Avalanche Forecast
Issued: Apr 28th, 2018 11:00AM
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
Cooling and showers are depositing shallow new snow above treeline, where wind deposited snow may create a localized increase in avalanche danger, mainly near ridges at higher elevations. You could still trigger Loose Wet avalanches on steep slopes where you find soft wet snow. Avoid being on or under cornices.
Summary
Detailed Forecast
Cool and showery weather will continue Sunday. Some shallow new snow is likely to accumulate above treeline Sunday and while expected winds should be light, any locally stronger winds may drift new snow and build some shallow wind slabs, mainly near ridges on lee slopes.
At lower elevations, daytime warming and possible rain showers will maintain shallow wet snow conditions Sunday where you could still trigger Loose Wet avalanches on steep slopes. While they are generally smaller and easier to predict, Loose Wet avalanches can be dangerous if they push you off cliffs, through rocks, or into gullies.
Cornices and Glide avalanches have been active in the past week. The cooling trend may take an additional few days to have an effect on these avalanche problems. Avoid stopping under known rock slabs and slopes with visible glide cracks. Continue to put a wide buffer of terrain between you and any cornices. They will often break surprisingly far back from the edge. Make sure you are well off of and out from under cornices as you travel through the mountains.
Snowpack Discussion
The cooling temperatures, clouds, and light precipitation this weekend is causing the previously wet and weak surface snow to gain strength as it freezes. A long stretch of warm weather and strong sun ushered in spring-like conditions last week. An extensive cycle of Loose Wet avalanches, cornice falls, and Wet Slabs occurred in the past week throughout the Cascades.
Generally, Wet Slabs or slab avalanches entraining wet snow in the past week have occurred in the upper snowpack with a few deeper releases reported. In the Mt Baker area and North Cascades National Park west of the Cascade Crest observers have reported a significant cycle of slabs 3-6 feet (1-2 meters) deep. One observer reported probing a widespread weak layer 3 meters below the surface on the Silver Glacier.
On Friday, Observer Matt Primomo toured near Pashastin Creek South of Leavenworth. Matt found a shallow snowpack of less than 5 feet deep and reported numerous recent Loose Wet avalanches. While the upper snowpack was moist to wet and not well frozen, it remained supportive to skis.
On Thursday, avalanche workers on Washington Pass triggered multiple slab avalanches up to size 2.5 on north and east aspects around 7,500ft.
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.
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 29th, 2018 11:00AM
