Even with some sunshine along the east slopes, freezing levels should be slow to rise Friday and help limit the avalanche danger to steeper slopes. Small loose avalanches are possible on steeper slopes involving snow received during the week. Only loose wet avalanches will be listed, but loose dry avalanches will be possible on non-solar aspects at higher elevations. Be especially wary of loose wet avalanches near terrain traps Friday.
Summary
Detailed Forecast
Skies may start out partly sunny Friday with increasing high clouds likely in the afternoon as a frontal system heads into British Columbia. Even with some sunshine along the east slopes, freezing levels should be slow to rise on Friday and help limit the avalanche danger to steeper slopes. Small loose avalanches are possible on steeper slopes involving snow received during the week. Only loose wet avalanches will be listed, but loose dry avalanches will be possible on non-solar aspects at higher elevations.
Watch for wet snow deeper than your boot tops, especially on steep solar slopes during the late morning to afternoon hours. Even if skies remain mostly cloudy, strong solar input in mid-April will affect the snow surface on solar slopes.
Storm and winds slabs will not be listed due to the right-side up layering and quicker settlement rates seen during spring storms. However, be aware of the potential for isolated slab avalanches involving the new snow mainly on non-solar aspects of higher terrain.
The potential for cornice releases should be low Friday but will increase through the weekend. Cornice releases can be unpredictable during the spring so avoid areas below cornices and remember that cornices can break much further back than expected along ridges.
Although not listed as an avalanche problem, large, powerful glide avalanches can release unexpectedly so avoid areas below steep unsupported slopes or rock faces showing glide cracks.
Snowpack Discussion
Weather and Snowpack
We've had two big warm-ups and spring shed cycles since the end of March. As we transition into spring, colder snow still remains in the upper snowpack on non-solar aspects of higher terrain in the northeast Cascades while many solar aspects along the east slopes away from the Cascade crest have mostly or completely melted out.
On Tuesday afternoon and night a frontal system brought a dusting of new snow along the east slopes. A low pressure system passing into Oregon brought a more substantial round of snow Wednesday night and Thursday, with several inches of new snow seen on both sides of the Cascade crest and light snowfall still accumulating Thursday afternoon in the Washington Pass area. Wind transport of new snowfall has been minimal during the week.
Frequent March storms built unusually large cornices along many ridges. The potential for low probability/high consequence encounters, such as cornice failures and glide avalanches from steep unsupported slopes and smooth rock faces will continue through the spring.
Recent Observations
Public observations from last weekend reported large avalanches running down to closed SR 20 and also in the surrounding Washington Pass backcountry. These large avalanches were likely triggered by natural cornice fall during the end of the warm spell and may have stepped down to older weak layers or interfaces from late winter.
Avalanche professionals working in the Washington Pass area during the week reported a quiet week regarding natural avalanche activity. Colder snow remained intact on non solar aspects around 6000 feet. Glide cracks and moats near rocks were reported as large for this time of year, likely due to the unusual warmth seen earlier this month.
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: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Apr 15th, 2016 11:00AM
