Avalanche Forecast
Issued: Dec 27th, 2015 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Dry.
NWAC,
Northwest Avalanche Center
Be wary of wind slabs on NW through SE aspects Monday, mainly near and above treeline. Remember MODERATE danger means human triggered avalanches are possible. Recent unconsolidated storm snow is still very deep in places with possible large tree wells lurking, so stick near your partner in the trees.
Summary
Detailed Forecast
Wind slab will most likely be found on northwest to southeast aspects near and above tree line. NW winds will redistribute new snowfall to lee SE slopes near and above treeline Monday.
Wind and storm slab avalanches should stay within recent storm layers but colder temperatures along the east slopes of the Cascades will help slow bonding of newly formed wind slabs to the weaker layers underneath. Test for inverted strong over weak storm snow and give cornices a wide margin.
Beware of loose dry avalanches in steep non-wind affected terrain that could have unintended consequences such as knocking you off your feet and into a terrain trap.
There have been snow immersion fatalities in tree wells already this season at Snoqualmie and in Canada. Recent unconsolidated storm snow is still very deep in places with possible large tree wells lurking, so stick near your partner in the trees and maintain visual contact.
Snowpack Discussion
For the week ending on Dec. 24th, a snowy and cool storm cycle brought 2-4 feet of snowfall to the east slopes. Only light additional accumulations over the last few days have allowed the snowpack to gradually settle, but it remains deep out there with Washington Pass area snowdepths over 2 meters and fine riding conditions reported! The upper snowpack should generally be right side up with increasing hardness with depth. Moderate southerly transport winds in the alpine with Sunday's system may have built new wind slab on NW through E aspects.
A snow pit report from 5700-6500 foot range in Stemilt Basin near Mission Ridge also on Tuesday indicated a buried surface hoar at 45 cm. This layer gave some cracking and whumping. There have not been any recent reports from the Mission Ridge area.
Storm instabilities reported last week have likely settled out, but the North Cascades Mountain Guides reported on Christmas Day, a buried surface hoar layer in the Washington Pass area around 40 cm down that gave sudden planar (pop!) results in compression tests, but was not found to be involved in any natural or skier triggered avalanches. This layer probably needs more of a cohesive slab above it before it becomes apparent whether this is an PWL issue or not.
The southeast zone has a relatively shallower snowpack versus areas further north but after a snowy few weeks, has more than enough snow to warrant an avalanche risk. No snowpack observations have been received from this zone.
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: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Loose Dry
Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.
Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.
Loose Dry avalanche with the characteristic point initiation and fan shape.
Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 28th, 2015 10:00AM
