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Archived

Avalanche Forecast

Dec 11th, 2015–Dec 12th, 2015
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
3: Considerable
The avalanche danger rating below treeline will be considerable
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

Regions: Olympics.

Expect deteriorating snow stability as the day progresses and the storm really takes hold. Travel conservatively by staying off steeper slopes and avoiding wind loaded northerly terrain, especially later in the day,

Detailed Forecast

Snow will begin by mid-morning and increase in intensity throughout the day. In addition, there should be a gradual warming trend as snow levels peak between around 4500 feet in the late afternoon. Combined with increasing southerly transport winds in the afternoon, there will be an increasing avalanche risk through the daylight hours. 

Storm slabs should become more sensitive and likely to trigger in the afternoon. Northerly lee slopes will continue to be loaded throughout the day as well.  

Expect deteriorating snow stability as the day progresses and the storm really takes hold. Travel conservatively by staying off steeper slopes and avoiding wind loaded northerly terrain, especially later in the day,

Be aware of early season hazards below treeline. The heavy rains from last week have left many creeks open at lower elevations. 

Snowpack Discussion

After a wet week that culminated with heavy rain Monday and Tuesday knocking the Waterhole snowdepth (5000 ft) down to around 18 inches, we have returned to colder snow levels with new snow. The NPS ranger at Hurricane Ridge reported a snowdepth of 34 inches Friday morning with 14 new inches received since Tuesday. 

Avalanche and Snowpack Observations

NWAC pro-observer Matt Schonwald was at Hurricane Ridge Friday, taking the first set of snowpack observations in the Olympic zone for NWAC this season.  Matt found a snowdepth averaging around 1 meter (over 3 ft) with slightly more snow in less wind affected areas. The most recent snow has bonded well to the old surface and there were no significant storm snow instabilities. However, there were signs of significant wind loading onto open lee NE slopes, mainly applicable to the near and above treeline elevation bands.  Below treeline should still contain ample terrain anchors, but above 4000'-4500' there is enough snow to allow avalanches in specific areas. 

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, North West.

Elevations: All elevations.

Likelihood: Likely

Expected Size: 1 - 1

Storm Slabs

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Likely

Expected Size: 1 - 1