Cloud cover and a slow rise in freezing levels Friday should help limit the avalanche danger to steeper slopes. Small loose wet avalanches are possible on steeper slopes involving snow received during the week. Be especially wary of loose wet avalanches near terrain traps Friday.
Summary
Detailed Forecast
Cloud cover and a slow rise in freezing levels Friday should help limit the avalanche danger to steeper slopes. Small loose wet avalanches are possible on steeper slopes involving snow received during the week. 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 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. Over this time period the snowpack has undergone significant settlement with lower elevation snow beginning to fade away and with an overall transition to a spring snowpack.
On Tuesday afternoon and night a frontal system brought a few inches of snow, with locally 7-12 inches in the Mt. Baker area. A low pressure system brought another round of snow Wednesday night and Thursday, with generally several inches of new snow seen on both sides of the Cascade crest. New snow has generally settled rapidly with both these systems and wind transport of new snowfall has been minimal.
After a relatively cool week, the upper snowpack has generally drained and refrozen near and above treeline and on non-solar aspects. However it's quite variable how supportable the underlying crust is throughout the terrain. At lower elevations and on solar aspects, wet grains remain dominant in the upper snowpack.
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.
The mid and lower snowpack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.
Recent Observations
An observation from the NWAC observation page Wednesday April 13th from the Mt. Baker area showed some skier triggered loose wet avalanches involving the new storm snow and running well.
DOT professionals in the Chinook Pass area reported small natural avalanches on Wednesday involving the new storm snow. Below 5000 feet on solar aspects, the snowpack had not refrozen and loose wet avalanches could easily gouge down to deeper wet grains on steeper slopes.
NWAC observers Dallas and Lee were out on Thursday April 14th. Dallas was in the Skyline area of Stevens Pass area and found the 7 inches of new snow from Thursday morning had quickly settled to 3 or 4 inches. Recent snow was well bonded to the previous crust. Dallas was able to ski trigger small loose wet avalanches on steep rollovers even on non-solar aspects.
Lee was in the Bagley and Anderson Creek drainages in the Mt. Baker area and found sun crusts on solar aspects and up to 30 cm of new snow on lee aspects above 5000 feet. The new snow was also well bonded here, with no significant storm snow instabilities noted.
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
