Very warm temperatures and sun will create dangerous avalanche conditions on Friday. You can trigger large wet avalanches and cornices. Plan your travel to be out of avalanche terrain before afternoon.
Summary
Detailed Forecast
The avalanche danger will increase to Considerable by late Friday morning. Due to very warm temperatures and strong sun, you can trigger large wet avalanches. Plan your day to be out of avalanche terrain before the snow becomes wet and unsupportive.
You can easily trigger Loose Wet avalanches on most slopes over 35 degrees. Use extra caution on large, steep, and unsupported slopes where you may be able to trigger a large avalanche. If you sink in wet snow up to the top of your boots, avoid slopes over 35 degrees. Wet Slabs can be dangerously large and are difficult to predict. These avalanches may be difficult for a single person to trigger, but could occur naturally with cornice fall. Wet snow resting on a weak layer or crust is a sign that you can expect Wet Slab conditions. Limit your exposure to large start zones and overhead avalanche paths suspect for Wet Slab avalanches.
Give cornices a wide buffer. They will break naturally and you could easily trigger them. Cornices will often break surprisingly far back from the edge. Make sure you are well off and out from under cornices, especially as the sun is shining on them. Cornice fall could be big enough to kill you and could trigger other avalanches.
Snowpack Discussion
A long stretch of warm weather and strong sun has ushered in spring-like avalanche concerns. An extensive cycle of Loose Wet avalanches and cornice falls occurred this week throughout the Cascades. As the warm temperatures have continued loose avalanches have been moving to higher elevations and more polar (northerly) aspects. Slab avalanche activity due to warming has been occurring, though not as widespread or showing major patterns.
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. On Thursday, avalanche workers on Washington Pass triggered multiple slab avalanches up to size 2.5. on north and east aspects around 7,500ft. Nearby, 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. There have not been similar reports on the east side of the crest.
On Thursday, Forecaster Dallas Glass reported a natural cornice fall as well as easily triggering Loose Wet avalanches near the Alpental Valley. Dallas reported that a thin surface crust quickly softened in the morning.
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
Wet Slabs
Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slab avalanches can be very destructive.
Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty
A Wet Slab avalanche. In this avalanche, the meltwater pooled above a dusty layer of snow. Note all the smaller wet loose avalanches to either side.
Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Apr 27th, 2018 11:00AM
