Avalanche Forecast
Issued: Dec 7th, 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 lot is going to start happening on Thursday and regional timing of weather and avalanche conditions will be tricky.
Summary
Detailed Forecast
A lot is going to start happening on Thursday and regional timing of weather and avalanche conditions will be tricky.
Southeast winds at exposed locations in the Cascades should continue to increase Thursday in advance of an incoming front.
Watch for wind transport and new wind slab on unusual southwest to northeast aspects on Thursday due to southeast and east winds. Regionally it doesn't sound like there is much if any old wind slab out there so the southwest to northeast aspects along the east slopes will be highlighted for new wind slab on Thursday. But watch for firmer wind transported snow on all aspects since confidence is not high on which aspects to highlight. The slight warming trend could enhance the formation of new wind slab on Thursday.
The front with increasing snow and a slight warming trend should begin to lift south to north over the Northwest starting in the south at Mt Hood by Thursday afternoon and reaching the Olympics and Washington Cascades by Thursday night. Be prepared to change your plans if the front and significantly increasing snow arrives earlier than expected along the east slopes during the day on Thursday.
In steep sheltered terrain Thursday continue to 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, you will need to watch for early season travel hazards such as barely covered rocks and open creeks.
Further new wind and storm slab likely Thursday night. The warming trend should enhance the formation of these layers.
Snowpack Discussion
Weather and Snowpack
Active snowy weather has been seen so far in December. Most stations along the east slopes had about 1-1.5 feet of snow so far in December through Tuesday morning with an overall cooling trend.
Cold fair weather has been seen the past couple days. There have been many reports of right side up, favorable density profiles with lower density snow nearer the surface. We've been hearing the term "as good as it gets" quite a lot lately!
Given the favorable snowpack profile, good bonds of older snow layers and a lack of deeper layers of concern, avalanche problems should be confined to the most recent storm snow at this time.
Cold fair weather like this usually causes some new surface hoar layer and near surface faceted snow. This snow can make for great skiing or riding. But these layers will need to be watched if loaded by wind transported snow or new snow.
Recent Observations
During midweek last week 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.
The most recent observations from near Washington Pass Monday and Tuesday indicate great skiing, favorable snowpack profiles and good bonds of older snow layers.
While coverage in most 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. But the lower elevations in many areas lack good coverage and open creeks and other early season hazards remain.
In general, more snow is needed in the Mission Ridge area and southeast Cascades to present an avalanche danger. In wind-loaded terrain near and above treeline, no weak layers of note have been reported lately by the Mission pro-patrol.
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, South West, West, 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 8th, 2016 10:00AM
