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Archived

Avalanche Forecast

Dec 4th, 2016–Dec 5th, 2016
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Olympics.

In the Hurricane Ridge area, avoid easily triggered slab avalanches on wind loaded slopes mainly near and above treeline by sticking to lower angled slopes and windward aspects.

Detailed Forecast

Sunday morning update: 

In the Hurricane Ridge area, avoid easily triggered slab avalanches on wind loaded slopes mainly near and above treeline by sticking to lower angled slopes and windward aspects. In areas where the new snow does not have a slab structure, watch for small loose dry avalanches near terrain traps like cliff bands where even small avalanches can have big consequences. 

Previous Discussion: 

A strong frontal system will quickly sweep through the Olympics Saturday night. This system should deliver around 6 inches of snow to Hurricane Ridge. A cooling trend beginning Saturday night will continue through Sunday as shower activity quickly wanes later Sunday morning. 

Despite a favorable cooling trend with this storm, westerly winds will build shallow wind slab on lee aspects near and above treeline. Look for wind slab development further downslope than you might expect due to locally strong winds and persistent loading. Watch for firm wind transported or hollow sounding snow. 

Avoid ridges that have a cornice and slopes below cornices.

In most of the lower part of the below treeline band there is still insufficient snow for avalanches but watch for early season hazards such as terrain traps, rocks and creeks.

Snowpack Discussion

Sunday morning update: 

New snow at the manual snowdepth plot at Hurricane Ridge measured 13" this morning. The new snowfall was low density but wind transport and new cornice building was visible along ridgelines. NPS rangers reported up to 2 natural slab avalanches in the Hurricane Ridge area that likely occurred overnight during the storm. One was in Maggie's Bowl, on an east or southeast aspect and ran at least 100 ft. The slab depth was estimated around 10". 

Weather and Snowpack

A front crossed the Olympics Friday with an inch of new snow reported by NPS rangers at Hurricane Ridge through Saturday morning. Post-frontal showers were generally light to moderate on Saturday and ridgetop winds were moderate out of the south. With the mild snow levels seen so far this season, most avalanche problems have been confined to the most recent storm layers found in the upper snowpack.  

Recent Observations

NWAC pro observer Matt Schonwald visited Hurricane on Thursday and reported widespread wind effects but a generally well bonded snowpack and snow profiles in pits that lacked a slab structure. The snow was found to be unconsolidated at lower elevations making terrain traps and creek crossings a hazard.

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, East, South East, North West.

Elevations: All elevations.

Likelihood: Likely

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.

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