Avalanche Forecast
Issued: Dec 6th, 2016 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
A slow improvement should continue, but locally increasing east winds near the crest may begin forming new sensitive slabs on lee, mostly westerly facing terrain, especially below ridges. In the steep wind sheltered terrain, watch for small loose dry avalanches near terrain traps like cliff bands where even small avalanches can have big consequences.
Summary
Detailed Forecast
Partly cloudy conditions with cold temperatures are expected through midday Wednesday. This weather should allow for continued very slow settlement of any wind slabs recent formed, allowing for a slow decreasing trend in the avalanche danger overall.
By midday Wednesday and into the afternoon, however, expect increasing easterly winds nearer the crest. Watch for locally increasing winds later Wednesday as the very low density surface snow will be easily transported to lee slopes or cross loaded features and shallow wind slabs may form quickly in exposed terrain.
In particular, watch for lingering or newly forming wind slab on lee aspects near and above treeline that may be sensitive to human trigger.
In steep terrain where the new snow lacks a slab structure, watch for small loose dry avalanches near terrain traps like cliff bands where even small avalanches can have big consequences.
Even though the lower part of the below treeline band is filling in, watch for early season travel hazards such as barely covered rocks and open creeks.
Snowpack Discussion
Weather and Snowpack
A front crossed the Cascades Friday, but likely only produced an inch or two of new snow near the Cascade crest with no accumulations further east. Moderate westerly ridgetop winds continued on Saturday.
A much stronger front crossed the area Saturday night, depositing 8-10 inches recorded from the Harts Pass and Lyman Lake Snotels respectively, 7 inches at Holden and Blewett Pass and 3-4 inches at Mission Ridge through Sunday morning. This system also ushered in the coldest air mass of the season. Cool light showers Monday through early Tuesday only added very light additional snow amounts, not affecting the danger.
Avalanche problems should be confined to the most recent storm snow and we are not tracking any weak layers in the lower or middle of the snowpack at this time.
Recent Observations
The most recent observations from near Washington Pass Sunday and again Monday, indicated a favorable snowpack profile with no triggered avalanches noted and good bonding to the older snow layers. Some minor wind effects were noted but most areas upper snowpack lacked a slab character, with mostly low cohesion powder providing excellent conditions!
During midweek NWAC pro-observer Jeff Ward was in the Washington Pass area and found generally good ski conditions and a stable snowpack. Stubborn and shallow wind slab existed near and above treeline.
While some of the coverage below treeline remains thin, the upper elevations of this lowest band are filling in quite impressively and we are bumping the overall danger to moderate in this band as a result. The lower elevations in many areas lack good coverage and open creeks and other early season hazards remain.
On Friday Jeff was out again at Jove Peak east of Stevens Pass, which can be representative of the eastern slopes closer to the Cascade crest. He saw no signs of instability, but noted minor wind transport along the ridge.
In general, more snow is needed in the Mission Ridge area to present an avalanche hazard. In wind-loaded terrain near and above treeline, no weak layers of note have been reported by the pro-patrol through Saturday.
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, 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 7th, 2016 10:00AM
