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 near 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 with 2-8 inches of new snow reported at NWAC stations through early Saturday morning. The new snow was generally on the heavier side and was accompanied by a slight warming trend Friday.
A much stronger front Saturday night delivering a welcome 1-2 feet of snow across the west slopes through mid-day Sunday. Showers, with further cooling Monday have deposited an additional 5-10 inches of snow as of Monday evening. Light snow showers Monday night and early Tuesday with light winds added a few more inches of low density snow to the already low density snowpack! The most recent snow has not changed the overall danger, that continues to slowly improve.
These latest storm systems have ushered in the coldest air mass of the season. The recent storm snow amounts since Saturday remain rather impressive, with 2-3 feet or more in most areas! We've been hearing the term, "as good as it gets", frequently!
Given the favorable snowpack profile, lack of deeper layers of concern as well as good bonding to the older snow, avalanche problems should be confined to the most recent storm snow at this time.
With deep unconsolidated snow in most areas, there is an increased risk for tree well and snow immersion suffocation at this time. Ride or ski with a partner and keep them in sight at all times!
Recent Observations
Multiple professional reports Monday and again Tuesday from the Stevens and Snoqualmie Pass areas, and above Paradise on Mt Rainier, indicating that the storm related sensitivity experienced Sunday had improved greatly by Monday and following that trend again Tuesday. No slab like character was noted with a favorable density profile providing excellent surface snow conditions. These are areas where sensitive human triggered soft slab conditions were reported Sunday, indicating an improving trend in conditions.
Despite the lower density snowfall and a generally right-side up layering of the new snow, sensitive and widespread soft storm slabs were reported Sunday by the Baker pro-patrol throughout the terrain near and below treeline in the Mt. Baker area. Crystal patrol reported widespread loose dry avalanches Sunday running fast on steep terrain, both natural and human triggered. Wind effects were relegated to immediately below ridgelines but there was plenty of new snow available for transport. Stevens Pass pro-patrol found sensitive slabs on lee aspects near treeline during morning control Sunday. Most slabs released on storm layers averaging 10-12" deep. Alpental pro-patrol found 12-18" soft slabs on a few wind-loaded slopes near treeline Sunday afternoon. These conditions have been improving markedly over the past two days, but remain cautious in terrain near ridges where wind slabs could still be triggered.
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
