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 have allowed shallow new snow to accumulate above treeline, where wind deposited snow may create a localized increase in avalanche danger, mainly near ridges at higher elevations. The snowpack has consolidated and stabilized in lower elevations, but watch for shallow wet snow conditions during the afternoon hours.
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.
A high degree of uncertainty exists in the expected new snow amounts, so watch for heavier showers that may quickly build new snow layers and be prepared to change travel plans and remain on lower angled slopes, avoiding exposure to steeper terrain above.
At lower elevations watch for generally shallow wet snow conditions that may make small triggered Loose Wet avalanches possible on some steep slopes, generally over 35 degrees.
Snowpack Discussion
The snowpack is now well drained and has transitioned to spring-like conditions following last weeks very warm temperatures. The cooling and showery weather this weekend has allowed for consolidation and re-freezing surface snow, limiting the avalanche potential in most areas.
Some shallow wet surface snow conditions continue at lower elevations that lack a refreezing crust, especially during the warmest part of the day.
Sunshine and very warm temperatures last week over the Mt. Hood region have continued the transition to a fully spring-like snowpack of well drained large melt-freeze snow grains. Daytime high temperatures were in the 50's and 60's at mid-elevation stations much of last week through Friday.
While numerous layers can still be found within the snowpack at higher elevations, there are no significant layers of concern at this time.
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
