigger Wind Slab avalanches on slopes over 35 degrees near ridges or in cross-loaded terrain features. Watch for clues like blowing snow, variable snow height, drifts, cornices, and stiff snow that produces cracking. These are all indicators that you should avoid avalanche terrain today.
Rain will fall below treeline Tuesday afternoon. Expect natural Loose Wet avalanches where rain is falling on previously dry snow. On large, steep slopes some of these avalanches could be big enough to bury you. You may be able to trigger Storm Slabs in the morning on steep slopes near and below treeline where the storm snow slides easily. For indicators to either of these types of avalanches use quick tests and small test slopes to check how easily the new snow slides.
It has been more than two weeks since the last reported avalanche involving older weak layers. Deep Persistent Slab avalanches are now very unlikely. The weak snowpack structure could be become a problem later in the spring.
Snowpack Discussion
Expect the most new snow on Tuesday in the Snoqualmie and Stevens Pass areas. Similar avalanche conditions will exist near Paradise, while the Crystal Mountain area will see less snow and less elevated danger. Most areas picked up 6-9" of snow by 5p Monday. This past weekend, about 1 foot (35cm) of new snow fell near Crystal, Paradise, and Stevens and Snoqualmie Passes. Observers reported a range of avalanche activity over the weekend including triggered avalanches in wind loaded terrain near Mt Baker and the Crystal backcountry and numerous small natural slab avalanches in the upper storm snow near Snoqualmie Pass.
Snow from the past week sits on the 3/22 interface. On sunny and lower elevation slopes you will find a crust at this interface. In some locations (especially near Crystal Mountain) you may find this week's snow resting on a layer of small facets or surface hoar. Below the top 2 feet (60cm) the snowpack is generally well-rounded, strong, and lacking interfaces of concern.
An old, thin layer of sugary facets (2/13) can still be found in some locations just above a very firm and wide spread crust (2/8). The overall depth (1.5-2 meters) and significant rounding (melt-freeze cycles in some locations) has allowed this structure to drop off the official list of avalanche problems. NWAC professionals will continue to track this weak layer as it could be a problematic later in the spring.
Observations
Stevens Pass
On Monday NWAC Forecaster Josh Hirshberg traveled in the Stevens Pass backcountry. Josh reported 4 inches (10-15cm) of new snow by mid-day. Over a foot of snow sat on a crust formed by rain last week. Snow from the past week was generally well bonded below treeline. Josh observed minor cracking in sheltered areas, but no new avalanches.
Snoqualmie Pass
NWAC staff were in the Snoqualmie Pass backcountry Saturday. They found 12" (35cm) of low density new snow. Wind affected snow was only observed in isolated features near ridge tops. Good visibility in the afternoon allowed them to see across the area. Numerous small slab avalanches were reported below treeline in steep terrain.
Crystal Backcountry
On Monday, NWAC Observer Jeremy Allyn investigated recent avalanches on SE and NW aspects around 6400ft in the Crystal backcountry. Jeremy reported that both avalanches ran on thin weak layers just over a foot below the surface. A nearby profile on a west aspect produced propagating test results.
On Saturday NWAC Forecaster Dallas Glass reported multiple natural and triggered Wind Slab avalanches on North, East, and Southwest aspects above 5,500 ft. The largest was almost 2 feet deep and involved all of the snow that fell since Wednesday. Dallas also reported sudden test results on the 2/13 facets over 6 feet below the snow surface.
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: Below Treeline.
Likelihood
Expected Size
Valid until: Mar 27th, 2018 12:03PM
