Avalanche Forecast
Regions: Olympics.
A lot is going to start happening on Thursday and regional timing of weather and avalanche conditions will be tricky.
Detailed Forecast
A lot is going to start happening on Thursday and regional timing of weather and avalanche conditions will be tricky.
South to southeast winds over the Olympics should continue to increase Thursday in advance of an incoming front.
Watch for wind transport and new wind slab on southwest to northeast aspects Thursday due to south to southeast winds. Regionally it doesn't sound like there is much if any old wind slab out there so just the southwest to northeast aspects in the Olympics will be highlighted for new wind slab. 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 during the day on Thursday.
Avoid ridges that may have a cornices and avoid slopes below cornices seen lately in the vicinity of Hurricane Ridge.
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.
Further new wind and storm slab formation is 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. The NWAC station at Hurricane indicates about 3 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 in the Cascades 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
The new snowfall received Saturday night through Monday was very low density but wind transport helped form some cornices along ridgelines. NPS rangers reported up to 2 natural slab avalanches in the Hurricane Ridge area that likely occurred Saturday night during the storm. One was in Maggie's Bowl, on an east or southeast aspect and ran at least 100 feet. The slab depth was estimated around 10 inches.  Later Sunday NPS rangers reported most of the avalanche activity revolved around loose dry avalanches in steep less wind affected areas below ridges. There were isolated reports of human triggered soft slab releases in wind loaded terrain that released down to the old snow surface.Â
Wind slabs formed during recent storms have now had a few days to slowly settle and stabilize. We have been seeing these isolated wind slabs become less reactive to trigger in the Cascades over the past few days and we expect similar recent improvement in the Olympics.
Avalanche 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: Possible
Expected Size: 1 - 1
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East, East, South East.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
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: Possible
Expected Size: 1 - 1